Chain-stereoregularity

As already mentioned, this high stereochemical order is a case of kinehc control, as all these polymers are the product of highly speciahzed enzymes. Shll, chain stereoregularity in polypeptides and/or polynucleohdes has some addihonal features of interest for the held of the origin of life. This is covered in the next sechons. 

In agreement with the data on poly-a-olefins and poly-vinyl-ethers having the side chain asymmetric carbon atoms in the y-position with respect to the main chain, stereoregularity does not exert a remarkable influence on the rotatory power of poly-acrylates and poly-methacrylates. In fact, according to the quantitative data reported by H. Sobue, K. Matsuzaki, S. Nakano (135), and to the qualitative indications given by Liu, Szuty and Ullmann (64), concerning respectively poly-menthyl-meth-acrylate and poly-(l-methyl-benzyl)-methaciylate, the specific optical activity of the polymers does not vary by more than 30% when varying, within a wide interval, the isotactic triads content of the polymers (Table 18). 

Whether or not the stereogenic tertiary carbon atoms in isotactic and syndiotactic polypropylene are chirotopic depends on the model chosen for their representation [16]. Stereochemical analysis, which followed Natta s discovery, involved two models of the polymer chain. Stereoregular polymers were considered, on the one hand, as the extrapolation towards high molecular weights of well-studied organic molecules, e.g. such as trihydroxyglutaric acid ... 

One of the cases of such complementarity may be the atactic polymer which can be considered as a copolymer of isotactic and syndiotactic links. Indeed, the composition and properties of the polycomplex depend on the chain stereoregularity, for example in the case of poly(methyl methacrylate) stereocomplexes16 and polyethylene glycol) with isotactic or atactic PAA complexes33. ... 

In all cases, however, the potentiometric plots of PMA reveal an anomaly at around a 0.15, which is absent in the titration curve of poly(acrylic acid) or other vinyl polyelectrolytes so far studied. The anomalies have been attributed by Leyte and Mandel to a conformational transition of cooperative character taking place in the PMA chains over a narrow range of a-values (5). This anomalous behavior of PMA samples of different t5q)e and amount of chain stereoregularity over a given range of degree of neutralization has been confirmed using... 

In this section, kinetic data relating to reactions between vinyl polyelectrolytes and simple compounds in aqueous solution will be briefly discussed. Only a few recent data, some of which may illustrate the dependence of polyion reactivity on chain stereoregularity and on hydrophobic bonding, with some simple substrates will be reviewed. 

The degree of PP chain stereoregularity can be modified from totally absent to almost perfect (in either direction, isotactic or syndiotactic) by altering the monomer enantioface selectivity of each active site of the catalyst in the case of metallocenes, this means varying the substitution pattern of the two cyclopentadienyl ligands. Metallocenes are by far the most versatile systems for the production of PPs of different chain stereoregularity and molecular masses. 

For example, Lu and Jiang showed [100], based on a statistical thermodynamic derivation in which some of the parameters of the model were calibrated by using experimental data, that the approximation in Equation 6.20 can be made for polymers with a vinyl-type chain backbone, where Coo(Tg) is the value of the characteristic ratio at Tg. C, which will be discussed in Chapter 12, depends both on polymer chain stereoregularity and on the temperature. Direct applications of the method which will be presented in Chapter 12 for its prediction as a function... 

Tip 2 Chain stereoregularity and active sites. In free radical polymerization, polymer chain configuration and MWD are often independent of initiator type and initiation mechanism, depending strongly on reaction temperature, initiation rate, and monomer concentration. One can, therefore, often predict chain stereoregularity and MWD without a detailed knowledge of the initiation mechanism. 

The copolymers of ethylene with norbornene produced with metallocene catalysts [199] are interesting thermoplastic materials characterized by high glass transition temperatures, good chemical and thermal resistance and excellent transparency [200,201]. The properties of these copolymers depend on the comonomer composition, the distribution of the comonomers and the chain stereoregularity. 

Stereoregular ds-transoidal polyacetylenes can take a regular helical conformation. The main-chain stereoregularity is important in inducing a single-handed helical conformation. Rhodium catalysts such as [Rh(norbor-nadiene)Cl]2 are often used to obtain a stereoregular polymer. The helical structure of polyacetylenes is... 

Iribarren, J.I., Ocampo, C., Armelin, E., Liesa, F., and Aleman, C. (2008) Poly(3-alkyllfaiophene)s as anticorrosive additive for paints influence of the main chain stereoregularity. /. Appl. Polym. Sci., 108, 3291-3297. 

Few publications have made reference to the supermo-lecular structure of sPP. Marchetti and Martuscelli [196] and Lovinger et al. [82, 197], and Rodriguez-Arnold et al. [198, 199] published mostly papers in which the morphology was systematically studied with regard to chain stereoregularity and crystallization conditions. 

Material properties of polymers are determined by their chain miaostmctures. For polymers made from a single monomer type, the above-discussed molecular weight and distribution, chain stereoregularity, head-tail and trans-cis configurations, and so on all play important roles. For copolymers that contain multiple monomer types, chain composition, sequence, as well as their distributions, are added to the important microstmc-ture property list. With these new parameters, almost unlimited number of polymer types can be produced for better balance of properties for commercial applications. Outstanding commercial examples include acrylonittile-butadiene-styrene (ABS), SBS, Acrylan (acrylonittile-vinyl acetate), styrene-butadiene (SBR), butyl mbber (isobutylene-isoprene), Vinylite (vinyl chloride-vinyl acetate), and styrene-maleic anhydride (SMA). 

Chain stiffness Chain polarity Chain stereoregularity Stereocopolymers iso/syndio cisltranslvm Crosslinking Ciystallinity... 

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