Poly(vinyl ester) star polymers via xanthate-mediated living radical polymerization: From poly(vinyl alcohol) to glycopolymer stars

Poly(vinyl ester) stars have been synthesized via different macromolecular design via interchange of xanthate (MADIX)/reversible addition-fragmentation chain transfer (RAFT) polymerization methodologies. Two approaches were investigated. The first method involved attaching the xanthate functionality to the core via a nonfragmenting covalent bond (Z-group approach). The second approach involved attaching the xanthate functionality to the core via a fragmenting covalent bond (R-group approach). The R-group approach yielded well-defined poly(vinyl acetate), poly(vinyl pivalate), and poly(vinyl neodecanoate) stars with narrow polydispersities (PDI <= 1.4). In contrast, the molecular weight distributions of poly(vinyl acetate) stars prepared using the Z-approach tended to broaden at moderate to high conversions. We attribute this broadening to steric congestion around the xanthate functionality, restricting the access of monomer to the C=S bonds. The R-group approach was also found to be superior for preparing precursor stars suitable for hydrolysis to poly(vinyl alcohol). Hydrolysis of stars generated by the Z-group approach resulted in destruction of the architecture, as the process also cleaved the xanthate linkage at the nexus of the arms and core. Preliminary experiments on using the R-group approach to mediate the star-polymerization of vinyl-functional glycomonomers demonstrated the possibility of generating complex glycopolymer architectures. However, some significant problems were observed, and this synthetic approach requires further optimization.