Tertiary and Quaternary Phosphonium Borane Bifunctional Catalysts for CO₂/Epoxide Copolymerization: A Mechanistic Investigation Using In Situ Raman Spectroscopy

Abstract

Tertiary and quaternary phosphonium borane catalysts are employed as catalysts for CO2/epoxide copolymerization. Catalyst structures are strategically modified to gain insights into the intricate structure–activity relationship. To quantitatively and rigorously compare these catalysts, the copolymerization reactions were monitored by in situ Raman spectroscopy, allowing the determination of polymerization rate constants. The polymerization rates are very sensitive to perturbations in phosphonium/borane substituents as well as the tether length. To further evaluate catalysts, a nonisothermal kinetic technique has been developed, enabling direct mapping of polymerization rate constant (k p) as a function of polymerization temperatures. By applying this method, key intrinsic attributes governing catalyst performance, such as activation enthalpy (ΔH ‡), entropy (ΔS ‡), and optimal polymerization temperature (T opt), can be extracted in a single continuous temperature sweep experiment. In-depth analyses reveal intricate trends between ΔH ‡, ΔS ‡, and Lewis acidity (as determined using the Gutmann–Beckett method) with respect to structural variations. Collectively, these results are more consistent with the mechanistic proposal in which the resting state is a carbonate species, and the rate-determining step is the ring-opening of epoxide. In agreement with the experimental results, DFT calculations indicate the important contributions of noncovalent stabilizations exerted by the phosphonium moieties. Excitingly, these efforts identify tertiary phosphonium borane analogues, featuring an acidic phosphonium proton, as leading catalysts on the basis of k p and T opt. Mediated by phosphonium borane catalysts, epoxides such as butylene oxide (BO), n-butyl glycidyl ether (BGE), 4-vinyl cyclohexene oxide (VCHO), and cyclohexene oxide (CHO) were copolymerized with CO2 to form polyalkylene carbonate with >95% chemo-selectivity. The tertiary phosphonium catalysts maintain their high activity in the presence of large excess of di-alcohols as chain-transferring agents, affording well-defined telechelic polyols. The results presented herein shed light on the cooperative catalysis between phosphonium and borane.

Bifunctional organoboron–phosphonium catalysts for coupling reactions of CO2 and epoxides

“…A more detailed analysis of the reaction mechanism consistent with this interpretation may be found in ref. 62 .…”

A one-component phosphonium borane Lewis pair serves as a dual initiator and catalyst in the ring-opening alternating copolymerization of anhydrides and epoxides

“…The first step involves an S N 2 substitution reaction between triphenylphosphine (PPh 3 ) and alkenyl halides to facilitate full conversion of phosphonium salts with terminal CvC double bond, which showed good agreement with the reported literature. [55][56][57] The phosphonium salts having allyl and 3-butenyl-group were characterized by X-ray diffraction analysis (Fig. S70 and S71 †).…”

Ultra-Productive Upcycling CO2 into Polycarbonate Polyols via Borinane-Based Bifunctional Organocatalysts

“…To address this issue, our group developed the recycling process of both the initiator and TEB, and more recently, direct recycling only through precipitation . In a recent addition, 9-borabicyclo(3.3.1)­nonane (9-BBN)-based bifunctional catalysts − were reported as showing high activity in CHO and CO 2 copolymerization, but only moderate performance toward PO and CO 2 copolymerization. Crucially, for an effective synthesis of polycarbonate polyols in the presence of CTAs, the catalyst selected should exhibit two imperative features: demonstrate a very high activity and be tolerant of CTAs.…”

“…Inspired by the remarkable synergy found in bifunctional metal catalysts ,, and particularly in dinuclear functional 9-BBN-based catalysts, , the present work explores the potential of bifunctional organocatalysts based on covalently linked 2, 3, and 4 Lewis acidic boron centers to one Lewis basic ammonium salts. We indeed report a series of borinane-based bifunctional catalysts (Scheme ) for organocatalytic ROCOP of CO 2 and epoxides carried out in the presence of large quantities of CTAs (with a ratio of 1000 between CTA and the catalyst).…”