Stereoregularity, determining factors

The polymers obtained from unsymmetrically terminally disubstituted butadiene such as 4-methyl-1,3-pentadiene are made up of 1,2 monomeric units only, irrespective of the catalyst used this is due to the presence of two methyl substituents at the C4 atom in the monomer. Two stereoregular polymers have so far been obtained from 4-methyl-l,3-pentadiene, one with a 1,2-isotactic structure and one with a 1,2-syndiotactic structure. The isotactic polymer has been yielded by heterogeneous Ziegler-Natta catalysts, e.g. TiCU—AlEt3 and a — TiCl3— [AlEt3 [182]. The factors that determine the orientation of the coordinating monomer in this case are not, however, completely clear [41]. 

The properties of polyelectrolytes are essentially determined by the functional groups and factors like, e.g., stereoregularity, extent of crosslinks, etc. Occasionally electrochemistry offers a unique opportunity to create a polyelectrolyte from an originally neutral polymer by oxidation or reduction (see -> polymer-modified electrodes). 

The relative stability of the two structures seems to be determined by entropic factors. The increase in the size of the olefin substituent [32-35] or of the number of substituents, e.g., cychc olefins such as norbornene [36] or dicyclopentadiene [4], leads to the stabilization of the spiroketal structure, which can survive even in solution. However, a precise determination of the relative stabihty has not been reported. As far as the growing of the copolymer chain is concerned, the mechanistic role, if any, of the spiroketal structure is still not very clear [4, 30]. It is noteworthy that the copolymers are, for the most part, isolated in the spiroketal structure when the copolymerization reaction is regio- and stereoregular. 

Since the first preparation of stereoregular poly(methyl methacrylate) by Fox et al. and Miller et al. in 1958, a large number of papers have been published on the steieospecific polymerization of methyl methacrylate, while the NMR technique for the determination of microstructure developed by Bovey and Tiers and Nishioka et al. enabled us to accumulate the extensive information on this polymerization. Mostly anionic initiators have been used for the pdymerization. A review on the polymerization by lithium compounds was presented by Bywater In a recent review by Pino and Suter were discussed some of the factors which can influence the stereoregulation in the polymerization of vinyl monomers including a-substituted acrylate. A variety of magnesium and aluminum compounds can be utilized as stereospecific initiators. Besides methyl methacrylate, not only methacrylates with various ester groups, but also a-substituted acrylates, such as a-ethyl- or o-phenyl-acrylate, were also subjected to the stereospecific polymerization by anionic initiator. The stereospecificity in the copolymerization between the monomers described above is also a matter of interest. 

The second factor that determines the steric order of the monomer in the polymer is the regularity of the attack. In a stereoregular polymerization attack occurs on a particular carbon atom. The question arises then, does reaction occur from the same side d or 1- attack) or does it alternate from one side (d- or 1-) to the other (I- or d-) The consequences of these two modes of attack, accompanied by cis or trans opening of the double bond for the cis isomer, are shown in Fig. 15. If the attack is constant from one side, then an isotactic structure must be formed. If it... 

The factors determining the stereospecificity in the polymerization of a-olefins have not been singled out with certainty. We tackled this problem from three different points of view, i.e. analysis of the sequence distribution in ethylene-propylene copolymers, microstructural analysis of partially stereoregular propylene polymers, microstructural analysis of ethylene-propylene copolymers. 

A more subtle struetural feature of polymer ehains, called stereoregularity, plays an important factor in determining polymer properties and is explained as follows. In a polymer molecule, there is usually a backbone of carbon atoms linked by covalent bonds. A certain amount of rotation is possible around any of these backbone covalent bonds and, as a result, a polymer molecule can take several shapes. Figure 1.3a shows three possible arrangements of the substituents... 

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