Organo-catalyzed/initiated ring opening co-polymerization of cyclic anhydrides and epoxides: an emerging story

This review deals with recent organo-catalyzed/initiated developments of co-polymerization of cyclic anhydrides and epoxides to access polyesters.

Homoconjugated Acids as Low Cyclosiloxane-Producing Silanol Polycondensation Catalysts

Polycondensation of α,ω-disilanols is a foundational technology for silicones producers. Commercially, this process is carried out with strong Brønsted acids and bases, which generates cyclosiloxane byproducts. Homoconjugated acids (a 2:1 complex of acid:base or a 1:1 complex of acid:salt), a seldom used class of silanol polycondensation catalysts, were evaluated for their ability to polymerize α,ω-disilanols while forming low levels of cyclosiloxane byproducts. Homoconjugated acid catalysts were highly active for silanol polycondensation, even when made from relatively mild acids such as acetic acid. Both the acid and base (or cation) component of the homoconjugated species was important for activity and avoiding cyclosiloxane byproduct formation. Stronger acids and bases were found to positively affect reactivity, and the pK a of the acid was found to correlate with cyclosiloxane byproduct formation. The individual components of the homoconjugated species (the acid and base) were ineffective as catalysts by themselves, and compositions with fewer than 2 mol of acid to 1 mol of base were much less reactive. Homoconjugated trifluoroacetic acid tetramethylguanidinium and tetrabutylphosphonium complexes were found to be privileged catalysts, able to give high-molecular-weight siloxanes (M n > 60 kDa) while generating less than 100 ppm of octamethylcyclotetrasiloxane byproduct. Finally, a mechanism has been proposed where silanols are electrophilically and nucleophilically activated by the homoconjugated species, leading to silanol polycondensation.

Tetradentate iminophenolate copper complexes in rac-lactide polymerization

Copper(II) nitrate complexes of 2-(((2-((2-aminoethyl)amino)ethyl)imino)methyl)phenol, 2-(((2-((2-aminoethyl)amino)ethyl)imino)methyl)-4,6-dichlorophenol, 2-(((2-(piperazin-1-yl)ethyl)imino)methyl)phenol and (2,4-di-tert-butyl-6-(((2-(piperazin-1-yl)ethyl)imino)methyl)phenol, as well as a copper(II) acetate complex of 2-(((2-(piperidin-1-yl)ethyl)imino)methyl)phenol, have been prepared and characterized by X-ray diffraction studies. In combination with benzyl alcohol, all complexes are active in rac-lactide polymerization at 140 °C in molten monomer to provide moderately heterotactic polylactic acid. Most complexes showed complicated reaction kinetics, indicative of two interconverting active species. Molecular weight control was poor and a strong tendency toward intramolecular transesterification led to oligomeric products. There was no indication that the basic site of the ligand is participating in the polymerization reaction by deprotonation of the alcohol nucleophile.

Squaramide and amine binary H-bond organocatalysis in polymerizations of cyclic carbonates, lactones, and lactides

Multiple combinations of six squaramides and eight amines as co-catalysts were success in ROPs of cyclic monomers by H-bond donor and acceptor binary catalysis that established a general protocol.