Synthesis of Branched Methacrylic Copolymers: Comparison between RAFT and ATRP and Effect of Varying the Monomer Concentration

The statistical copolymerization of methyl methacrylate (MMA) with varying amounts of a disulfide-based dimethacrylate (DSDMA) branching comonomer in toluene at 90 degrees C can lead to highly branched soluble methacrylic copolymers under appropriate conditions. This model system is utilized in order to examine the following points: (i) the relative merits of using reversible addition-fragmentation chain transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP) in such syntheses; (ii) the effect of varying the number of I)SDMA units per primary chain; (iii) the effect of varying the initial monomer concentration. Kinetic Studies of the linear RAFT and ATRP homopolymerizations conducted in the absence of any DSDMA confirmed their living character Lit 10, 30, and 50 wt % [MMA](0), where the former monomer concentration approximately corresponds to the critical overlap concentration, c*, for linear poly(methyl methacrylate) (PMMA) chains with a mean degree of polymerization of 50. HPLC analysis of the monovinyl and divinyl comonomers confirms that there is statistical incorporation of the DSDMA brancher into the growing poly(methyl methacrylate) chains. Cleavage of both RAFT- and ATRP-synthesized branched copolymers prepared at 50 wt % [MMA](0) using tributylphosphine affords linear primary chains with narrow molecular weight distributions; thus these retro-syntheses demonstrate the retention of living character tinder branchingeonditionsand Suggest little or no chain transfer to polymer. In principle, macroscopic gelation can be avoided provided that the number of fully reacted divinyl branching comonomers per primary chain is less than unity. Taking into account the respective efficiencies of the RAFT chain transfer agent and the ATRP initiator, this hypothesis holds for both ATRP and RAFT branching copolymerizations conducted in the presence of DSDMA at 50 wt % [M MA](, but fails at 10 wt % [MMA](0). Thus, soluble branched copolymers can be prepared at 10 wt % [M MA]() containing Lip to five fully reacted DSDMA units per primary chain using RAFT chemistry and tip to three fully reacted DSDMA units per primary chain with the ATRP formulation; no gelation is observed even when the overall conversion of vinyl groups exceeds 96%. These observations strongly Suggest that intramolecular cyclization is prevalent at this lower monomer concentration, regardless of the precise nature of the polymerization chemistry. In contrast, intermolecular branching between primary chains is evidently favored at 50 wt % [MMA](0), since this concentration substantially exceeds c*. In summary, although there are no doubt some subtle differences between branched copolymers synthesized via RAFT and ATRP chemistry, physical factors are arguably much more important than the precise nature of the living radical polymerization chemistry; in particular, systematic variation of the monomer concentration clearly leads to fundamentally different behavior.