Stereo-irregular

Expressions for the optical anisotropy AT of Kuhn s random link (an equivalent to the stress-optical coefficient) of stereo-irregular and multirepeat polymers are derived on the basis of the additivity principle of bond polarizabilities and the RIS approximation for rotations about skeletal bonds. Expressions for the unperturbed mean-square end-to-end distance , which are required in the calculation of Ar, are also obtained. 

On using vanadium-based homogeneous catalysts, polymers consisting of syndiotactic stereo-blocks and stereo-irregular blocks are obtained... 

The results suggest that the insertion of propylene to V-,3CH3 bonds is primary and that the subsequent monomer addition proceeds in a stereo-irregular fashion.122... 

With V-based catalysts, e.g., VCl4/AlCl(C2H5)2/aniso, polymers are obtained that consist of syndiotactic stereoblocks and stereo-irregular blocks. Interesting changes occur... 

In 1953, Ziegler [38] first employed aluminum trialkyl-titanium tetrachloride (RsAl-TiCU) catalyst to prepare stereo-irregular polypropylene. Later in 1954, Natta [39,40] refined the Ziegler catalyst with aluminum trialkyl-titanium trichloride (RsAl-TiCls) catalyst and successfully prepared highly stereoregular polypropylene. The new catalyst rendered polypropylene the possibility of practical usefulness. In 1957, it led to the construction of the first production facility for polypropylene by Montecatini Co. in Italy. 

The absence of crystallinity in isosorbide polyesters can be attributed to the lack of symmetry in this building block. Storbeck showed that polyesters based on the symmetrical isoidide are semi-crystalline [13]. Under normal polymerization conditions the presence of the two different OH groups in isosorbide gives rise to the formation of stereo-irregular polyesters (Figure 9.4). 

Figure 9.4 Random /stereo-irregular poly(isosorbide terephthalate) (PIT)...

The conformation of the stereo-irregular chains of PAN, which are able to crystallize in a pseudo-hexagonal form has been a matter of intense debates in the literature for years [39-48,95-97,177]. 

Comonomers (either chemical, geometrical or stereo-irregular sequences)... 

In studying polymer stereochemistry, the form of the NMR spectrum gives a rapid, unequivocal identification of the structure of stereoregular vinyl polymers. For stereo-irregular polymers, the resonance peaks are frequently split into several lines from sequences of a few monomer residues of different stereochemistry. The relative peak intensities give statistical information on... 

Experimental results clearly indicate that stereo-irregular polymers do indeed crystalhze as though they were copolymers. For example, specific volume-temperature curves for isotactic poly(propylene) display all the characteristics expected for a random type copolymer. The results of such a study by Newman (44)... 

Considering only a single repeat unit the 1,2 and 3,4 structures are identical and both polymers may be considered as 1,2-polybutadienes. However, as with polypropylene, polyvinyl chloride, polystyrene and many other thermoplastics, 1,2-polybutadiene possesses an asymmetric carbon atom so that both isotactic and syndiotactic stereoregular forms as well as the stereo-irregular atactic form are possible. 

The melting of stereo-irregular poly(propylenes), illustrated in Fig. 4.9, follows a similar pattern. The melting temperature and level of crystallinity decrease as the crystallizing isotactic content decreases. Concomitantly, the fusion process becomes very much broader. It is often difficult to recognize the crystallinity and the fusion of copolymers that have high contents of stmctural irregularities. 

The presence of an asymmetric chiral carbon atom in PLA leads to the existence of stereoisomers (Figure 11.11). Therefore, levorotatory (r), dextrorotatory (d), or racemic (dl) forms of PLA can be obtained. Stereoregular PLAs result from polymerization of optically pure lactides. Optically pure PLAs - poly(L-lactide), PLLA, and poly(D-lactide), PDLA-are therefore usually semicrystalline, while if there is an LL/DD motif distribution in PLA chains, then PLAs are stereo-irregular and can be regarded as copolymers whose crystallization ability decreases with optical purity [114, 115],... 

The structural parameters, determined by infra-red spectroscopic measurements (Table 1) show that the synthetic polyisoprenes produced using the Ziegler catalyst systems are closely similar to each other and are almost as structurally pure as natural rubber. Recent studies using the NMR technique indicate that the natural rubber hydrocarbon is at least 99-6% c/.y-l,4-polyisoprene. The small stereo-irregularities present in the synthetic polymers are sufficient to cause a reduced tendency for the synthetic polymers to crystallise either at low temperature or induced by applied strains. This difference in the rate of crystallisation, or perhaps the magnitude of the crystallites formed, is suggested to influence both processing and vulcanisate properties. The alkyl/lithium catalysed rubbers... 

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