Isotactic productivity

Tacticity of products. Most solid catalysts produce isotactic products. This is probably because of the highly orienting effect of the solid surface, as noted in item (1). The preferred isotactic configuration produced at these surfaces is largely governed by steric and electrostatic interactions between the monomer and the ligands of the transition metal. Syndiotacticity is mostly produced by soluble catalysts. Syndiotactic polymerizations are carried out at low temperatures, and even the catalyst must be prepared at low temperatures otherwise specificity is lost. With polar monomers syndiotacticity is also promoted by polar reaction media. Apparently the polar solvent molecules compete with monomer for coordination sites, and thus indicate more loosely coordinated reactive species. 

Another example is the polymerization of penta-1,3-diene with an optically inactive initiator of the corresponding racemate (see Example 2.1), which leads to an isotactic product. 

Most vinyl monomers give a predominance of the isotactic product. Typically, the more exposed the catalytic site, the less the stereoregularity of the resulting polymer. The isotactic polypropylene (iPP) is produced using this technique, as is HDPE. 

The temperature-dependent stereoregular MAO-promoted polymerization of a-olefins502 has been explained by /i-hydrogen interactions in the olefin insertion and formation of a six-membered — 0 —Cp—D — Al—O—A1 ring TS. The stereoselective isotactic product formation occurs as a result of the substituent orientation at the -carbon (R1 vs CH2CHR2R4 in the conformationally restricted 426 equation 255). 

Much of the interest in polymerization initiated by lithium compounds is caused by the formation of highly specific products in nonpolar solvents. Under these conditions a highly cis-1,4-polyisoprene is formed, and methyl methacrylate is polymerized to a largely isotactic product. It is reported that isotactic polystyrene can be formed at low temperatures (2, 9), but this seems to form only in the presence of lithium hydroxide formed by catalyst destruction (28). 

Typical of the second-generation catalysts is a modified solid phase TiCl3 or TiCl3 A1C13, adjusted during production or later by a donor, usually diethyl ether. These catalysts are more active with more efficient control of monomer addition to yield an isotactic product so that the unwanted atactic fraction only amounts to about 2%. 

D Hondt and Lenz polymerized optically active a-phenyl-a-ethyl-/ --propiolactone anionically a racemic and an isotactic product were formed [99]... 

Using a similar catalyst, Kashiwa 83 n8> also noticed an increase of the isotactic production rate after the addition of EB, although the total productivity was lowered. Moreover, he found that the molecular weight of the isotactic polymer increased, while the number of the isospecific sites did not change, and hence concluded that the effect of the Lewis base was to increase the propagation rate constant (kp) of the stereospecific sites. 

The increase in isotacticity seems to be essentially connected to the decrease of the initial rate, as practically no change in the isotacticity index with polymerization time was detected. Moreover, while the atactic productivity decreases monotonically with the EB/TEA ratio in both systems, the isotactic productivity has a more complex behavior with the binary catalyst it remains almost unchanged up to EB/TEA s 0.25 and then falls, whereas with the ternary catalyst it increases up to EB/TEA 0.2 and then rapidly drops. On the grounds of these results, Spitz suggested that the reversible adsorption on the catalytic surface of the TEA EB complex (which is supposed to be very fast) changes the non specific centers into stereospedfic, though less active, centers, while the slower adsorption of free EB reversibly poisons both types of sites. The differences between the binary and the ternary catalysts would arise mainly from the presence, in the latter, of a larger number of potential stereospecific sites. 

With similar binary and ternary catalysts (but using EB as internal and MPT as external donor), however, rather different results were obtained in our laboratories. With the binary catalyst, a two-step increase of the isotacticity was noticed, the first step (up to MPT/TEA 0.2) being associated mainly with a strong decrease in the atactic productivity, the second (at MPT/TEA > 0.2) with a slightly selective decrease in both the atactic and the isotactic productivity (Fig. 38). 

A similar behavior was found with the ternary catalyst, with the difference that in the first step an increase of the isotactic productivity was noticed (Fig. 40). This effect, however, appears to arise mainly from a marked change in the strong dependence of the isotacticity on polymerization time which is peculiar of this catalyst (Fig. 41) and presumably due to the extraction of the inside donor by interaction with the Al-alkyl, as discussed in the previous section. 

