Fast and selective ring-opening polymerizations by alkoxides and thioureas

• Published in: Nature chemistry Vol. 8; no. 11; pp. 1047 - 1053
• Main Authors: Zhang, Xiangyi, Jones, Gavin O., Hedrick, James L., Waymouth, Robert M.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.11.2016; Nature Publishing Group
• Subjects: 119/118; 140/131; 140/58; 639/638/455/941; 639/638/77/889; Alcohol; Analytical Chemistry; Biochemistry; Carbonates; Carbonyl compounds; Chemistry; Chemistry/Food Science; Competition; Hydrogen; Inorganic Chemistry; Molecular weight; Organic Chemistry; Physical Chemistry; Polymerization; Polymers; Potassium; United States > US; California
• ISSN: 1755-4330; 1755-4349; 1755-4349
• DOI: 10.1038/nchem.2574

Ring-opening polymerization of lactones is a versatile approach to generate well-defined functional polyesters. Typical ring-opening catalysts are subject to a trade-off between rate and selectivity. Here we describe an effective catalytic system combining alkoxides with thioureas that catalyses rapid and selective ring-opening polymerizations. Deprotonation of thioureas by sodium, potassium or imidazolium alkoxides generates a hydrogen-bonded alcohol adduct of the thiourea anion (thioimidate). The ring-opening polymerization of L -lactide mediated by these alcohol-bonded thioimidates yields highly isotactic polylactide with fast kinetics and living polymerization behaviour, as evidenced by narrow molecular weight distributions ( M w / M n  < 1.1), chain extension experiments and minimal transesterifications. Computational studies indicate a bifunctional catalytic mechanism whereby the thioimidate activates the carbonyl of the monomer and the alcohol initiator/chain end to effect the selective ring-opening of lactones and carbonates. The high selectivity of the catalyst towards monomer propagation over transesterification is attributed to a selective activation of monomer over polymer chains. By simply deprotonating a neutral hydrogen-bond donor thiourea it is possible to generate a class of highly efficient and tunable thioimidates that can simultaneously activate a pro-nucleophile and an electrophile. These bifunctional thioimidates exhibit fast kinetics and high selectivity for ring-opening polymerizations of cyclic lactones and carbonates.