Research Areas

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Center for Macromolecular Engineering

CRP Consortium

ATRP Consortium
(1996–2000)

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• Homopolymers
  
• Linear Copolymers
  
  • Statistical copolymers
    
  • Gradient copolymers
    
  • Alternating copolymers
    
• Copolymerization of Complexed Monomers
  
  • Preparation of alternating copolymers
    
  • Tacticity control
    
• Block Copolymers
  
  • AB and ABA
    
  • Multiblock
    
• Graft Copolymers
  
  • a) Grafting through
    
  • b) Grafting from
    
  • c) Grafting to
    
• Graft Copolymers with Complex Architectures
  
  • Densely grafted copolymers
    
    • Linear macromolecular brushes
      
    • Star brush copolymers
      
    • Brush block copolymers
      
    • Heterografted brush copolymers
      
    • Double grafted copolymers
      
• Star Copolymers
  
• Hyperbranched Structures
  
• Networks/Gels
  
• Cyclic

Well-defined Copolymers

The spectrum of polymers that can be prepared by a well controlled ATRP presently includes polymers with any desired distribution in monomer units along the backbone and within any specific segment in a copolymer. This includes homopolymers, random copolymers, gradient copolymers block, graft and star copolymers; each of which is discussed below.

Homopolymers

There are still some limitations to the range of monomers that can be homopolymerized in an ATRP. The limitation is related to the requirement for repeated reactivation of the dormant species by the transition metal complex. With the current spectrum of catalysts there has to be an alpha-stabilizing substituent adjacent to the transferable atom or group in order that the dormant chain end can be reactivated. The initial range of monomers that could be polymerized by ATRP included styrenes, (meth)acrylates, meth(acrylamides), acrylonitrile and recently an improved ATRP for 4-vinyl pyridine, (1) and monomers containing an -OH group, such as HEA and HEMA, (2) glycidyl acrylate and ionic monomers. (3) Non-polar monomers have also been incorporated into ATRP reactions. (4, 5) See "Starting Points". The major differences between the polymers prepared by ATRP and prior art polymers prepared by a free radical polymerization are the additional degree of control over architecture, molecular weight, PDI and telechelic functionality provided by CRP.

A prior limitation on the maximum molecular weight of the (co)polymers that can be prepared by an ATRP has been surmounted by the development of ARGET ATRP which minimized outer sphere electron transfer (OSET) catalyst based side reactions (6) allowing the production of higher molecular weight polymers such as polyacrylonitrile with less apparent tailing. (7)

Linear Copolymers

A wide spectrum of copolymers can be prepared via spontaneous or sequential ATRP of two or more monomers with precise control of molar mass, composition and functionality. (8-10) The use of a difunctional initiator allows, for the first time in a radical process, preparation of functional homo-telechelic polymers. (11)

The reactivity ratio of comonomers in a CRP are very similar to the values found in free radical copolymerization although there some factors that affect CRP processes with intermittent activation, where differences in repetitive activation or deactivation and time for complete re-equilibration of the system, can result in different rates of consumption of comonomers. (12)

Random or statistical copolymers: can be prepared by one-pot ATRP of two monomers when there is essentially random incorporation of monomers into the copolymer. This type of copolymer is formed in radical copolymerization when the reactivity ratio of each comonomer is close to one. (9, 10) The chemical composition of statistical copolymers prepared by CRP is similar to that formed in a standard free radical copolymerization. (12) Since ARGET ATRP reduces catalyst based side reactions this has also enabled the synthesis of high molecular weight styrene/acrylonitrile copolymers (13) and improved incorporation of alpha-olefins into an acrylate polymerization. (14)

When there is preferential incorporation of one monomer in a CRP then spontaneous gradient copolymers are formed in a one pot reaction.

Gradient copolymers: are a special class of copolymer that rose to prominence with the development of ATRP (15, 16), since ATRP was the first CRP to allow copolymerization of a range of monomers of differing reactivity. In contrast to standard radical copolymerization where differences in comonomer reactivity ratios results in variation in instantaneous copolymer composition as the polymerization progresses in CRP this variation in the rate of incorporation of the monomers into the copolymer is reflected as a change, or tapering in composition of the monomer units along the main chain of the copolymer. Gradient copolymers can be prepared by one-pot copolymerization of a selected mole ratio of monomers with sufficiently reactivity ratios or by controlled feed of one or more monomers in a manner that forces the composition to be tapered. (15-19)

The shape of the gradient can also be changed by use of a difunctional initiator, which leads to a symmetrical "skipping rope" distribution of monomers along the backbone of the copolymer. Gradient copolymerization can be extended to the formation of graft copolymers with a gradient distribution of grafts, see below. (19, 21) Gradient copolymers can also be prepared in biphasic systems by spontaneous copolymerization of appropriate comonomers such as an acrylate and a methacrylate or by controlled addition of one monomer to an active miniemulsion (22) or ab initio ATRP emulsion polymerization where the added comonomer diffuses from the monomer droplets to the active micelle. (23) The rate of diffusion affects the gradient obtained in the copolymer.

Alternating copolymers: may also be prepared by CRP processes. They can be obtained from comonomers that have a spontaneous tendency for alternation: such as a strong electron accepting monomer (maleic anhydride or N-substituted maleimides) and an electron donating monomer (styrene). (24-26) Monomers without this inherent tendency towards alternation may also be copolymerized in an alternating fashion by performing CRP in the presence of Lewis acids such as diethyl aluminum chloride or ethyl aluminum sesquichloride. (27, 28) Polymerization of complexed monomers is discussed in greater detail below.

(1) “(1) "Rational Selection of Initiating/Catalytic Systems for the Copper-Mediated Atom Transfer Radical Polymerization of Basic Monomers in Protic Media: ATRP of 4-Vinylpyridine," Tsarevsky, N. V.; Braunecker, W. A.; Brooks, S. J.; Matyjaszewski, K. Macromolecules 2006, 39, 6817-6824.

(2) "Synthesis of poly(2-hydroxyethyl methacrylate) in protic media through atom transfer radical polymerization using activators generated by electron transfer," Oh, J. K.; Matyjaszewski, K. Journal of Polymer Science, Part A: Polymer Chemistry 2006, 44, 3787-3796.

(3) "Controlled synthesis of polymers with ionic or ionizable groups using atom transfer radical polymerization" Tsarevsky, N. V.; Matyjaszewski, K. ACS Symposium Series 2006, 937, 79-94.

(4) "Alternating copolymers of methyl acrylate with isobutene and isobutyl vinyl ether using ATRP," Coca, S.; Matyjaszewski, K. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1996, 37, 573-574.

(5) "Olefin Copolymerization via Controlled Radical Polymerization: Copolymerization of Methyl Methacrylate and 1-Octene," Venkatesh, R.; Klumperman, B. Macromolecules 2004, 37, 1226-1233.

(6) "Electron transfer reactions relevant to atom transfer radical polymerization," Tsarevsky, N. V.; Braunecker, W. A.; Matyjaszewski, K.; J. Organomet. Chem. 2007, 692, 3212-3222.

(7) "Well-Defined High-Molecular-Weight Polyacrylonitrile via Activators Regenerated by Electron Transfer ATRP," Dong, H.; Tang, W.; Matyjaszewski, K. Macromolecules 2007, 40, 2974-2977.

(8) "How to make polymer chains of various shapes, compositions, and functionalities by atom transfer radical polymerization"; Gaynor, S. G.; Matyjaszewski, K., ACS Symp. Ser. 1998, 685, 396.

(9) "Statistical, gradient, block, and graft copolymers by controlled/living radical polymerizations"; Davis, K. A.; Matyjaszewski, K., Advances in Polymer Science 2002, 159, 2-166.

(10) "Synthesis of Block, Statistical, and Gradient Copolymers from Octadecyl (Meth)acrylates Using Atom Transfer Radical Polymerization", Qin, S. et. al. Macromolecules 2003, 36, 8969-8977.

(11) "Synthesis of well-defined azido and amino-end functionalized polystyrene by atom transfer radical polymerization," Shipp, D. A.; Wang, J.-L.; Matyjaszewski, K. Macromolecules 1998, 31, 8005-8008.

(12) "Factors Affecting Rates of Comonomer Consumption in Copolymerization Processes with Intermittent Activation", Matyjaszewski, K., Macromolecules 2002, 35, 6773-6781.

(13) "Synthesis of High Molecular Weight Poly(styrene-co-acrylonitrile) Copolymers with Controlled Architecture, Pietrasik, J.; Dong, H.; Matyjaszewski, K. Macromolecules 2006, 39, 6384-6390.

(14) "Controlled Copolymerization of n-Butyl Acrylate with Nonpolar 1-Alkenes Using Activators Regenerated by Electron Transfer for Atom-Transfer Radical Polymerization," Tanaka, K.; Matyjaszewski, K. Macromolecules, 2007, 40, 5255-5260.

(15) "Gradient copolymers-a new class of materials," Greszta, D.; Matyjaszewski, K. Polymer Preprints, 1996, 37, 569-570.

(16) Matyjaszewski, K. et. al. US Patents 5807937; 6887962 & WO 9718247.

(17) "Gradient copolymers by atom transfer radical copolymerization" ; Matyjaszewski, K.; Ziegler, M. J.; Arehart, S. V.; Greszta, D.; Pakula, T., J. Phys. Org. Chem. 2000, 13, 775-786

(18) "Nitroxide-mediated miniemulsion polymerization of n-butyl acrylate: synthesis of controlled homopolymers and gradient copolymers with styrene." Farcet, C., Charleux, B., Pirri, R.: Macromolecular Symposia 2002, 182: 249-260.

(19) "Molecular Brushes with Spontaneous Gradient by Atom Transfer Radical Polymerization," Lee, H.-I.; Matyjaszewski, K.; Yu, S.; Sheiko, S. S. Macromolecules 2005, 38, 8264-8271.

