Dialkylaluminum 2-substituted 6,6-dimethylcyclopentylpyridin-7-oxylates toward structural-differentiation of the ring-opening polymerization of ε-caprolactone and l-lactides

Herein, a series of dialkylaluminum 2-substituted 6,6-dimethylcyclopentyl pyridin-7-oxylates Al1-Al7 were synthesized and characterized by 1H- and 13C-NMR spectroscopy and elemental analysis. The molecular structure of Al3 was proven to be a dimer of an aluminum complex. These aluminum complexes could efficiently initiate the ring-opening polymerization (ROP) of ε-caprolactone (CL), and the structural differentiations of the resultant PCL were strongly dependent on the amount of BnOH (PhCH2OH) used. In the absence of BnOH, the resultant PCL showed a cyclic structure, whereas BnO-capped linear PCL was obtained in the presence of >2.0 equivalents of BnOH; the resultant PCL was a mixture of linear and cyclic PCLs in the presence of 1.0 equivalent of BnOH. Moreover, these aluminum complexes exhibited high efficiency towards the ROP of l-lactide (LLA); however, the activities were lower than those for the ROP of ε-CL. Without BnOH, the resultant PLLA showed a highly linear structure with the alkyl-end group from aluminum complexes; on the other hand, PLLA displayed a major cyclic structure and minor BnO-capped linear PLLA if 1.0 equivalent of BnOH was employed, and the percentage of BnO-capped linear PLLA was increased by increasing the amount of BnOH.

Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs

The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or regioselectivity, as well as its unique application in materials chemistry. These advances made in LPP are comprehensively reviewed, with the scope of monomers focusing on heteroatom-containing polar monomers, while the polymerizations mediated by main-group LAs and LBs separately that are most relevant to the LPP are also highlighted or updated. Examples of applying the principles of the LPP and LP chemistry as a new platform for advancing materials chemistry are highlighted, and currently unmet challenges in the field of the LPP, and thus the suggested corresponding future research directions, are also presented.

Lewis Pair Catalysts in the Polymerization of Lactide and Related Cyclic Esters

Polyesters, especially poly(lactide) (PLA), are used widely as biodegradable and biocompatible materials, yet their controllable synthesis, especially the stereoselective synthesis of polyesters, is still a challenge. Recently some excellent Lewis pair catalysts for ring-opening polymerization (ROP) of lactide and related cyclic esters have emerged. This review article will highlight the key advances in the ROP catalyzed by Lewis pair compounds with the aim of encouraging the wider application of Lewis pair catalysts in the polymerization of lactide and related cyclic esters.