Metallocene catalysts chain transfer reactions

Recent advances in the development of well-defined homogeneous metallocene-type catalysts have facilitated mechanistic studies of the processes involved in initiation, propagation, and chain transfer reactions occurring in olefins coordi-native polyaddition. As a result, end-functional polyolefin chains have been made available [103].For instance, Waymouth et al.have reported about the formation of hydroxy-terminated poly(methylene-l,3-cyclopentane) (PMCP-OH) via selective chain transfer to the aluminum atoms of methylaluminoxane (MAO) in the cyclopolymerization of 1,5-hexadiene catalyzed by di(pentameth-ylcyclopentadienyl) zirconium dichloride (Scheme 37). Subsequent equimolar reaction of the hydroxyl extremity with AlEt3 afforded an aluminum alkoxide macroinitiator for the coordinative ROP of sCL and consecutively a novel po-ly(MCP-b-CL) block copolymer [104]. The diblock structure of the copolymer... 

Some of the drawbacks of the metallocene catalysts are their limited temperature stability and the production of lower-molecular-weight materials under commercial application conditions. It follows that they have a limited possibility for comonomer incorporation due to termination and chain-transfer reactions prohibiting the synthesis of block copolymers by sequential addition of monomers. This led to the development of half-sandwich or constrained geometry complexes, such as ansa-monocyclopentadienylamido Group IV complexes (67) 575,576... 

The synthetic procedure of PE-fo-PCL using hydroxyl terminated polyethylene was reported [39]. Terminally hydroxylated polyethylene was prepared during a metallocene-catalyzed polymerization using controlled chain transfer reaction with alkylaluminum compounds. PE-fo-PCL block copolymer was synthesized from terminally hydroxylated PE and e-caprolactone (e-CL) using Sn(Oct)2 as a catalyst for ring opening polymerization. 

Occasional regioerrors appear significantly to inhibit the polymerisation of a-olefins by methylaluminoxane-activated metallocene catalysts [114, 138, 253— 261], In order to reduce the number of secondary Zr-CH(R)-CH2 species, and therefore to accelerate the polymerisation, advantage has been taken of the chain transfer reaction with hydrogen ... 

Chain transfer reactions in homogeneous olefin polymerisation systems with metallocene catalysts, which terminate individual polymer chains, in some instances can also terminate the polymerisation kinetic chain. For example, chain transfer with the monomer in propylene oligomerisation or polymerisation, which involves a bond metathesis reactions between the Mt-C species of the growing polymer chain and the C H species of methyl [scheme (45)] or vinyl [scheme (46)] groups in the monomer, gives rise to temporally inactive metal allyl or metal-vinyl species respectively [177, 241, 264] ... 

Mulhaupt and coworkers have reported the details of several studies related to the preparation of block copolymers from thiol, maleic acid and hydroxy-functional polypropylene prepared by a metallocene catalyst [157, 158]. The same group also reported the transformation of metallocene-mediated olefin polymerization to anionic polymerization by a novel consecutive chain-transfer reaction for the preparation of polypropylene-based block copolymers [159]. The latter were also... 

All chain transfer reactions are influenced by the nature of the metallocene catalyst (steric/electronic factors) and polymerization conditions (temperature/pressure). Table 10.1 summarizes the effect of the ligand structure in the metallocene active site on chain transfer mechanisms and the structure of the growing polymer chain in propylene polymerization. 

In metallocene-catalyzed olefin polymerization, the propagation reaction is terminated usually by chain transfer. It is generally believed that three major chain-transfer reactions exist in homogeneous Ziegler-Natta catalysts (Scheme 3) [8] ... 

Process (i) is a unimolecular process, while (ii) is a bimolecular process and the rate depends on monomer concentration. Frequent chain-transfer reactions bring about low molecular weight polyolefins. If chain transfer is negligible or very slow, the polymerization can be living , as observed in group 5 metallocene-diene complexes [30, 31]. j3-Methyl elimination is also reported in bis(pentamethylcyclopentadienyl)metallocene catalysts [32,33]. 

D-Limonene and ot-pinene have been used as renewable solvents and chain transfer agents in metallocene-methylaluminoxane (MAO) catalysed polymerization of ot-olefins. Chain transfer from the catalyst to the solvent reduces the achieved in limonene compared with toluene and also reduces the overall catalyst activity. This was confirmed, as in the ROMP studies, by performing identical reactions in hydrogenated limonene. However, an increase in stereospecificity was seen when D-limonene was used as the solvent. This is measured as the mole fraction of [mmmm] pentads seen in NMR spectra of the polymer. 100% isotactic polypropylene would give a value of 1.0. On performing the same propylene polymerization reactions in toluene and then in limonene, the mole fraction of [mmmm] pentads increased from 0.86 to 0.94, indicating that using a chiral solvent influences the outcome of stereospecific polymerizations. Unfortunately, when a-pinene was used, some poly(a-pinene) was found to form and this contaminates the main polymer product. 

The chain-transfer and -release reactions occurring with Ti-based heterogeneous Ziegler-Natta catalysts are discussed in Section 4.09.3. In the following, the most important chain-release reactions occurring at metallocene and other singlecenter group IV catalysts are summarized. Chain transfer to ethylene is also addressed in Sections 4.09.4.1 and 4.09.4.2. 

Transfer to A1 was reported to be operative with several non-metallocene catalysts. It is the only chain-release mechanism operative with the diamido complexes MCl2 ArN(CH2) NAr catalysts, as well as with the mono-and tris(benzamidinate) catalysts, since no olefinic resonances were observed in the H or 13C NMR spectra of these polymers.275 276 This chain-release reaction is also dominant with bis(phenoxy-imine)zirconium cat-... 

That is, in terms of reaction rates, the molecular weight of polyolefins is given by the ratio between the overall rate of propagation (Rp) and the sum of all rates of chain release (Rr) reactions this means that the molecular weight is dependent on the type of catalyst and the kinetics of the process, that is, the polymerization conditions (polymerization temperature, monomer concentration, catalyst/cocatalyst ratio). Hence, understanding the details of the mechanisms of chain release reactions is the key to molecular weight control in metallocene-catalyzed olefin polymerization. Here, chain release reactions (usually referred to as termination or transfer reactions) are all those steps that cause release of the polymer chain from the active catalyst, with the formation of a new initiating species (see section... 

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