Abstract
Even after several decades of intense research, mechanistic studies of olefin polymerization by early transition metal catalysts continue to reveal unexpected elementary reaction steps. In this mini-review, the recent discovery of two unprecedented chain termination processes is summarized: chain transfer to solvent (CTS) and chain transfer to monomer (CTM), leading to benzyl/tolyl and allyl type chain ends, respectively. Although similar transfer reactions are well-known in radical polymerization, only very recently they have been observed also in olefin insertion polymerization catalysis. In the latter context, these processes were first identified in Ti-catalyzed propene and ethene polymerization; more recently, CTS was also reported in Sc-catalyzed styrene polymerization. In the Ti case, these processes represent a unique combination of insertion polymerization, organic radical chemistry and reactivity of a M(IV)/M(III) redox couple. In the Sc case, CTS occurs via a σ-bond metathesis reactivity, and it is associated with a significant boost of catalytic activity and/or with tuning of polystyrene molecular weight and tacticity. The mechanistic studies that led to the understanding of these chain transfer reactions are summarized, highlighting their relevance in olefin polymerization catalysis and beyond.
Highlights
Starting with the pioneering work by Cossee and Arlman in the 1960s [1,2], mechanistic studies of olefin polymerization catalysis have unveiled an intricate series of chain initiation, propagation and termination processes presiding over polymer formation
We summarize the efforts to elucidate the mechanisms of these novel and unconventional chain termination reactions by combining polymerization experiments, spectroscopic studies and density functional theory (DFT) modeling
The recent discovery of chain transfer to solvent (CTS) and chain transfer to monomer (CTM) exemplifies the intriguing complexity of the olefin polymerization mechanism, which still needs to be fully unveiled
Summary
Starting with the pioneering work by Cossee and Arlman in the 1960s [1,2], mechanistic studies of olefin polymerization catalysis have unveiled an intricate series of chain initiation, propagation and termination processes presiding over polymer formation. Instead, usually derive from chain transfer to a main group metal cocatalyst (e.g., Al and Zn alkyls) [31,32] or reaction with molecular hydrogen (Scheme 1) [33,34]; they can derive from chain reinitiation via monomer insertion into metal-hydride and metal-alkyl species, generated after BHE/hydrogenolysis and BHTM/BME/chain transfer to the cocatalyst, respectively [5] These chain termination mechanisms and their correlations with polymer microstructure have been extensively studied, and are discussed in depth in a number of reviews [3,4,5,8]. The potential relevance of this reactivity for the production of functionalized polymers, as well as for small molecule activation and synthesis of metal allyl complexes, is discussed
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