The theory of selective and reversible poisoning proposed by most authors can be satisfactorily applied in all those cases where a simultaneous decrease of both the atactic and isotactic productivities has been observed. Furthermore, it is in agreement with the fact that the Lewis base actually forms complexes with the catalyst, as seen in Section 5.2. Nevertheless, this explanation appears oversimplified to explain all the effects the base produces on the kinetics, as has been revealed by Spitz45,97 . Moreover, it seems completely inadequate to account for the increase of productivity of isotactic polymer which has been detected in a few cases 45,112, U8). 

A comparison between the productivity trends of binary and ternary catalysts in the presence of an external donor proves to be quite interesting. While the atactic fraction productivity has a similar behavior for both catalysts, the binary catalyst shows a decrease of isotactic productivity with increased D/Al ratio the ternary catalyst, on the other hand, shows a maximum (see Fig. 40). This is still in agreement with the Burfield model. In fact, these results can be accounted for by assuming that the internal donor gives rise to the formation of a new type of isospecific center, characterized by kA and kD values different from those present in binary catalysts. The presence of a maximum in productivity is consistent with the formation of complexes between the aluminum alkyl and the donor and the resulting modification of the adsorption equilibrium, as suggested by Burfield1S6>. 

Approach of methyl methacrylate monomer is possible in only one orientation because of the steric hindrance of the gegen ion. As a result, at the critical bond-forming step the substituents of the incoming monomer are always oriented in the same way, which results in isotactic product. The outcomes for the other examples tabulated can be rationalized in similar ways. 

Using metallocene catalysts it has proved possible to tailor the microstructure of the polymers by fine-tuning of the ligands. Besides polyethylene, it is possible to co-polymerize ethylene with a-olefins such as propylene, but-l-ene, pent-l-ene, hex-l-ene, and oct-l-ene, in order to produce LLDPE. In addition, many kinds of co-polymers and elastomers, and new structures of polypropylenes, polymers and co-polymers of cyclic olefins can be obtained. Furthermore, catalysts with chiral centers can be beneficial in stereospecific polymerization to build the desired isotactic products. 

The polymerization of propene with the racemic tetrahydroindenyl zirconium complex (30) in the presence of m thylaluminoxane gives a highly isotactic product which is more soluble than conventional material. The structures of different fractions of the polymer were determined by Analogous chiral... 

FIGURE 13.4 Structure of l,l -ethylenedi-r) -indenylzirconium dichloride, a common metallocene catalyst. Only one enantiomer is shown both enantiomers produce isotactic product for this particular catalyst. 

Wide Mw/Mn = 3-6 Controllable range 160-164°C High melting point 162°C Isotactic products (iPP)only... 

The polymerization probably results from an insertion mechanism, as would be expected particularly in the case of polymerization of apolar monomers in hydrocarbons (see Section 19.1). This mechanism explains why only limited branching can occur and why the same catalyst system leads to isotactic products in the higher a-olefins, as was first discovered by Natta for the polymerization of propylene with Ziegler catalysts. 

Soum and Fontanille report that di-s-butyl magnesium generates living polymer from 2-vinylpyridine without the involvement of the side-reactions that afflict the polymerization initiated by alkali metal alkyls the resulting polymer has an isotacticity index of 0.9. Arai et al. have synthesized styrene-butadiene-4-vinylpyridine triblock copolymers. Hogen-Esch et a/. have continued their study of the stereochemistry of the anionic polymerization of 2-vinylpyridine in THF solution. Oligomers were synthesized by addition of alkali salts of 2-ethylpyridine to 2-vinylpyridine termination was effected by reaction with methyl iodide. Highly isotactic products were obtained with U and Na as counterions but with K or Rb there was no stereoselection. Epimerization resulted in the expected statistical mixtures of stereoisomers and it was concluded that stereoselection is kinetically controlled. 

On the other hand, with the same initiator, the stereoselectivity is very different depending on the nature of the monomer. Almost purely isotactic products are obtained with t-butyl-thiirane while less than 30 % of crystalline fraction is isolated in the case of propylene oxide (table I). 

I.P. isotactic productivity=yield in grams of isotactic polymer/grams of Ti hour... 

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