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.

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.

The solution structure and dynamics of three prototypical bis-indenyl ansa-zirconocenium methyl cations (Me 2 Si-(2-methyl-4-phenyl-6-tert-butylindenyl) 2 ZrMe + , [1Me] + ; Me 2 Si(2,4dimethylindenyl) 2 ZrMe + , [2Me] + ; Me 2 Si(indenyl) 2 ZrMe + , [3Me] + ) paired with the weakly coordinating B(C 6 F 5 ) 4− anion have been investigated by NMR spectroscopy in aromatic hydrocarbons with different polarities (toluene, ε r = 2.38; chlorobenzene, ε r = 5.69; 1,2difluorobenzene (ODFB), ε r = 13.38). These highly electrophilic cations are stabilized by solvent coordination, as evidenced by the unequivocal identification of [1Me•C 7 H 8 ] + B(C 6 F 5 ) 4 − , a rare example of a toluene-stabilized ion pair that has been fully characterized in solution. Combining spectroscopic and DFT methods allowed us to evaluate how solvent coordination modulates the dynamic processes typical of this class of catalysts. An exchange between the two faces of coordinated toluene (face inversion, FI) occurs without solvent decomplexation (ΔH ⧧ FI = 14.6 kcal mol −1 ; ΔS ⧧ FI = 3 cal mol −1 K −1 ; ΔG ⧧ FI (298) = 13.7 kcal mol −1 ) and is about 20 times faster than the exchange between coordinated and free solvent (solvent decomplexation, SD, ΔH ⧧ SD = 17.9 kcal mol −1 ; ΔS ⧧ SD = 10 cal mol −1 K −1 ; ΔG ⧧ SD (298) = 14.9 kcal mol −1 ) and ion pair symmetrization (IPS, ΔH ⧧ IPS = 18.6 kcal mol −1 ; ΔS ⧧ IPS = 12 cal mol −1 K −1 ; ΔG ⧧ IPS (298) = 14.9 kcal mol −1 ). For the ion pairs [1− 3Me•solvent] + B(C 6 F 5 ) 4 − , IPS rate constants (k IPS ) do not correlate with the solvent polarity (k IPS (ODFB) > k IPS (toluene) > k IPS (chlorobenzene)). For the more coordinating toluene and chlorobenzene solvents, ΔG ⧧ IPS nicely tracks with the amount of positive charge at the metal, increasing as the positive charge increases (q ZrMe 2 ,CM5 ; 1Me 2 > 3Me 2 > 2Me 2 ). In contrast, in the less coordinating ODFB, differences in the IPS barriers for [1−3Me•ODFB] + B(C 6 F 5 ) 4− appear to correlate better with steric parameters, as measured by the percent buried volume on the open quadrants (%V Bur,open ) for the corresponding neutral precursors (%V Bur,open = 34.8, 35.5,34.6% for 1Me 2 , 2Me 2 , and 3Me 2 , respectively).

The control of the stereochemistry of macromolecules is a very important goal, and coordination-insertion polymerization is superior with respect to the other polymerization methods for its achievement. In this contribution, we focus on Pd(II) homogeneous catalysts for the stereocontrolled synthesis of CO/vinyl arene polyketones. We developed a library of aldo-and keto-iminopyridine ligands N-N′ that feature an αor β-naphthyl or anthracenyl moiety on the imino nitrogen atom (N imm ). With such ligands, the Pd(II) monocationic complexes [Pd(CH 3 )(CH 3 CN)-(N-N′)][PF 6 ] were synthesized. NMR spectroscopy shows that in solution, each complex exists as an equilibrium mixture of cis and trans stereoisomers, the latter having the CH 3 ligand opposite to the Pd−N imm bond. The isomeric population depends on the N-N′ ligand: an almost 1:1 ratio is found for the ketimine complexes, whereas those with the aldimines show a preference for the trans geometry. These complexes generate very efficient catalysts for the CO/vinyl arene copolymerization. Catalyst performances depend both on the nature of N-N′ and of the vinyl arene comonomer. The ketimine-based catalysts are more stable and more productive than the aldimine counterpart, leading to prevailingly syndiotactic macromolecules of high M w (up to 280 kDa). The aldimine derivatives produce copolymers with isotactic and syndiotactic stereoblocks of different lengths depending on the vinyl arene. The effect of the prochiral monomer on the copolymer tacticity is steric in nature as demonstrated by the stereochemistry of the obtained CO/4-fluorostyrene polyketone, whose synthesis is reported here for the first time. As a conclusion, we have now demonstrated that when catalysts with nonsymmetric ancillary ligands are used, and stereoisomers are present, the stereochemistry of the copolymerization is driven by both the catalyst isomeric distribution and the prochiral comonomer.