Bi-functional and mono-component organocatalysts for the ring-opening alternating co-polymerisation of anhydride and epoxide

Bi-functional organocatalysts constituted by a (thio-)urea moiety and an iminophosphorane moiety were synthesized and optimised for the ring-opening alternating co-polymerisation of phthalic anhydride with three different epoxides: cyclohexene oxide, propylene oxide and butylene oxide. The most effective catalyst featured a cyclohexyl urea moiety, an iminophosphorane moiety with three 2,4-dimethyl-3-methoxy phenyl substituents, and a short spacer length of two carbon atoms between them. All tested epoxides reached quantitative conversion within 24 hours with ester-selectivities up to >97%. NMR and DFT experiments reveal that the catalysts exist in solution as dimers that dissociate during the initiation of the polymerisation. During the polymerisation, the catalyst is coordinated to the growing chain and further modulates its reactivity through reversible protonation/deprotonation suppressing transesterification side reactions even at prolonged polymerisation times without the need for a co-catalyst. The rate-determining step of the polymerisation is the ring-opening of the epoxide by the carboxylate chain end, and accordingly, higher temperatures (up to 150 °C) and higher concentrations of epoxide and catalyst increase polymerisation rates. The polyester synthesis from anhydrides and epoxides requires their strictly alternating incorporation achieved by a catalyst that is capable of mediating the reactivity of carboxylate and alcohol chain ends.