Tacticity crystallinity

The polymers feature two chiral centers per monomer unit and therefore are ditactic. While polymers produced by achiral Pd catalysts seem to be atactic, using chiral metallocene catalysts highly tactic crystalline materials can be produced, featuring extraordinary high melting points (in some cases above the decomposition temperature) and extreme chemical resistance. 

IV to VIII metals and base metal alkyls of Group II or III metals (Penczek and Premia, 2012 Boor, 1979 Ciardelli, 1992). It arose from the spectacular discovery of Ziegler et al. (1955) that mixtures of titanium tetrachloride and aluminum alkyls polymerize ethylene at low pressures and temperatures and from the equally spectacular discovery by Natta (1955) that the Ziegler catalysts can stereospecifically polymerize monoolefins to produce tactic, crystalline polymers. As can be imagined, these systems can involve many combinations of catalyst components, not all of which are catalytically active or stereospecific. However, we shall be concerned here only with polymerizations involving the commercial elastomers, principally polyisoprene, polybutadiene (Duck and Locke, 1977 Zohuri et al., 2012 Teyssie et al., 1988), and the ethylene-propylene copolymers (Schobel et al., 2012 Ver Strate, 1986 Davis et al., 1996 Noordermeer, 2003 Baldwin and Strate, 1972). 

The study of PLA hydrolysis has been performed in aqueous media, sueh as phosphate-buffered solutions or water, at 37 °C, to simulate its degradation in body fluids at the appropriate temperature. Studies have also been done at higher temperatures, in acidic, alkaline or buffered solutions, to determine the hydrol5Aic effects of PLA under severe and accelerated conditions. Simultaneously, different sample structural parameters have been studied during the hydrolytic degradation of PLA, such as shape and history of the specimen, chemical structure, molecular weight, tacticity, crystallinity and molar distribution, purity and morphology. We summarize and discuss in detail the above-mentioned parameters in the next parts of this chapter. 

Cyclic monomers can be divided into achiral and prochiral types. Polymerization of both types by chiral metallocenes may yield tactic, crystalline homopolymers.The melting points (TmS) of... 

In practice TMA is mostly used for measurement of the glass transition temperature by measuring CLTE and on occasion volume expansion. Therefore, it is appropriate to review those chemical factors (effects of pendant groups, molecular mass, tacticity, crystallinity, crosslinking, etc.) that have the largest influence on the glass transition temperature and the expansion of a polymer. 

Besides the molecular weight, the most Important parameter to control for the production of commercial polypropylene is the stiffness of the polymer. A modern catalyst system must be able to control the stiffness of the product over a vide range independently of the other polymer properties. The stiffness of the polypropylene is closely related to its intrinsic tacticity but this is not the only parameter. The stiffness is the result of a combination of tacticity, crystallinity and HVD of the resin. The effect of HVD has already been pointed out. The relationship between tacticity, mean molecular weight and the stiffness of the polypropylene produced vith the SB 12 catalyst is illustrated hereafter. 

More recent developntients have also to be pointed out, in particular that of tactic crystalline 1,2-polybutadienes having high melting points the use of the syndiotactic product has been strongly promoted in Japan for films production, and even the amorjtous 1.2 polymer seems to have desirable properties as a oomponertin elastomers blends. Finally, very recent reports, also from Japan, claim improved ihy-... 

The polymers obtained by 100% double bond opening feature two chiral centers per monomer unit and therefore are ditactic. Chiral metallocene catalysts produces tactic, crystalline homopolymers with extremely high melting points (for poly(cydobutene) 485 °C, for polynorbornene about 600 °C) and decomposition occurs before melting. Whereas the atactic cydic olefin polymers can be dissolved in hydrocarbon solvents at least to some extent, the tactic pol5miers are hardly soluble. " ... 

As Robertson noted,his concept of chain folding in atactic polymers was suggested because of the known presence of folding in tactic crystalline polymers. 

Polymers of different tacticity have quite different properties, especially in the solid state. One of the requirements for polymer crystallinity is a high degree of microstructural regularity to enable the chains to pack in an orderly manner. Thus atactic polypropylene is a soft, tacky substance, whereas both isotactic and syndiotactic polypropylenes are highly crystalline. 

In order to generate stereoregular (usually isotactic) polymers, the polymerization is conducted at low temperatures ia nonpolar solvents. A variety of soluble initiators can produce isotactic polymers, but there are some initiators, eg, SnCl, that produce atactic polymers under isotactic conditions (26). The nature of the pendant group can influence tacticity for example, large, bulky groups are somewhat sensitive to solvent polarity and can promote more crystallinity (14,27). 

Advances in VP cationic polymerization hold out the possibiUty of tacticity, and the study of this route to crystalline homologues continues to be of interest. 

Polypropylene. There is an added dimension to the catalytic polymerization of propylene, since in addition to the requirement that the catalyst be sufficiently active to allow minute amounts of catalyst to yield large quantities of polymer, it must also give predominantly polypropylene with high tacticity that is, a highly ordered molecular stmcture with high crystallinity. The three stmctures for polypropylene are the isotactic, syndiotactic, and atactic forms (90) (see Olefin polya rs, polypropylene). 

FIGURE 6.3 The crystallinity and composition continuum for ethylene- and propylene-dominated polyolefins. Note the dispersion for the propylene-dominated polyolefins due to much-greater prevalence of blends and the presence of tacticity derived changes in crystallinity. 

How does the type and level of tacticity of the polypropylene chain affect the degree of crystallinity of the solid polymer ... 

Many of the properties of a polymer depend upon the presence or absence of crystallites. The factors that determine whether crystallinity occurs are known (see Chapter 2) and depend on the chemical structure of the polymer chain, e.g., chain mobility, tacticity, regularity and side-chain volume. Although polymers may satisfy the above requirements, other factors determine the morphology and size of crystallites. These include the rate of cooling from the melt to solid, stress and orientation applied during processing, impurities (catalyst and solvent residues), latent crystallites which have not melted (this is called self-nucleation). 

On well characterised non-stabilized PP samples [48] having molar mass within 45-180 kg/mol with differing tacticity and crystallinity, we can see that the increasing molar mass leads to an increase of induction time and reduction of the maximum chemiluminescence intensity (Figure 14). The polymer with higher average molar mass appears to be more stable than that with lower molar mass. This may be ascribed to the effect of increased concentration of more reactive terminal groups, which promote initiation of thermal oxidation. 

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