Ionicity factor

Table 1.15 Relationship among mean polyhedral linear thermal expansion (aao-iooo) Pauling s bond strength, and ionicity factor (adapted from Hazen and Finger, 1982). ...

The term S] is defined by the authors as an ionicity factor and assumes a value of 0.5 for oxides and silicates, 0.75 for halides, 0.40 for calcogenides, 0.25 for phosphides and arsenides, and 0.2 for nitrides and carbides (Z is the anion charge). Equation 1.94 is based on the thermal expansion data listed in table 1.15. 

Hazen and Finger (1979) extended equation 1.110 to mean polyhedral compressibility (mean compressibility of a given coordination polyhedron within a crystal structure), suggesting that it is related to the charge of ions in the polyhedron through an ionicity factor, analogous to what we have already seen for thermal expansion—i.e.. 

Relative initial rates of polymerization of aliphatic aldehydes catalyzed by [Me2A10CPhNPh]2 catalyst followed the order of MeCHO > EtCHO > nPrCHO > i PrCHO (Fig. 9). It is apparent from the order that the rate of polymerization is lower when the alkyl group of aldehyde monomer is bulkier. It is conceivable that the steric factor, not the ionic factor, is predominant for the relative rates of polymerization, because the frequencies of IR bands assigned to the carbonyl group are nearly constant for these aldehydes. 

Since the discovery of steric control and the development of Ziegler-Natta catalysts, there has been considerable research on the mechanism of steric control. An attempt to review all of the pertinent literature on this topic would be a gigantic job, completely out of the scope of this paper. In the reviews by Kennedy and Langer (1), Roha (2), Bawn and Led with (3) and Bier (4) much of the past information has already been summarized and discussed, including some of the roles played by ionic factors. 

This review attempts to cover the more pertinent information which shows the correlation of the ionic factors of the polymerization process with control of the stereochemistry of the product. It is apparent that the keys for steric control in coordinate polymerizations are the effects... 

In order to provide the basis for the mechanism of steric control, it is important to consider the various monomers in relation to the catalysts and ionic factors which produce stereoregular polymers from those monomers. 

Hie study of effects of the catalyst components also help clarify the ionic factors in the steric control of isotactic polyvinylethers. Dall Asta and Bassi (15) studied the polymerization of butylvinylether with various alkylaluminum halides. They found that diethylaluminum chloride and ethylaluminum dichloride were the most effective catalysts for the production of isotactic polymer. Ethylaluminum dibromide and ethoxyaluminum dichloride were of questionable effectiveness, while diethylaluminum fluoride was completely ineffective. 

An interesting effect of the ionic factors of the polymerization was found by Kuntz (59). He has shown that the homopolymerization of styrene using butyllithium catalysts is six times as rapid as that of butadiene. However, in copolymerization, butadiene polymerized initially at its own rate with relatively small amounts of the styrene being consumed. Only after 90% of the butadiene had been consumed, the styrene began to polymerize at its own rate. THF increased the rate of the polymerization but had little effect on the rate of butadiene to styrene which is polymerized. The butadiene structure is little influenced by copolymerization. The homopolymer contained 44% cis-1.4, 7% 1.2 and 49% trans-1.4 while the butadiene units of the butadiene copolymers contained 40% cis 1.4, 7% 1.2 and 53% trans-1.4 groups. 

An analysis of the ionic factors for the polymerization of dienes to cis and trans structures is possible in the same way as for isotactic mono-enes. The mechanism which controls the steric structure of poly 1,4 dienes is parallel to that we have already seen for the mono-olefins. Roha (2) listed the catalysts which polymerize dienes according to the polymer structures produced. It was shown that the highly anionic as well as the highly cationic catalyst systems with increasing ionic separation produced trans-poly-1,4-dienes. This is analogous to the production of syndiotactic polyolefins. 

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