Styrene, syndiospecific polymerisation

Syndiotactic polystyrene was first obtained only recently by Ishihara et al. [5] in polymerisation with a homogeneous catalyst derived from a transition metal compound such as monocyclopentadienyltitanium trichloride and methylalu-minoxane in toluene. Since then, several authors have reported on the synthesis of syndiotactic polystyrene promoted by different catalysts based on metal hydrocarbyls such as benzyl compounds, half-sandwich metallocenes (e.g. monocyclopentadienyl, monopentamethylcyclopentadienyl and monoindenyl metal derivatives), metal alkoxides, metallocenes and some other compounds. These catalysts are commonly derived from titanium or zirconium compounds, either activated with methylaluminoxane or aluminium-free, such as those activated with tris(pentafluorophenyl)boron, and promote the syndiospecific polymerisation of styrene and substituted styrenes [5-10,21,48-70], Representative examples of the syndiospecific polymerisation of styrene using catalysts based on various titanium compounds and methylaluminoxane are shown in Table 4.2 [6,52,53,56,58],... 

Table 4.2 Syndiospecific polymerisation of styrene in the presence of homogeneous catalysts based on a titanium compound and methylaluminoxane0...

It is now realised that almost all catalysts based on Ti(III) or Ti(IV) compounds and methylaluminoxane, soluble in aromatic solvents, could polymerise styrene into a highly syndiotactic polymer. The syndiotacticity measured by 13C NMR spectroscopy can be greater than 98%. Syndiospecific polymerisation of styrene with homogeneous catalysts is characterised by a narrow molecular weight distribution (Mw/Mn can reach a value of 2). 

Monocyclopentadienyl titanium derivatives are the most active precursors for catalysts possessing high syndiospecific polymerisation activity for styrene and ring-substituted styrenes. The polymerisation activity of biscyclopentadie-nyl titanium compounds activated with methylaluminoxane is lower than that of other soluble titanium-based catalysts [73]. 

Styrene enchainment in the syndiospecific polymerisation of styrene with homogeneous catalysts, both containing and not containing a cyclopentadienyl or cyclopentadienyl-like ligand, is through a cis insertion [76,77], This was evidenced by H NMR analysis of copolymers of perdeuterostyrene and... 

As regards a comparison between the electrophilic properties of methylalu-minoxane-activated titanium-based homogeneous catalysts for syndiospecific polymerisation of styrene, half-sandwich titanocene-based catalysts are stronger electrophiles than non-cyclopentadienyltitanium-based catalysts, the former catalysts thus being more active in the polymerisation [55]. 

One of the best catalysts for the syndiospecific polymerisation of styrene appeared to be that derived from CpTiCl3 and methylaluminoxane. The polymerisation rate for this system decreases with increasing polymerisation time such behaviour is very similar to that of other Ziegler Natta catalysts. A maximum polymerisation rate is achieved at 50 °C [6]. Other catalysts such as CpTi(OBu)3 [Al/Me/O] are also reported to exhibit a very high activity and syndiospecificity in the polymerisation of styrene [50,51]. 

It was found that substituted cyclopentadienyltitanium trichloride, in a combination with methylaluminoxane, exhibited higher catalytic activity for syndiospecific polymerisation of styrene than CpTiCl3 [52,53]. The efficiency of half-sandwich titanocenes as methylaluminoxane-activated precatalysts for the syndiospecific polymerisation of styrene increases in the following order CpTi(OMe)3 < Me4(Me3Si)CpTi(OMe)3 < Cp Ti(OMe)3. Thus, electron-donating substituents on the cyclopentadienyl ligand lead to increased catalyst activity and stability, stereospecificity and polymer Mw. 

The syndiospecific polymerisation of styrene has also been found [85] to proceed in the presence of the methylaluminoxane-activated trisiloxane-bridged dinuclear titanocene complex hexamethyltrisiloxanediylbis(cyclopentadienylti-tanium trichloride) [Cl3TiCpSi(Me)2OSi(Me)2OSi(Me)2CpTiCl3], which also polymerised ethylene. 

