Stereo regularity

Polybutadiene was first prepared in the early years of the 20th century by such methods as sodium-catalysed polymerisation of butadiene. However, the polymers produced by these methods and also by the later free-radical emulsion polymerisation techniques did not possess the properties which made them desirable rubbers. With the development of the Ziegler-Natta catalyst systems in the 1950s, it was possible to produce polymers with a controlled stereo regularity, some of which had useful properties as elastomers. 

It may be prepared in two stereo-regular forms, cis- and trans-. The cii-polymer, which crystallises in zig-zag form, has a of 235°C, whilst the fran -polymer, which crystallises in helical form, melts at the much lower temperature of 145°C. Tensile strengths of both forms are reportedly similar to that of Penton whilst the tensile modulus of 2300 MPa is about twice as high. Unfortunately the material is rather brittle with an impact strength only about half that of polystyrene although this may be improved by orientation. 

Reduction in the stereo-regularity by adding co-monomers, reducing the crystallization tendency of the polychloroprene. 

The presence in these copolymers of hetero-substituted monomeric units randomly dispersed along the phosphazene skeleton brings about the extreme difficulty of the polymeric chains to be packed in regular structures. They lose, therefore, the original stereo-regularity of the parent phosphazene homopolymers (microcrystalline materials), and show only amorphous structures, with sharp decrease in the values of the Tg (collapsed up to about -90 °C) and with the onset of remarkable elastomeric properties [399,409,457]. 

Kohnkc, F.H., Mathias, J.P., and Stoddart, J.F. Substrate-Directed Synthesis The Rapid Assembly of Novel Macropolycyclic Structures via Stereo regular Diels-Alder Oligomerizations. 165, 1-69 (1993). 

The stereo-regularity of a polymer can be reduced by a variety of defects. For example, regio-errors arise when a 2,1-monomer insertion occurs instead of a 1,2-in-sertion the latter is usually 10 -10 times faster. Misinsertions can lead to chain termination and formation of butenyl end groups, they can be incorporated into... 

Matsumura S, Shimura Y, Terayama K, Kiyohara T (1994) Effects of molecular weight and stereo regularity on biodegradation of poly(vinyl alcohol) by Alcaligenes faecalis. Biotechnol Lett 16 1205-1210... 

Prepoly- merization Silicon compound added in -polymerization Activity ofTi Stereo- regularity... 

When butadiene and 2,3-dimethylbutadiene are included in the channels of urea and thiourea, respectively, 1,4 addition invariably results to yield polymers with chemical and stereo regularities (Scheme 39). Note that addition in the 1,2 fashion is prevented sterically by the narrow channel. Similarly, high selectivity was obtained when butadiene, vinyl chloride, and styrenes were polymerized in the channels of cyclophosphazenes. Syndiotac-tic polymer alone is obtained from vinyl chloride included in urea channels this is apparently the first example of inclusion polymerization of a vinyl polymer in which control is exerted over the steric configuration of the developing tetrahedral carbon atom (Scheme 39). Highly isotactic polymer is obtained from 1,3-pentadiene when it is included in a perhydrotriphenylene matrix (Scheme 39). Note that addition could occur at either end (i.e., Q to... 

The substituted carbon atoms in the polymer chain are asymmetric. Stereoregular polymers are produced if all these carbon atoms have the same configuiation (all d or all I) or if the d and 1 configurations occur alternately pronounced stereo-regularity is seldom achieved in radical polymerizations except perhaps at very low temperatures. When dienes are polymerized by a radical mechanism, the resulting polymers contain several distinct types of monomer unit, thus butadiene can give rise to -CH2 C(CH CH2)-, -CH CH CH CH2- cis, and -CH CH.CH CH2- trans. 

A decrease in the catalyst Al/Mt ratio and an increase in polymerisation temperature both result in a lower average polymer molecular weight and, in the case of stereospecific a-olefin polymerisation, decreased stereo regularity [22,107]. 

In later studies on the homopolymerization of E-l,3-pentadiene with NdO/ TIBA/DEAC crystalline polymers with cis- 1,4-contents in the range 84-99% and a high isotacticity were obtained. It was found that the cis- 1,4-content increases when the polymerization temperature is decreased from room temperature to -30°C. The polymerization of E-2-methyl-l,3-pentadiene resulted in polymers which almost exclusively comprised cis- 1,4-units and no dependence of the cis- 1,4-content on polymerization temperature was observed. The obtained poly(2-methyl-l,3-pentadiene) was composed of various polymer fractions with different stereo regularities [165,166]. 

PP is composed of linear hydrocarbon chains and therefore its properties quite closely resemble those of PE. The properties of isotactic PP are particularly useful. The stereo regularity of the macromolecule chain construction and the related high crystallinity give PP its outstanding characteristics. Large scale commercially produced PP is up to 95 % isotactic in nature. 

Nishioka, Watanabe, Abe, and Sono (48) carried out an extensive study of the Grignard reagent catalyzed polymerization of methyl methacrylate in toluene with respect to tactidty of the resulting polymers. The tactidty of the polymer was determined quantitatively by nuclear magnetic resonance analysis. It was found that the stereo-regularity depended on the nature of the R group of the Grignard... 

Polymers with Tg/Tm ratios above 0.67 are unsymmetrical. They can also be highly crystalline if they have long sequences of methylene groups or are highly stereo regular all have a much more complex structure than the polymers with ratios below 0.5. 

Table III. Effect of Stereo regularity on Crystallization Kinetics of Propylene Oxide Polymers...

Applied difunctional organocyclotetrasiloxanes, both pure cis- and tram-isomers and their mixtures with isomers in various ratios, were of interest as model compounds for synthesizing stereo-regular poly(organocyclotetrasiloxanes) (POCS-4) trans-tactic, cis-tactic and with strict regular alternation of cis- and /ram-scqucnccs. 

Figure 4. DSC curves for PMCS-4 (Table 2.3) of various stereo regular structure 1 - atactic, 2, 2 - transtactic polymer 2 (2- heating, 2 - cooling) 3 - heating of polymer 3 4—heating of polymer 4...

Rare earth halide (in particular NdCl3)-based compounds are very active catalysts for polymerization of dienes. The products are polymers with improved elastic and thermoplastic properties caused by high stereo regularity. The polymerization process is highly efficient (i.e.) 60 000 moles of butadiene are polymerized by one atom of rare earth. Hence the demand for rare earth compounds as polymerization catalysts is low. 

The fluorescence intensity of quinoline derivatives has been found to increase dramatically with an increase in the molecular weight of the host polystyrene. " This is attributed to a decrease in the free volume in the polymer matrix restricting molecular rotation/motion of the fluorophore. Similar effects have been observed for juliodinemalononitrUe in different stereo-regular poly(methyl methacrylates), and temperature effects on the luminescence properties of indole and coumaric acid derivatives in different polymer matrices showed abrupt changes in emission intensity at temperatures which correspond to the onset of local relaxation processes in the polymer. ... 

Sakurada et al. (57) have investi ted the influence of chain stereo-regularity on the rate of the polysulphonic-add-catalyzed hydrolysis of simple alkyl esters. They suggest, on the basis of limited data, that the... 

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