Selective transformations of carbon dioxide and epoxides into biodegradable polycarbonates by the alternating copolymerization of the two monomers represent some of the most well-studied and innovative technologies for potential large-scale utilization of carbon dioxide in chemical synthesis. For the most part, previous studies of these processes have focused on the use of aliphatic terminal epoxides or cyclohexene oxide derivatives, with only rare reports concerning the synthesis of CO(2) copolymers from epoxides containing electron-withdrawing groups such as styrene oxide. Herein we report the production of the CO(2) copolymer with more than 99% carbonate linkages from the coupling of CO(2) with epichlorohydrin, employing binary and bifunctional (salen)cobalt(III)-based catalyst systems. Comparative kinetic studies were performed via in situ infrared measurements as a function of temperature to assess the activation barriers for the production of cyclic carbonate versus copolymer involving two electronically different epoxides: epichlorohydrin and propylene oxide. The relative small activation energy difference between copolymer versus cyclic carbonate formation for the epichlorohydrin/CO(2) process (45.4 kJ/mol) accounts in part for the selective synthesis of copolymer to be more difficult in comparison with the propylene oxide/CO(2) case (53.5 kJ/mol). Direct observation of the propagating polymer-chain species from the binary (salen)CoX/MTBD (X = 2,4-dinitrophenoxide and MTBD = 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene) catalyst system by means of electrospray ionization mass spectrometry confirmed the perfectly alternating nature of the copolymerization process. This observation in combination with control experiments suggests possible intermediates involving MTBD in the CO(2)/epichlorohydrin copolymerization process.
The first highly active phosphine (P)/borane (B) Lewis pair polymerization is promoted unexpectedly by P-B adducts. The P and B site cooperativity is essential for achieving effective polymerization, as shown by this study examining the reactivity of a library of P/B Lewis pairs toward polymerization of a renewable acrylic monomer.
Highly active catalyst systems based on N-heterocyclic olefin (NHO) as initiator were developed for polymerizing various polar monomers such as methyl methacrylate (MMA), n-butyl methacrylate (BMA), N,N-dimethylacrylamide (DMAA) and N,N-diphenylacrylamide (DPAA) at ambient temperature. The preactivation of these polar monomers by Lewis acid such as Al(C6F5)3 or AlCl3 is a prerequisite for their rapid transformation, since a stable adduct easily forms by the interaction of nucleophilic NHO and Lewis acidic activator. The formation of NHO/Al(C6F5)3 adduct was confirmed by 1H NMR spectroscopy and X-ray single crystal analysis. The length of AlAl(C6F5)3 –CNHO bond is in the range of 2.002–2.018 Å, dependent on the substitute groups of N-heterocyclic ring. Highly molecular weight (M n = 7.50 × 105 g/mol) and narrow molecular weight distribution (M w /M n = 1.04–1.27) were achieved with dissymmetry NHO as initiator and Al(C6F5)3 as activated agent. Although binary NHO/Al(C6F5)3 catalyst system could polymerize both MMA and BMA with high activity, the attempt to synthesize their block copolymers proved to be unsuccessful. Electrospray ionization time-of-flight mass spectrometry (ESI–TOF MS) study provided important information on the polymer-chain ends. It was found that NHO as the initiation group bounded to one end of a polymer chain and an unexpected six-membered lactone ring appeared at another chain end. The formation of lactone end is ascribed to the nucleophilic backbiting of the polymeric anion to the carboxyl carbon of the adjacent unit, in companion with the release of the methoxyl group. The low initiation efficiency of NHO is attributed to the formation of the stable NHO/alane adduct during the polymerization, while the production of lactone end results in complete deactivation in polymer chain propagation.
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