Abstract
Controlling polymer composition starting from mixtures of monomers is an important, but rarely achieved, target. Here a single switchable catalyst for both ring-opening polymerization (ROP) of lactones and ring-opening copolymerization (ROCOP) of epoxides, anhydrides, and CO2 is investigated, using both experimental and theoretical methods. Different combinations of four model monomers-ε-caprolactone, cyclohexene oxide, phthalic anhydride, and carbon dioxide-are investigated using a single dizinc catalyst. The catalyst switches between the distinct polymerization cycles and shows high monomer selectivity, resulting in block sequence control and predictable compositions (esters and carbonates) in the polymer chain. The understanding gained of the orthogonal reactivity of monomers, specifically controlled by the nature of the metal-chain end group, opens the way to engineer polymer block sequences.
Highlights
Discovering selective transformations using monomer mixtures to yield polymers of homogeneous compositions, including block sequences, are highly desirable both to reduce costs and to enhance chemical complexity.[1]
Mixtures of cyclohexene oxide, εcaprolactone, and carbon dioxide were first polymerized by ring-opening copolymerization (ROCOP) to produce a perfectly alternating polycarbonate block and, once the carbon dioxide was consumed/removed, the same catalyst selectively polymerized the lactone, by ring-opening polymerizations (ROP), producing block copoly(estercarbonates)
A detailed study comparing the products of polymerizations conducted using mixtures selected from four generic types of oxygenated precursors was undertaken (Scheme 2)
Summary
Discovering selective transformations using monomer mixtures to yield polymers of homogeneous compositions, including block sequences, are highly desirable both to reduce costs and to enhance chemical complexity.[1]. The selectivity arises by controlling the chemistry of the metal−polymer chain end group, with metal alkoxides and carbonates having orthogonal reactivities toward the monomers.[17] Using this method, mixtures of cyclohexene oxide, εcaprolactone, and carbon dioxide were first polymerized by ROCOP (epoxide/CO2) to produce a perfectly alternating polycarbonate block and, once the carbon dioxide was consumed/removed, the same catalyst selectively polymerized the lactone, by ROP, producing block copoly(estercarbonates) This surprising result was rationalized by kinetic phenomena: i.e., relatively faster rate of carbon dioxide insertion into the zinc alkoxide intermediate, which is common to both catalytic cycles. We combine experimental and theoretical studies to examine the thermodynamic factors under-pinning the selective transformations
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