Synthesis of copolymers containing acetals

The polymerization of cyclic acetals such as dioxolane and dioxepane, initiated by triflic acid in the presence of poly(ethylene oxide)glycol (PEOG), exhibits special features: One can produce triblock copolymers of PEOG and of one cyclic acetal; the molecular weight of the copolymer increases linearly with time like in a living system; a hydroxyl group is found at both ends of the copolymer obtained; macrocycle formation, usually inevitable in large quantity when a classical onium mechanism operates, can be very much reduced and even practically suppressed depending on the experimental conditions. When we tried to prepare a pentablock from the triblock by adding a second acetal under similar conditions (after recovery and purification of the triblock copolymer) we obtained fast transacetalization instead, leading to random placements of the two cyclic acetals, while the PEOG remained as one block, and that only a very small amount of cycles was formed if any. These results are interpreted in terms of multiple transfer reactions of hydroxyl groups onto activated monomer molecules or onto small growing oligomers.