trans- and cis-DACH-Based Bifunctional Squaramides Catalyzed Ring-Opening Polymerizations of Asymmetric Substituted Glycolides

A broad scope of the library of squaramide organocatalysts was prepared starting from two geometric isomers (trans- and cis-(1,2-diaminocyclohexane) (DACH)) with various numbers of −CH2– linkages between the squaramide core and the aromatic group, different numbers of electron-withdrawing −CF3 groups on the aromatic ring, and different sizes of cyclic tertiary amines from piperidine to pyrrolidine on the DACH ring. Then, the ring-opening polymerizations of asymmetrically substituted glycolides (ASGs) such as isobutyl glycolide (IBG), isobutyl methyl glycolide (IBMG), and isobutyl ethyl glycolide (IBEG) were achieved with bifunctional squaramide organocatalysts at ambient temperature (20 °C) in dichloromethane (DCM) or in 1,2-dichloroethane (DCE) at 50 °C. Predictable molecular weights of poly­(substituted glycolide) (PSG) homopolymers in accordance with monomer/initiator ratios, high conversions up to 100%, narrow polydispersity (PDI) values as low as 1.04, and high yields up to 93% were acquired. IBG, IBMG, and IBEG monomers showed much better polymerization activity in the presence of trans-DACH-based catalysts when compared to cis-DACH-based derivatives. It was revealed that the change of chemical shifts in the NHCH 2 and NH Cy amine peaks in the trans catalyst were 4.6 and 2.2 times higher than that of the cis catalyst upon addition of the monomer according to 1H NMR titration analyses, which proves that the monomer interacts better with the trans-DACH-based catalyst, probably due to the proximity of the trans-organocatalyst to the monomer, thus opening the ring more easily. The relative reactivities within three monomers toward polymerization are closely related to the sterically crowded groups of alkyl substituents on monomers. For instance, the IBG monomer having less sterically crowded groups led to polymerization faster when compared to IBMG and IBEG monomers. Thus, all trans catalysts 19–25 were very efficient in the polymerization of IBG at 20 °C. On the other hand, the existence of the steric group in IBMG and IBEG monomers made the catalyst selectivity important in their polymerizations. The most effective trans- and cis- catalysts in the polymerization of both IBMG and IBEG monomers were found to be trans 22 and cis 34 with two −CF3 groups and no −CH2– groups probably due to their high acidity, respectively. One or two different asymmetric centers leading to structural isomers in the chains (H–H, T–T, and H–T segments) in PIBG, PIBMG, and PIB