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Properties of Well-defined Novel Copolymers Prepared by CRP

  • Evaluation of the Properties of Materials Prepared by ATRP
  • Gradient Copolymers
  • Gradient block copolymers
  • Block Copolymers
  • Polar thermoplastic elastomers
  • Surfactants
  • Molecular assembly of block copolymers
  • Graft Copolymers
  • Brush Macromolecules
  • Stimuli responsive brushes
  • SuperSoft elastomers
  • Physical crosslinking
  • Ionic conductors
  • Surfaces
  • Flat surfaces
  • Control of surface properties
  • Antibacterial Surfaces

Evaluation of the Properties of Materials Prepared by ATRP

Another objective of the research conducted in the Matyjaszewski group is to develop an understanding of the properties of materials prepared for the first time by CRP. However since the properties of the materials prepared by CRP can be “adjusted” by many subtle and presently unexplored ways we use the word “evaluation” in the title of this section. We hope the following brief overview might give you some ideas on how to construct materials with the spectrum of properties required by your target application.

Many of the following slides were provided by Professor Tadeusz Pakula from the Max-Planck-Institute for Polymer Research, Mainz, Germany who collaborated with the group. The slides are taken from a presentation made at the March 2005 CRP Consortium Meeting. The slides have been selected to show how the bulk properties of a material depend on the structure, size scales and heterogeneity of the material, and the dynamics, or time scales, of the test; and how the response rates of the polymer to stress ultimately depend on molecular structure. They illustrate how different molecular elements on a macroscopic continuum are affected by the physics of processing and how they interact with the chemical parameters controlled by synthesis. They are presented here in order to provide a foundation from which to understand or learn from many of the figures presented later on this page, and discussed in greater detail in papers co-authored by Professor Pakula and Professor Matyjaszewski. The first slide illustrates how the macroscopic continuum of phase separation in a polymer can be influenced at different levels by physical or chemical manipulation of the material.

The next slide illustrates how the physical properties of a material are dependent on the composition or morphological elements of the material and how the scale of the dynamics of molecular motion change with temperature.

In many of the following figures changes in polymer dynamics, over several orders of magnitude, have been measured using mechanical spectroscopy. The final spectrum is constructed from mechanical spectroscopy measured over a fairly narrow frequency range at different temperatures. When the curves are fitted together they provide an illustration of mechanical response over a wide range of physical environments.

The following figure shows the spectrum of a linear polyisoprene. It clearly shows the transition from the glassy state through segmental relaxation, transitioning into a rubbery state, seen as a plateau, prior to chain relaxation and polymer flow.

Professor Pakula has done a great deal of work on modeling such systems particularly Monte Carlo and Molecular Dynamics during his interactions with a spectrum of synthetic chemists around the world but we will focus solely on the results from materials uniquely prepared by CRP at CMU.

There are a multiplicity of materials that could be discussed in each of the following topical sub-headings but we will focus on a very few examples that were selected in the hope they show how materials prepared by CRP differ from materials prepared using earlier polymerization processes.

Gradient Copolymers

Gradient copolymers could not be prepared until a robust Controlled Radical Copolymerization was developed. [1] The cross-propagation kinetics in “living” ionic processes did not allow synthesis of such materials. The initial paper on properties of gradient copolymers (2) provides a comparison between the properties of a styrene-methyl acrylate gradient copolymer (G2), a random copolymer (R1) and a block (B1) copolymer, each with the same overall composition. The following figures show the DSC thermograms and the temperature dependencies of the storage (G') and loss moduli (G'') for the three materials. The extraordinary broad temperature range for the segmental relaxation in the gradient copolymer is due to a wide spectrum of compositions within the chain.

It is possible to manipulate the phase transitions in a gradient copolymer by controlling the molecular weight, composition and shape of the gradient along the polymer backbone. Potential applications for gradient copolymers include reinforcing agents, blend compatibilizers (3) or, because their behavior at an interface can be controlled, as impact modifiers and sound or vibration dampeners. In addition, because of the ability to prepare materials with a broad and selectively adjustable range of Tg's, gradient copolymers will find application as pressure sensitive adhesives, and wetting or leveling additives for coatings or inks. (4)

In a paper discussing the “Cooperative Motion Algorithm” (5) Professor Pakula indicated that smoothing of the chain-end distributions are most efficient for gradient copolymers. A later joint paper demonstrated that the volume fraction of a gradient copolymer that resides within the interphase of a phase separating copolymer can be modified by changing the gradient along the polymer backbone resulting in a very broad segmental relaxation. Consequentially the composition changes continuously across the interphasial boundary and the volume fraction of the material within the boundary layer can be controlled thereby modifying the bulk properties of the material. (6)

Gradient block copolymers As a transition to the next section the following figures are taken from reference (2) and show how the properties of an ABA block copolymer can be changed by incorporation of a low concentration of a second monomer into the A blocks. The polymers are GM13, MMA131-BA507-MMA131, and GM14, (MMA115/BA15)-BA527-(MMA115/BA15). The presence of a gradient of composition along the backbone of the A-blocks leads to a block copolymer with reduced ultimate stress but much higher ultimate elongation than the comparable block copolymer with pure MMA A-blocks.

The reason for this behavior is that a higher mole fraction of the A block copolymer segment resides in the interphasial layer resulting in smaller domains of the “A” blocks, compared to the pure block copolymer.

1. “Synthesis of novel gradient copolymers via atom transfer radical polymerization.” PhD Thesis Greszta, D., 1997, p 276 pp.
2. “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.
3. “Application of styrene/methyl methacrylate gradient copolymer.” Wang, T., Liu, F., Luo, N., Ying, S., Liu, Q.: Hecheng Xiangjiao Gongye 2001, 24: 328-330.
4. “Controlled polymerisation techniques for the design of novel polymeric additives.” Goebelt, B.: FATIPEC Congress 27th: 2004, 303-312.
5. “Copolymers with controlled distribution of comonomers along the chain, 1. Structure, thermodynamics and dynamic properties of gradient copolymers. Computer simulation” Tadeusz Pakula, Krzysztof Matyjaszewski; Macromol. Theory Simul., 1996, 5, 987-1006
6. “Synthesis and properties of copolymers with tailored sequence distribution by controlled/living radical polymerization.” Lutz, J.-F., Pakula, T., Matyjaszewski, K.: ACS Symposium Series, 2003, 854: 268-282.

