The Effect of Counterion/Ligand Interplay on the Activity and Stereoselectivity of Palladium(II)–Diimine Catalysts for CO/p‐Methylstyrene Copolymerization

Abstract

The catalytic activity and stereoselectivity of complexes [Pd(eta(1),eta(2)-C(8)H(12)OMe)(Ar--N==C(R')--C(R')==N--Ar)]X in the copolymerization of CO and p-methylstyrene have been correlated with their interionic structure in solution and in the solid state, as determined by (19)F,(1)H-HOESY NMR spectroscopy and X-ray diffraction studies, respectively. The highest productivity is obtained with unhindered diimine ligands bearing electron-donating substituents and with the least coordinating counterion. Copolymers with a microstructure ranging from atactic to predominantly isotactic are obtained. The degree of isotacticity increases as the steric hindrance in the apical positions and the coordinating ability of the counterion increase. The counterion is located close to the diimine in both solution and the solid state but it moves toward the palladium as the steric hindrance in the apical positions decreases. When the latter is small the counterion competes with the substrate for apical coordination, and consequently it affects the productivity. In the case of ortho-dimethyl-substituted ligands the counterion is confined in the back, above the N==C(R')--C(R')==N moiety, and does not affect the productivity. However, it contributes to increasing the stereoregularity of the copolymer by making the aryl moieties more rigid. With R'=Me and Ar=o-Me(2)C(6)H(3) an ll of 81 % and 72 % was obtained with X(-)=CF(3)SO(3) (-) or BArF(-), respectively. The isotacticity of the copolymers produced by ortho-monosubstituted catalysts depends greatly on the counterion and ranges from 30 % to 59 % with X(-)=BArF(-) and X(-)=CF(3)SO(3) (-), respectively, with Ar=o-EtC(6)H(4) and R'=Me. Based on the interionic structural results, this effect can be explained by a greater reduction of the copolymerization rate of C(s)-symmetric isomers with respect to their C(2)-symmetric counterparts.

Unprecedented Comonomer Dependence of the Stereochemistry Control in Pd‐Catalyzed CO/Vinyl Arene Polyketone Synthesis

“…[23][24][25][26] CO/ vinyl arene polyketones with isotactic stereoblocks have been synthesized by Pd catalysts with a-diimine ligands. [27][28][29] Up to now,t he tacticityo ft he produced polyketones was always dictated by the ligand bondedt ot he catalytic center and it was not affected by the variation of the substituent in para position on the vinyl arene comonomer.I no ther words, no effect on polymer stereochemistry was observed on changing the vinyl arene from styrene (S) to its 4-substituted derivatives such as 4-methyl styrene (MS) and 4-tert-butyl styrene (TBS). Only if the 2-substituted styrene was used as ac omonomer,t he [Pd(phen)(MeCN) 2 ][BF 4 ] 2 catalyst yieldeda na tactic polyketonei nsteado ft he expected syndiotactic macromolecule.…”

Analogies and Differences in Palladium‐Catalyzed CO/Styrene and Ethylene/Methyl Acrylate Copolymerization Reactions

“…[18,19] To obtain fully isotactic, optically active copolymers, catalysts having modified bis-oxazolines or imidazolines are required. [20][21][22][23] If catalysts having bis(aryl)-a-diimines with an acenaphthene (Ar-BIAN) or a 1,4-diaza-1,3-butadiene (Ar-DAB) skeleton are applied, polyketones with either an atactic microstructure [24,25] or isotactic stereoblocks [26][27][28] are produced depending if meta-or ortho-substituted aryl rings are present on the ligand itself. On the other hand, for the ethylene/polar vinyl monomer copolymerization the involved N-donor ligands are exclusively a-diimines with both the acenaphthene or the 1,4-diaza-1,3butadiene skeleton.…”

A ligand exchange strategy for one-pot sequential synthesis of (hyperbranched polyethylene)-b-(linear polyketone) block polymers

“…14,15 However, it is ineffective for catalyzing alternating polymerization of olefin and CO, 16 which also requires cationic Pd(II) catalysts. 17 In 1, the bulky ortho-substituents on the aryl rings of the diimine ligand block the axial coordination sites of the metal center.…”