Z-RAFT star polymerizations of acrylates: Star coupling via intermolecular chain transfer to polymer

Six-arm star polymers of methyl acrylate (MA), butyl acrylate (BA), and dodecyl acrylate (DA) were generated in bulk at 60 degrees C via reversible addition-fragmentation chain transfer (RAFT) polymerization. A hexafunctional trithiocarbonate was employed as mediating compound, in which the active RAFT-agent moieties are interlinked to the central core molecule via the stabilizing Z-group. Well-defined star polymers with predictable number-average molecular weights of more than 1 x 10(6) g.mol(-1) were obtained by this Z-RAFT star polymerization of acrylates. Narrow and monomodal molecular weight distributions were found up to intermediate monomer conversions. At higher monomer conversions, an unexpected high molecular weight component occurred with increasing extent when going from MA over BA to DA polymerization. This high molecular weight material was assigned to a star-star couple containing two living cores, which formation is not in accordance to the basic Z-RAFT star polymerization mechanism, but most likely arises from an intermolecular chain transfer to polymer reaction. The amount of star-star couples, which is an indication for the extent of long-chain branching, was quantified as a function of the monomer conversion and subsequently modeled via kinetic simulations. By this approach, the rate coefficient of intermolecular transfer to polymer at 60 degrees C was estimated to be k(tr)(P,inter) = 0.33 L.mol(-1).s(-1) in BA polymerization and k(tr)(P,inter) = 7.1 L.mol(-1).s(-1) in DA polymerization. The living process was found to be very effective up to high monomer conversions, indicating that steric congestion next to the star polymer core, where the actual RAFT process takes place, is not significantly hampering the Z-RAFT star polymerization of acrylates.