Block Copolymers

Innumerable block copolymers with novel compositions in each segment have been prepared by CRP, particularly after the initial expansion of the range of copolymerizable monomers that occurred when ATRP was developed. The following discussion focuses on a few examples that show properties significantly different from the properties of materials that could be prepared before 1995. The first example is a polar thermoplastic elastomer whose synthesis became much simpler with the development of CRP. The second example is the synthesis of an ABC block copolymer where each block or segment is selected to accomplish a specific function in the final application.

Polar thermoplastic elastomers The major benefit of polar thermoplastic elastomers (TPEs) is that they are oil resistant and recyclable, i.e. it is possible to injection mold the materials and minimize waste by immediately recycling sprues and runners in addition to providing long term recycleability as required for automotive applications in Europe. The potential market for such materials has been estimated to be $2.7 billion/yr market by 2007 (Global Information Inc. (GII), Press release 12/19/03) Polar TPEs are materials that resistant to hydrocarbon solvents (e.g., fuels). TPEs provide the softness, flexibility, resilience of elastomers with the processability of thermoplastics. The most common examples of commercial TPEs are poly(styrene-b-butadiene-b-styrene) (e.g., Kraton®) and polyurethane-polyether and polyester-polyether multiblock copolymers. However the materials that can now be prepared by CRP provide several additional benefits including greater versatility in monomer choice, which provides materials where the Tg and phylicity of hard and soft block can be selectively chosen, or tailored to attain specific properties required for the application, and additional functionality can be incorporated into one or both blocks. Other architectures can be considered (e.g., star-blocks, grafts, brushes with block side-chains, etc.) since polymer topology affects properties.

The desire to prepare well defined thermoplastic elastomers led to the development of “halogen exchange”. The use of “halogen exchange” in a polymerization increases the rate of initiation in the critical cross propagation chain extension step, from the added macroinitiator, compared to the rate of propagation of the second monomer and is required for the preparation of well defined P(MMA-BA-MMA) block copolymers. (7) The properties of a TPE can be readily modified in a CRP by incorporation of copolymer blocks into the design of the material. However rather than discuss this linear polar TPE macromolecule we will discuss the properties of PBA-b-PAN 3-Arm Star Block Copolymers. These materials are polar TPE's with easily adjustable properties, based on composition, that retain their useful properties over a temperature range: -50 to +250 C!

Stress/strain relationship dependence on %PAN
for Mn (PBA) = 117 000

The compositions of the star block copolymers discussed below are given in the following tables.


  • all samples are glassy below the Tg of pBA, (~227K), with G´~ GPa;
  • above the Tg of pBA the materials remain elastic with a G´ plateau extending up to the softening temperature of pAN (~370K). The height of the plateau is dependent on the pAN content:
    G´~0.1 MPa when pAN fraction < 0.1 and
    G´~100 MPa when pAN fraction is 0.3;
  • above the temperature of softening of the pAN block the material remains elastic with a lower G´ plateau (0.1 - 1 MPa) extending up to temperature of ~550K. This behavior probably indicates some chemical effects such as chemical crosslinking take place above the melting temperature. (Note similar materials are used as precursors for nano-structured carbons.) {Link}

In the following figures the mechanical spectrum resulting from quasi-static stress-strain
cycles with successively increasing maximum strain are shown in different colors. The PAN1 sample exhibits nearly ideal elastic behavior: no residual strains after unloading. Whereas with the samples with slightly higher mole fractions of PAN the elasto-plastic behavior shows large residual strains after unloading the plastic deformation which is probably associated with fragmentation of the pAN cylindrical domains.

Under quasi-static stretching the modulus increases with pAN content and the draw ratio at break seems to increase with the length of pBA segment and be reduced with increasing pAN content.

High temperature annealing indicated a hardening of the annealed samples which probably results from improved morphological order. A sample annealed above 300°C remains elastic at temperatures above the softening point of pAN.

This example shows how the properties of a thermoplastic elastomer can be readily modified by changing the volume fraction of the A-blocks in the final product.

7. “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.


Two examples will be provided where the components of each segment in a block copolymer was selected to give the final material a set of properties required to accomplish a specific task.

a) Block copolymer specifically designed as a surfactant for dispersion polymerization Since the preparation of polymers with terminal functionality and block copolymers or segmented copolymers is “easy” by CRP techniques it would be expected that materials suitable as surfactants have been prepared and used in various applications including emulsion polymerization (8) and dispersion polymerization in super-critical CO2, (9) which required the synthesis of a specific fluorinated polymeric surfactant stabilizer. (10) This has indeed been the case however we will discuss a more recent example of a surfactant specifically designed for a particular application. This is the synthesis and use of a poly(_-caprolactone)-b-poly(octadecyl methacrylate)-b-poly(dimethylaminoethyl methacrylate) block/random copolymer which was expressly prepared to act as a surfactant in the dispersion polymerization of L-lactide (11) to form biodegradable nanoparticles of controlled dimensions. (12) The poly(_-caprolactone) block was prepared first by an anionic ring opening polymerization of _-caprolactone initiated with hydroxyethyl 2-bromoisobutyrate/ tin(II) hexanoate system followed by sequential ATRP of octadecyl methacrylate then dimethylaminoethyl methacrylate.

The ABC triblock copolymers functioned as surfactants for the dispersion polymerization of cyclic esters and microspheres (particles with diameters above 0.5 mm) with functional groups in the surface layer could be obtained, either directly in the presence of surfactants containing polyester blocks and blocks with the required terminal chemical groups or by post-treatment (partial hydrolysis) of the earlier synthesized polyester microspheres without functional groups.

