The preparation of ABA type block copoly(ester-b-carbonate-bester) from a mixture of ε-caprolactone, cyclohexene oxide, and carbon dioxide monomers and using a single catalyst is presented. By using a dinuclear zinc catalyst, both the ring-opening polymerization of ε-caprolactone and the ringopening copolymerization of cyclohexene oxide and carbon dioxide are achieved. The catalyst shows high selectivity, activity, and control in the ring-opening copolymerization, yielding poly(cyclohexene carbonate) polyols, i.e., α,ω-dihydroxyl end-capped polycarbonates. It also functions efficiently under immortal conditions, and in particular, the addition of various equivalents of water enables the selective preparation of polyols and control over the polymers' molecular weights and dispersities. The catalyst is also active for the ring-opening polymerization of ε-caprolactone but only in the presence of epoxide, generating α,ω-dihydroxyl-terminated polycaprolactones. It is also possible to combine the two polymerization pathways and, by controlling the chemistry of the growing polymer chain-metal end group, to direct a particular polymerization pathway. Thus, in the presence of all three monomers, the selective ring-opening copolymerization occurs to yield poly(cyclohexene carbonate). Upon removal of the carbon dioxide, the polymerization cycle switches to ring-opening polymerization and a triblock copoly(caprolactone-b-cyclohexene carbonate-b-caprolactone) is produced. The ABA type block copolymer is fully characterized, including using various spectroscopic techniques, size exclusion chromatography, and differential scanning calorimetry. The copolymers can be solvent cast to give transparent films. The copolymers show controllable glass transition temperatures from −54 to 34°C, which are dependent on the block compositions.
■ INTRODUCTIONAliphatic polycarbonates and polyesters are important polymers for both commodity and medical applications. 1−18 Combining both carbonate and ester linkages into copolymers is an attractive means to moderate macroscopic properties and widen the range of applications. In this context, block copolymers are particularly desirable due to the ability to use their chemistry and composition to precisely control the morphology on the nano-and micrometer scales. Block copolymers are useful products in fields spanning microelectronics, advanced plastics, controlled release, and engineering materials. 19−22 Recent successes from the groups of Yang and Hedrick have demonstrated the potential for aliphatic polycarbonates, and related copolymers, in a range of important biomedical applications, including as vectors for the delivery of drugs, as antimicrobial surfaces, or as materials for cell proliferation/growth. 23−33 The preparation of multiblock copolymers is best accomplished using controlled polymerization methods. In the context of polyesters and -carbonates, controlled polymerizations include the metal catalyzed ring-opening polymerization (ROP) of cyclic esters 14,34−38 or cyclic carbonates ...
