Two dinuclear aluminum complexes bearing dinaphthalene bridging Schiff bases have been synthesized and investigated as catalysts in ring-opening polymerization (ROP) of cyclic esters such as rac-lactide (rac-LA) and ε-caprolactone (ε-CL) and in the copolymerization (ROCOP) of phthalic anhydride (PA) with cyclohexene oxide (CHO) and limonene oxide (LO). The polymerizations of cyclic esters were living, producing polymers with narrow molar mass distributions. Kinetic studies showed that the polymerizations were first order with respect to the monomers. Cooperative effects between two metal centers, located in proximal positions, are invoked to rationalize the high activities toward both monomers, although the rigid backbone of the complexes enhances the reactivity of less encumbered caprolactone in comparison to lactide. Good activities were achieved also in the copolymerization of phthalic anhydride with cyclohexene epoxide and with the bioderived limonene oxide.

A 6-(2,6-dimethylphenyl)-2-(2,6-diisopropylphenyl)iminopyridine dibromo nickel(II) complex was synthesized, characterized by X-ray diffraction analysis and tested in ethylene polymerization using diethylaluminumchloride as the cocatalyst. Low molecular weight (M_n ∼ 10³ g mol⁻¹) polyethylene oils were obtained under a variety of reaction conditions. Detailed NMR analysis showed the formation of hyperbranched macromolecules (branching density >100 branches per 1000 carbons) with a high fraction of “branches on branch” and one unsaturation per chain, resulting in polymer features comparable to those of polymers produced by α-diimine Pd(II) catalysts. The DFT model of the catalytic species showed that the ortho-2,6-dimethylphenyl substituent of the pyridine group destabilizes the ethylene coordination to the metal centre but does not encumber both axial coordination site. So the polymerization performance of 1 can be addressed to the catalytic pocket generated by the coordinated ligand that favors both chain transfer and chain walking over propagation.

The introduction of polar functional groups into the polyolefin skeleton is a challenging goal of high interest, and coordination-insertion polymerization represents the most powerful and environmentally friend approach to achieve it. Until now the most considerable catalysts are based on Pd(II) complexes and only a few examples on Ni(II) derivatives have been reported. We have now investigated a series of Ni(II) complexes with four pyridylimino ligands, both aldimines and ketimines, differing for the substituent present in position 6 on the pyridine ring (either a methyl group or a 2,6-dimethyl-substituted phenyl ring). These complexes generated active catalysts for the copolymerization of ethylene with methyl acrylate, yielding low-molecular weight, hyperbranched copolymers with the polar monomer content ranging between 0.2 and 35 mol % and inserted in a variety of modes, some of which were never observed before. The way of incorporation of the polar monomer goes from “in-chain only” to “everywhere but in-chain”, and it is dictated by both the activation mode and the solvent used to dissolve the nickel precatalyst.

Poly(lactic acid) (PLA) is currently by far the most commercially relevant biodegradable polymer derived from bio-renewable sources. The industrial production of high molecular weight isotactic PLA is based on the ring-opening polymerization (ROP) of lactide (the dimeric lactone of lactic acid) promoted by Sn(II) 2-ethylhexanoate under solvent-free conditions at high temperatures (190–200 °C) in the presence of excess alcohol. Although a huge number of alternative catalysts have been reported in the literature, searching for better performance and lower toxicity, most studies were performed under conditions unsuitable for industrial processes, for example, at lower temperatures in solution. We report here the synthesis of two Zn(II) complexes supported by bulky monoanionic guanidinate ligands, which have been tested as catalysts for the ROP of l-lactide under both mild and industrially relevant conditions, that is, at 190 °C using technical grade l-lactide at a monomer/catalyst ratio up to 10⁴ and excess alcohol. Interestingly, the Zn(II) catalysts also showed stability under the latter harsh conditions, showing activity comparable to that of the Sn catalyst and even lower monomer racemization. Moreover, they are able to copolymerize l-lactide with ε-caprolactone, affording either block or random copolymers depending on the reaction conditions.