Hydrolysis introduced carboxyl and hydroxyl groups into surface layer of these particles. The microspheres were transferred (via ethanol) to water and did not aggregate due to stabilization by the DMAEMA blocks containing amino groups.

b) Block copolymer specifically designed for destruction of DNAPL. DNAPL or Dense Nonaqueous Phase Liquids are mainly chlorinated solvents, e,g trichloroethylene, tetrachloroethylene or chloroform that have accumulated in aquifiers. A series of block copolymers were designed to deliver iron nanoparticles to DNALP in underground reservoirs. They have an anchoring segment, and both a hydrophobic and a hydrophilic block. The amphiphilic ABC triblock copolymer precursors consisting of poly((tBMA)-b-(MMA)-b-(St)) and poly((tBMA)-b-(BMA)-b-(St)) were synthesized via ATRP. Sulfonation of the polystyrene block and hydrolysis of t-butyl ester groups were conducted simultaneously by reaction with acetyl sulfate. (13)

The resulting block copolymers were capable of self-attachment to iron nanoparticles by mixing commercially available iron nanoparticles with the triblock copolymer followed by sonication in water. A slight shift in size distribution of the iron particles was clearly observed but the size distribution remained monomodal making the composite structures suitable for transportation through aqueous media to hazardous DNAPLs in aquifers. When contact is made with a hydrophobic medium the hydrophobic block swells and reases the iron nanoparticle which could then destroy the DNAPL via reductive dechlorination reactions.

Hydrophylic block, Hydrophobic block, Anchoring Block

In bench top experiments the modified iron particles migrate to the interface between water and organic solvents. This behavior is strongly dependent in the composition of the copolymer but the narrow size distribution of emulsion droplets is typical of stable Pickering emulsions and the emulsions are stable more than 6 months. Therefore a targeted reactive nano-iron delivery system for the remediation of chlorinated solvent-contaminated groundwater was obtained and is being examined in real soil.

8. “Amphiphilic block copolymers prepared via controlled radical polymerization as surfactants for emulsion polymerization.” Burguiere, C., Pascual, S., Coutin, B., Polton, A., Tardi, M., Charleux, B., Matyjaszewski, K., Vairon, J.-P.: Macromol. Symp., 2000, 150: 39-44.
9. “Atom Transfer Radical Polymerization in Supercritical Carbon Dioxide.” Xia, J., Johnson, T., Gaynor, S. G., Matyjaszewski, K., DeSimone, J.:. Macromolecules, 1999, 32: 4802-4805.
10. “Molecularly engineered surfactants for CO2.” DeSimone, J. M., Betts, D., Johnson, T., McClain, J. M., Wells, S. L., Dobrynin, A., Rubinstein, M., Londono, D., Wignall, G., Triolo, R.: Polym. Prepr. , 1999 40: 435-436.
11. “Block and random copolymers as surfactants for dispersion polymerization. I. Synthesis via atom transfer radical polymerization and ring-opening polymerization.” Jakubowski, W., Lutz, J.-F., Slomkowski, S., Matyjaszewski, K.: Journal of Polymer Science, Part A: Polymer Chemistry, 2005, 43: 1498-1510.
12. “Biodegradable nano- and microparticles with controlled surface properties.” Slomkowski, S., Gadzinowski, M., Sosnowski, S., De Vita, C., Pucci, A., Ciardelli, F., Jakubowski, W., Matyjaszewski, K.: Macromolecular Symposia, 2005, 226: 239-252.
13. “Preparation of amphiphilic block copolymers for the stabilization and delivery of iron nanoparticles for remediation of DNAPLs.” Ok, J., Dufour, B.; Sarbu, T.; Matyjaszewski, K.; Polymer Preprints 2005, 46, 347.

Molecular assembly of block copolymers A multiplicity of novel block copolymers can be synthesized as a result of the development of CRP. Amongst them are materials prepared by dual polymerization mechanisms such as materials prepared by a polycondensation process followed by ATRP. (14) Rather unexpectedly the bulk morphology of the tri-block copolymer led to the development unique properties. The simple schematic of the block copolymer disguises the repercussions of the formation of a supramolecular structural unit that impacts the bulk structure, and hence the physical properties of the material. (15)

Block Polymer Supramolecular unit Bulk Structure

Changes in the copolymer architecture influence the range of frequencies (or temperatures) in which various properties appear. The fabricated material behaves in the manner expected from a pBA with a molecular weight in the millions due to the requirement for cooperative molecular relaxation.

This requirement for cooperative molecular relaxation is perhaps more dramatically illustrated by an examination of the relaxation modes of a melt of bottle-brush macromolecules. When the relaxation modes of a poly(n-butylacrylate) bottle-brush macromolecules with a main chain length of 400 and a side chain length of 60 is examined one can discern an additional relaxation resulting from the total molecular relaxation at very low modulus. Because of the spatial correlations of the supramolecular units and the dense packing of polymer chains in bulk materials the motion must occur cooperatively and therefore constitutes the slowest structural relaxation. This phenomenon is also seen in single bottle brush macromolecules which display comparable complexity and indeed were the first materials in which the supersoft state was identified. (16)

The side chain length and backbone length can be independently varied and each influences the viscoelastic behavior of the bottlebrush copolymers. The backbone length has a major influence on the flow controlled by global rearrangements. The properties are discussed in detail below.

Another aspect of molecular assembly of block copolymers is discussed in the section on precursors for carbon nano-structures where phase separated block copolymers containing PAN are converted to carbon nanostructures.

