Sheng-Hsuan Wei

Perfectly Alternating Copolymerization of CO2 and Epichlorohydrin Using Cobalt(III)-Based Catalyst Systems

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.

Alternating copolymerization of CO2 and styrene oxide with Co(III)-based catalyst systems: differences between styrene oxide and propylene oxide

A detailed study of the difference in reactivity of the copolymerization reactions of styrene oxidevs.propylene oxide with carbon dioxide utilizing binary (salen)cobalt(III) catalyst systems to provide perfectly alternating copolymers is reported. This investigation focuses on the discrepancy exhibited by these two terminal epoxides for the preference for C–O bond cleavage during the ring-opening process. It was found that the nucleophilic ring-opening of styrene oxide occurs predominantly at the methine Cα–O bond which leads to an inversion of configuration at the methine carbon center. This tendency results in a significantly lower reactivity as well as a deterrent for synthesizing stereoregular poly(styrene carbonate) when compared to the propylene oxide/CO2 process. The chiral environment about the metal center had a notable effect on the regioselectivity of the ring-opening step for styrene oxide, with the methylene Cβ–O bond being preferentially cleaved. Using a binary catalyst system composed of an unsymmetrical (S,S,S)-salenCo(III) complex in conjunction with the onium salt PPNY (PPN = bis(triphenylphosphine)iminium, and Y = 2,4-dinitrophenoxy), a highly regioregular ring-opening step was observed with a concomitant 96% retention of configuration at the methine carbon center.

Base initiated depolymerization of polycarbonates to epoxide and carbon dioxide co-monomers: a computational study†

High-accuracy CBS-QB3(+) calculations were used to obtain the free energy barriers for several polycarbonates of interest to undergo alkoxide back-biting to give the corresponding epoxide and carbon dioxide. Free energy barriers to epoxide formation were modest for most polymeric alkoxides (12.7–17.4 kcal mol−1), and they were higher than for the same starting material to give cyclic carbonate (10.7–14.6 kcal mol−1). Poly(cyclopentene carbonate) differs: epoxide formation has a lower free energy barrier (13.3 kcal mol−1) than cyclic carbonate formation (19.9 kcal mol−1). These results explain why poly(cyclopentene carbonate) depolymerizes to cyclopentene oxide when treated with a strong base, whereas propylene and styrene polycarbonates depolymerize to their respective cyclic carbonates. Recycling via regeneration of the monomer represents the ideal method for producing material of the highest quality.

Iron complexes containing electrochemically active diazocycle-bis(di-tert-butyl-phenol) ligands

Quatro ligantes N, O-doadores contendo unidades centrais diazociclicas e grupos bis(di-terc-butilfenol) foram empregados na sintese de complexos de ferro(III), resultando em quatro complexos mononucleares e um binuclear. Os ligantes apresentados neste estudo diferem entre si na unidade diazociclica, sendo elas: piperazina (H2L1), diazepam/homopiperazina (H2L2), hexaidropirimidina (H2L3) ou hexaidropirimidin-5-ol (H3L4). As estruturas moleculares dos complexos [FeL2Cl], 2, e [Fe2(L4)(HL4)Cl], 4, foram elucidadas por difratometria de raios X de monocristal. Estudos eletroquimicos mostram que, alem de processos redox centrados no metal, os complexos apresentam processos redox atribuidos aos ligantes. Estudos coulometricos acoplados a espectroscopia eletronica no UV-Vis confirmam a formacao da especie radicalar para o complexo 2, enquanto dados de espectroscopia de ressonância paramagnetica eletronica (EPR) mostram a formacao do radical para os complexos 2, 4 e 5. De relevância e a observacao de que o ligante que sofre oxidacao em menor potencial eletroquimico e aquele cujo centro metalico apresenta a menor acidez de Lewis.