Three dinuclear aluminum alkyl complexes of the general formula LAlMe, where L are salen ligands with an alkyl backbone of different lengths between the nitrogen atoms (1,3-propylene (1), 1,5-pentylene (2) and 1,12-dodecaylene (3)), have been prepared through alkane elimination reactions between each ligand and two equivalents of AlMe. The related hemi-salen aluminum complex 4 was prepared by an analogous reaction between a phenoxy-imine ligand and a single equivalent of AlMe. The activities of these aluminum complexes in the ring-opening polymerization (ROP) of rac-lactide and of several epoxides have been investigated and compared. The dinuclear complex 1, bearing the salen ligand with the shortest alkyl bridge, was the most active in the ROP of LA producing isotactic enriched PLA. Otherwise, the other complexes (2 and 3), in which the metal centers are remote, produced atactic PLA with inferior activity. Analogous differences in terms of activity emerged in the ROP of epoxides. The comparison of the catalytic behavior of the dinuclear complexes as well as their mononuclear counterparts suggests the cooperation between the two aluminum metal centers of the dinuclear species in which these are close enough.

The preparation and characterization of new Zn(II) complexes of the type [(PPP)ZnR] in which R = Et (1) or N(SiMe(3))(2) (2) and PPP is a tridentate monoanionic phosphido ligand (PPP-H = bis(2-diphenylphosphinophenyl)phosphine) are reported. Reaction of ZnEt(2) and Zn[N(SiMe(3))(2)](2) with one equivalent of proligand PPP-H produced the corresponding tetrahedral zinc ethyl (1) and zinc amido (2) complexes in high yield. Homoleptic (PPP)(2) Zn complex 3 was obtained by reaction of the precursors with two equivalents of the proligand. Structural characterization of 1-3 was achieved by multinuclear NMR spectroscopy ((1)H, (13)C, and (31)P) and X-ray crystallography (3). Variable-temperature (1)H and (31)P NMR studies highlighted marked flexibility of the phosphido pincer ligand in coordination at the metal center. A DFT calculation on the compounds provided theoretical support for this behavior. The activities of 1 and 2 toward the ring-opening polymerization of ε-caprolactone and of L- and rac-lactide were investigated, also in combination with an alcohol as external chain-transfer agent. Polyesters with controlled molecular parameters (M(n), end groups) and low polydispersities were obtained. A DFT study on ring-opening polymerization promoted by these complexes highlighted the importance of the coordinative flexibility of the ancillary ligand to promote monomer coordination at the reactive zinc center. Preliminary investigations showed the ability of these complexes to promote copolymerization of L-lactide and ε-caprolactone to achieve random copolymers whose microstructure reproduces the composition of the monomer feed.

New 3-coordinated Zn ROP catalysts afford lactide stereo-block copolymers with variable block lengths and steric structures and diblock ε-caprolactone-lactide copolymers at room temperature and in short reaction times.

The development of efficient homogeneous catalysts for the synthesis of functionalized polyolefins is a challenging topic. Palladium(II) complexes with -diimine ligands having a phenanthrene skeleton and 2,6-disubstituted aryl rings (Ar-BIP) were synthesized, characterized and tested as precatalysts in the copolymerization of ethylene with methyl acrylate. The direct comparison with analogous complexes having the corresponding diimines with an acenaphthene skeleton (Ar-BIAN) was performed. X-ray characterization in solid state and NMR analysis in solution of both neutral, [Pd(Ar-BIP)(CH 3)Cl], and monocationic [Pd(Ar-BIP)(CH 3)(NCCH 3)][PF 6 ] complexes, indicate that the Ar-BIP ligands have a higher Lewis basicity and are more strongly coordinated to the metal center than the Ar-BIAN counterparts. Therefore, the Pd-(Ar-BIP) cationic complexes can be regarded as electronrich metal cations. In addition, they create a higher steric congestion around palladium than Ar-BIAN, regardless of the substituents on the aryl rings. The monocationic species generate active catalysts for the ethylene/methyl acrylate copolymerization leading to copolymers with M n values up to 37000 and a content of polar monomer of 5.3 mol %. The detailed study of the catalytic behavior points out that Pd-(Ar-BIP) catalysts show a good affinity for the polar monomer, a good thermal stability and favor the cleavage of the catalyst resting state, leading to copolymer with Mw values higher than that of the macromolecules produced with the corresponding Pd-(Ar-BIAN) under the same reaction conditions. NMR characterization of the produced copolymers points out that the polar monomer is inserted both at the end of the branches and into the main chain, with a more selective enchainment than that achieved when the copolymerization is carried out in dichloromethane. In situ NMR investigations allowed us to detect relevant intermediates of the catalytic cycle and shed light on the nature of possible deactivation species.

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 series of new phenoxy-thioether (OS) proligands have been synthesized. They were found to readily react with 1 equiv of AlMe 3 to afford the corresponding Al chelate complexes {4,6-tBu 2 -OC 6 H 2 -2-CH 2 S(2-R-C 6 H 4 )}AlMe 2 (R = H (1), Br (2), CH 3 (3), CF 3 (4)) in quantitative yields. All the aluminum methyl complexes are stable monomeric species. In the solid state, as determined from X-ray crystallographic studies, complex 2 consists of a four-coordinate aluminum species in which the metal center is chelated by the sulfur and oxygen atoms of the bidentate ligand. All complexes promote the ring-opening polymerization of ε-caprolactone and L-and rac-lactide. Upon addition of methanol, efficient binary catalytic systems for the immortal ring-opening polymerization of the cyclic esters are produced (in detail, 300 equiv of ε-CL were converted in 20 min at 50°C and 100 equiv of rac-LA were converted in 1 day at 80°C). Kinetic studies show that polymerizations promoted by 1−4 are first order with respect to monomer concentration. The steric and electronic characteristics of the ancillary ligands have moderate influence on the polymerization performance of the corresponding aluminum complexes. However, the introduction of a substituent at the ortho position of the thiophenol aryl ring showed an opposite effect on the catalytic activities of the two different cyclic esters, increasing the activity in the ε-caprolactone polymerization and decreasing it in the polymerization of lactide.

The reaction of MgBu and ZnEt or Zn{N[Si(CH)]} with a tridentate monoanionic pyrrolylpyridiylimino [N,N,N] proligand gave homoleptic species, as exclusive products, in high yields. The complexes were characterized in solution by 1D and 2D NMR analysis and by single crystal X-ray crystallography. The new homoleptic complexes were tested as initiators in the polymerization of ε-caprolactone and lactide in the presence of an exogenous alcohol. For both complexes, the polymerizations proceed via an "activated monomer" mechanism that, in the case of the magnesium complex, was correlated with the coordinative flexibility of the ligands, resulting in extremely high productivities under mild conditions.

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.