14. “Step-Growth Polymers as Macroinitators for \"Living\" Radical Polymerization: Synthesis of ABA Block Copolymers.” Gaynor, S. G., Matyjaszewski, K.: Macromolecules; 1997, 30: 4241-4243.
15. “Structure and properties of poly(butyl acrylate-block-sulfone-block-butyl acrylate) triblock copolymers prepared by ATRP.” Zhang, Y., Chung, I. S., Huang, J., Matyjaszewski, K., Pakula, T.: Macromolecular Chemistry and Physics, 2005, 206: 33-42.
16. “Polymers, supersoft elastomers and methods for preparing the same.” Pakula, T., Matyjaszewski, K.: PCT Int. Appl.: 2004014963.

Graft Copolymers

Once again we will only discuss a few of many possible graft copolymers that have been, or could be prepared by applying ATRP to materials synthesis. An early review on the preparation of graft copolymers by ATRP utilizing both “grafting through” and “grafting from” provided examples of poly(N-vinlpyrrolidone-g-styrene); poly(methyl methacrylate-g-dimethylsiloxane), discussed in greater detail below; poly(methyl methacrylate-g-lactide); poly(n-butyl acrylate-g-methylmethacrylate); and poly(n-butyl acrylate-g-ethylene) prepared by grafting through, and examples prepared by grafting from functionalized poly(isobutene), polyethylene, polyvinyl chloride. (17)

As noted in the chemistry discussion the reactivity ratio of monomers and macromonomers may also be affected by micro-inhomogeneity of the reaction mixture and reaction mechanism. This has often been observed in macromonomer copolymerization, or “grafting through” copolymerization for preparation of graft copolymers that can undergo phase separation. For example, when MMA was copolymerized with a poly(dimethylsiloxane)-methacrylate macromonomer, the composition of the product was strongly affected by the reaction conditions. (18) As shown in the following cartoons copolymerization by conventional FRP led to the preparation of copolymers that displayed a broad distribution of chain length and composition due to the continuous change in the instantaneous feed ratios of the comonomers remaining in the reaction medium and the effect of continuous initiation of copolymerization throughout the reaction. At the other extreme ATRP performed in 30% xylene solution at 90 0C utilizing a compatible macroinitiator resulted in preparation of a uniform comb-like graft copolymer due to similar reactivity of both comonomers, r ~1 in the diluted reaction solution. However, conducting ATRP or RAFT in only 3% xylene generated gradient copolymers, due to monomer feed variation throughout the reaction because of viscosity effects on macromonomer reactivity resulting in a higher reactivity ratio for MMA, r~2, compared to the macromonomer. The following schematics illustrate the differences in the microstructure and also the dramatic affect the differences in microstructure had on tensile properties. The tapered, or gradient graft copolymer has a tensile elongation of only ~30%, compared to ~120% for the irregular product formed by FRP, or the~280% for a regular comb copolymer. (19) These dramatic differences in physical properties occur even though all three copolymers have approximately the same overall composition (~50/50) and the same overall molecular weight Mw~100,000.

All 3 samples have similar Mn~100,000 & contain ~50 w% PDMS. They differ in the distribution of grafts along the copolymer backbone. (see above slide)

DSC results:
ATRP -112.4°C
FRP -116.8°C
RAFT -120.7°C

Topological control can also be extended to heterograft copolymers (20) where graft copolymers with different grafts can be prepared in a one step or two step CRP grafting through procedure with two different macromnomers forming copolymers with different gradient distribution of grafts. In the following set of schematics the different reactivity of a PLA macromonomer and a PDMS macromonomer are shown to result in the preparation of a gradient copolymer with a gradient of both macromonomers along the polymer backbone. The PLA-M is preferentially incorporated while the PDMS-M is incorporated later in the copolymerization process forming a copolymer with a higher concentration of PLA grafts at one chain end and a higher concentration of PDMS grafts at the other. This situation is amplified when a two step copolymerization is conducted.

An example of a graft copolymer prepared by ATRP in a “grafting from” reaction is a polyethylene-g-poly(n-butyl acrylate) which used a polyethylene macroinitiator with distributed _-bromoisobutyrate functionality. The copolymers were prepared using a less active BA6TREN based ligand that was soluble in the reaction medium to controllably polymerize butyl acrylate in a solution of the polyethylene macroinitiator. (21-22) Depending on the mole ratio of the segments such a material could find application as an impact modifier for polyethylene, a compatibilizer in polyethylene blends, as a surfactant or as a surface modifier to provide better adhesive properties. The macroinitiator was prepared by copolymerization of ethylene with undecenol using a zirconocene based catalyst. The pendant hydroxyl groups were esterified ml 2-bromopropionyl bromide for a butyl acrylate grafting from reaction.

There is strong incompatibility between the components and the final copolymer, containing 32-67% butyl acrylate, showed both clear melting and crystallization peaks attributable to polyethylene and a glass transition from the butyl acrylate. At lower mole fractions of butyl acrylate the matrix was based on polyethylene and displayed good extensibility as well as molecular and structural orientability while the material with a poly(butyl acrylate) matrix displayed a loss of resistance to tensile deformation. Below the glass transition the PBA phase seems to play a role of reinforcement in the PE matrix so that the modulus of the material can even exceed the values determined for the pure polyethylene. Above the glass transition of the PBA, the soft phase of grafts influences the modulus of the polymers to a degree dependent on composition. A strong effect of the presence of the grafted chains was also seen in the molten state in which the grafted polymers behave as elastomers with remarkably higher modulus than determined for the entangled polyethylene. The observed behavior should be related to the two phase structure of the systems with PBA microdomains probably dispersed in the PE matrix and networked by the PE backbone chains. A PE-g-MMA demonstrated good performance as a compatibilizer between the two homopolymers when the blends were studied by TEM. (23)

(a) PE/PMMA(4/6 w.r.) (b) PE/PMMA/PE-g-PMMA (4/6/1 w.r.)

