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

Abstract

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.

Insight into the reaction mechanisms between CO2 and epoxides over Zn(II) phenoxide catalytic system – A DFT study

“…It was worth noting that the activation energy barrier difference between the methyleneeoxygen bond cleavage and the methineeoxygen bond cleavage of SO was over 12 kcal/mol. Therefore, the ring-opening of SO occurred predominantly at the methine C CH-R eO bond, which was in agreement with experimental studies [39]. As for ECH, the energy barrier difference between the methyleneeoxygen bond cleavage (C CH2 eO) and the methineeoxygen bond (C CH-R eO) cleavage was indistinctive, but the methyleneeoxygen bond cleavage product 2X-P was much lower in Gibbs free energy than that of the methineeoxygen bond cleavage product 2X-І.…”

Dinuclear Metal Complex-Mediated Formation of CO2-Based Polycarbonates

“…In addition, the catalyst afforded epichlorohydrin/CO 2 copolymers (>99% carbonate linkages). A decrease in the reaction temperature, from 25 to 0 C, enabled a high selectivity for polymer formation (>99% vs. cyclic carbonate), whereas under the same conditions, a binary cobalt salen/PPNX system led to only 66% selectivity for polymer [178].…”

Recent advances in carbon dioxide based copolymers

“…Later homogeneous catalysts including metal-porphyrin [33][34][35][36][37], zinc phenolate [38][39][40], discrete b-diiminate zinc [41][42][43][44][45], and binary or single bifunctional catalyst systems based on metal-Salen or -Salan complexes have been developed [46][47][48][49][50][51][52][53][54][55][56][57][58]. Recently, highly active bifunctional SalenCo catalytic systems (TON > 20,000; TOF > 20,000 h À1 ) are introduced for CO 2 /PO copolymerization [59,60], producing high molecular weight copolymers (M n > 300,000 g mol À1 ) in high selectivity.…”