Let us recall that the origin of stereospecificity in the syndiospecific polymerisation of styrene lies in a chain end stereochemical control mechanism [52,70]. Key features of the stereoregulation mechanism are stereorigid rf coordination of the growing chain end and diastereoselective rj2 coordination of the styrene... 

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]. 

These compounds exhibited fairly high catalytic activity for the syndiospecific polymerisation of styrene. 

It is worth mentioning that a rar.- / ,v -titanocene methylaluminoxane catalyst, such as rac.-Ph2C(Cp)(Ind)TiCl2—[Al(Me)0]x, which yields an isotactic polymer in propylene polymerisation, promotes the syndiospecific polymerisation of styrene [73,100]. This is the first example where two different stereoregular polymers, isotactic and syndiotactic, can be obtained using the same catalyst in the case of two different monomers. 

Heterogeneous non-supported or supported catalysts that have been used successfully for the syndiospecific polymerisation of styrene are presented in Tables 4.3 and 4.4 [62,63,66-69,101-103],... 

In the case of chlorine-free catalysts such as Mg(OH)2/Ti(OBu)4—[A1 (Me)0]x and Si02/Ti(0Bu)4—[Al(Me)0]x, heterogeneous species are assumed to promote the syndiospecific polymerisation of styrene [67,68]. In a polymerisation system with the latter catalyst, best results were achieved when treating the carrier with [AI(Mc)0]x prior to supporting Ti(OBu)4 (no further activation with methylaluminoxane was needed). The polymerisation rate reaches a maximum at an Al/Ti molar ratio of 20 this is much lower than the value of the Al/Ti molar ratio required to reach the maximum polymerisation rate in the respective homogeneous system, i.e. the system without a carrier [54]. 

As AI2O3 contains considerable amounts of acidic centres on the surface, it was suggested that the syndiospecific polymerisation of styrene takes place via polyinsertion involving Ti C cationic species. Non-cationic species with the Ti C bond on the alumina surface are believed to promote isospecific polymerisation. 

Similarly, the same catalysts that promote the syndiospecific polymerisation of styrene also polymerise ethylene and a-olefins [106,107], ring-substituted styrenes [6] and conjugated dienes [44,74,108-110], These monomers can also be copolymerised with each other [111-114], Substituted styrenes, which yield syndiotactic polymers by polymerisation run with syndiospecific catalysts, form copolymers with styrene the polymerisation rate increases with increasing nucleophilicity of the comonomer. The random copolymers formed are co-syndiotactic [6,111,112]. 

Name and characterise coordination catalysts for the isospecific and syndiospecific polymerisation of styrene. 

In the late 1980s and in the 1990s, homogeneous metallocene-based Ziegler Natta and related aluminium-free catalysts as well as other non-metallocene-based homogeneous single-site catalysts, which are active in the syndiospecific polymerisation of styrene, were found also to promote the polymerisation of conjugated dienes [16,38 13],... 

Let us recall that half-sandwich metallocene-based catalysts, either activated with methylaluminoxane or aluminium free, such as those activated with tris(per-fluorophenyl)boron, also promote the syndiospecific polymerisation of styrene. 

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],... 

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...

Catalysts of the Ti(OR)4—[Al(Me)0]x type show greatly inferior activity and syndiospecificity in the polymerisation of styrene by comparison with catalysts of the CpTi(OR)3—[Al(Me)0]x type [54,70]. The activity and syndiospecificity of Ti(OR)4—[Al(Me)0]x catalysts increases when the Al/Ti molar ratio in the polymerisation system is increased. The maximum activity of Ti(OR)4—[A1 (Me)0]x catalysts is observed at an Al/Ti molar ratio of ca 100 [54,55]. It is worth mentioning that, under the same polymerisation conditions, these catalysts yield syndiotactic polystyrene with a higher molecular weight than does the CpTiCl3—[Al(Me)O]x catalyst [71],... 

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