Styrene, polymerisation

The styrene-diene triblocks, the main subject of this section, are made by sequential anionic polymerisation (see Chapter 2). In a typical system cc-butyl-lithium is used to initiate styrene polymerisation in a solvent such as cyclohexane. This is a specific reaction of the type... 

Anionic polymerisation mechanism is illustrated in Figure 26 with the example of styrene polymerisation. 

Styrene derivatives show a higher tendency to polymerise under the action of protonic acid in comparison to aliphatic olefins, a-methyl styrene polymerises in ethyl chloride when catalysed by sulphuric acid at -78.5°C but isobutene does not polymerise. 

For the polymerisation of styrene (SnC -F O-PhNC -CC at 0°) kjkv for anisole was found [85] to be 1.62. It is highly probable that the big difference between this and the value for isobutene reflects mainly the difference between the ps for the two monomers. The very low value of kjkv in the polymerisation of isobutene - or the very large kv of isobutene - accounts for the observation [86] that, whereas styrene polymerising cationically in the presence of preformed poly-p-methoxystyrene will form grafts by reacting with pendent rings, isobutene will not do so. 

X (SbF6") = 1.81-10 3 S nr-mol 1, which agrees well with the experimental values from the styrene polymerisations, and it means that iCjj 1, i.e., the ionisation is virtually complete. 

This absorption is in fact due to the ions derived from l-methyl-3-phenylindane (the cyclic dimer of styrene) and its higher homologues (oligostyrenes with indanyl end groups). There can be no doubt that the ions formed at the end of the polymerisation of styrene belong to the same families of compounds (indanyl and various phenyl alkyl carbonium ions [7]). Our evidence showed that the 1-phenylethyl cation is absent from the ions formed from styrene by excess of acid its dimeric homologue, the l,3-diphenyl- -butyl cation, is a minor component of the ion mixture. We refer to this mixture of ions formed rapidly from styrene by excess acid, or at the end of a styrene polymerisation, as SD (styrene-derived) ions. 

Boodhoo, K. V. K. and JACHUCK, R. J. J. Appl. Therm. Eng. 20 (2000) 1127. Process intensification spinning disc reactor for styrene polymerisation. 

In the presence of sonication there is an overall increase in conversion with increase potential, whilst the opposite is true for copolymerisation performed in silent conditions. The lower degree of conversion at the lower potential is not without precedent, since simple mechanical stirring retards styrene polymerisation [82]. 

Signer and Especially purified styrene polymerised Weiler thermally below 150° C. Absence of... 

Technical Information, Polystyrene Initiators for Styrene Polymerisation, Akzo Chemie, Deventer, the Nethedands, Nov. 1985. 

Boodhoo KVK, Jachuck RJJ, Ramshaw C. Spinning disc reactor for the intensification of styrene polymerisation. In Semel J, ed. 2nd International Conference on Process Intensification in Practice, Antwerp, Oct. 1997. 

It may be of interest that isotactic polystyrene formed by styrene polymerisation with Ziegler Natta catalysts [13] did not appear to be a polymer that could exhibit significantly better usable properties compared with atactic polystyrene produced in free radical styrene polymerisation processes. 

Styrene polymerisation with heterogeneous Ziegler Natta catalysts activated by alkylaluminium compounds generally produces a mixture of isotactic and non-stereoregular polymer. For example, polystyrene produced with the... 

Styrene polymerisation in both isospecific and non-stereospecific homogeneous catalyst systems is highly regioselective and involves a secondary (2,1) insertion of the monomer ... 

Although the mechanism of steric control that operates in the isospecific styrene polymerisation system with homogeneous nickel-based [(MeAll)(Cod)Ni]+[PF6] /P(Chx)3 catalyst is not completely clear, it is evident that re-benzyl nickel species play an important role in determining the mode of the styrene insertion. 

Also, divalent TiPh2 activated with [Al(Me)0]x appeared to be a catalyst for syndiospecific styrene polymerisation [71]. Even 5-TiCh or (Acac)3TiCl3, when activated with [Al(Me)0]x, could yield a mixture of isotactic and syndiotactic polystyrenes. Some other catalysts, such as rare-earth coordination catalysts, have been successfully used to obtain syndiotactic-rich polystyrenes [72],... 

As regards a comparison of the relative effectiveness of titanium-and analogous zirconium-based catalysts in syndiospecific styrene polymerisation, the latter in general are less active than the former. Usually, polymer yields are lower, and a higher polymerisation temperature and reaction time as well as higher [Al(Me)01/transition metal compound ratios are required. Among the few zirconium compounds examined, only tetrabenzylzirconium activated with methylaluminoxane has relatively higher syndiospecific activity [10,48,56],... 

Although there is dispute about the exact oxidation state of titanium in the active species [Ti(III) or Ti(IV)], it was suggested, from the results of ESR measurements, that Ti(III) species form highly active sites for producing syndiotactic polystyrene in styrene polymerisation systems with the TiBz4—[Al(Me)0]x catalyst [50]. The moderately low catalyst activity is attributable to the stability of the benzyl transition metal derivatives towards reduction. 

A much lower stereospecificity of the ZrBz4—[Al(Me)0]x catalyst for styrene polymerisation (ca 58% of the hot acetone-insoluble polymer fraction [56]) compared to that for the respective Ti-based catalyst should be noted. This can be explained in terms of the larger radius of zirconium than titanium, thus resulting in the impossibility of sufficiently effective chain end stereocontrol. 

Zirconium-based cationic complexes derived from ZrBz4 and B(CeF5)3 or [Me2N(Ph)H]+[B(C6F5)4]- do not provide active syndiospecific catalysts for styrene polymerisation [70],... 

Figure 4.1 Proposed structure for the polymerising species in syndiospecific styrene polymerisation promoted by the Cp TiR.3—B(C6Fs)3 catalyst...

Considering the above stereochemical model for syndiospecific styrene polymerisation, one may conclude reasonably that tf coordination of the monomer at the active site could hardly be possible, and r 2 coordination would always be involved in the syndiospecific polymerisation of this monomer [87]. One should note that preliminary concepts concerning the stereoregulation mechanism of syndiospecific styrene polymerisation assumed the styrene monomer to undergo only t]4 coordination at the titanium centre, the propagating chain being anchored via a benzylic bond as an t]3 ligand at the titanium [44,55,70]. 

Catalysts derived from alkoxytitanium chlorides and methylaluminoxane have also appeared to promote styrene polymerisation [58]. 

When a-cyclodextrin-supported Cp TiCl3 is used as the catalyst precursor, [AI(Mc)0]x should be used as an activator in order to obtain a suitable catalyst for the stereospecific polymerisation of styrene (polymerisation syndiospecifi-city ca 93%) with AlMe3 as the activator, a-cyclodextrin-supported Cp TiCl3 had less activity for styrene polymerisation even if a-cyclodextrin was treated with [AI(Mc)0]x prior to supporting Cp TiCl3 [103]. 

Chien, J. C. W., Advances in Ziegler Catalysts. Syndiotactic Styrene Polymerisation , in Ziegler Catalysts, Springer-Verlag, Berlin, 1995, pp. 209-210. 

Explain the role of methylaluminoxane as an activator for the catalyst precursor in styrene polymerisation systems. 

What are the advantages of half-sandwich metallocene-based catalysts as compared with heterogeneous Ziegler-Natta catalysts in styrene polymerisation What are the possible consequences of this for developing industrial processes ... 

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