17. Graft copolymers by atom transfer polymerization; Boerner, H. G.; Matyjaszewski, K. Macromolecular Symposia 2002, 177, 1-15
18. Improving the Structural Control of Graft Copolymers by Combining ATRP with the Macromonomer Method; Shinoda, H., Miller, P. J., Matyjaszewski, K.; Macromolecules 2001, 34: 3186-3194
19. 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.
20. “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.
21. 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.
22. “Preparation of polyethylene block copolymers by a combination of postmetallocene catalysis of ethylene polymerization and atom transfer radical polymerization.” Inoue, Y. and Matyjaszewski, K. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 496
23. “Graft Copolymers from Linear Polyethylene via Atom Transfer Radical Polymerization”. Inoue, Y.; Matsugi, T.; Kashiwa, N.; Matyjaszewski, K. Macromolecules 2004, 37, 3651-3658.

Brush Macromolecules

As the solution and bulk physical properties of these materials with complex intramolecular structure have begun to be examined a number of potential applications presented themselves. These include impact resistant materials, solid stable replacements for hydrogels, stabilizers, surface-modifying agents, dispersants, emulsifiers, compatibilizers and even materials that provide environments for intramolecular chemistry such as directed mineralization of inorganic nanocrystals (24) and for the formation of high aspect ratio nanowires. (25)
The solution properties of bottle-brush macromolecules were measured under good solvent conditions with small-angle neutron scattering and static light scattering. (26) The systems under investigation are densely grafted brushes, synthesized via the grafting-from route, built from a poly(alkyl methacrylate) backbone to which poly(butyl acrylate) side chains were attached in a grafting from reaction. The aim of the work is to study how the systematic variation of structural parameters such as the side chain length and backbone length change the conformation of the polymer brushes in solution. All spectra can be consistently described by a model, considering the bottle-brush polymers as flexible rods with internal d. fluctuations.

The correlation between side chain length and persistence length of the backbone in molecular brushes is exponential. At longer side chain lengths (DP > 60), the persistence length approaches the length of the molecule; i.e. the backbone is fully extended. Parameters discussed are the contour length per main chain monomer unit, lb = 0.253+-0.008 nm is found to be independent of the side chain length and equal to the value found for the bare main chain, indicating a strongly stretched conformation for the backbone due to the presence of the side chains. The fractal dimension of the side chains, Ds, were detected to be Ds = 1.75+-0.07 which is very close to the value of 1/0.588~1.70 expected for a three-dimensional self-avoiding random walk (3D-SAW) under good solvent conditions. On larger length scales the overall brush appears to be a 3D-SAW itself (D = 1.64+-0.08) with a Kuhn-step length of lk=70+-4 nm. The value is independent of the side chain length and 46 times larger than the Kuhn length of the bare backbone (lk = 1.8+-0.2 nm).

In LB films, this translates to parallel organization over short length scales. A conformational collapse of the brush molecule can be induced by increasing the surface pressure on a Langmuir monolayer of the molecules on a water subphase. (27)

Cylindrical Coexistence Globular

Transitions from extended to globular conformation and persistence lengths in extended conformation show strong dependence on grafting density. This transition was later examined in real time by monitoring the change in conformation by adsorption of ethanol or water from the contacting atmosphere. (28)

Stimuli Responsive Molecules The first stimuli responsive water soluble molecular brush was prepared by Manfred Schmidt (29) who prepared brushes with poly(N-isopropylacrylamide) (PNIPAM) side chains. This is one of the few published examples of the preparation of water-soluble brushes via a “grafting from” process. The polymers underwent a thermally induced collapse from extended cylindrical structure to spheres. We prepared temperature-responsive brushes with tunable LCST's by statistical copolymerization of DMA and n-butyl acrylate from a macroinitiator backbone. A similar behavior, collapse of the extended cylinder to a sphere was envisioned.

However when a 1.0 w% solution of the brush was warmed through the LCST an increase in volume was observed as a function of temperature, with DH = 57 nm at 20 °C leading to DH = 104 nm at 55 °C. The broad nature of the transition is common for statistical copolymers that demonstrate LCST behavior and is most likely a result of composition polydispersity. The increase in size for species in solution is typical of an LCST transition and is the result of intermolecular aggregation. Interestingly, when the concentration of the copolymer in the solvent was decreased by an order of magnitude, to 0.1 w%, the opposite behavior was observed in that the hydrodynamic diameter decreased as a function of temperature (DH = 47 nm at 55 °C).

This was attributable to higher solution concentrations favoring dehydration and subsequent intermolecular aggregation, typical for (co)polymers that demonstrate LCST behavior but, when the solution is diluted, side chains dehydrate on heating, and intramolecular collapse is favored, such that individual brushes transition from extended cylinders to compact spheres. (30)

24. “Cascade of Coil-Globule Conformational Transitions of Single Flexible Polyelectrolyte Molecules in Poor Solvent .” Kiriy, A.; Gorodyska, G.; Minko, S.; Jaeger, A.; Stepanek, P.; Stamm, M., JACS, 2002, 124, 13454-13462.
25. “Amphipolar Core-Shell Cylindrical Brushes as Templates for the Formation of Gold Clusters and Nanowires.” Djalali, R.; Li, S-Y; Schmidt, Manfred. ; Macromolecules, 2002, 35, 4282-4288.
26. P.-G. de Gennes in “Scaling Concepts in Polymer Physics”, Cornell University, Ithaca 1979.
27. “Single Molecule Rod-Globule Phase Transition for Brush Molecules at a Flat Interface”. Sheiko, S. S., Prokhorova, S. A., Beers, K. L., Matyjaszewski, K., Potemkin, I. I., Khokhlov, A. R., Moeller, M.: Macromolecules, 2001, 34: 8354-8360.
28. “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.
29. “New Perspectives for the Design of Molecular Actuators: Thermally Induced Collapse of Single Macromolecules from Cylindrical Brushes to Spheres.” Li, C., Gunari, N., Fischer, K., Janshoff, A., Schmidt, M.: Angew. Chem. Int. Ed., 2004, 43: 1101 -1104.
30. “Temperature-responsive water-soluble molecular brushes prepared by atom transfer radical polymerization.” Sumerlin, B. S., Pietrasik, J., Matyjaszewski, K.: Polymer Preprints ACS, Div. Polym. Chem., 2005, 46: 470-471.

