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Research
SURFACE INITIATED RAFT POLYMERIZATION
The recent advent of controlled/‘living’ free radical polymerization techniques has provided a number of advantages, when compared to traditional free radical techniques, for polymer brush synthesis. These advantages include, control over the thickness of the polymer brush both by control of molecular weight and narrow polydispersities, and the ability to prepare block copolymers by the sequential activation of the dormant chain ends in the presence of different monomers. Probably the most common controlled/‘living’ free radical polymerization technique used to produce polymer brushes is atom transfer radical polymerization (ATRP). However, the recently discovered reversible addition fragmentation chain transfer (RAFT) process has yet to be fully utilized in the preparation of polymer brushes. RAFT polymerization is arguably the most versatile of the controlled/‘living’ free radical polymerization techniques due to its ability to polymerize a wide variety of functional monomers in the preparation of homopolymers and block copolymers, no protecting groups are required, and the polymerizations can be conducted under non-stringent conditions in simple solvents. Surprisingly, the use of RAFT polymerization to produce polymer chains covalently attached to surfaces has not been widely reported.
Current research takes advantage of the versatility of RAFT polymerization techniques to form highly functional polymer brushes. As the mechanism of a successful RAFT polymerization requires that all chains are initiated at the same time, preferably by chain transfer agent (CTA), arguably the best way to prepare polymer brushes via RAFT is by the immobilization of CTAs. By utilizing a combination of techniques to modify silica surfaces with RAFT CTAs, thin films of functional polymers can be formed, which are attractive for antifouling and biomedical applications. To achieve this, a traditional ATRP initiator was first immobilized on the surface and the converted to a RAFT CTA using a modification of an atom transfer addition reaction. Following immobilization of the RAFT CTAs, both homopolymer and diblock copolymer brushes were prepared via RAFT polymerization.
To determine the efficiency of the RAFT CTA surface towards surface initiated polymerizations, a PMMA-b-PSty diblock brush were synthesized. Shown below are representative XPS spectra of (a) a silica wafer modified with a widely used bromo-isobutyrate surface initiator, (b) conversion to a surface-immobilized CTA, (c) formation of a PMMA homopolymer brush, and (d) subsequent extension to form a PMMA-b-PSty diblock copolymer brush. The XPS data with the immobilized bromo-silane initiator having an atomic percentage of bromine (Br3d) of 1.4%, and following the ATA conversion, the bromine atomic percentage decreases to <0.1%, with a respective increase of sulfur (S2s) to 2.7%, comparing directly with the theoretical 2:1 sulfur to bromine ratio. XPS analysis of the PMMA brush indicated the presence of carbon, oxygen, and a small atomic percentage of sulfur. Wide-scan XPS spectra of the PMMA-b-PSty diblock copolymer brush illustrates a decrease in the oxygen atomic percentage, from 27.5% to 4.1%, with a respective increase in the carbon (C1s) atomic percentage, from 71.6% to 95.6%, after the formation of the PSty outer block.
The GATR-FTIR spectrum, shown below, of the immobilized bromo-silane initiator after reaction with the DTBDS indicates few discernable differences. The GATR-FTIR spectra for the PMMA brush demonstrated characteristic peaks at approximately 1720 cm -1, associated with the carbonyl stretch, and at approximately 2920 cm -1 and 2850 cm -1 which are attributed to the asymmetric CH2 stretching and the symmetric CH 2 stretching, respectively. For the formation of the PMMA-b-PSty brush, the GATR-FTIR spectra indicates an increase in intensity of C-H stretches around 3050 cm -1, the appearance of the C=C doublet at 1420-1480 cm -1, and a decreased intensity of the carbonyl stretch representative of the PMMA block, all of which confirm the presence of PSty.
Work is currently focused on stimuli responsive polymeric brushes formed by surface initiated RAFT and surface modification of nanoparticles with biocompatible and stimuli responsive copolymers for use in biomedical and antifouling materials.
