Incorporation of Functional Groups into
Polymers Prepared by ATRP
- Direct polymerization of functional monomers
- Post-polymerization modification of monomer units
- Use of functional ATRP initiators
- End-group transformation chemistry
Functional groups increase the utility of polymers and are fundamental to the development of many aspects of structure-property relationships. The functionality present on the monomer units determines the solubility of the polymer in a given solvent. One can control the hydrophilicity/phobicity, or polarity, the elasticity or modulus of a material by selecting appropriate monomers. End functionalized polymers are used for blend compatibilization during reactive processing, in all thermosetting compositions e.g. epoxy-functional polymers, and functional materials form the basis of the majority of products prepared for dispersant, coating, adhesive, and sealant, etc. applications.
Four synthetic strategies can be employed for the synthesis of well-defined polymers with site specific functional groups using ATRP.
- direct polymerization of functional monomers
- post-polymerization modification of monomer units
- use of functional ATRP initiators
- end-group transformation chemistry.
These approaches are summarized in the following scheme.
The most obvious way to incorporate functionality is the use of functional initiators:
ATRP is generally tolerant of various functional polar groups and this route has been successfully used in many instances to form homopolymers, random or gradient copolymers and block copolymers with controlled distribution of functional monomer units along the backbone. However in some cases, especially when strongly coordinating basic, nucleophilic or acidic monomers are used the monomers can react with either the ATRP catalyst (leading to its partial or complete deactivation) or with either the alkyl halide-type initiator or dormant polymeric species killing the active chain ends. While this limitation is being addressed by continued research targeting stable catalyst complexes the current synthetic strategy of choice in these cases is to use monomers with protected groups that can be transformed into the desired polar functionality after the polymerization:
This approach has been illustrated by the preparation of well-defined polymers with distributed tetrazole groups. (1) Indeed "Click" chemistry-type post-polymerization modifications have been used to introduce various functional groups into polymers prepared by ATRP (see below).
The most obvious way to incorporate a single functionality at the polymer chain end is the use of functional initiators:
This approach can be used to prepare either homo- or hetero-telechelic polymers. Use of a mono-functional initiator with desired functionality leads to direct "-functionalization of the polymer and no post-polymerization modification is required. In the vast majority of ATRP reactions the active, or growing, polymer chains are halogen-terminated, and can be further used as macroinitiators in chain-extension reactions or as precursors of end-functionalized polymers through end group transformations:
A number of nucleophilic substitution reactions have been employed to achieve this goal of post-polymerization functionalization making ATRP an attractive technique for the synthesis of well-defined end-functionalized polymers. This topic has been the subject of detailed review articles. (2-4)
1. "Well-Defined (Co)polymers with 5-Vinyltetrazole Units via Combination of Atom Transfer Radical (Co)polymerization of Acrylonitrile and \"Click Chemistry\"-Type Postpolymerization Modification." Tsarevsky, N. V.; Bernaerts, K. V.; Dufour, B.; Du Prez, F. E.; Matyjaszewski, K. Macromolecules 2004, 37, 9308.
2. "Functional polymers by atom transfer radical polymerization." Coessens, V.; Pintauer, T.; Matyjaszewski, K. Prog. Polym. Sci. 2001, 26, 337.
3. "Statistical, gradient, block, and graft copolymers by controlled/living radical polymerizations." Davis, K. A.; Matyjaszewski, K. Adv. Polym. Sci. 2002, 159, 2.
4. Controlled/living radical polymerization: Features, developments, and perspectives, Braunecker, W. A.; Matyjaszewski, K. Progress in Polymer Science 2007, 32, 93-146.
i) Direct polymerization of functional monomers
- Direct functionalization of the polymer backbone
- No post-polymerization modification
- High degree of functionality (DPfunctional monomer)
- Arrangement dependent on (co)polymer architecture
- Plethora of monomers with different functionality available
A number of monomers containing polar functional groups have been successfully polymerized by ATRP. They include acrylonitrile (AN), (5-8) (meth)acrylamides, (1, 10) 4-vinyl pyridine (4VP), (11, 12) dimethylaminoethyl methacrylate (DMAEMA), (13) and monomers containing an -OH group such as 2-hydroxyethyl acrylate (HEA) (14) and 2-hydroxyethyl methacrylate (HEMA). (15) Glycidyl acrylate (16) has also been polymerized by ATRP yielding well-defined polymers containing the reactive glycidyl group, which can be used as precursors for other functional groups. Water-soluble monomers (both neutral and ionic) can be polymerized in controlled fashion by ATRP directly in protic (aqueous) media. (17, 18) Some examples of monomers, including some with polar groups that have been polymerized by ATRP, are shown below.