SuperSoft elastomers Common elastomeric materials have a typical Young's modulus value (at small strains) of the order of 106 Pa, with reversible extensibility reaching 1000%. They are approximately five orders of magnitude softer, and three orders of magnitude more deformable than typical solids. Weakly cross-linked rubbers preserve the modulus of the rubbery plateau seen for the melt of linear entangled polymers (Mc remains similar to the order of Me), whereas, for highly cross-linked systems the modulus increases.

It is not easy to move in the opposite direction and prepare stable soft rubbers, i.e. rubbers with moduli lower than the bulk plateau modulus of a given polymer. However, the plateau modulus does decrease considerably in polymer solutions, (31-32) and soft gels can be obtained by swelling weakly cross-linked systems with a good solvent for the matrix material. One example of an application for such a material would be soft contact lenses. These solvent swollen states are not stable in environments in which the solvent can evaporate and such gels can also appear unstable when external forces are exerted on the gel. In water-swollen networks of hydrophilic polymers (e.g. hydrogels), shear moduli on the order of 103 Pa can be obtained at low cross-link densities and relatively high degrees of swelling. However, higher levels of swelling result in an increase in modulus, because of strong extension of the network chains. (33)

These unusual mechanical properties that had been observed in aqueous gels are now observed for the first time in bulk polymeric materials. A phenomenological explanation is that the molecular network of crosslinked brush molecules consists of backbone segments diluted by short tethered side chains. The side chains do not entangle, as they are below the critical entanglement MW, but since they are covalently attached to the matrix they provide stability against evaporation or deformation, while preventing the networks from collapsing. They behave as a cross-linked network of “thick” molecules, i.e. polymers with a multiple short chains attached to a polymer backbone, or as a stable, swollen cross-linked network, where the “solvent” is attached to the backbone. The meanings of “thick” and “attached solvent” will become apparent in the following description.

The first materials recognized as precursors of super soft elastomers were bottle brush macromolecules with a very long backbone and densely grafted PnBA side chains which displayed an ultra low modulus plateau in the soft gel range. When they were transformed to a network by chemical cross-linking, the material became a super-soft rubber (34) which instead of the expected elastomeric global flow range displayed a plateau in G' extending towards low frequencies. This plateau indicates elastic properties for such polymers.

Linear bottle bush macromolecule Crosslinked bottle-brush macromolecules

However in this case the plateau modulus is much lower than that seen for typical polymeric rubbers, which has to be attributed to the large fraction of the short dangling chains in the system acting as intrinsic plasticizers, making the material extremely soft.

Physical crosslinking A SuperSoft thermoplastic elastomer can be formed when the backbone is selected to be an ABA block copolymer with phase separable A blocks and a bottle brush B segment. The properties of one such block copolymer are shown below.

Frequency dependencies of the storage (G') and loss (G'') modulus for the triblock copolymer with a brush-like middle block (master curves at the reference temperature of 254K). A discontinuity seen in G' corresponds to melting in the pOMA microphase taking place at 292K.

The bulk tactile response can be modified by selecting the composition of the “dangling chains or hairs” and can vary from hydrophilic to hydrophobic and encompass attached oleophilic and oleophobic diluents that respond to environmental pressures. The bulk properties can be further modified, as discussed above, by changing the degree of cross-linking of the backbone matrix to form a material with the desired modulus for the targeted application. Higher levels of cross-linking, or increased stiffness of either the backbone or tethered dangling graft or “hair” will increase the modulus while still providing polymers significantly softer than current elastomers.

Ionic Conductors Grafted PEO brush polymers were examined as potential solvent-free, lithium ion conducting materials. Analysis of the structure and mechanical properties of these systems in bulk showed that the specific architecture of the copolymers completely suppresses crystallization of the PEO graft segments. Consequently, amorphous homogeneous materials are obtained in which both high local mobility and sufficient macroscopic mechanical stability are achieved at the same time. When melts of linear brush polymers were optimally doped with CF3SO3-Li+ ionic conductivity reached 10-3 S/cm. (35)

31. “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, 124904/124901-124904/124913.
32. “Viscoelasticity of an Entangled Polymer Solution with Special Attention on a Characteristic Time for Nonlinear Behavior.” T. Inoue, Y. Yamashita, K. Osaki, Macomolecules, 2002, 35, 1770.
33. U.P. Schröder and W. Oppermann, in “Physical properties of polymer gels” Ed.: J.P. Cohen Addad, John Wiley & Sons, Chichester 1996.
34. “Polymers, supersoft elastomers and methods for preparing the same;” Pakula, T.; Matyjaszewski, K. In PCT Int. Appl.; WO 2004014963, 2004, p 65 pp.
35. “Super soft elastomers as ionic conductors.” Zhang, Y., Costantini, N., Mierzwa, M., Pakula, T., Neugebauer, D., Matyjaszewski, K.: Polymer, 2004, 45: 6333-6339.


Flat Surfaces In a grafting from a flat surface the use of ATRP is particular advantageous as the thickness of the polymer brush can be precisely controlled by systematic variation of grafting density and DPn of the tethered polymers. (See section on initiation from surfaces) Furthermore, the properties of these polymer brushes can be tuned by the tethering of block copolymers, where the composition and DPn of each polymer segment directly affects the morphology and behavior of these nanocomposite materials. Modification of surfaces with thin polymer films can be used to tailor the surface properties such as hydrophylicity/phobicity, biocompatibility, adhesion, adsorption, corrosion resistance and friction. Nanoscale organization of the functional surface can be directed by photolithography and micro and nanoscale printing. (36) The chemical nature of the underlying material becomes hidden by the presence of a film a few Angstroms thick and the interaction of the whole system with the surrounding environment is governed by these coatings.

