Molecular weight crystallizable polymers

The homopolymer has a crystalline melting temperature of 168-170°C a byproduct is the non-crystallizable variant, atactic polypropylene, (contaminated with low-molecular-weight crystalline polymer), which has little commercial value. This was inevitably isolated in the diluent-based polymerization, but recent manufacture, with more discriminating catalysts, seeks to exploit the whole product. The early polypropylene was of very high molecular weight and was almost impossible to process there was even... 

The case of PEO as a crystallizable component within block copolymers, as well as isolated droplets, is atypical in the sense that it has been studied in a wide variety of block copolymers and in the widest possible volume range if micron-size droplets are also considered (Figs. 4, 5, Tables 2, 3). Nevertheless, the observed change in crystallization temperature is particularly large, not only in comparison with low molecular weight materials, as previously mentioned, but also with other polymers. 

Sinn, Lundborg and Kirchner (61) have reported that the homogeneous polymerization of styrene in benzene using lithium alkyls at 50° produces a relatively low molecular weight polymer a portion of which is crystallizable by treatment with boiling heptane. 

Another method for the determination of polymer crystallinity was discussed by Duswalt (159). It is based on the ability of the instrument to cool a molten sample rapidly and reproducibJy to a reselected temperature where isothermal crystallization is allowed to occur. A number of crystallization curves for polyethylene obtained isothermalJy at different, preset crystallization temperatures are shown in Figure 7.57. Differences in polymer crystallizability that may be caused by branching, nucleation, or molecular weight effects can be observed. The sensitivity and speed of the method allow pellet-to-pellet variations in a lot of polymer to be examined. 

Three of the block polymers were DuPont Hytrels 4055, 5555, and 6355 (17, 18, 19). These materials have a crystallizable PTMT hard block and a poly(tetramethylene glycol) (PTMG) soft block with a molecular weight of about 1000. Hytrels 4055, 5555, and 6355 contained about 42.6, 35, and 27% PTMG, respectively. The fourth polymer was Uniroyal TPR-19 which has a... 

MAJOR APPLICATIONS The polymers referred to in this chapter include those families of homopolymers of propylene which are known to have elastomeric recovery properties at reasonable molecular weight and for which properties have been attributed to a crystallizable-noncrystallizable (e.g., isotactic-atactic) stereoblock structure, or to a major component with a stereoblock structure, whether or not the compositions are homogeneous by solvent fractionation tests. Copol)rmers and blends are not deliberately included in the data presented, but are described in some of the references. (See also some of the closely related elastomeric polymers presented in the entry on Polypropylene, atactic in this handbook.) The criterion of multiple crystallizable blocks per polymer chain may be met in significant fractions of low-tacticity, low-stereoregularity polymers of very high molecular weight. 

The preparation of crystallizable polypropylene, as practiced in the earlier days of polypropylene, involved the preparation of catalyst, polymerization, purification, solvent recovery, and, finally, compounding. Typically, yields were 500 1000 lb of polymer per pound of catalyst [6]. Each manufacturer practiced one s own version of these process steps in an effort to produce a uniform product with regard to molecular weight and molecular weight distribution, ash content, color and color stability, and atactic, noncrystallizable polymer content. As discussed earlier, some systems used solvents that kept the atactic polymer in solution. 

Over the past several decades, polylactide - i.e. poly(lactic acid) (PLA) - and its copolymers have attracted significant attention in environmental, biomedical, and pharmaceutical applications as well as alternatives to petro-based polymers [1-18], Plant-derived carbohydrates such as glucose, which is derived from corn, are most frequently used as raw materials of PLA. Among their applications as alternatives to petro-based polymers, packaging applications are the primary ones. Poly(lactic acid)s can be synthesized either by direct polycondensation of lactic acid (lUPAC name 2-hydroxypropanoic acid) or by ring-opening polymerization (ROP) of lactide (LA) (lUPAC name 3,6-dimethyl-l,4-dioxane-2,5-dione). Lactic acid is optically active and has two enantiomeric forms, that is, L- and D- (S- and R-). Lactide is a cyclic dimer of lactic acid that has three possible stereoisomers (i) L-lactide (LLA), which is composed of two L-lactic acids, (ii) D-lactide (DLA), which is composed of two D-lactic acids, and (iii) meso-lactide (MLA), which is composed of an L-lactic acid and a D-lactic acid. Due to the two enantiomeric forms of lactic acids, their homopolymers are stereoisomeric and their crystallizability, physical properties, and processability depend on their tacticity, optical purity, and molecular weight the latter two are dominant factors. 

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