Catalyst molar ratio microstructure

Figure 3. Effect of EtsA l i-BusA l molar ratio on microstructure. Polymerization conditions monomer concentration, 11 wt % in hexane catalyst concentration, 7.5 X 10 5 mol/L molar ratio Nd(vers), Et3Al2Cls AIRS, 1 1 30 polymerization time, 2 h and 60°C.

Polymerization Temperature. The stereoregularity of polybutadienes prepared with the BuLi-barium t-butoxide-hydroxide catalyst in toluene is exceedingly temperature dependent. Figure 6 compares the trans-1,4 dependence for polybutadiene prepared with BuLi, alone, and with the BuLi-barium t-butoxide-hydroxide complex in toluene (the molar ratio of the initial butadiene to BuLi concentration was 500). The upper curve demonstrates that the percent trans content increased rapidly from 627. to 807. trans-1,4 as the temperature decreased from 75°C to 22°C. From 22°C to 5°C, the microstructure does not change. The increase in trans-1,4 content occurred with a decrease in cis-1,4 content, the amount of vinyl unsaturation remaining at 5-87.. For the polybutadienes prepared using BuLi alone, there is only a very slight increase in the trans-1,4 content as the polymerization temperature is decreased. 

Variations of the amount of cocatalyst which are usually expressed by the molar ratio W Nd have a significant influence on polymerization rates, molar masses, MMDs and on the microstructures of the resulting polymers. These aspects are addressed in the following sections with a special emphasis on ternary catalyst systems. For ternary systems it has to be emphasized, however, that in many reports the ratio Ai/ Nd only accounts for the amount of aluminum alkyl cocatalyst and not for other Al-sources such as alkyl aluminum halides. Variations of the Ai/ Nd-ratios are also used for defined control of molar mass. This aspect is addressed in separate sections (Sects. 2.2.8 and 4.5). 

To our knowledge, there is only one study in which the influence of magnesium alkyl cocatalysts on microstructure of poly(diene)s was investigated. Duvakina et al. used the catalyst system Nd(d3/TBP/Mg( C4H9)( Q H17) for BD polymerization and investigated the influence of the molar ratio Mg/ Nd in the range of 6-60. This increase of Mg/ Nd led to a decrease in trans- 1,4-contents from 95 to 88-85% while the 1,2-content increased [135,136]. 

Halide donors constitute an essential component of ternary catalyst systems (Sect. 2.1.3). In these systems variations of the molar ratios x/ Nd (X = halide) affect catalyst activities, molar masses, MMDs and the microstructures of the poly(diene)s. 

The dependence of the polymer microstructure on the ratio of catalyst components is related to the nature of these components. The structure of polybuta-diene obtained with an aluminum triisobutyl (AIBU3)-titanium tetrachloride catalyst system is a function of the Al/Ti molar ratio (Table II). Polybutadiene prepared at Al/Ti ratios of 0.5 to 8 in benzene or heptane and at 3° or 25° C. contain at least 90% 1,4- units. Polymerizations carried out at ratios of 1.0 and less at 25° C. in heptane and at ratios of 1.25 or less at 3° C. in heptane or benzene give crystalline polymers containing more than 96% trans-, A- structure (6). A similar temperature dependence of polymer structure has been reported in the polymerization of butadiene with a diethylcadmium-titanium tetrachloride catalyst system (3). Polybutadiene obtained with a triethylaluminum-titanium tetrachloride catalyst system at a 0.9 Al/Ti ratio at 30° C. in benzene is reported to contain 67% cis-1,4- units (19). 

Ethylene homopolymerization using Phillips catalyst PC600 calcined at 600°C followed by activation with DEAE cocatalyst during the slurry polymerization process was carried out with Al/Cr molar ratios of 7.5, 15.0, and 22.5 [84]. As shown in Fig. 14, a typical single-type polymerization kinetics corresponding to type b in Fig. 10b was observed, which was completely different from the kinetics with the same catalyst activated by TEA at the same conditions (as shown in Fig. 13). This t3 pe of polymerization kinetics could be ascribed to one type of active site (Site-B) formed in two ways. One was similar with the PC600 activated by TEA some chromate Cr(VI) species were reduced to Cr(II) species by ethylene monomer and coordinated with formaldehyde, then formaldehyde-coordinated Cr(ll) sites were transformed to DEAE-coordinated Cr(II) sites by substitution, as shown in Scheme 8. On the other hand, some chromate Cr(VI) species were reduced by DEAE, and then the Al-alkoxy product coordinated with the Cr(Il) sites. Site-B had relatively low activity and high stability. Based on the microstructure analysis, the relative amount of SCBs of polymers obtained from the DEAE systems was even more than that from TEA catalyst systems. This can be explained as follows. Firstly, the reduction ability of DEAE was weaker than that of TEA. More Cr(VI) species... 

The synthesis of a synthetic high molecular weight ci5-l,4-poly-isoprene is of both academic and industrial significance. The preparation of a polymer with a microstructure close to that of natural rubber was first disclosed in 1954 (British Patent 827 365). An AlRa/TiCU system with an Al/Ti molar ratio of about 1 was used and subsequent work has confirmed the preference for this system. The activity of the catalyst appears to be affected by the method of preparing the catalyst, in particular the catalyst preparation temperature. Although this does not seem to influence microstructure some reported results would appear contradictory. 

A recent study performed by Mello et al. focuses on a comparison of n-hexane and cyclohexane in the polymerization of BD with the catalyst system NdV/D I BAH/f BuCl. In this study Mello et al. use the pure solvents and mixtures of n-hexane and cyclohexane [423]. Cyclohexane yields BR with a significantly lower molar mass than n-hexane. According to Mello et al. this effect is due to the thermodynamically better solvent quality of cyclohexane. The authors found no strong influence of the cyclohexane/n-hexane ratio neither on catalyst activity nor on microstructure. 

Poly(2-phenylbutadienes) with a high cis content are also produced with the triisobuthyl-aluminum/titanium tetrachloride catalysts [349]. Phenyl-1,3-butadienes can also be considered as vinyl-substituted styrenes, which explains the effects on activities and microstructures. Poly(2-phenyl butadienes) occur in trans-, A, cw-1,4, 3,4, and 1,2 structures. Maximum conversions are achieved with a molar Al/Ti ratio of 1, which leads to the formation of 73% cis-, A and 27% 1,2 structures. At higher Al/Ti ratios the d -1,4 content goes up to 96%. The molecular weights are low, ranging from 2000 to 18,000. 

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