Control of surface properties In addition to “grafting from” a functionalized surface it is also possible to prepare tethered block copolymers by the “grafting to” approach. However, because of kinetic and thermodynamic restrictions, grafting-onto approach leads to formation of polymer brushes with relatively low grafting density but this does not mean surface properties are not affected. We have used ATRP to modify surface properties, create nano-patterns and design stimuli-responsive, mainly temperature responsive, materials. Ellipsometry, contact angle, XPS and AFM are used to assess whether tethered (co)polymers possess precise molar mass and composition.

The grafting to approach is exemplified by the preparation of a novel silylating agent/designed ABC block copolymer which was prepared by living anionic polymerization of D3 to prepare a poly(dimethylsiloxane) (pDMS) macroinitiator followed by hydrosilation reactions to incorporate 2-bromoisobutyrate end groups for initiation of ATRP. The ATRP of styrene using the pDMS macroinitiator yielded a diblock copolymer. Chain extension of the pDMS-b-pS macroinitiator with 3-(dimethoxymethylsilyl)-Pr acrylate (DMSA) by ATRP yielded an ABC triblock copolymer. The reactive DMSA segment was covalently attached to silanol groups on a silicon wafer. (37) Due to the low grafting densities inherent to this approach, tethered copolymer chains retained adequate degrees of freedom enabling segment selective behavior when the surfaces were exposed to various solvents. The resulting tethered ultrathin film can present either a hard surface or a soft-lubricating surface to the environment dependent of treatment with selective solvents.

a polymer brush with a glassy surface a brush with a rubbery surface

The presentation of either glassy pSt or flexible pDMS segments of the brushes attached to the surface was reversibly controlled by treatment with solvents selected for preferential salvation of each segment.

A similar approach was applied to the synthesis of a diblock copolymer containing poly(N,N-dimethylaminoethyl methacrylate) which was extended with a poly(trimethoxysilylpropyl methacrylate) block. Surface properties of the copolymer ultrathin films could be reversibly controlled by treatment with selective solvents or temperature, since pDMAEMA exhibits a lower critical solution temperature (LCST). Due to the temperature dependent solubility the tethered chains could change their conformation from extended to globule as the temperature was increased through the LCST.

The grafting-from approach, or grafts generated from a surface immobilized initiator, generally leads to the formation of a higher density of polymer brushes than the grafting-onto approach. By tuning parameters including grafting density, chemical composition or type of substrate, products with different properties can be achieved. Silicon wafers are among the most commonly used flat surfaces for these grafts and initiators can be attached to both kinds of silicon substrates, namely oxidized (Si-OH) or hydrogen-terminated silicon (Si-H). (39-40) Tethering a chlorosilane (mono- or trichloro) functionalized initiator to an oxidized substrate is the most frequently documented route for the preparation of a surface for a grafting from ATRP.

The preparation of block copolymer brushes using a “grafting-from” ATRP approach was first reported by tethering polystyrene-block-poly(t-butyl acrylate) (pSt-b-ptBA) to Si wafers. (39) Hydrolysis of the t-butyl groups yielded a polystyrene-block-poly(acrylic acid) brush demonstrating a versatile approach to tune film properties and wettability.
The concentration of initiating groups on the surface can be varied by varying the molar ratio between two chlorosilanes (one an active initiator the other a “dummy” initiator) attached through silanol groups on the surface of silicon wafers. For oxidized wafer surfaces, chlorotrimethylsilane was used as a “dummy” initiator and 1-(chlorodimethylsilyl)propyl 2-bromoisobutyrate as an active ATRP initiator.

A similar approach was used to attach initiators to/passivate Si-H substrates. A non-functionalized 1-alkene was used as the “dummy initiator” to allow variation of the concentration of initiating groups on the hydrogen-passivated silicon substrate. The initiating groups were attached to freshly hydrogen-passivated silicon wafers by a hydrosilylation reaction which was conducted by exposing the substrate to UV light in the presence of 1- alkene and allyloxytrimethylsilane (TMS). In a second step, the TMS group was converted to 2-bromoisobutyrate ester (an ATRP initiator) using the corresponding acid bromide in the presence of tetrabutylammonium fluoride in dry tetrahydrofuran.

When conducting an ATRP from a surface it is normal to add sacrificial initiator at the beginning of the reaction since this allows controlled chains growth. (38) It is needed because the low concentrations of initiating groups cannot generate a sufficient concentration of persistent radical (deactivator) to provide a controlled grafting from reaction. Alternatively, excess of persistent radical (CuII species) can be added to the contacting monomer solution. (39) Tapping mode AFM images of pDMAEMA grafts prepared by grafting from a modified Si-OH surface imaged in air under ambient conditions are shown below in images a-c. Images of surfaces with tethered grafts formed with higher then 20% initiator coverage (Image a) revealed the presence of dense, relatively smooth films, with occasional patchy defects. (Silicon surfaces modified with ATRP initiators can be transformed to allow for controlled polymerization by RAFT or NMP.)

At lower initiator coverage, the tethered grafts were increasingly patchy, and at 1% coverage and below, they appeared to be highly uniform, isolated patches. AFM images of grafts grown from the Si-H substrates are shown in images d-f. It appears that, in comparison with oxidized silicon substrates with the same assumed initiator coverage, films grown from Si-H substrates were denser, showing fragmentation into patches only at the lowest initiator coverage (Image f). The use of either substrate (Si-OH vs. Si-H) seems to yield equally uniform graft layers when the concentration of initiating group on the surface is &Mac179;20%, and neither approach appears to offer any clear advantage for initiation of ATRP.

When poly(N-isopropylacrylamide) was synthesized by surface initiated polymerization the phobicity of the resulting surface changed as the temperature was increased through the LSCT however in contrast to “free” poly(N-isopropylacrylamide) the surface does not respond to external stimuli within seconds. The following images show an air bubble in water in contact with the modified surface at different temperatures. The contact angle increased from 62 degrees to 79 degrees as the tethered NIPAM chains were transformed at the LCST.

