The preparation of perfectly alternating and regioslective copolymers derived from the copolymerization of carbonyl sulfide (COS) and epoxides by metal-free Lewis pair catalysts composed of a Lewis base (amidine, guanidine, or quaternary onium salts) and a Lewis acid (triethyl borane) is described. Colorless and highly transparent copolymers of poly(monothiocarbonate) were successfully obtained with over 99 % tail-to-head content and high molecular weight (up to 92.5 kg mol ). In most instances, oxygen-sulfur exchange reactions (O/S ERs), which would generate random thiocarbonate and carbonate units, were effectively suppressed. The turnover frequencies (TOF) of these Lewis pair catalyzed processes were as high as 119 h at ambient temperature.

Polyester synthesis from the alternating copolymerization of epoxides with cyclic anhydrides via a metal-free route remains a key challenge. This work reports the development of a highly active organocatalytic route for the copolymerization of a spectrum of epoxides and cyclic anhydrides. Fully alternating polyesters were synthesized by a variety of organic Lewis acid–base pairs including organoboranes and quaternary onium salts. The effect of the acidity, type, and size of Lewis pairs on the catalytic activity and selectivity of the copolymerization is presented. The undesirable transesterification and etherification were effectively suppressed even in the case of complete conversion of the cyclic anhydride. This could be ascribed to the formation of a unique tetracoordinate bond-carboxylate (or alkoxide) anion. The Lewis pairs are highly active, with a turnover frequency of 102 and 303 h–1 for the copolymerization of propylene oxide with maleic anhydride and phthalic anhydride, respectively, at 80 °C. Block polyester with narrow polydispersity of 1.05 was achieved via a sequential addition strategy. This work provides robust organocatalysts for the selective copolymerization of epoxides with cyclic anhydrides.

A semicrystalline poly(trimethylene monothiocarbonate) (PTMMTC) has been synthesized via the selective and alternating copolymerization of carbonyl sulfide and oxetane. This reaction was catalyzed by (salen)CrCl accompanied by organic bases over a wide range of temperatures from 40 to 130 °C. PTMMTC is shown to exhibit similar crystallization behavior to high-density polyethylene (HDPE), i.e., being spherulite and possessing melting temperatures (T m ) up to 127.5 °C and a degree of crystallinity (X c ) of up to 71%. Moreover, PTMMTC has a wide processing temperature window of ca. 100 °C.

This work discloses a zwitterionic approach for selective copolymerization of carbon dioxide (CO 2 ) and propylene oxide (PO), producing poly(propylene carbonate) (PPC), a biodegradable polymer with broad applications. Small-molecule catalysts composed of triethylamine (TEA) and trialkyl boranes are effective for CO 2 / PO copolymerization with an alternating degree of >99% and a productivity of 171 g PPC/g catalyst. A diamine N,N,N′,N′-tetraethyl ethylenediamine (TEED) paired with trialkyl borane exhibited improved activity and productivity (up to 216 g PPC/g catalyst). By adjusting the Lewis acid−base pair, the PPC selectivity can be regulated to 99%. In addition, PPCs have medium regioregularity with a head-to-tail diad content of 80−82% and number-average molecular weights of up to 56.0 kg/mol with narrow polydispersity (below 1.2). The overall catalytic performance of these readily available simple molecules is better than that of previously reported organic catalysts for CO 2 /PO copolymerization. Successive insertion of PO and CO 2 into the Lewis pair leads to the formation of an end-to-end zwitterion featuring a TEB-masked anion and an onium cation, which is highly selective to the alternating copolymerization, as demonstrated by quantum mechanical calculations.

Metal-free and controlled synthesis of sulfur-containing polymer is still a big challenge in polymer chemistry. Here, we report a metal-free, living copolymerization of carbonyl sulfide (COS) with epoxides via the cooperative catalysis of organic Lewis pairs including bases (e.g.: phosphazene, amidine, and guanidine) and thioureas as hydrogen-bond donors, afford well-defined poly(monothiocarbonate)s with 100% alternating degree, >99% tail-to-head content, controlled molecular weights (up to 98.4 kg/mol), and narrow molecular weight distributions (1.13–1.23). The effect of the types of Lewis pairs on the copolymerization of COS with several epoxides is investigated. The turnover frequencies (TOFs) of these Lewis pairs are as high as 112 h−1 at 25 °C. Kinetic and mechanistic results suggest that the supramolecular specific recognition of thiourea to epoxide and base to COS promote the copolymerization cooperatively. This strategy provides commercially available Lewis pairs for metal-free synthesis of sulfur-containing polymers with precise structure.