Proton-transfer Polymerization of Acrylate Monomer Containing Two Carboxyl Groups for Synthesis of Branched Polymer

This paper reports proton-transfer polymerization of an acrylate monomer containing two carboxyl groups. The monomer was synthesized by reaction of DL-malic acid with acrylic anhydride in the presence of a catalytic amount of acryloyl chloride. The polymerization was carried out in the presence of ributylphosphine as an initiator at 100°C. The product was isolated as a fraction insoluble in a mixture of n-hexane and diethyl ether. The structure of the polymer was determined by the 1H and 13C NMR spectra, as well as IR spectrum to be the polyester having a phosphonium end group. The molecular weights were estimated by GPC analyses with water as an eluent using poly(ethylene glycol) standards, which were 5500–11200 with the polydispersities of 1.6–2.6. Based on the analytical data, the following zwitterionic mechanism is proposed. The first step is formation of phosphonium-carboxylate zwitterion species by reaction of the monomer with the initiator. Then, the propagation proceeds via the Michael-type addition between the zwitterion species, followed by the proton-transfer process to give the polymer having ester linkages. Based on the polymerization mechanism, proposed here, the carboxyl groups in the monomer and polymer are considered as the latent propagating species. Therefore, the branched structure of the obtained polymer can be assumed