(20) "Synthesis of Molecular Brushes with Gradient in Grafting Density by Atom Transfer Polymerization"; Boerner, H. G.; Duran, D.; Matyjaszewski, K.; da Silva, M.; Sheiko, S. S., Macromolecules 2002, 35, 3387-3394.

(21) "Tadpole Conformation of Gradient Polymer Brushes" Lord, S. J.: et. al., Macromolecules 2004, 37, 4235-4240.

(22) "Preparation of gradient copolymers via ATRP using a simultaneous reverse and normal initiation process. I. spontaneous gradient;" Min, K. E.; Li, M.; Matyjaszewski, K. Journal of Polymer Science, Part A: Polymer Chemistry 2005, 43, 3616-3622.

(23) "Preparation of gradient copolymers via ATRP in miniemulsion. II. Forced gradient," Min, K.; Oh, J. K.; Matyjaszewski, K. Journal of Polymer Science, Part A: Polymer Chemistry 2007, 45, 1413-1423.]

(24) Matyjaszewski, K. et. al. In PCT Int. Appl.; (CMU, USA). WO 9718247, 1997.

(25) "Radical copolymerization of maleic anhydride and substituted styrenes by reversible addition-fragmentation chain transfer (RAFT) polymerization." Davies, M. C., Dawkins, J. V., Hourston, D. J.: Polymer 2005, 46: 1739-1753.

(26) "Alternating copolymers of methyl acrylate with isobutene and isobutyl vinyl ether using ATRP," Coca, S.; Matyjaszewski, K. Polym. Prepr. 1996, 37, 573-574.

(27) "Synthesis of Well-defined Alternating Copolymers Poly(Methyl Methacrylate-alt-Styrene) by RAFT Polymerization in the Presence of Lewis Acid", Kirci, B.; Lutz, J-F.; Matyjaszewski, K., Macromolecules, 2002, 35, 2448.

(28) "Properties of well-defined alternating and random copolymers of methacrylates and styrene prepared by controlled/living radical polymerization," Denizli, B. K.; Lutz, J.-F.; Okrasa, L.; Pakula, T.; Guner, A.; Matyjaszewski, K. Journal of Polymer Science, Part A: Polymer Chemistry 2005, 43, 3440-3446.

Copolymerization of Complexed Monomers

Preparation of alternating copolymers: Until recently CRP has been less successful at attaining control over chain microstructure in terms of sequence distribution and tacticity since, due to the radical nature of the propagation step, the chemoselectivity (reactivity ratios), regioselectivity (proportions of head to head units) and stereoselectivity (tacticity) of polymers formed in a CRP are similar to those in conventional radical polymerization. Recently the controlled alternating copolymerization of a donor monomer (styrene) and an acceptor monomer (alkyl methacrylate or alkyl acrylate) complexed with a Lewis acid were examined using several CRP processes, including ATRP, NMP, iodide degenerative transfer polymerization (IDTP), and RAFT.

Complex formation increases the electron-accepting ability of the acceptor monomer, which strongly enhances the cross-propagation rate constants and thereby increases the tendency for alternating copolymerization. RAFT polymerization was found to be the most versatile system. The combination of RAFT and Lewis acids complexation techniques allowed synthesis of well-defined poly(styrene-alt-methyl methacrylate) copolymers with controlled mol. wt. (up to Mn = 70,000 g mol-1), low polydispersities (Mw/Mn < 1.3), and controlled comonomer sequences (approx.90% of alternating triads).

These results were obtained in the presence of diethylaluminum chloride and ethylaluminum sesquichloride. Moreover, the alternating copolymers obtained retain chain end functionality and were used as macroinitiators for the synthesis of well-defined diblock copolymers poly(methyl methacrylate-alt-styrene)-b-polystyrene. (29)

Tacticity control: Generally, free radical addition reactions or homolytic chain propagation reactions are not stereoselective, however Okamoto determined that addition of Lewis acids can provide stereocontrol in the radical polymerization of acrylic monomers. (30, 31) As noted above, conditions for CRP in the presence of Lewis acids had been developed. Therefore, all three major controlled/living radical polymerization processes, ATRP, RAFT and NMP, were investigated for the polymerization of N,N-dimethylacrylamide in the presence of Lewis acids known to enhance isotacticity, such as yttrium trifluoromethanesulfonate (Y(OTf)3) and ytterbium trifluoromethanesulfonate (Yb(OTf) 3). (32)

Poly(N,N-dimethylacrylamide) with controlled MW, low polydispersity, and a high proportion of meso dyads (approx.85%) was prepared using ATRP (methyl 2-chloropropionate/CuCl/Me6TREN) and RAFT (with cumyl dithiobenzoate transfer agent) in the presence of Y(OTf)3. The combination of NMP (using N-tert-butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide, SG1) and a Lewis acid complexation technique led to less precise control over chain architecture and microstructure (approximately 65% meso dyads), than RAFT/Y(OTf)3 or ATRP/Y(OTf)3. The latter two systems were used for the first one-pot synthesis of stereoblock copolymers by radical polymerization. Well-defined stereoblock copolymers, atactic-b-isotactic poly(N,N-dimethylacrylamides), were obtained by adding Y(OTf)3 to either an ongoing RAFT or ATRP polymerization, started in the absence of the Lewis acid. (33)

This was later extended to iron catalyzed polymerization of acrylamides in the presence of Lewis acids then to the use of triple hydrogen bonding for stereospecific polymerization of a DAD monomer which provided simultaneous control of tacticity and molecular weight. (34)

(29) "Synthesis of Well-Defined Alternating Copolymers by Controlled/Living Radical Polymerization in the Presence of Lewis Acids." Lutz, J.-F., Kirci, B., Matyjaszewski, K.: Macromolecules, 2003, 36, 3136-3145.

(30) "Stereocontrol in radical polymerization of acrylic monomers," Okamoto, Y., Habaue, S., Isobe, Y., Nakano, T.: Macrom. Symp., 2002, 183, 83-88.

(31) "Iron-catalyzed radical polymerization of acrylamides in the presence of Lewis acid for simultaneous control of molecular weight and tacticity." Sugiyama, Y.; Satoh, K.; Kamigaito, M.; Okamoto, Y. Journal of Polymer Science, Part A: Polymer Chemistry 2006, 44, 2086-2098.

(32) "Stereoblock Copolymers and Tacticity Control in Controlled/Living Radical Polymerization"; Lutz, J.-F.; Neugebauer, D.; Matyjaszewski, K., J. Am. Chem, Soc., 2003, 125, 6986-6993.

(33) "Controlled/living radical polymerization of methacrylic monomers in the presence of Lewis acids: Influence on tacticity"; Lutz, J.-F.; Jakubowski, W.; Matyjaszewski, K., Macromolecular Rapid Communications 2004, 25, 486-492.

(34) "Triple Hydrogen Bonding for Stereospecific Radical Polymerization of a DAD Monomer and Simultaneous Control of Tacticity and Molecular Weight." Wan, D.; Satoh, K.; Kamigaito, M. Macromolecules 2006, 39, 6882-6886.

Block Copolymers

are normally prepared by controlled polymerization of one monomer, followed by chain extension with a different monomer. (9) Multifunctional initiators can also be used in the process to prepare ABA or AB-star multi-armed block copolymers. (35, 36) Macroinitiators can be prepared by any polymerization process including free radical polymerization (37) and other controlled polymerization processes including cationic, (38) anionic, (39) ROP, (40) condensation (41) and post-metallocene catalysis (42) as long as the terminal functionality is or can be converted into an ATRP initiating moiety. Any type of ATRP catalyst/initiation system can be employed. (43) Targeted applications range from thermoplastic elastomers (44) to precursors for carbon nanostructures. (45) The properties of the materials targeting thermoplastic elastomers were significantly affected by the mode of (co)polymerization of the second block. (44) In a continuous ATRP copolymerization the fraction of first comonomer available for incorporation into the second block significantly changes the properties of the final material by preparation of a gradient copolymer segment in the second block that affected phase separation. (46) {link to properties}

The term "normally" was used in the first sentence to allow for the fact that another method for preparation of multi-segmented block copolymers has been developed based on "click" chemistry. (47, 48) ","-Diazido-terminated polystyrene-b-poly(ethylene oxide)-b-polystyrene was coupled with propargyl ether in DMF in the presence of a CuBr/N,N,N",N"",N""-pentamethyldiethylenetriamine (PMDETA) catalyst. The same catalyst could be used for both the formation of the first precursor block copolymer and the chain extended multiblock copolymer as shown below.

The strategy involves one or two chain extensions in a normal ATRP block copolymerization followed by end group transformation (47) and click coupling to obtain copolymers with up to 25 polymer segments in a single chain. (48) Formation of a product of higher molecular weight and broader molecular weight distribution was verified by triple-detection size exclusion chromatography, which revealed that typically 5"7 ABA block copolymers were linked together during the click chain extension reaction. Differential scanning calorimetry and dynamic mechanical analysis revealed that the amphiphilic block copolymer behaves as a viscoelastic fluid, while its corresponding multiblock copolymer is an elastic material. This would suggest that the polystyrene domains aggregate to form chemical cross-links in the swellable gel.

(35) "Preparation of Linear and Star-Shaped Block Copolymers by ATRP Using Simultaneous Reverse and Normal Initiation Process in Bulk and Miniemulsion," Li, M., Jahed, N, M., Min, K., and Matyjaszewski, K., Macromolecules, 2004, 37, 2434.

(36) "The Synthesis of Functional Star Copolymers as an Illustration of the Importance of Controlling Polymer Structures in the Design New Materials," Krzysztof Matyjaszewski, Polymer Int., 2003, 52, 1559.