ATRP catalysts with strongly binding ligands should be used for copolymerization of monomers containing functional groups (mostly substituted amides, amines, or pyridines) to avoid, or reduce, competitive complex formation between the monomer or polymer and the copper center. In many cases, catalyst destabilization can be suppressed by selection of the proper ligand or addition of excess ligand. For example, in the ATRP of sodium 4-styrenesulfonate in aqueous solution, there is a rapid disproportionation of the ATRP catalyst, resulting in loss of control. However, this disproportionation reaction is prevented in the presence of excess pyridine (a ligand substitute) or added bromide ions.(19) This is shown in the following table where water and a 1:1 water/pyridine mixture was used as the solvent for a CuBr / bpy polymerization of NaSS using a MePEOBiB initiator at 100:1 ratio at 30 C.
The reaction was clearly controlled in the presence of a large molar excess of the pseudo ligand pyridine.
While the ATRP of several types of polar monomers, particularly acidic ones, has proved quite challenging progress continues to be made. (20-21)
Other Functional Monomers
- Derivatized styrenics, (meth)acrylates, (meth)acrylamides
- Acidic monomers remain a challenge
- Functionality often dictates ATRP conditions (solvent, temp, catalyst, etc.)
- Macromonomers allow copolymerization of functional monomers that are generally polymerized by different mechanisms
5. "Synthesis of Well-Defined Polyacrylonitrile by Atom Transfer Radical Polymerization." Matyjaszewski, K.; Jo, S. M.; Paik, H.-j.; Gaynor, S. G. Macromolecules 1997, 30, 6398.
6. "An Investigation into the CuX/2,2'-Bipyridine (X = Br or Cl) Mediated Atom Transfer Radical Polymerization of Acrylonitrile." Matyjaszewski, K.; Jo, S. M.; Paik, H.-j.; Shipp, D. A. Macromolecules 1999, 32, 6431.
7. "Synthesis of Styrene-Acrylonitrile Copolymers and Related Block Copolymers by Atom Transfer Radical Polymerization." Tsarevsky, N. V.; Sarbu, T.; Goebelt, B.; Matyjaszewski, K. Macromolecules 2002, 35, 6142.
8. "Nanostructured Carbon Arrays from Block Copolymers of Polyacrylonitrile." Kowalewski, T.; Tsarevsky, N. V.; Matyjaszewski, K. J. Am. Chem. Soc. 2002, 124, 10632.
9. "Atom transfer radical polymerization of (meth)acrylamides." Teodorescu, M.; Matyjaszewski, K. Macromolecules 1999, 32, 4826.
10. "Controlled polymerization of (meth)acrylamides by atom transfer radical polymerization." Teodorescu, M.; Matyjaszewski, K. Macromol. Rapid Commun. 2000, 21, 190.
11. "Atom Transfer Radical Polymerization of 4-Vinylpyridine." Xia, J.; Zhang, X.; Matyjaszewski, K. Macromolecules 1999, 32, 3531.
12. "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.
13. "Controlled/\"Living\" Radical Polymerization of 2-(Dimethylamino)ethyl Methacrylate." Zhang, X.; Xia, J.; Matyjaszewski, K. Macromolecules 1998, 31, 5167.
14. "Polymerization of acrylates by atom transfer radical polymerization. Homopolymerization of 2-hydroxyethyl acrylate." Coca, S.; Jasieczek, C. B.; Beers, K. L.; Matyjaszewski, K. J. Polym. Sci.: Part A: Polym. Chem. 1998, 36, 1417.
