Homogeneous metallocenes

Monomer sequence length distribution and penultimate effect in ethylenc-cycloolefln copolymers synthesized over homogeneous metallocene catalysts... 

Figure 19 (a) Peak melting temperature as a function of the branch content in ethylene-octene copolymers (labelled -O, and symbol —B (symbol, ) and -P (symbol, A) are for ethylene-butene and ethylene-propylene copolymers, respectively) and obtained from homogeneous metallocene catalysts show a linear profile, (b) Ziegler-Natta ethylene-octene copolymers do not show a linear relationship between peak melting point and branch content [125]. Reproduced from Kim and Phillips [125]. Reprinted with permission of John Wiley Sons, Inc. 

The features of homogeneous metallocene/M AO catalysts distinguishable from the classical heterogeneous Ziegler-Natta catalysts can be summarized as follows ... 

Sivaram S, Srinivasa RS (1995) Homogeneous metallocene-methylaluminoxane catalyst systems for ethylene polymerization. Prog Polym Sci 20 309-367... 

Figure 10.5. The first homogeneous metallocene catalyst for ethene polymerisation...

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... 

Supported metallocenes generally have lower activity than do homogeneous metallocenes because of steric hindrance by the support. At the same time, less MAO is required because... 

Polymerization using Ziegler-Natta catalysts yields a mostly saturated polymer with 2,3 linkages (17). The polymerization with heterogeneous Ziegler-Natta catalysts is accompanied by ROMP, whereas homogeneous metallocene, Ni, and Pd catalysts promote addition polymerization (18). 

The Mw/Mn ratio is usually equal to 5-10 for polyethylene [49,64,66,67, 123,244-247], A much lower polydispersity is displayed by polymers obtained in polymerisation with homogeneous metallocene catalysts the Mw/Mn ratio usually does not significantly exceed a value of 2 [22,95,101,112,138,140], By polymerising propylene with soluble vanadium-based Ziegler-Natta catalysts at low temperature, a very narrow molecular weight distribution of the polypropylene has been found (the Mw/Mn ratio usually reaches values of 1.15-1.25) and a linear increase in its Mn with time has been observed, indicating a noticeable living character of the polymerisation [75,76,241],... 

In the case of olefin polymerisation in the presence of homogeneous metallocene-based catalysts, the individual polymerisation stages have not been very thoroughly investigated. However, kinetic studies have helped, among others, to define the nature of the active centres and to establish the occurrence of some polymerisation elementary steps in a quantitative way. 

At the end of considerations of olefin polymerisation with homogeneous metallocene catalysts, it should be added that it is necessary to differentiate between the soluble catalyst system itself and the polymerisation system. For... 

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