(37) "Polymerization of Vinyl Acetate Promoted by Iron Complexes," Xia, J.; Paik, H.-j.; Matyjaszewski, K. Macromolecules 1999, 32, 8310-8314.

(38) "Block copolymers by transformation of \"living\" carbocationic into \"living\" radical polymerization. II. ABA-type block copolymers comprising rubbery polyisobutene middle segment," Coca, S.; Matyjaszewski, K. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 3595-3601.

(39) "Block copolymers by transformation of living anionic polymerization into controlled /\"living\" atom transfer radical polymerization," Acar, M. H.; Matyjaszewski, K. Macromol. Chem. Phys. 1999, 200, 1094-1100.

(40) "Block Copolymers by Transformation of \"Living\" Ring-Opening Metathesis Polymerization into Controlled/\"Living\" Atom Transfer Radical Polymerization," Coca, S.; Paik, H.-j.; Matyjaszewski, K. Macromolecules 1997, 30, 6513-6516.

(41) "Step-Growth Polymers as Macroinitators for \"Living\" Radical Polymerization: Synthesis of ABA Block Copolymers," Gaynor, S. G.; Matyjaszewski, K. Macromolecules 1997, 30, 4241-4243.

(42) "Preparation of polyethylene block copolymers by combination of post-metallocene catalysis of ethylene polymerization and atom transfer polymerization," Yoshihisa Inoue, Y.; and Matyjaszewski, K., J. Polym. Sci Polym. Chem. Ed., 2004, 42, 496"504.

(43) "Use of an Immobilized/Soluble Hybrid ATRP Catalyst System for the Preparation of Block Copolymers, Random Copolymers and Polymers with High Chain End Functionality", Hong, S. C.; Lutz, J-F.; Inoue, Y.; Strissel, C.; Nuyken, O.; and Matyjaszewski, K., Macromolecules, 2003, 36, 107.

(44) "Simple and effective one-pot synthesis of (meth)acrylic block copolymers through atom transfer radical polymerization"; Matyjaszewski, K.; Shipp, D. A.; McMurtry, G. P.; Gaynor, S. G.; Pakula, T., J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 2023-2031.

(45) "Nanostructured Carbon Arrays from Block Copolymers of Polyacrylonitrile"; Kowalewski, T.; Tsarevsky, N. V.: Matyjaszewski, K., J. Am. Chem. Soc., 2002, 124, 10632-10633.

(46) "Gradient copolymers by atom transfer radical copolymerization," Matyjaszewski, K., Ziegler, M. J., Arehart, S. V., Greszta, D., Pakula, T.; J. Phys. Org. Chem., 2000, 13: 775-786.

(47) "Catalyst Performance in \"Click\" Coupling Reactions of Polymers Prepared by ATRP: Ligand and Metal Effects," Golas, P. L.; Tsarevsky, N. V.; Sumerlin, B. S.; Matyjaszewski, K. Macromolecules 2006, 39, 6451-6457.

(48) "Multisegmented Block Copolymers by 'Click' Coupling of Polymers Prepared by ATRP," Golas, P. L.; Tsarevsky, N. V.; Sumerlin, B. S.; Walker, L. M.; Matyjaszewski, K. Australian Journal of Chemistry 2007, 60, 400-404.

Graft copolymers: belong to the general class of segmented copolymers and have been used as impact resistant materials, thermoplastic elastomers, compatibilizers or emulsifiers for the preparation of stable blends or alloys but their use has now expanded with the development of CRP. Well-defined graft copolymers are most frequently prepared by either a) a "grafting through" or b) a "grafting from" controlled polymerization process; however the development of "click" chemistry (49) has led to a third approach, c) based on site specific "grafting to" chemistry.

Graft copolymers: belong to the general class of segmented copolymers and have been used as impact resistant materials, thermoplastic elastomers, compatibilizers or emulsifiers for the preparation of stable blends or alloys but their use has now expanded with the development of CRP. Well-defined graft copolymers are most frequently prepared by either a) a "grafting through" or b) a "grafting from" controlled polymerization process; however the development of "click" chemistry (49) has led to a third approach, c) based on site specific "grafting to" chemistry.

Typically a low molecular weight monomer is radically copolymerized with a (meth)acrylate functionalized macromonomer. This method permits incorporation of macromonomers prepared by other controlled polymerization processes into a backbone prepared by a CRP. Macromonomers such as polyethylene, (50, 51) poly(ethylene oxide), (52) polysiloxanes, (53) poly(lactic acid), (54) polycaprolactone (55) have been incorporated into a polystyrene or poly (meth)acrylate backbone. Moreover, it is possible to design well-defined graft copolymers by combining the "grafting through" macromonomers prepared by any controlled polymerization process and CRP. (56) This combination allows control of polydispersity, functionality, copolymer composition, backbone length, branch length and branch spacing by consideration of mole-ratio of the MM in the feed and reactivity ratio. Branches can be distributed homogeneously or heterogeneously and, as shown in the properties section, this has a significant effect on the physical properties of the materials. (53, 54)

A series of segmented poly(alkyl methacrylate)-g-poly(D-lactide)/poly(dimethylsiloxane) terpolymers, with different topologies were prepared by employing a combination of the "grafting through" technique and CRP. (57) Two synthetic pathways were used. The first was a single-step approach in which a methacrylate monomer (methyl methacrylate or butyl methacrylate) was copolymerized with a PLA macromonomer and a PDMS macromonomer. The second strategy was a two-step approach in which a graft copolymer containing one macromonomer was chain-extended by a copolymerization of the second macromonomer and the low-molecular weight methacrylate. The molecular structure of the terpolymers was investigated by 2D GPC showing that well-defined terpolymers with controlled branch distribution were prepared via both pathways. The topologies of the graft terpolymers prepared by different combinations of the two step approach are displayed in the following schematic diagrams.

While properties have yet to be fully explored, it is likely that phase separation is modified by polymer topology and hence the properties of the material will differ even though compositions were held constant. A spontaneous gradient graft copolymer was prepared by grafting through two different PEO macromonomers (58) Selection of a PEO methacrylate with a methyl end-group (PEOMeMA, DPPEO = 23) and a PEO acrylate end-capped by a phenyl ring (PEOPhA, DPPEO = 4) for the copolymerization led to a spontaneous gradient of PEO grafts along the copolymer backbone. A gradient copolymer was formed because of significantly different reactivities of the two PEO macromonomers. The resulting copolymer has PEOMeMA at one end of the polymer chain, gradually changing through hetero-sequences to predominately PEOPhA at the other chain end. An increase in the initial feed ratio of PEO acrylate reduced the rate of change in the shape of the gradient.

(49) Click chemistry: diverse chemical function from a few good reactions, Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004-2021.

(50) "Polyolefin Graft Copolymers via Living Polymerization Techniques: Preparation of Poly(n-butyl acrylate)-graft-polyethylene through the Combination of Pd-Mediated Living Olefin Polymerization and ATRP;" Hong, S. C.; Jia, S.; Teodorescu, M.;Kowalewski, T.; Matyjaszewski, K.; Gottfried, A. C.; Brookhart, M.; J. Polym. Sci., Polym. Chem. Ed., 2002, 40, 2736-2749.

(51) "Synthesis of Block and Graft Copolymers with Linear Polyethylene Segments by Combination of Degenerative Transfer Coordination Polymerization and Atom Transfer Radical Polymerization." Kaneyoshi, H., Inoue, Y., Matyjaszewski, K.: Macromolecules 2005 38: 5425-5435.

(52) "Densely-Grafted and Double-Grafted PEO Brushes via ATRP. A Route to Soft Elastomers"; Neugebauer, D.; Zhang, Y.; Pakula, T.; Sheiko, S. S.; Matyjaszewski, K., Macromolecules 2003, 36, 6746-6755.

(53) "Structural Control of Poly(methyl methacrylate)-g-poly(dimethylsiloxane) Copolymers Using Controlled Radical Polymerization: Effect of the Molecular Structure on Morphology and Mechanical Properties"; Shinoda, H.; Matyjaszewski, K.; Okrasa, L.; Mierzwa, M.; Pakula, T. Macromolecules 2003, 36, 4772-4778.

(54) "Structural Control of Poly(Methyl Methacrylate)-g-poly(Lactic Acid) Graft Copolymers by ATRP"; Shinoda, H.; Matyjaszewski, K. Macromolecules 2001, 34, 6243-6248.

(55) "Living free radical polymerization of macromonomers. Preparation of well defined graft copolymers"; Hawker, C. J. et. al. Macromol. Chem. Phys. 1997, 198, 155-166.

(56) "Hydrogels by atom transfer radical polymerization. I. poly(N-vinylpyrrolidinone-g-styrene) via the macromonomer method"; Beers, K. L.; Kern, A.; Gaynor, S. G.; Matyjaszewski, K. et. al. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 823-830.

(57) "Preparation and characterization of graft terpolymers with controlled molecular structure." Lutz, J.-F., Jahed, N., Matyjaszewski, K.; Journal of Polymer Science, Part A: Polymer Chemistry, 2004, 42, 1939-1952.

(58) "Gradient graft copolymers derived from PEO-based macromonomers," Neugebauer, D.; Zhang, Y.; Pakula, T. Journal of Polymer Science, Part A: Polymer Chemistry 2006, 44, 1347-1356.

b) "Grafting from" the primary requirement for a successful "grafting from" reaction is a preformed macromolecule with distributed initiating functionality.

Grafting from reactions have been conducted from polyethylene, (59-61) polyvinylchloride, (62, 63) and polyisobutylene. (64, 65) The only requirement for a multifunctional ATRP grafting from macroinitiator is distributed radically transferable atoms along the polymer backbone. The initiating sites can be incorporated by copolymerization, (58) be inherent parts of the first polymer, (63) or incorporated in a post-polymerization reaction. (64) Indeed a pre-functionalized isobutylene rubber is available commercially from Exxon, (EXXPRO 3035) and has been employed in a grafting from ATRP of MMA forming a spectrum of materials ranging from elastomers to impact resistant poly(MMA). (64)

Densely grafted copolymers are also prepared by a "grafting from" polymerization and are discussed in the following section on bottle-brush macromolecules and on the "nanocomposites" page.

