Copolymerisation with Heterogeneous

1 Copolymerisation with Heterogeneous Ziegler-Natta Catalysts 

Styrene undergoes copolymerisation with ethylene and various a-olefins in the presence of heterogeneous Ziegler-Natta catalysts. Its reactivity in the copolymerisation is quite low, which is illustrated by the values of the relative reactivity ratios, r and r2, presented in Table 4.5 [118]. One may note, however, a considerably high relative reactivity of styrene in copolymerisation with vinyl-cyclohexane. The copolymerisation of styrene with small amounts of a-olefin, such as 1-octene or 1-decene, yields copolymers of reduced crystallinity and thus reduced brittleness compared with the homopolymer of styrene. 

Styrene/a-olefin copolymers containing a predominant amount of styrene units can be easily formed through the monomer isomerisation-copolymerisation of styrene and /i-olefin such as m-2-butene in the presence of heterogeneous Ziegler-Natta catalysts such as TiCl3—AlEt3. Styrene appeared to be a favourable comonomer for monomer isomerisation-copolymerisation with internal olefins, since only the isomerisation of /i-olefin to a-olefin, and not the isomerisation of styrene (in contrast to the olefin), occurs in the presence of Ziegler Natta catalysts [119]  

By adding NiCl2 to the TiCl3—AlEt3 catalyst (Ni/Ti/Al molar ratio 1 1 3), the contents of 1-butene units in the copolymer were increased at the same comonomer feed as compared with the copolymerisation system without added 

It is interesting that copolymers consisting almost entirely of styrene units are produced easily via monomer isomerisation-copolymerisation of styrene and czx-2-butene, contrary to the ordinary copolymerisation of styrene and 1-butene [119]. 

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In copolymerisations carried out with heterogeneous catalysts, the higher a-olefin is preferentially incorporated at sites producing shorter chains higher a-olefins are thus enriched in the low-molecular fractions of the copolymers obtained. This increases the content of extractables and tends to make these copolymers sticky, two generally undesirable properties [30]. 

Functionalised a-olefins capable of undergoing insertion polymerisation with Ziegler-Natta catalysts are, in principle, monomers in which the heteroatom (X) does not electronically interact with the double bond to be polymerised in such monomers, the heteroatom is separated from the double bond CH2=CH-(CH2)x X [326,384,518,522-528], Monomers with the heteroatom directly bound to the double bond, i.e. those of the CH2=CH-X type, may also undergo polymerization, but when the heteroatom is silicon or tin (X= Si, Sn) [522-526], Representative examples of the insertion polymerisation of functionalised a-olefins and their copolymerisation with ethylene and a-olefins in the presence of heterogeneous Ziegler-Natta catalysts are shown in Table 3.7 [2,241,326,384,518,522-528],... 

Styrene, which can be treated formally as an a-olefin branched in the 3-position, forms copolymers with ethylene and a-olefins (as well as with /i-olefins, involving isomerisation copolymerisation). Both heterogeneous Ziegler-Natta catalysts and a single-site metallocene catalyst promote the copolymerisation. 

Table 4.5 Relative reactivity ratios for copolymerisation of styrene (ri) and a-olefins (r2) with heterogeneous Ziegler-Natta catalysts...

With the sole exception of the random ethylene-propylene copolymers, for industrial applications heterogeneous catalysts have been used for alkene polymerisations. Ethylene-propylene statistical copolymerisation has been carried out using homogeneous vanadium-based catalysts [28]. 

The extent of comonomer selectivity of the heterogeneous Ziegler-Natta catalysts is particularly evident in the copolymerisation of the two enantiomers of a racemic a-olefin. In fact, with TiCl3—AIR2CI [374] or MgCk/TiCU AlEh [375], macromolecules containing a prevalence of monomeric units arising from... 

Typical values of comonomer relative reactivity ratios in ethylene/propylene copolymerisations run with various heterogeneous and homogeneous Ziegler-Natta catalysts are listed in Table 3.5 [30, 72, 454]. 

Table 3.7 Insertion homopolymerisation and copolymerisation of functionalised a-olefins with ethylene and a-olefins in the presence of heterogeneous Ziegler-Natta catalysts...

Statistical copolymerisation kinetics with the crosslinker lessens the heterogeneity of receptor sites. 

A special aspect of (emulsion) copolymerisation compared to (emulsion) homopolymerisation is the occurrence of composition drift. In combination with the instantaneous heterogeneity (statistical broadening around the average chemical composition), this phenomenon is responsible for the chemical heterogeneity of the copolymers formed. Composition drift is a consequence of the difference between instantaneous copolymer composition and overall monomer feed composition. This difference is determined by (a) the reactivity ratios of the monomers (kinetics) and (b) the monomer ratio in the main loci of polymerisation (viz., latex particles) that can differ from the overall monomer ratio of the feed (as added according to the recipe), which in turn is caused by monomer... 

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