Synthesis of Highly Branched Methacrylic Copolymers: Observation of Near-Ideal Behavior using RAFT Polymerization

We report the synthesis of model highly branched methacrylic copolymers by copolymerizing methyl methacrylate (MMA) with a disulfide-based dimethacrylate (DSDMA) branching comonomer via reversible addition-fragmentation chain transfer (RAFT) in toluene at 90 degrees C using 1,1'-azobiscyclohex-anecarbonitrile initiator and a cumyl dithiobenzoate (CDB) chain transfer agent. Selective cleavage of the disulfide bonds in the DSDMA branching comonomer using tributylphosphine leads to the formation of low polydispersity primary chains, as judged by gel permeation chromatography. The molecular weight distribution of these degraded chains is comparable to a RAFT-synthesized linear poly(methyl methacrylate) homopolymer prepared in the absence of any DSDMA brancher. This confirms that good control over the RAFT copolymerization is achieved under branching conditions and that the polydisperse highly branched chains simply comprise randomly coupled near-monodisperse primary chains, as expected. Moreover, HPLC analysis of the copolymerizing solution confirms that the consumption of DSDMA comonomer is close to that expected for a statistical copolymerization. The CDB efficiency is estimated to be 90% by GPC and H-1 NMR spectroscopy. Taking this into account and allowing for the incomplete comonomer conversions (typically 96-97%), our systematic variation of the proportion of DSDMA per primary chain indicates that this RAFT formulation conforms closely to classical Flory-Stockmayer theory. This near-ideal behavior is in marked contrast with earlier literature reports of strongly nonideal behavior, presumably because of significant levels of intramolecular cyclization. Our hypothesis is that this unwanted side reaction, which consumes the DSDMA brancher without leading to inter-molecular branching, is suppressed in the present study because of the relatively high comonomer concentration (50% w/w) used in our RAFT syntheses.