(59) "Graft Copolymers from Linear Polyethylene via Atom Transfer Radical Polymerization"; Inoue, Y.; Matsugi, T.; Kashiwa, N.; Matyjaszewski, K. Macromolecules 2004, 37, 3651-3658.

(60) "Morphology and thermomechanical properties of well-defined polyethylene-graft-poly(n-butyl acrylate) prepared by atom transfer radical polymerization"; Okrasa, L.; Pakula, T.; Inoue, Y.; Matyjaszewski, K. Colloid and Polymer Science 2004, 282, 844-853.

(61) "Polyethylene-g-poly(meth)acrylate copolymers"; Kaneyoshi, H. et. al. Polymeric Materials: Science and Engineering 2004, 91, 41-42.

(62) "Synthesis and characterization of graft copolymers of poly(vinyl chloride) with styrene and (meth)acrylates by ATRP;" Paik, H. J. ; Gaynor, S. G.; Matyjaszewski, K. Macromol. Rapid Commun. 1998, 19, 47-52.

(63) "Metal-catalyzed living radical graft copolymerization of olefins initiated from the structural defects of poly(vinyl chloride);" Percec, V., Asgarzadeh, F.: J. Polym. Sci., Part A: Polym. Chem. , 2001 39: 1120-1135..

(64) "Preparation of polyisobutene-graft-poly(methyl methacrylate) and polyisobutene-graft-polystyrene with different compositions and side chain architectures through atom transfer radical polymerization (ATRP)" Hong, S. C.; Pakula, T.; Matyjaszewski, K. Macromolecular Chemistry and Physics 2001, 202, 3392-3402.

(65) Matyjaszewski, K. et. al. In PCT Int. Appl.; (CMU, USA). WO 9840415, 1998, p 230 pp.

c) "Grafting to" has become a more efficient method for the preparation of graft copolymers with the rise of various "click" chemistries. This approach has been used for well defined star molecules, (66) loosely grafted copolymers (67) and as noted in the following section densely grafted structures, (68, 69).

In order to prepare a copolymer suitable for "grafting to" click linking chemistry the epoxide ring in a glycidyl butyrate monomer unit that had been incorporated into well-defined copolymers of glycidyl methacrylate (<40 mol %) and Me methacrylate prepared by ATRP was efficiently opened with sodium azide in the presence of ammonium chloride in DMF at 50 0C. This click-type reaction led to the formation of distributed units of the corresponding 1-hydroxy-2-azido functional groups in high yields. These azide-containing copolymers were functionalized in a second click reaction, the room temp. CuBr/N,N,N',N'',N''-pentamethyldiethylenetriamine-catalyzed 1,3-dipolar cyclo-addition of poly(ethylene oxide) methyl ether pentynoate to yield loosely grafted polymeric brushes with hydrophilic PEO side chains.

(66) "Synthesis of 3-arm star block copolymers by combination of \"core-first\" and \"coupling-onto\" methods using ATRP and click reactions," Gao, H.; Min, K.; Matyjaszewski, K. Macromolecular Chemistry and Physics 2007, 208, 1370-1378.

(67) "Graft Copolymers by a Combination of ATRP and Two Different Consecutive Click Reactions," Tsarevsky, N. V.; Bencherif, S. A.; Matyjaszewski, K.; Macromolecules 2007, 40, 4439-4445

Graft Copolymers with Complex Architecture

The accepted nomenclature is that one transitions from "graft" copolymers to "brush"copolymers when the density of the tethered side chains starts to affect the ability of the backbone of the copolymer to assume a random coil configuration.

Densely grafted copolymers

Linear macromolecular brushes Star brush copolymers Gradient brush Copolymers Brush block copolymers Heterografted brush copolymers Double grafted brush copolymers

AFM: M. Moeller & S. Sheiko (64)

This aspect of the group"s research focuses on the synthesis of macromolecules with advanced intramolecular architectural control, particularly the synthesis and properties of macromolecular brushes, also called linear bottle-brush copolymers. The synthesis of polymers with this type of complex molecular architecture by ATRP was initially an intellectual and synthetic challenge, perhaps subsequently stimulated by the visual images of individual macromolecules on adsorbing surfaces, (70) but has resulted in the preparation of materials with interesting bulk physical properties. (71) The molecules provide environments for intramolecular chemistry, such as the formation of nanowires. (72, 73)

The materials can be prepared by several techniques including "grafting through" and "grafting to" (68, 69, 74) but the following discussion focuses on "grafting from" well defined multifunctional macroinitiators (75) since this is the best approach to well defined densely grafted bottle-brush copolymers with pre-selectable topology, although the other approaches will be mentioned. In order to render a cylindrical shape, as shown in the majority of schematic images of brush macromolecules, the backbone should be much longer than the side chains

Grafting to reactions require the coupling of individual preformed tele-functional side chain precursor to a common backbone polymer with complementary functionality.

Grafting through consists of the polymerization of macromonomers.

Grafting from involves preparing the precursor to the backbone polymer with monomer units that contain functionalities ultimately capable of initiating polymerization of a second monomer.

Macromolecular brushes belong to the general class of graft copolymers. However, in this case the grafting density is very high, at least in some segments of the copolymer. Indeed, polymers with one graft per backbone repeat unit have been prepared. This leads to an extremely crowded environment along the backbone which causes the macromolecules to adopt unusual conformations due to steric repulsion of the densely packed side chains which force the backbone from the normal Gaussian random coil conformation into a chain extended conformation with increased persistence length as side chain graft density increases. (76)

Graft Copolymer Densely Grafted Brush Copolymer

The congested structure of the brushes arises because of the confined mobility of the side chains.

Linear Macromolecular Brushes: Synthesis of macromolecular brush copolymers takes advantage of all of the characteristics of CRP synthesis in that the molecular weight and composition of the backbone and attached side chains are independently controlled. Additionally, as shown for other polymers prepared by CRP, polymer brushes can be prepared as random graft copolymers, block graft copolymers, (77) gradient brush copolymers (78, 79) and molecules with double grafted side chains (a brush of brushes). (80)

The first step in a typical preparation of a densely grafted bottle brush macromolecule is the preparation of a linear macroinitiator, or linear macroinitiator segment in a copolymer, with an initiating moiety on a high fraction of backbone monomer units. A macroinitiator for ATRP has generally been prepared by polymerization of a monomer with a protected functional group, such as 2-(trimethylsilyloxy)ethyl methacrylate (HEMA-TMS), followed by cleavage of the TMS protective groups and esterification with 2-bromopropionyl bromide to yield poly(2-(2-bromopropionyloxy)ethyl methacrylate) (PBPEM). (74) This approach to brush copolymers is shown in the following schematic where the multifunctional macroinitiator is prepared and used to initiate ATRP of various monomers from each repeat unit by a grafting from mechanism

Macroinitiators with a backbone length of 50 to over 6,000 monomer units have been prepared by CRP processes and multiple graft chains, also via CRP, with 20 to over 400 monomer units have been prepared by grafting from reactions. Note that the molecular weight of a butyl acrylate based polymer with a backbone of 6,000 monomer units and 170 monomer units in each grafted side chain is well over 107. Note in the image above, a brush macromolecule with a high DP of the backbone adopts a chain extended random coil conformation when a dilute solution of the brush molecule is deposited on an adsorbing surface despite the congested steric environment along the polymer backbone.

Generally the side chain density is controlled by controlling the distribution of initiating sites along the backbone by copolymerization of monomers with precursors of the initiating groups and non-initiating comonomers. The distribution of initiating sites can be further controlled by control over monomer feed and selection of monomers with favorable reactivity ratios of the initiating monomers and non-initiating comonomers. However, initiation efficiency also affects graft density. Grafting efficiency was generally assumed to be high because of the appearance of the final polymers. However by comparing the molecular weight of the cleaved side chains to their theoretical molecular weights, calculated assuming quantitative initiation, grafting efficiency was detected as a function of monomer conversion. (81, 82)

In the early stages of the polymerization the initiation efficiency is low but gradually increases to 87% at 12% monomer conversion. The results were compared to an analogous linear ATRP conducted under identical conditions, except that ethyl 2-bromopropionate was employed as the initiator. The initiation efficiency for the linear polymerization was consistently higher than that observed for the brushes. The difference was attributed to the congested environment, high local concentration of initiation sites, encountered in the case of grafting from a macroinitiator backbone, which leads to slower deactivation of growing radicals at low conversion. The initiation efficiency was enhanced by increasing the rate of deactivation of the growing species or decreasing the rate of propagation (82) by either adding more Cu(II) or less initiator to the reaction. A low [P*] suppresses intra- and inter-molecular termination and the activation / deactivation equilibrium mitigates polymerization allowing all side chains to grow simultaneously. Controlled copolymer synthesis and post-polymerization modification allows the molecular composition of the side chains to be tailored to the target application.

