The anionic ring-opening polymerization and copolymerization of cyclic carbonates

Abstract

Cyclic carbonates are eligible to ring-opening polymerization using a wide variety of initiators such as carbanionic or alcoholate species as well as initiators known to be effective for the ring-opening polymerization of lactones and for the group transfer polymerization of vinyl monomers. Depending on the catalyst, high molecular weight polymers may be obtained in high yields (kinetically controlled regime) or a ring-chain equilibrium is observed upon end-biting, back-biting and transesterification reactions (thermodynamically controlled regime). The polymerizability of the cyclic carbonates is strongly dependent on their structure. Five-membered cycles generally cannot be polymerized, whereas six-membered cycles can be polymerized and copolymerized in an ideal manner. The polymerizability of higher cyclics, in particular when containing aromatic ring systems, is highly dependent on the substitution pattern of the aromatics. Since the active species in the polymerization of aliphatic cyclic carbonates was disclosed to be of alcoholate type, a copolymerization with ϵ-caprolactone is easily achieved, the reactivity of the cyclic carbonate, however, being by far larger than that of the lactone. On the other hand, the copolymerization with pivalolactone exerts a different behaviour, since the active species of the growing pivalolactone chain after a few steps assumes the character of a carboxylate anion which is unable to promote the ring-opening polymerization of cyclic carbonates. Since carbanionic species may be used as initiators for the ring-opening polymerization of cyclic carbonates, polystyryl, polybutadienyl, and polyisoprenyl anions may be used as initiators to achieve the corresponding block copolymers. To obtain block copolymers with poly(methyl methacrylate) blocks a group transfer polymerization of the respective acrylate has to be performed, followed by the polymerization of the cyclic carbonate. The latter, however, rather proceeds by a metal- free anionic process than by a group transfer process. The ring-opening polymerization and copolymerization of cyclic carbonates allows the preparation of a broad variety of new polymers with remarkable properties.