In the following series of images we show schematics of a surface modified with dual responsive tethered polymers. The fundamental chemical nature of the underlying material becomes hidden by the presence of a film a few Angstroms thick. The interaction of the whole system with the surrounding environment is governed by these coatings. Scanning Probe Microscopy topography images and wetability experiments for hybrid polymer layers consisted of PS and PVP show how the response of the surface is modified by prior environments changing from a hydrophobic surface to a more hydrophilic surface. (40)

Antibacterial Surfaces Potentially harmful bacteria abound and pathogenic bacteria are deposited onto surfaces touched by many people thereby spreading infection. It would be beneficial to have surfaces that kill bacteria on contact. Stable non-leaching antimicrobial surfaces are required since leaching systems are eventually exhausted, rendering the material ineffective and posing potential environmental risks. Quaternary ammonium ion-containing polymers are known to effectively kill cells and spores by disrupting cell membranes. Monomers, such as 2-dimethylaminoethyl methacrylate (DMAEMA), 4-vinyl pyridine (4-VP) and N-substituted acrylamides that can be quaternized providing biocidyl activity can be polymerized by ATRP. This means that antimicrobial surfaces can be prepared by grafting from (43) or grafting onto surfaces. (44)

In order to conduct a grafting from reaction the only requirement is tethered initiators and this is readily accomplished by reacting a surface hydroxyl group with 2-bromoisobutyryl bromide. ATRP of DMAEMA followed by quaternization with ethyl bromide provided effective tethered biocidal functionality. When paper was treated the surfaces were extremely effective at killing E.Coli, reducing the number of cells by four orders of magnitude, from 1.6 x 109 to 4.9 x 105, in one hour. The surface also showed activity against B. subtilis spores. Activity of a biocidal film on a glass surface survived repeated washing with aqueous detergent solution. Permanent, nonleaching biocidal surface treatment would find utility in food packaging, household items and military applications.

While grafting from is an efficient method of tethering quaternizable polymers to a substrate a more convenient approach for household, and even hospital use, would be a “consumer friendly” “grafting onto” approach such as spraying a solution of a reactive copolymer onto a surface. In order to demonstrate the grafting onto approach PDMEAMA/PTMSPMA block copolymers of different molecular weights and different backbone topology were prepared via ATRP and immobilized on a glass surface by simple immersion of the glass slide in an aqueous solution of the copolymer. The efficiency of the tethering reaction was measured by addition of 1% of the sodium salt of fluorescein to the DMAEMA block prior to immersing the glass slide in the solution. Desorption of the dye after washing provided a measurement of the concentration of grafted chains on the surface. Grafting density was dependent on concentration and time and molecular weight of the copolymer, with higher molecular weight providing lower graft density. Antibacterial activity only depended on concentration of quaternary ammonium units tethered to the surface. The interaction between bacteria and poly(quaternary ammonium) was investigated and the ability of the modified surfaces to kill bacteria was tested. Treated glass samples were incubated with a suspension of E. coli. The test results showed that the treated glass killed 104 ~ 106 of the contacting bacteria. The biocidal efficiency increased with the amount of quaternary ammonium on the surface, rather surprisingly introduction of hydrophobic units into poly(quaternary ammonium) led to 100 times enhancement of biocidal activity with a log/kill of 7.0. (42)

Modification of surfaces with polymers is applicable to a variety of surfaces. Control of surface morphology can be achieved by patterning the initiator layer and subsequent graft polymerization. Further variation of surface morphology can be achieved by phase separation on the nanoscale thereby providing precision surface modification.

36. “Polymer brushes by atom transfer radical polymerization”, Pyun, J.; Kowalewski, T.; Matyjaszewski, K. Polymer Brushes 2004, 51-68.
37. “Synthesis and surface attachment of ABC triblock copolymers containing glassy and rubbery segments.” Pyun, J.; Jia, S.; Kowalewski, T.; Matyjaszewski, K. Macromolecular Chemistry and Physics 2004, 205, 411-417.
38. “Controlled Synthesis of Polymer Brushes by \"Living\" Free Radical Polymerization Techniques,” Husseman, M.; Malmstroem, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 1424-1431.
39. “Polymers at Interfaces: Using Atom Transfer Radical Polymerization in the Controlled Growth of Homopolymers and Block Copolymers from Silicon Surfaces in the Absence of Untethered Sacrificial Initiator,” Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716-8724.
40. “Bidisperse Mixed Brushes: Synthesis and Study of Segregation in Selective Solvent.” Minko, S.; Luzinov, I.; Luchnikov, V.; Mueller, M.; Patil, S.; Stamm, M., Macromolecules, 2003, 36, 7268-7279.
41. “Polymers at Interfaces: Using Atom Transfer Radical Polymerization in the Controlled Growth of Homopolymers and Block Copolymers from Silicon Surfaces in the Absence of Untethered Sacrificial Initiator.” Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716-8724.
42. “Controlled Grafting of Well-Defined Polymers on Hydrogen-Terminated Silicon Substrates by Surface-Initiated Atom Transfer Radical Polymerization.” Yu, W. H.; Kang, E. T.; Neoh, K. G.; Zhu, S. J. Phys. Chem. B 2003, 107, 10198-10205.
43. “Permanent, Nonleaching Antibacterial Surfaces. 1. Synthesis by Atom Transfer Radical Polymerization”; Lee, S. B.; Koepsel, R. R.; Morley, S. W.; Matyjaszewski, K.; Sun, Y.; Russell, A. J. Biomacromolecules 2004, 5, 877-882.
44. “Self-Decontaminating Surfaces via “Grafting onto” Surface Technique“; Huang, J., Matyjaszewski, K.; Sun, Y.; Russell, A. J. in progress

Click to go to section 12: Starting Points for Conducting an ATRP Reaction