Recently, (68, 69) an efficient grafting to approach has been successfully used for the preparation of densely grafted brushes. Linear poly(2-hydroxyethyl methacrylate) (PHEMA) polymers were synthesized first by ATRP. After esterification reactions between pentynoic acid and the hydroxyl side groups, polymeric backbones with alkynyl side groups on essentially every monomer unit (PHEMA-alkyne) were obtained. Five different kinds of azido-terminated polymeric side chains (SCs) with different chemical composition and molecular weight were used in the grafting to reactions, including poly(ethylene glycol)-N3 (PEO-N3), polystyrene-N3, poly(Bu acrylate)-N3, and poly(Bu acrylate)-b-polystyrene-N3. All click coupling reactions between alkyne-containing polymeric backbones (PHEMA-alkyne) and azido-terminated polymeric SCs were completed within 3 hours. The grafting density of the molecular brushes obtained was affected by several factors, including the molecular weight and the chemical structure of the linear polymers used in the grafting to reaction, as well as the initial molar ratio of linear chains to alkynyl groups. When linear polymers with "thinner" structure and lower molecular weight, e.g., PEO-N3 with Mn = 775 g/mol, were reacted with PHEMA-alkyne (DP = 210) at a high molar ratio of linear chains to alkynyl groups in the backbone, brush copolymers with the highest grafting density were obtained (efficiency of grafting = 88%). This result indicates that the average number of tethered side chains was ~ 186 per brush.

Star Brush Copolymers: The use of a multifunctional initiator for the synthesis of the first macroinitiator backbone copolymer can lead to the preparation of bottle bush polymers displaying different topologies. For example, if a tetra-functional initiator is used to polymerize HEMA-TMS, the resulting polymer can be functionalized to yield a 4-arm macroinitiator. This product can, in turn, be used to prepare 4-arm star macromolecular brushes. (83) An AFM image of such a copolymer with poly(n-butyl acrylate) side chains is shown below.

Gradient Brush Copolymers: Combining gradient copolymerization with macroinitiator synthesis gives access to brushes with a controlled gradient of graft density along the back bone. (77, 78) Gradient macroinitiators can be formed by spontaneous copolymerization of monomers with different reactivity ratios or by controlled addition of one monomer to a copolymerization of monomers with relatively similar reactivity ratios. When a macroinitiator with a gradient of initiating sites along the backbone is used in a grafting from copolymerization, the resulting molecule has a higher density of side chains at one end of the backbone than it does at the other. AFM examination of gradient graft macromolecules shows a coexistence of two conformational phases within individual molecules adsorbed on a mica substrate. These observations were made by compressing monolayers on the surface of water and then transferring a sample of this monolayer to a mica substrate for AFM studies. Upon compression, the rod-globule transition occurs at the end where the brush is densely grafted, leaving a molecule with a globular "head" and an extended "tail" a so-called tadpole conformation. (78)

Brush Block Copolymers: Topologically different materials can be described as brush block copolymers including a) a brush macromolecule with block copolymer grafted side chains and b) a "standard block copolymer" where one or more segments along a polymer backbone are brush copolymers of different composition.

a) Block copolymer side chains. The use of a macromolecular brush as a macroinitiator to polymerize a second monomer allows the synthesis of densely grafted copolymers with block copolymer side chains (76)

The resulting nanostructured macromolecules can form soft/hard core/shell systems (e.g. poly(n-butyl acrylate) / polystyrene) and potentially also macromolecular channels, stable worm-like micelles or inverse micelles, and other complex architectures. These materials have served as templates for the preparation of gold nanowires (72).

The resulting nanostructured macromolecules can form soft/hard core/shell systems (e.g. poly(n-butyl acrylate) / polystyrene) and potentially also macromolecular channels, stable worm-like micelles or inverse micelles, and other complex architectures. These materials have served as templates for the preparation of gold nanowires (72).

b) Block copolymer backbone. A second approach to prepare a material that can also be describes as a block brush copolymer, is the preparation of a macroinitiator precursor followed by chain extension with another monomer. This leads to "standard" block copolymer architecture along the backbone of the brush. The second block can be used to form a phase separable segment to physically link the bottle brush copolymer macromolecules, e.g. in the following schematic and AFM image show association behavior between two ODMA units driven by crystallization.

AFM image of a brush copolymer with linear block copolymer architecture along the backbone. The first block has pendant n-BA chains while the second block is composed of ODMA monomer units and causes end-to-end phase separation and physical linking of the brush macromolecules as the polymer is cast from dilute solution.

A star block copolymer would be expected to form a network.

AFM image of a brush copolymer with linear block copolymer architecture along the backbone. The first block has pendant n-BA chains while the second block is composed of ODMA monomer units and causes end-to-end phase separation and physical linking of the brush macromolecules as the polymer is cast from dilute solution.

A star block copolymer would be expected to form a network.

Double Grafted Brush Copolymers: Another combination of grafting from and grafting through can lead to the preparation of a type of copolymer named "double grafted" brush copolymers. This occurs when macromonomers are polymerized by grafting from a linear macroinitiator.

Responsive Bottle-Brush Macromolecules

Heterografted environmentally responsive brush copolymers: Another interesting brush topology is demonstrated by the materials, called hetero-grafted brush copolymers. This type of brush copolymer with grafts of different identity along the backbone was prepared by a random grafting through copolymerization of a macromonomer with a low molecular weight comonomer that was a precursor for a CRP initiator. (87)

The protecting groups on the precursor monomer units were derivatized to yield moieties capable of initiating ATRP, followed by a grafting from polymerization of a second monomer from the brush copolymer backbone. A brush copolymer with poly(ethylene oxide) grafts and poly(butyl acrylate) grafts prepared as described above displays interesting behavior. The materials undergo a reversible collapse in the presence of ethanol or water vapors which results in a change of the surface properties of mica, favoring either adsorption or desorption of one of the graft polymer segments. (88) When extended, tightly adsorbed poly(butyl acrylate) brush molecules are exposed to ethanol vapor, the macromolecules swell and contract to form compact globules. Exchanging the ethanol vapor to a humid atmosphere caused the molecules to extend again to a wormlike two-dimensional conformation.

Polymer brush molecules with responsive units in the graft chains can be prepared by copolymerization.

Photoresponsive:- Molecular brushes with trans-4-methacryloyloxyazobenzene (MOAB) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) units in the side chains were successfully synthesized by grafting from a poly(2-(2-bromopropionyloxy)ethyl methacrylate) (pBPEM) macroinitiator. The molecular weight, PDI, and DP of the side chains containing different distributions of DMAEMA and MOAB were detected by gel permeation chromatography (GPC) and 1H NMR spectroscopy. The number average molecular weight (Mn) of the brushes ranged from 4.7x105 to 1.1x106 depending on molecular architecture. UV-vis spectra of the brush copolymers in either chloroform or aqueous solution showed reversible isomerization of azobenzene units in the side chains upon irradiation with UV (365 nm) or visible light (442 nm). (88)

The transmission spectra of the aqueous solutions of the brush copolymers at 600 nm were measured as a function of temperature and showed that the lower critical solution temp. (LCST) can be affected by photo-irradiation.

UV-Vis spectra of Brush II upon a) UV (365 nm) and b) visible (442 nm) irradiation in chloroform solution (0.1 wt %).

Temperature Responsive:- Molecular brushes, with side chains consisting of two copolymers: 2-(dimethylamino)ethyl methacrylate with Me methacrylate, and N,N-dimethylacrylamide with Bu acrylate were prepared by a grafting from polymerization (90) Poly(2-(2-bromoisobutyryloxy)ethyl methacrylate) and poly(2-(2-bromopropionyloxy)ethyl methacrylate) were used as macroinitiators. Dynamic light scattering (DLS) studies were performed on aqueous solutions of the brush macromolecules below and above the lower critical solution temperature (LCST), and an unusual concentration-dependent LCST was observed. Due to the compact structure of molecular brushes, intra-molecular collapse can occur when the average distance between the molecules on solution is larger than the hydrodynamic dimensions of the individual macromolecules. However, if the concentration of the solution of molecular brushes is increased to the level in which the separation distance is comparable with the brush hydrodynamic dimensions, inter-molecular aggregation occurs, as typically observed for solutions of linear polymers. (90)

(68) "Highly Efficient \"Click\" Functionalization of Poly(3-azidopropyl methacrylate) Prepared by ATRP, Sumerlin, B. S.; Tsarevsky, N. V.; Louche, G.; Lee, R. Y.; Matyjaszewski, K. Macromolecules 2005, 38, 7540-7545."

(69) "Synthesis of Molecular Brushes by \"Grafting onto\" Method: Combination of ATRP and Click Reactions," Gao, H.; Matyjaszewski, K.; Journal of the American Chemical Society 2007, 129, 6633-6639.

(70) "The Synthesis of Densely Grafted Copolymers by Atom Transfer Radical Polymerization;" Beers, K. L., Gaynor, S. G., Matyjaszewski, K., Sheiko S. S. and Moeller. M.; Macromolecules 1998, 31(26), 9413"9415.

(71) "Densely-Grafted and Double-Grafted PEO Brushes via ATRP. A Route to Soft Elastomers;" Neugebauer, D.; Zhang, Y.; Pakula, T.; Sheiko, S. S.; Matyjaszewski, K. Macromolecules 2003, 36, 6746-6755.

(72) "Amphipolar Core-Shell Cylindrical Brushes as Templates for the Formation of Gold Clusters and Nanowires;" Ramin Djalali, R., Li, S. and Schmidt, M;. Macromolecules 2002, 35, 4282.

(73) "Superparamagnetic hybrid nanocylinders," Zhang, M.; Estournes, C.; Bietsch, W.; Mueller, A. H. E. Advanced Functional Materials 2004, 14, 871-882.

(74) "Graft copolymers by atom transfer polymerization (Synthesis of Defined Polymer Architectures)," Borner, H.G. and K. Matyjaszewski, Macromolecular Symposia 2002, 177 1"15.

(75) "The Synthesis of Densely Grafted Copolymers by Atom Transfer Radical Polymerization;" Beers, K. L., Gaynor, S. G., Matyjaszewski, K., Sheiko S. S. and Moeller. M.; Macromolecules 1998, 31(26), 9413"9415.

(76) "On the shape of bottle-brush macromolecules: Systematic variation of architectural parameters." Rathgeber, S., Pakula, T., Wilk, A., Matyjaszewski, K., Beers, K. L.: Journal of Chemical Physics, 2005, 122: 124904, 13 pages.