15. "Atom Transfer Radical Polymerization of 2-Hydroxyethyl Methacrylate." Beers, K. L.; Boo, S.; Gaynor, S. G.; Matyjaszewski, K. Macromolecules 1999, 32, 5772.
16. "Polymerization of acrylates by atom transfer radical polymerization. Homopolymerization of glycidyl acrylate." Matyjaszewski, K.; Coca, S.; Jasieczek, C. B. Macromol. Chem. Phys. 1997, 198, 4011.
17. Matyjaszewski, K.; Tsarevsky, N. In PCT Int. Appl.; (CMU, USA). WO 0228913, 2002, p 64 pp.
18. "\"Green\" Atom Transfer Radical Polymerization: From Process Design to Preparation of Well-Defined Environmentally Friendly Polymeric Materials, Tsarevsky, N. V.; Matyjaszewski, K. Chemical Reviews, 2007, 107, 2270-2299.
19. "Deactivation Efficiency and Degree of Control over Polymerization in ATRP in Protic Solvents," Tsarevsky, N. V.; Pintauer, T.; Matyjaszewski, K. Macromolecules 2004, 37, 9768-9778.
20. "Atom transfer radical polymerization of protected methacrylic acids." Zhang, X.; Xia, J.; Matyjaszewski, K. Polym. Prepr. 1999, 40(2), 440.
21. "Atom Transfer Radical Polymerization of Dimethyl(1-ethoxycarbonyl)vinyl Phosphate and Corresponding Block Copolymers." Huang, J.; Matyjaszewski, K. Macromolecules 2005, 38, 3577-3583.
ii) Post-polymerization modification of monomer units
- Incorporate functionality incompatible with polymerization
- Characterization prior to further functionalization
- Facilitates grafting to or grafting from other polymers
Acidic monomers such as (meth)acrylic acid, isomeric vinylbenzoic acids, or unsaturated sulfonic or phosphonic acids protonate and therefore "destroy" the transition metal complexes with N-based ligands typically used as ATRP catalysts. Although some moderately successful attempts have been made to polymerize MAA by ATRP, in general, protected acids are preferred. Examples of protective groups include t-Bu (22, 23) or the TMSO group frequently used as the precursor of brush macromolecules.
Tetrazoles are both acidic, (resembling carboxylic acids), and coordinating compounds and hence the direct ATRP of vinyltetrazoles has not been reported. The preparation of well-defined tetrazole-containing polymers can be accomplished using ATRP to (co)polymerize AN followed by a "click" chemistry-type of chemical modification of the produced polyAN to yield the desired materials. (4)
A number of high yield "click" chemistry functionalization of monomer units in a copolymer backbone have been conducted as shown below. (24-28)
As mentioned above, HEA and HEMA are easy to (co)polymerize by ATRP, and the polymers containing the hydroxyl-group can be used in esterification reactions with various functionalized carboxylic acids to attach a different functional groups to the polymer backbone. This approach is frequently used to prepare multifunctional macroinitiators for the preparation of bottle-brush copolymers. However, esterifications are not always efficient, especially when a polymeric substrate is used. Alternatively poly(glycidyl acrylate) copolymers can be prepared by ATRP to serve as a precursor of functional polymers, since the pendant glycidyl group can react with nucleophiles and thereby serve as a precursor of functional polymers
In another example a novel monomer, 3-azidopropyl methacrylate (AzPMA), was polymerized via ATRP with good control of the polymer molecular weight distribution and retention of chain functionality. Poly(3-azidopropyl methacrylate) was coupled with propargyl alcohol, propargyl triphenylphosphonium bromide, propargyl 2-bromoisobutyrate, and 4-pentynoic acid via a highly efficient "click" reactions in the presence of a CuI catalyst. The azido-functionalized polymer demonstrated enhanced reactivity during "click" reactions compared to small molecules with comparable structures. The ability of the coupling reactions to be conducted at room temperature without significant excess of reagents makes this an attractive alternative approach for the preparation of (co)polymers with high degrees of functionalization. (28) To avoid the synthesis of the heat- and shock-sensitive azide-containing monomer, azido groups were introduced in polymers containing epoxide groups using a click-type ring-opening reaction. The azide-containing polymers thus prepared were further modified using alkyne-azide-type click reactions. (29)
22. "Atom Transfer Radical Polymerization of tert-Butyl Acrylate and Preparation of Block Copolymers." Davis, K. A.; Matyjaszewski, K. Macromolecules 2000, 33, 4039.
