Monometallic mechanism olefin polymerization

Zirconocenes and lanthanocenes active for olefin polymerization do, in fact, carry out well-controlled homopolymerizations of (meth)acrylic monomers, but polymerization takes place by an enolate mechanism in which the conjugated carbonyl group plays a crucial role in stabilizing the active center. Both monometallic and bimetallic mechanisms have been documented. Collins and co-workers developed a zirconocene group-transfer polymerization (GTP) technique for the polymerization of methyl methacrylate (MMA) which utilizes a neutral zirconocene enolate as an initiator and the conjugate zirconocene cation as a catalyst (Scheme 3). ... 

Although the exact mechanism of the polymerization of olefins by Ziegler-Natta catalysis is not delineated with the precision that would satisfy every one, it is possible to write a reasonable mechanism [1,2]. The first of these is a monometallic mechanism. The following mechanism is for a TiCl3/AlEt3 combination. We have seen above that titanium tetrachloride has been initially used in the polymerization of ethylene. It has been subsequently found that TiCls is better than TiCLj. 

The basic assumptions common to most mechanism studies relative to transition metal catalyzed polymerizations are as follows (i) The mechanism is essentially monometallic and the active center is a transition metal-carbon bond.13-15,18,19 (ii) The mechanism is in two stages coordination of the olefin to the catalytic site followed by insertion into the metal-carbon bond through a cis opening of the olefin double bond.13,20,21... 

Most reaction models which describe the mechanism of diene polymerization by Nd catalysts have been adopted from models developed for the polymerization of ethylene and propylene by the use of Ti- and Ni-based catalysts systems. A monometallic insertion mechanism which accounts for many features of the polymerization of a-olefins has been put forward by Cossee and Arlman in 1964 [624-626]. Respective bimetallic mechanisms date back to Patat, Sinn, Natta and Mazzanti [627,628]. The most important and generally accepted mechanisms for the polymerization of dienes by Nd-based catalysts are discussed in the following. 

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