(77) "Synthesis of Molecular Brushes with Block Copolymer Side Chains Using Atom Transfer Radical Polymerization." Boerner, H. G., K. Beers, K. Matyjaszewski, S. S. Sheiko and M. Moeller. Macromolecules 2001, 34, 4375"4383.

(78) "Synthesis of Molecular Brushes with Gradient in Grafting Density by Atom Transfer Polymerization." Boerner, H. G., Duran, D., Matyjaszewski, K., da Silva M., Sheiko, S. S.; Macromolecules 2002, 35, 3387"3394.

(79) "Tadpole Conformation of Gradient Polymer Brushes;" Lord, S. J.; Sheiko, S. S.; LaRue, I.; Lee, H.-I.; Matyjaszewski, K. Macromolecules 2004, 37, 4235-4240.

(80) "Architecture and solution properties of AB-type brush-block-brush amphiphilic copolymers via ATRP techniques" Ishizu, K., Satoh J., Sogabe. A., Journal of Colloid and Interface Science, 2004, 274, 472-479.

(81) "How dense are cylindrical brushes grafted from a multifunctional macroinitiator"" Neugebauer, D. Sumerlin, B. S., Matyjaszewski, K., Goodhart, B., Sheiko, S. S.: Polymer 2004, 45, 8173-8179.

(82) "Initiation Efficiency in the Synthesis of Molecular Brushes by Grafting from via Atom Transfer Radical Polymerization;" Sumerlin, B. S. D. Neugebauer, and K. Matyjaszewski. Macromolecules 2005, 38, 702-708.

(83) "Multiarm Molecular Brushes: Effect of the Number of Arms on the Molecular Weight Polydispersity and Surface Ordering;" Boyce, J. R., Shirvanyants, D., Sheiko, S. S., Ivanov, D. A., Qin, S., Boerner, H. and Matyjaszewski K.; Langmuir, 2004, 20, 6005-6011.

(84) "Conformational Switching of Molecular Brushes in Response to the Energy of Interaction with the Substrate;" Sun, F. et. al. Journal of Physical Chemistry A, 2004, 108, 9682-9686.

(85) "Molecular brushes as super-soft elastomers," Pakula, T.; Zhang, Y.; Matyjaszewski, K.; Lee, H.-i.; Boerner, H.; Qin, S.; Berry, G. C. Polymer 2006, 47, 7198-7206.

(86) "Heterografted PEO-PnBA brush copolymers." Neugebauer, D., Y. Zhang, T. Pakula and K. Matyjaszewski; Polymer, 2003, 44, 6863-6871.

(87) "Densely Heterografted Brush Macromolecules with Crystallizable Grafts. Synthesis and Bulk Properties," Neugebauer, D.; Theis, M.; Pakula, T.; Wegner, G.; Matyjaszewski, K. Macromolecules 2006, 39, 584-593.

(88) "Reversible collapse of brushlike macromolecules in ethanol and water vapours as revealed by real-time scanning force microscopy;" Gallyamov, M. O, Tartsch, B., Khokhlov, A. R., Sheiko, S. S., Boerner, H. G., Matyjaszewski, K., Moeller, M. Chemistry--A European Journal, 2004, 10, 4599-4605.

(89) "Phototunable Temperature-Responsive Molecular Brushes Prepared by ATRP," Lee, H.-i.; Pietrasik, J.; Matyjaszewski, K. Macromolecules 2006, 39, 3914-3920.

(90) "Solution behavior of temperature-responsive molecular brushes prepared by ATRP," Pietrasik, J.; Sumerlin, B. S.; Lee, R. Y.; Matyjaszewski, K. Macromolecular Chemistry and Physics 2007, 208, 30-36.

Star Copolymers

Star polymers consist of several linear polymer chains connected at one point. (91) Prior to the development of CRP, the nature of star molecules prepared by anionic polymerization had been examined. Their compact structure and globular shape predetermine their unique properties, such as low solution viscosity, and enable potential applications, including use as drug carriers. (92)

Star polymers can be divided into two topological categories: stars with a well defined core and a predetermined number of arms and multi-arm stars with a multifunctional core frequently prepared by an "arm first" approach. The arm-first approach can lead to two structural categories: homo- and mikto-arm star polymers. In the former case, the stars have arms with identical chemical composition, while in the mikto-arm case two or more than two different types of arms are combined in a single star molecule.

As suggested above, star polymers can be synthesized by one of three methods:

• "core-first", where the controlled polymerization is conducted from either a well defined initiator with a known number of initiating groups (36, 93-97) and less well defined multifunctional macromolecules, (98-101);
• There are two approaches to the "arm-first" synthesis of star polymers. (102-106) One is where a linear "living" copolymer chain, or added macroinitiator, is linked by continuing the copolymerization of the mono-functional macroinitiator with a difunctional monomer forming a crosslinked core;
• the other, is the direct copolymerization of a macromonomer with a difunctional monomer in the presence of a low molecular weight initiator. (107, 108)
• A combination of "arm-first" and "core-first" methods and is particularly useful in the synthesis of miktoarm star copolymers. The retained initiating functionality in the arm first core is used in a grafting out copolymerization. (109-113)
  

The preparation of stars using click chemistry could be applied to almost any of the above strategies. (114)

Core first: The following section on nano-composites formed by grafting from the surface of a functionalized particle is an evolution of the "core first" approach. However, initially a low molecular weight multi-functional molecule was used in a grafting from reaction to form star macromolecules with a well defined number of arms. Use of hexakis(chloromethyl)benzene as a well defined multifunctional initiator for the polymerization of styrene provided the first multi-arm polymer prepared using ATRP. (93, 95) The composition of the core and arms were quickly expanded. (36, 95-97)

Since the tethered chains retained their terminal functionality they could be chain extended to form star block copolymers and/or the radically transferable atoms on the chain ends (X) could be converted to other functional groups (F) suitable for post-polymerization functionalization reactions. (96)

Less well-defined multi arm star structures were prepared by grafting from a hyperbranched core. The core was prepared by polymerizing a molecule containing both a reactive group suitable to initiate a CRP and a polymerizable double bond, an initiator/monomer of inimer. Self-condensing vinyl polymerization using ATRP has been applied to inimers, i.e. monomers that also contain activated halogen atoms, such as "-bromoesters and benzyl halides. (90-93) For example, when 2-bromopropionyloxyethyl acrylate was polymerized in the presence of a copper catalyst with 4,4'-dinonyl-2,2'-bipyridine ligands, a hyperbranched polymer with degree of branching ~0.5 and DP = 78 was obtained. (100, 101)

This polymer contained 78 active bromopropionyloxy groups, and was used to initiate the polymerization of methyl acrylate forming a star with 78 arms.

Arm first: The "arm first" approach, which forms the core by coupling monofunctional "living" polymeric chains with a difunctional reagent, was first applied to living anionic polymerization. (102) A similar approach has also been successful with ATRP. (103, 104) There are several parameters in an ATRP that should be controlled carefully in order to maximize the yield of stars and prevent star"star coupling reactions. Some detailed studies have been carried out on the coupling of monofunctional polystyrenes and polyacrylates with divinylbenzene (DVB) and di(meth)acrylates to prepare star polymers and the following guidelines were developed: (105-108)

• The ratio of di-functional reagent to growing chains seems to be optimal in the range of 10"20.
• Monomer conversion (or reaction time) has to be carefully controlled and stopped before star"star coupling occurs. It seems that ~5% of arms can not be incorporated into the star macromolecules under typical conditions.
• Higher yields of stars are observed for polyacrylates than for polystyrenes. This may be attributed to a higher proportion of terminated chains (due to slower propagation and higher concentration of radicals) in styrene polymerization under "standard" ATRP conditions.
  

(The use of ARGET ATRP for the synthesis of the active growing polymer chains should change both these observations.)

• The choice of the difunctional reagent is important and reactivity should be similar to, or lower than that of the arm-building monomers.
• Halogen exchange slightly improves efficiency of star formation.
• Solvent, temperature, catalyst concentration should be also optimized.
  

Under typical conditions, ~50 arms are coupled into star structures. Apparent molecular weights, measured by GPC, show a 10"20-fold increase of molecular weight (from Mn = 10,000 to Mn ~ 150,000) but light scattering indicates that the stars have a much higher molecular weight (Mn ~500,000).

Another approach to arm first star copolymers is the CRP of higher molecular weight macromonomers in a pure homo-polymerization (108) with a small molecule initiator which usually leads to a brush molecule with a degree of polymerization of 10-25, which from a topological standpoint can be considered a star.

An alternate approach is a copolymerization of a macroinitiator with a divinyl crosslinking monomer. (109) This procedure allows the initiating functionality to be retained within the core which is required for the preparation of miktoarm star copolymers.

Mikto-arm star copolymers: The arm-first procedure involves the synthesis of a linear functional polymer chain polyA, which is used as a macroinitiator (MI) in a subsequent cross-linking reaction using a divinyl compound to produce a (polyA)n-polyX star polymer, where polyX represents the core of the star polymer and n is the number of polyA arms. However due to the "living" nature of the CRP polymerization, the initiating sites are preserved in the core of the star polymer (i.e. alkyl halide groups in ATRP) and the star polymer can be used as a multifunctional star initiator in the chain extension reaction with a different monomer, B, to yield a miktoarm star copolymer, (polyA)n-polyX-(polyB)m. This combination method for synthesizing miktoarm star copolymers was termed the "in-out" method. (1, 110, 112)

The efficiency of initiation of the second arms is dependent on the compactness of the first formed core with less densely crosslinked cores providing more efficient initiation of the grafting out polymerization. (113)

Normally, one seeks to, or has to, form a chemically stable core. However, it is possible to select a crosslinking agent with a degradable link between the two functional crosslinking groups and prepare a material with a degradable core.

