Free-radical copolymerization of methyl methacrylate with styrene in the presence of 2-mercaptoethanol II. Influence of methyl methacrylate/styrene ratio

The free-radical copolymerization of methyl methacrylate (MMA) with styrene (St) in the presence of 2-mercaptoethanol (ME) was investigated in order to obtain ω-hydroxy oligomers with random copolymer-type chains of various compositions and molecular weights. Polymerizations at three different MMA/St molar ratios were carried out, while keeping constant the ME/monomer ratio. Monomer mixtures richer in MMA than in St were employed in order to attempt preparing lower polydispersity oligomers with monomodal molecular weight distribution (MWD). The molecular weights of the resulting oligomers increased with both conversion and MMA fraction in the feed, while polydispersities increased with conversion and decreased with MMA concentration in the initial monomer mixture. For the lower MMA fractions in the monomer feed, bimodal MWDs resulted beyond a certain conversion due to the faster relative consumption of ME than of monomer. Based on the pseudo-kinetic rate constant method, apparent chain transfer constants corresponding to the three different compositions of the monomer feed were estimated. The values obtained were in good agreement with the evolution of molecular weights and polydispersities with conversion and MMA fraction in the monomer feed. The co-oligomers prepared displayed functionalities around unity, making them suitable for the synthesis of macromonomers.