23. "Preparation of block copolymers of polystyrene and poly(t-butyl acrylate) of various molecular weights and architectures by atom transfer radical polymerization." Davis, K. A.; Charleux, B.; Matyjaszewski, K. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 2274.
24. Huisgen, R., in 1,3-Dipolar Cycloaddition Chemistry, Padwa, E., Ed., Wiley, 1984, pp. 1-176
25. "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
26. "Efficiency and Fidelity in a Click-Chemistry Route to Triazole Dendrimers by the Copper(I)-Catalyzed Ligation of Azides and Alkynes," Wu, P. et al., Angew. Chem., Int. Ed., 2004, 43, 3928
27. "Reversible Redox Cleavage/Coupling of Polystyrene with Disulfide or Thiol Groups Prepared by Atom Transfer Radical Polymerization.".Tsarevsky, N. V., Matyjaszewski, K., Macromolecules, 2002, 35(24), 9009.
28. "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.
29. "Graft Copolymers by a Combination of ATRP and Two Different Consecutive Click Reactions," Tsarevsky, N. V.; Bencherif, S. A.; Matyaszewski, K. Macromolecules 2007, 40, 4439.
iii) Use of functional ATRP initiators
- Direct functionalization
- No post-polymerization modification
- Yields _-telechelic polymers
- Multiple applicable functionalities
Various functionalized ATRP initiators have been used to prepare hetero-telechelic polymers. A few examples of functional initiators used in styrene polymerization are shown below and a more extensive listing is provided in the review paper by Coessens et al. (1)
One route to alpha,omega-homo-telechelic copolymers is to couple a copolymer prepared using a mono-functional initiator in an atom transfer radical coupling reaction (ATRC). The atom transfer coupling reaction is driven to completion by continuous removal of the deactivator from the reaction medium. Initially this was accomplished by the addition of a transition metal in the zero oxidation state. (30, 31) However, a more environmentally benign approach was recently developed using ascorbic acid or reducing sugars to drive the coupling reaction.
The coupling efficiency of (meth)acrylates can be improved by the addition of one mole of styrene immediately prior to the addition of the coupling agent. (30, 32)
Difunctional initiators can also be employed to introduce functionality into the mid point of a chain and when coupled with the coupling reaction can lead to the formation of degradable polymers that degrade to oligo/polymeric fragments of known molecular weight and narrow PDI. This was exemplified recently by the use of an initiator with an internal disulfide bond to polymerize styrene. The disulfide bond could degrade under a reducing environment to yield the corresponding thiol-terminated polystyrene. The thiol end groups were efficiently coupled back to the starting disulfide by oxidation with FeCl3. (33) Polar monomers could also be polymerized using the same difunctional initiator. (34) The implication of this incorporation of a degradable link into the backbone of a polymer will be discussed fully in the section discussing degradable networks.
30. "Synthesis of Hydroxy-Telechelic Poly(methyl acrylate) and Polystyrene by Atom Transfer Radical Coupling." Sarbu, T.; Lin, K.-Y., Spanswick, J., Gil, R. R., Siegwart, D. J.; and Matyjaszewski K., Macromolecules 2004, 37, 9694-9700.
31. "Synthesis of telechelic oligomers via atom transfer radical polymerization, 1 study of styrene." Otazaghine, B., David, G., Boutevin, B., Robin, J. J., Matyjaszewski, K.: Macromolecular Chemistry and Physics, 2004, 205, 154-164.
32. "Synthesis of telechelic oligomers by atom transfer radical polymerization: A study of acrylate monomers." Otazaghine, B., Boyer, C., Robin, J.-J., Boutevin, B.: Journal of Polymer Science, Part A: Polymer Chemistry, 2005, 43: 2377-2394.
