Synthesis and properties of cationic photopolymerizable hyperbranched polyesters with terminal oxetane groups by the couple-monomer polymerization of carboxylic anhydride with hydroxyl oxetane

The hyperbranched polyesters with terminal oxetane groups were synthesized via couple-monomer methodology based on carboxylic anhydride and hydroxyl oxetane. Two competitive reactions, ring-opening reaction of carboxylic group with oxetane group and esterification of carboxylic group with hydroxyl group, occurred synchronously during the polymerization. The results showed that the hyperbranched polyesters, poly(SA-EHO) and poly(SA-EHPO), were synthesized successfully when succinic anhydride (SA) was selected to react with 3-Ethyl-3-(hydroxymethyl)oxetane (EHO) and 3-ethyl-3-((4-hydroxymethyl)phenoxymethyl)oxetane (EHPO), respectively. The degree of branching was determined to be about 0.7 for poly(SA-EHO) from the 1H NMR result, which was higher than those reported in literature, indicating that the esterification was dominant. The glass transition temperatures (Tg)s were measured as −11.5 °C for poly(SA-EHO) and 14.3 °C for poly(SA-EHPO) by DSC. The number average molecular weights were 1914 g/mol and 2108 g/mol with the polydispersity indices of 2.39 and 2.28 for poly(SA-EHO) and poly(SA-EHPO), respectively. Both poly(SA-EHO) and poly(SA-EHPO) were added to bisphenol A epoxy resin, EP828, with different contents and cured under UV exposure. From the DMTA and tensile results, the UV cured poly(SA-EHPO) showed higher Tg and tensile strength compared with poly(SA-EHO) because of the existence of phenyl group in poly(SA-EHPO) chain. Moreover, both the Tgs and mechanical strength decreased, whereas the elongation percents at break increased with poly(SA-EHO) and poly(SA-EHPO) increased. This indicates that the flexibility of cured films was improved by the addition of poly(SA-EHO) and poly(SA-EHPO) in the formulations.