This was accomplished by linking the first formed arms with a dimethacrylate containing a disulfide link between the methacrylate units. As shown in the scheme the mikto-arm star copolymer could be degraded in a reducing environment to form a mixture of an AB block copolymer and some residual A-homopolymer. The ratio between the block copolymer and homopolymer gave a direct measurement of the initiating efficiency of the constrained core initiating units; in this example only ~20%. (112)

A recent example of a combination of an "arm first" and a "core first" approach is "grafting onto" a functionalized core via "click" chemistry. (114) Three-arm and four-arm star polystyrene polymers were synthesized by a combination of ATRP and click coupling chemistry. The click reaction between an azido-terminated polystyrene (PS-N3) and an alkyne-containing multifunctional compd. proved to be fast and efficient. When an azido-terminated polystyrene polymer was reacted with a trialkyne-containing or tetraalkyne-containing compound, the yields of 3-arm star and 4-arm star polymers were around 90% and 83%, respectively. The influence of several parameters on the efficiency of the click coupling reaction was studied, including the molecular weight of the PS-N3 polymer, the presence of an added reducing agent, Cu(0), and the stoichiometry between the azido and alkynyl groups. The results indicated that the yield of the coupled product was higher when a lower mol. wt. PS-N3 was employed in conjunction with a small amt. of reducing agent, and the molar ratio of azido and alkynyl groups was close to 1.

(91) "Architectural control in \"living\" free radical polymerizations: preparation of star and graft polymers"; Hawker, C. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 1456-1459.

(92) "Polymers with Complex Architecture by Living Anionic Polymerization:" Hadjichristidis, N.; Pitsikalis, M.; Pispas, S.; Iatrou, H. Chem. Rev. 2001, 101, 3747.

(93) "Atom transfer radical polymerization (ATRP): A new approach towards well-defined (co)polymers;" Wang, J.-S.; Greszta, D.; Matyjaszewski, K. Polym. Mater. Sci. Eng. 1995, 73, 416.

(94) "Atom- or group-transfer radical polymerization and polymers produced by the process;" Matyjaszewski, K.; Wang, J.-S. PCT Int. Appl.; WO 9630421, (Carnegie Mellon University, USA). 1996, 129 pp.

(95) "Synthesis and Characterization of Star Polymers with Varying Arm Number, Length, and Composition from Organic and Hybrid Inorganic/Organic Multifunctional Initiators;" Matyjaszewski, K.; Miller, P. J.; Pyun, J.; Kickelbick, G.; Diamanti, S. Macromolecules 1999, 32, 6526.

(96) "Atom transfer radical polymerization of styrene using a novel octafunctional initiator: synthesis of well-defined polystyrene stars"; Angot, S.; Murthy, K. S.; Taton, D.; Gnanou, Y. Macromolecules 1998, 31, 7218.

(97) "Calixarene-Core Multifunctional Initiators for the Ruthenium-Mediated Living Radical Polymerization of Methacrylates"; Ueda, J.; Kamigaito, M.; Sawamoto, M. Macromolecules 1998, 31, 6762.

(98) "Synthesis of Branched and Hyperbranched Polystyrenes"; Gaynor S. G, Edelman S and Matyjaszewski K, Macromolecules 1996, 29, 1079"1081.

(99) "Preparation of Hyperbranched Polyacrylates by Atom Transfer Radical Polymerization. 1. Acrylic AB* Monomers in \"Living\" Radical Polymerizations"; Matyjaszewski K, Gaynor S.G., Kulfan, A. and Podwika, M., Macromolecules 1997, 30, 5192"5194.

(100) "Preparation of Hyperbranched Polyacrylates by Atom Transfer Radical Polymerization. 2. Kinetics and Mechanism of Chain Growth for the Self-Condensing Vinyl Polymerization of 2-((2-Bromopropionyl)oxy)ethyl Acrylate"; Matyjaszewski K., Gaynor S. G. and Mueller A. H. E., Macromolecules 1997, 30, 7034"7041.

(101) "Preparation of Hyperbranched Polyacrylates by Atom Transfer Radical Polymerization. 3. Effect of Reaction Conditions on the Self-Condensing Vinyl Polymerization of 2-((2-Bromopropionyl)oxy)ethyl Acrylate"; Matyjaszewski K., and Gaynor S. G., Macromolecules 1997, 30, 7042"7049.

(102) "Recent developments in the field of star-shaped polymers." Rein, D; Rempp, P. Lutz, P. J.; Makromolekulare Chemie, Macromolecular Symposia, 1993, 67, 237-49.

(103) "Synthesis of Star-Shaped Polystyrene by Atom Transfer Radical Polymerization Using an \"Arm First\" Approach"; Xia J, Zhang X and Matyjaszewski K, Macromolecules 1999, 32 4482"4484.

(104) "End-Functional Poly(tert-butyl acrylate) Star Polymers by Controlled Radical Polymerization": Zhang X, Xia J and Matyjaszewski K, Macromolecules 2000, 33 2340"2345.

(105) "Core-Functionalized Star Polymers by Transition Metal-Catalyzed Living Radical Polymerization. 1. Synthesis and Characterization of Star Polymers with PMMA Arms and Amide Cores"; Baek, K.-Y.; Kamigaito, M.; Sawamoto, M. Macromolecules 2001, 34, 7629-7635.

(106) "Structural Control in ATRP Synthesis of Star Polymers Using the Arm-First Method," Gao, H.; Matyjaszewski, K. Macromolecules 2006, 39, 3154-3160.

(107) "Low Polydispersity Star Polymers via Cross-Linking Macromonomers by ATRP," Gao, H.; Ohno, S.; Matyjaszewski, K. Journal of the American Chemical Society 2006, 128, 15111-15113.

(108) "Low-Polydispersity Star Polymers with Core Functionality by Cross-Linking Macromonomers Using Functional ATRP Initiators," Gao, H.; Matyjaszewski, K. Macromolecules 2007, 40, 399-401.

(109) "Synthesis of Miktoarm star polymers;" Hadjichristidis, N. J. Polym. Sci. Pol. Chem. 1999, 37, 857.

(110) "Synthesis and Characterization of Double-Hydrophilic Model Networks Based on Cross-linked Star Polymers of Poly(ethylene glycol) Methacrylate and Methacrylic Acid"; Georgiades, S. N.; Vamvakaki, M.; Patrickios, C. S. Macromolecules 2002, 35, 4903.

(111) "Star polymer, PCL-PS heteroarm star polymer by ATRP, and core-carboxylated PS star polymer thereof "; Du, J. Z.; Chen, Y. M. Macromolecules 2004, 37, 3588.

(112) "Synthesis of degradable miktoarm star copolymers via ATRP"; Gao, H.; Tsarevsky, N. V.; Matyjaszewski, K. Macromolecules 2005, 38, 5995.

(113) "Synthesis of Miktoarm Star Polymers via ATRP Using the \"In-Out\" Method: Determination of Initiation Efficiency of Star Macroinitiators," Gao, H.; Matyjaszewski, K. Macromolecules 2006, 39, 7216-7223.

(114) Synthesis of Star Polymers by a Combination of ATRP and the \"Click\" Coupling Method, Gao, H.; Matyjaszewski, K. Macromolecules 2006, 39, 4960-4965.

Hyperbranched Polymers: As noted above ill-defined stars were prepared from hyperbranched cores prepared by ATRP of AB* molecules, i.e. monomers that also contain an active halogen atom. (36, 98, 99) The reason for poor definition was that the structure of the branch distribution in the hyperbranched molecule depended on the catalyst employed, (KATRP) for the ATRP, and hence the number of monomer units added during each activation cycle. (115) Improved deactivation leads to a more compact structure. (116) Use of a polymeric AB* macromolecule leads to a hyperbranched or highly branched structure with rater low PDI, 2.6 and a significantly lower value for the Mark-Houwink exponent than a linear molecule. (117) A review article noted the importance of controlling the polymer structure when designing functional materials. (118)

Another route to hyperbranched macromolecules is the copolymerization of a monomer with a divinyl monomer. For example, Sherrington (119) and Armes et al. (120) used ATRP for the synthesis of branched polymers by copolymerization of methacrylate monomer and dimethacrylate cross-linker at a mole ratio close to 1:1. They concluded that highly branched polymers only occurred at high monomer conversions and it is essential to keep the molar ratio of cross-linker to initiator less than 1 in order to obtain branched sols instead of gels. (121)

(115) "Molecular Parameters of Hyperbranched Polymers Made by Self-Condensing Vinyl Polymerization. 2. Degree of Branching," Yan, D.; Mueller, A. H. E.; Matyjaszewski, K. Macromolecules 1997, 30, 7024-7033.

(116) "Controlling the degree of branching in ATRP of hyperbranched polyacrylates," Yoo, S. H.; Yoon, T. H.; Jho, J. Y. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1999, 40, 460-461.

(117) "Synthesis of hyperbranched poly(tert-butyl acrylate) by self-condensing atom transfer radical polymerization of a macroinimer," Cheng, G.; Simon, P. F. W.; Hartenstein, M.; Muller, A. H. E. Macromol. Rapid Commun. 2000, 21, 846-852.

(118) "The synthesis of functional star copolymers as an illustration of the importance of controlling polymer structures in the design of new materials," Matyjaszewski, K. Polymer International 2003, 52, 1559-1565.

(119) "Synthesis of densely branched poly(methyl methacrylate)s via ATR copolymerization of methyl methacrylate and ethylene glycol dimethacrylate," Bouhier, M.-H.; Cormack, P.; Graham, S.; Sherrington, D. C. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 2375.

(120) "Development of Branching in Living Radical Copolymerization of Vinyl and Divinyl Monomers," Bannister, I.; Billingham, N. C.; Armes, S. P.; Rannard, S. P.; Findlay, P. Macromolecules 2006, 39, 7483.

