Syndioselective polymerization, of styrene

Transition Metal Complexes for the Syndioselective Polymerization of Styrene 365... 

Titanium complexes with Schiff-base ligands (Figure 14.13,68-76) show low activities for the syndioselective polymerization of styrene when activated with MAO (2.26 x 10" g PS/(mol Ti mol... 

SCHEME 14.1 The role of TOA in the proposed mechanisms for the formation of the active species for atactic and syndioselective polymerizations of styrene with the Cp TiMe3/B(C6F5)3 catalyst system at low temperature. Dashed lines represent weak coordination interactions. 

A significant step forward was accomplished with the heteroleptic fluorenyl-benzylcalcium complex 11 (Fig. 3), which was obtained by reaction of the homoleptic benzylcalcium compound 9 with 9-Me3Si-fluorene. Compound 11 indeed allowed the syndioselective polymerization of styrene [31]. The PS obtained with the heteroleptic benzylcalcium initiator 11 under standard polymerization conditifMis (10 % of styrene in cyclohexane at 50 °C) was only slightly emiched in syndiotactic sequences. However, the syndiotacticity of the materials could be increased considerably by increasing the monomer concentration in fact, polymerization in neat styrene yielded PS with 85 % r-diads and 76 % rr-triads. 

In addition, a carbocationic polymerization mechanism has been proposed for the polymerization of styrene with the Cp TiMe3/B(C6p5)3 catalyst system. However, detailed studies confirmed that under normal polymerization conditions (room temperature and above, in toluene), the syndioselective styrene polymerization with the Cp TiMe3/B(C6F5)3 system proceeds through coordination-insertion mechanism instead of a carbocationic mechanism. Using this same catalyst system, a real carbocationic polymerization of styrene becomes significant at low temperature (below 0 °C) or in methylene chloride, and yields atactic polystyrene (Scheme 14.1). h73... 

In an attempt to combine the syndioselectivity of half-sandwich titanium catalysts with the living characteristics of anionic polymerization initiators, the use of half-sandwich calcium-based catalysts has been described.363 364 In neat styrene complex (152) affords 76% rr triad PS. However, polydispersities are still quite high (Mw/Mn > 2.2)... 

The synthesis of syndiotactic alternating styrene/CO co-polymers with bidentate nitrogen-ligated Pd complexes has already been reported, but thus far, there have never been any reports of a-olefm and CO-based congeners. Syndioselective alternating co-polymerization is believed to proceed via a chain-end control mechanism. 

The catalyst systems consisting of MAO and titanium alkoxides, such as Ti(OMe)4, Ti(OEt)4, and Ti(0-n-Bu)4, catalyze syndioselective styrene polymerization with moderate activities," - but are less active than mono(cyclopentadienyl)titanium-based systems. Nevertheless, higher molecular weight syndiotactic polystyrenes are obtained from these catalyst systems (Table 14.1, entries 3-6). 

Chien and coworkers observed that the styrene polymerization activities of a set of MAO activated mono-cyclopentadienyl Ti catalysts (Figure 14.4, 7-10) as well as their syndioselectivity, decreased with increasing steric bulk on the Cp-ring in the following order 7 > 8 > 10 (sPS% for complexes 7, 8, 10 are 99%, 98%, 75% respectively). In complex 9, the introduction of diphenylphosphino group caused a drastic decrease of catalyst activity. 

The most commonly applied variation of the a-ligands in mono-Cp H styrene polymerization catalysts is the introduction of alkoxy/aryloxy groups, which generally leads to catalysts with increased activity and syndioselectivity. Campbell et al. determined the order of deceasing... 

---