Synthesis and chemical degradation of branched vinyl polymers prepared via ATRP: Use of a cleavable disulfide-based branching agent

Highly branched poly(2-hydroxypropyl methacrylate) has been prepared by atom transfer radical polymerization (ATRP) in methanol at 20 degrees C using a disulfide-based dimethacrylate (DSDMA) branching agent. The mean degree of polymerization of the primary chains was fixed at 50; since ATRP has reasonably good living character, the molecular weight distribution of these primary chains is relatively narrow, which allows significantly better control than conventional radical polymerization. Varying the proportion of the DSDMA produced a series of soluble branched polymers, provided that there was on average less than one branching agent per primary chain. However, higher levels of DSDMA lead to macrogelation, as expected. The soluble branched polymers were characterized using triple detector gel permeation chromatography (GPC). The most highly branched copolymers had weight-average molecular weights of up to 540 000, with polydispersities of around 8.0 and Mark-Houwink alpha parameters as low as 0.21 being obtained. H-1 NMR spectroscopy confirmed that very high monomer conversions were obtained (> 99%), and the final branched copolymers contained little or no unreacted pendent vinyl groups. The disulfide bond in the DSDMA branching agent was readily cleaved using either dithiothreitol or benzoyl peroxide. GPC studies confirmed the progressive decrease in molecular weight and polydispersity during the chemical degradation of one of the branched copolymers with reaction time. Eventually, the final polydispersity of this degraded branched copolymer was comparable to that of linear poly(2-hydroxypropyl methacrylate) prepared in the absence of any disulfide-based dimethacrylate branching agent. Thus, all the disulfide bonds had been cleaved, reducing the branched copolymer to its near-monodisperse primary chains.