(121) Linear and Hyperbranched Glycopolymer-Functionalized Carbon Nanotubes: Synthesis, Kinetics, and Characterization, Gao, C.; Muthukrishnan, S.; Li, W.; Yuan, J.; Xu, Y.; Mueller, A. H. E. Macromolecules, 2007, 40(6), 1803-1815.

Networks/Gels

An extension of the observation of Sherrington and Armes (119, 120) is that if an excess of crosslinker vs. initiator is used in an ATRP, then a crosslinked network or gel will be formed. In contrast to copolymers prepared by FRP, the networks prepared by a CRP will have a more regular structure if the structure of the crosslinker and monomer are similar; i.e. reactivity ratio"s are close to 1 and conversion is driven to a point where the concentration of reacted pendant vinyl groups was larger than that of primary chains. Indeed Zhu et al. also studied the homopolymerization of ethylene glycol dimethacrylate (EGDMA) and copolymerization of methyl methacrylate (MMA) and dimethacrylate cross-linker using bulk ATRP. (122, 123) They found that most of the pendant vinyl groups were consumed and the cross-link density of the final gel network was close to the maximum values achievable for the added cross-linker amounts. These results suggested that the occurrence of gelation (gel point) in ATRP was determined by the initial molar ratio of cross-linker to initiator. When the concentration of initially added cross-linker was larger than that of initiator, insoluble gels were produced at high monomer conversions. (124)

The arm first approach to stars can also be adapted to formation of well defined network. This was accomplished by use of a difunctional initiator in an ATRP to form telefunctional polymers by linking of the end groups by addition of a divinyl crosslinker. (125) The amount of soluble polymer decreased with continued reaction time after addition of the crosslinker and the swelling ratio decreased to reach a plateau value as the dispersed crosslinked nodes formed. However, the cross-linking density was often far below that of the theoretical value. Possible unexamined variables included (a) the polymeric precursor concentration, (b) the molar mass of the polymeric precursor, and (c) the molar concentration ratio [cross-linking agent]/[polymer precursor]. This approach was also adapted to formation of gels with controlled topology using "click" chemistry. (126) ATRP was conducted from a bifunctional initiator then the bromine end groups of the resulting telechelic polymer were converted to azides, and crosslinking of this azido-telechelic macromonomer with multi-acetylene functionalized small molecule via copper-catalyzed azide-alkyne cycloaddition was employed to prepare model networks. Model networks derived from a trifunctional alkyne were found to be more completely cross-linked than those derived from a tetrafunctional alkyne, presumably due to less steric hindrance in the former system.

Functional networks: are prepared when functional groups are incorporated into the crosslinker. A degradable disulfide crosslinker was used to prepare a degradable gel were by the copolymerization of MMA and a disulfide-containing difunctional methacrylate, bis(2-methacryloyloxyethyl) disulfide. (127) The gels were reduced with tributylphosphine to form soluble low molecular weight linear poly(methyl methacrylate) fragments containing thiol groups at the chain end and along the backbone, originating from the disulfide difunctional initiator and monomer. The disulfide cross-linked gels were further used as \"supermacroinitiators\" for the bulk ATRP of styrene at 90 0C, forming gels with core/shell structure that swelled more than the starting polymethacrylate gels in both THF and toluene.

When this synthetic strategy is employed to copolymerize water soluble monomers in an inverse miniemulsion ATRP functional nano-sized colloidal particles are formed. (128) Dynamic light scattering (DLS) and atomic force microscopy (AFM) measurements indicated that these particles possessed excellent colloidal stability. ATRP in inverse miniemulsion led to materials with several desirable features. The colloidal particles preserved a high degree of halogen chain-end functionality, which enabled further functionalization. Cross-linked nanogels with a uniformly cross-linked network were prepared that were degraded to individual polymeric chains with relatively narrow Mw/Mn < 1.5 in a reducing environment. Higher colloidal stability, higher swelling ratios, and better controlled degradability indicated that the nanogels prepared by ATRP were superior to their corresponding counterparts prepared by conventional FRP in inverse miniemulsion. The colloids were examined for drug delivery as detailed in page 11.

In addition to chemically crosslinked networks physically crosslinked gels were also prepared. Indeed among the first gels prepared by a CRP/FRP were physically crosslinked copolymers with a water soluble backbone prepared by standard FRP grafting through well-defined vinyl macromonomers of polystyrene prepared using vinyl chloroacetate as an initiator. Since styrene and vinyl chloroacetate do not copolymerize, no branching or incorporation of the initiator into the backbone was observed. The macro-monomers used were of sufficiently high molecular weight to form physical crosslinks in solvents which favor the hydrophilic NVP, such as water, which prevent the copolymer from dissolving and cause it to swell. These materials, therefore, formed hydrogels of swell-abilities in water exceeding 95%, depending on the amount of styrene that was incorporated into the copolymer. (129)

(122) "Branching and gelation in atom transfer radical polymerization of methyl methacrylate and ethylene glycol dimethacrylate," Wang, A. R.; Zhu, S. Polymer Engineering and Science 2005, 45, 720-727.

(123) "Kinetic behavior of atom transfer radical polymerization of dimethacrylates," Yu, Q.; Zhang, J.; Cheng, M.; Zhu, S. Macromolecular Chemistry and Physics 2006, 207, 287-294.

(124) Atom transfer radical crosslinking polymerization of methacrylates and network density, Qin, Z.-q.; Zhou, M.; Sun, Y.; Yu, Q. Jiangsu Gongye Xueyuan Xuebao 2007, 19, 26-29.

(125) "Synthesis of Polymer Networks by \"Living\" Free Radical Polymerization and End-Linking Processes ," Asgarzadeh, F.; Ourdouillie, P.; Beyou, E.; Chaumont, P. Macromolecules 1999, 32, 6996-7002.

(126) "Synthesis of Degradable Model Networks via ATRP and Click Chemistry," Johnson, J. A.; Lewis, D. R.; Diaz, D. D.; Finn, M. G.; Koberstein, J. T.; Turro, N. J. Journal of the American Chemical Society 2006, 128, 6564-6565.

(127) "Combining Atom Transfer Radical Polymerization and Disulfide/Thiol Redox Chemistry: A Route to Well-Defined (Bio)degradable Polymeric Materials," Tsarevsky, N. V.; Matyjaszewski, K. Macromolecules 2005, 38, 3087-3092.

(128) "Inverse Miniemulsion ATRP: A New Method for Synthesis and Functionalization of Well-Defined Water-Soluble/Cross-Linked Polymeric Particles," Oh, J. K.; Tang, C.; Gao, H.; Tsarevsky, N. V.; Matyjaszewski, K. Journal of the American Chemical Society 2006, 128, 5578-5584.

(129) "Hydrogels by atom transfer radical polymerization. I. poly(N-vinylpyrrolidinone-g-styrene) via the macromonomer method," Matyjaszewski, K.; Beers, K. L.; Kern, A.; Gaynor, S. G. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 823-830.

Cyclic: During the preparation of higher molecular weight polymers by the step growth "click" coupling of homo- and hetero-telechelic polystyrenes in DMF solution it was noted that a lower molecular weight polymer was formed. This was assumed to be the result of intramolecular "click"cyclization. (130, 131) ","-Diazido-terminated polystyrene oligomers were prepared by ATRP of styrene with dimethyl 2,6-dibromoheptadioate as an initiator followed by nucleophilic displacement of the bromine end groups with NaN3. The resulting difunctional homotelechelic "monomer" was polymerized with propargyl ether at room temperature to afford higher molecular weight polystyrene. Because the click reaction was conducted in N,N-dimethylformamide (DMF), no additional ligand was necessary to solubilize the CuBr catalyst. "-Alkyne-"-azido-terminated polystyrene was prepared by ATRP of Sty with propargyl 2-bromoisobutyrate as an initiator and subsequent post-polymerization nucleophilic substitution of the bromine end groups by reaction with NaN3. The resulting heterotelechelic polystyrene was click coupled in DMF with CuBr as the catalyst. A one-pot, two-step ATRP"nucleophilic substitution"click coupling process was also successful. For all three approaches, click coupling of the telechelic polystyrene "monomers" (Mn = 960 " 2590 g/mol) yielded moderate to high molecular weight polySty (Mn up to 21500 g/mol) with molecular weight distributions characteristic of step growth polymers (Mw/Mn = 2 - 5). Intramolecular click coupling also occurred in DMF producing cyclic structures, as evidenced by the formation of product with lower hydrodynamic volume than the starting material. The cyclization reaction was found to be concentration dependant and more dilute solutions provided a higher yield of cyclized product. A modification of this approach was taken by Grayson (132) who effectively maintained a low concentration of the "-alkyne-"-azido-terminated polymer the Cu(I)Br and 2,2'-bipyridine catalyst in a warm DMF solution by using a syringe pump to add a 2 mM solution of the l-PS-N3 in DMF was added over 25 hours. The product, c-PS, was isolated by extraction and precipitation.

(130) ""Click" coupling of azide- and alkyne-functionalized well-defined polymers prepared by atom transfer radical polymerization ," Tsarevsky, N. V.; Sumerlin, B. S.; Golas, P. L.; Matyjaszewski, K. Polymer Preprints 2005, 46, 179-180.

(131) "Step-Growth "Click" Coupling of Telechelic Polymers Prepared by Atom Transfer Radical Polymerization," Tsarevsky, N. V.; Sumerlin, B. S.; Matyjaszewski, K. Macromolecules 2005, 38, 3558.

(132) "An Efficient Route to Well-Defined Macrocyclic Polymers via "Click" Cyclization," Laurent, B. A.; Grayson, S. M. Journal of the American Chemical Society 2006, 128, 4238-4239.

Click to go to section 08: Incorporation of Functionality into Polymers Prepared by ATRP

Carnegie Mellon University | Mellon College of Science | Department of Chemistry