Air-Stable Single-Component Pd-Catalysts for Vinyl-Addition Polymerization of Functionalized Norbornenes

Well-defined single-component and tolerant Ru-, Mo-,
and W-catalysts
have been earlier developed for ring-opening metathesis polymerization
of norbornene and its derivatives. Another way of norbornene polymerizationvinyl-addition
polymerization (VAP)is a more thermodynamically favorable
process and gives polymers with saturated and rigid main chains. Single-component
catalysts for VAP are required, which combine good activity, tolerance
to functional groups, resistance to oxygen, and air moisture and are
capable of catalyzing living VAP of norbornenes. Herein, we have shown
that cationic Pd-complexes with N-heterocyclic carbene ligands (NHC
ligands) and weak coordinating anion ligands ([(NHC)Pd(allyl)L]+A–, where allyl is an allyl group, L = CO,
CH3CN, CH2Cl2, and A– = SbF6– or BARF–)
are highly efficient single-component catalysts for VAP of both norbornene
and various substituted norbornenes. These complexes, being stable
in the solid state and in solutions, have exhibited several important
features as VAP catalysts. They are capable of catalyzing VAP of norbornenes
in air and are tolerant to functional groups. The complexes have shown
high catalytic activity and have allowed polymerizing monomers at
very low catalyst loadings, as low as 2 ppm. The versatility of [(NHC)Pd(allyl)L]+A– complexes as catalysts of VAP has been
demonstrated by successful polymerization of monomers with polar and
bulky groups, containing alkyl, vinyl, alkylidene, ether, ester, imide,
or organosilicon motifs, affording high-molecular-weight products
in good and high yields. Tuning the nature of NHC and allyl ligands
in [(NHC)Pd(allyl)L]+A– complexes let
perform living VAP, yielding block copolymers.

先前已成功研发出定义明确的单组分Ru、Mo和W催化剂，适用于环氧化开环共轭聚合反应中诺伯伦及其衍生物的聚合。诺伯伦聚合的另一种途径——乙烯基加成聚合（VAP）——是一种在热力学上更为有利的反应过程，能够生成具有饱和且刚性的主链聚合物。VAP的单组分催化剂需具备良好的活性、对官能团的耐受性、抗氧性和抗空气湿度性，并能够催化诺伯伦的活性VAP。本研究中，我们展示了阳离子Pd-配合物，其中含有N-杂环卡宾配体（NHC配体）和弱配位阴离子配体（[(NHC)Pd(allyl)L]+A–，其中allyl代表烯丙基，L = CO、CH3CN、CH2Cl2，A– = SbF6–或BARF–），作为诺伯伦及其多种取代衍生物VAP的高效单组分催化剂。这些配合物在固态和溶液中均保持稳定性，作为VAP催化剂展现出多项重要特性。它们能够在空气中催化诺伯伦的VAP，并对官能团具有耐受性。这些配合物表现出高催化活性，并允许在极低催化剂负载量下进行单体聚合，低至2 ppm。[(NHC)Pd(allyl)L]+A–配合物作为VAP催化剂的通用性已通过成功聚合带有极性和体积庞大官能团的单体得到证实，包括烷基、乙烯基、烷基乙烯基、醚、酯、亚胺或有机硅基团，以良好的收率和高产率提供高分子量产品。通过调节[(NHC)Pd(allyl)L]+A–配合物中NHC和allyl配体的性质，实现了活性VAP，从而获得嵌段共聚物。