33. "Reversible Redox Cleavage/Coupling of Polystyrene with Disulfide or Thiol Groups Prepared by Atom Transfer Radical Polymerization," Tsarevsky, N. V.; Matyjaszewski, K. Macromolecules 2002, 35, 9009-9014.
34. "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.
iv) End-group transformation chemistry
- Incorporate functionality incompatible with polymerization
- Characterization prior to further functionalization
- Prepare omega- and alpha,omega-telechelic polymers
- Block copolymers, immobilized to surfaces, etc.
The halide end-functionality, frequently present on the active chain end(s) of polymers prepared by ATRP, particularly polystyrenes or polyacrylates, can participate in nucleophilic substitution reactions. This strategy has been used for the synthesis of a plethora of end-functional well-defined polymeric materials. The advantages are that one can incorporate functionality incompatible with the polymerization process, and if desired the material can be characterized prior to further functionalization. This procedure allows the preparation of omega- and alpha,omega-telechelic polymers and the selection of functionality suitable for further specific reactions such as attachment to bio-materials or materials that can be immobilized to surfaces, etc.
Sample conditions for azidation of the terminal halides on the three major classes of monomers are:
An early example is the reaction of halogen-capped polymers with sodium azide. (35-37) A diazido-terminated polySty prepared in this way could be further reduced with tributylphosphine in THF in the presence of water (moisture) to yield a well-defined diamino-terminated polymer, that, in turn, could be used in a step-growth process with terephthaloyl chloride leading to polyamides with controlled-length polystyrene segments. (36).
Azido-terminated polymers can also be used in click chemistry modifications with acetylene derivatives to incorporate various functional groups. Polymers with phopshonium salt end-groups were prepared from bromine-terminated polystyrene or polyacrylates, and Bu3P. (38) Mercapto-terminated polystyrene was prepared by the reaction of the corresponding bromo-compound with either thiodimethylformamide or thiourea followed by reaction with a nucleophilic compound. (39)
Azido-terminated polymers can also be used in click chemistry modifications with acetylene derivatives to incorporate various functional groups. Polymers with phopshonium salt end-groups were prepared from bromine-terminated polystyrene or polyacrylates, and Bu3P. (33) Mercapto-terminated polystyrene was prepared by the reaction of the corresponding bromo-compound with either thiodimethylformamide or thiourea followed by reaction with a nucleophilic compound.
This approach can easily be adapted to provide a step growth coupling reaction from telechelic copolymers prepared by ATRP (40)
The effect of ligand and solvent on copper-catalyzed click reactions were recently reported, and catalysts based on metals other than copper, such as platinum and palladium were studied. (41)
Many other end-group transformations using polymers prepared by ATRP are described in extensive review papers. (1, 42)
35. "Synthesis of well-defined azido and amino-endfunctionalized polystyrene by atom transfer radical polymerization." Matyjaszewski, K.; Nakagawa, Y.; Gaynor, S. G. Macromol. Rapid Commun. 1997, 18, 1057.
36. "Synthesis of azido end-functionalized polyacrylates via atom-transfer radical polymerization." Coessens, V.; Nakagawa, Y.; Matyjaszewski, K. Polym. Bull. 1998, 40, 135.
37. "End group transformation of polymers prepared by ATRP, substitution to azides." Coessens, V.; Matyjaszewski, K. J. Macromol. Sci., Pure Appl. Chem. 1999, A36, 667.
38. "Synthesis of polymers with phosphonium end groups by atom transfer radical polymerization." Coessens, V.; Matyjaszewski, K. J. M. S. - Chem. 1999, A36, 653.
39. "Step-Growth "Click" Coupling of Telechelic Polymers Prepared by Atom Transfer Radical Polymerization." Tsarevsky, N. V., Sumerlin, B. S., Matyjaszewski, K.: Macromolecules 2005, 38: 3558-3561.
40. "Combining Atom Transfer Radical Polymerization and click chemistry: A versatile method for the preparation of end-functional polymers." Lutz, J.-F., Boerner, H. G., Weichenhan, K.: Macromolecular Rapid Communications 2005, 26: 514-518.
41. "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.
42. "Atom Transfer Radical Polymerization." Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921.
Click to go to section 09: Nanostructures Materials