Free-radical cross-linking polymerization of unsymmetrical divinyl compounds 2. Steric effect on gelation in the copolymerizations of allyl methacrylate with several alkyl methacrylates

As an extension of our continuing studies concerned with the free-radical cross-linking polymerization and copolymerization of multivinyl compounds, this article deals with the gelation in the copolymerization of allyl methacrylate (AMA), a typical unsymmetrical divinyl compound containing two types of vinyl groups such as methacryloyl and allyl ones having quite different reactivities, with several alkyl methacrylates, especially focusing on the steric effect of long-chain alkyl groups. Thus, AMA was copolymerized in bulk with methyl methacrylate (MMA), butyl methacrylate (BMA), lauryl methacrylate (LMA), and stearyl methacrylate (SMA) using azo-initiator at 50 °C in the presence of lauryl mercaptan as a chain transfer agent. The actual gel point was compared with the calculated one according to Stockmayer's equation by tentatively supposing equal reactivity of both vinyl groups. In the copolymerizations with LMA and SMA, the suppressed occurrence of intermolecular cross-linking reaction caused by the steric effect of long-chain alkyl groups was clearly observed as a reflection of delayed gelation, whereas the copolymerizations with MMA and BMA were roughly governed by the predominant occurrence of intermolecular cross-linking. These were supported by the conversion dependencies of the molecular-weight distribution profiles, the correlation curve of molecular weight versus elution volume, and the correlation of intrinsic viscosity versus molecular weight of resulting precopolymers.