Stereochemistry of propagation

Fig. 1. Effect of ion pairing on stereochemistry of propagation for alkyl vinyl ethers, where L is large substituent S, small substituent.

The M[ZnR4] and M[A1R4]2 ate complexes (M = Ca, Sr, Ba) polymerize vinyl and conjugated diene monomers including styrene, methylmethacrylate, acrylonitrile, vinyl-ketones, isoprene and butadiene . The bimetallic complexes produce polymers of differing microstructure to that formed by simple alkaline-earth metal initiators, so that the presence of Zn or A1 influences the stereochemistry of propagation. Thus, polybutadiene obtained in benzene from a Ba-Zn initiator contains more than 90% 1,4-bonds (trans-1,4 75-81 %) ... 

The solvent and counter-ion also can influence the stereochemistry of propagation and these effects will be considered in Section 2.8. 

Different molecular microstructures arise from there being several possible modes of propagation. The possibility of head-to-tail and head-to-head placements of the repeat units has been encountered already, with the observation that for both steric and energetics reasons the placement is almost exclusively head-to-tail for most polymers. Therefore in the subsequent sections dealing with the stereochemistry of propagation only head-to-tail placements will be considered. 

Combining control over architecture with control over the stereochemistry of the propagation process remains a holy grail in the field of radical polymerization. Approaches to this end based on conventional polymerization were described in Chapter 8. The development of living polymerization processes has yet to substantially advance this cause. 

The explanation for the existence of a stereo-block polymer is that after a mistake this mistake will propagate as the chain end controls the stereochemistry of the new centre to be formed. Thus after the mistake has occurred, the polymer switches the stereochemistry from s to r. 

The stereoselective polymerization of various acrylates and methacrylates has been studied using initiators such as atkyllithium [Bywater, 1989 Pasquon et al., 1989 Quirk, 1995, 2002]. Table 8-12 illustrates the effects of counterion, solvent, and temperature on the stereochemistry of the anionic polymerization of methyl methacrylate (MMA). In polar solvents (pyridine and THF versus toluene), the counterion is removed from the vicinity of the propagating center and does not exert an influence on entry of the next monomer unit. The tendency is toward syndiotactic placement via chain end control. The extent of syndiotacticity... 

Although the treatment described above has been limited to polymerizations proceeding by meso and racemic placements, the various models can also be adopted to describe the stereochemistry of 1,4-propagation in 1,3-dienes. 

One problem encountered in the field is the apparent irre-producibility of the results of different workers, even those in the same laboratory. This is particularly the case with molar mass distribution and steric triad composition. The explanation of these apparent inconsistencies comes with the realization that the mechanisms are eneidic and the polymer properties are primarily determined by independent active centres of different reactivities and stereospecificities whose relative proportions are set at the initiation step, which is completed in the first seconds of the polymerization. The irreproducibilities arise from irreproducibilities in the initiation step which had not been thought relevant. Ando, Chfljd and Nishioka (12) noted that these rapid exothermic reactions tend to rise very significantly above bath temperature (we have confirmed this effect) and allowed for this in considering the stereochemistry of the propagation reaction. However our results show that the influence on the initiation reactions can have a more far-reaching effect. 

In alkyllithium initiated, solution polymerization of dienes, some polymerization conditions affect the configurations more than others. In general, the stereochemistry of polybutadiene and polyisoprene respond to the same variables Thus, solvent has a profound influence on the stereochemistry of polydienes when initiated with alkyllithium. Polymerization of isoprene in nonpolar solvents results largely in cis-unsaturation (70-90 percent) whereas in the case of butadiene, the polymer exhibits about equal amounts of cis- and trans-unsaturation. Aromatic solvents such as toluene tend to increase the 1,2 or 3,4 linkages. Polymers prepared in the presence of active polar compounds such as ethers, tertiary amines or sulfides show increased 1,2 (or 3,4 in the case of isoprene) and trans unsaturation.4. 1P U It appears that the solvent influences the ionic character of the propagating ion pair which in turn determines the stereochemistry. 

The titanium trichloride-diethylaluminum chloride catalyst converted butadiene to the cis-, trans,-trans-cyclododecatriene. Professor Wilke and co-workers found that the particular structure is influenced by coordination during cyclization between the transition metal and the growing diene molecules. Analysis of the influence of the ionicity of the catalyst shows effects on the oxidation and reduction of the alkyls and on the steric control in the polymerization. The lower valence of titanium is oxidized by one butadiene molecule to produce only a cis-butadienyl-titanium. Then the cationic chain propagation adds two trans-butadienyl units until the stereochemistry of the cis, trans, trans structure facilitates coupling on the dialkyl of the titanium and regeneration of the reduced state of titanium (Equation 14). 

As mentioned in section 4.1, the kinetics of the living polypropylene synthesis have been interpreted in terms of a coordination polymerization mechanism represented by Eq. (22). We discuss here the mechanism of chain propagation on the basis of the structure and stereochemistry of the synthesized polypropylenes. 

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