Effect on polymer properties

Free-radical polymerization is the preferred iadustrial route both because monomer purification is not required (109) and because initiator residues need not be removed from polymer for they have minimal effect on polymer properties. 

Another way to recover the catalyst from the dormant site is the copolymerisation of ethene, but this is slower and less attractive than the use of hydrogen. Furthermore the use of ethylene inevitably results in the formation of propylene-ethylene copolymers with all the consequent effects on polymer properties. 

Methyl branching also occurs in the preparation of many polymers. Ordinarily methyl branches are present in only trace amounts, barely detectable by the most sensitive 13C-NMR(69), and they have no effect on polymer properties. But if the polymerization is carried out at atmospheric pressure or less, methyl branching becomes much more common (69). It may originate from isomerizarion at the active site. This isomerization is probably not favored but does occur very rarely (Scheme 2). Low pressure decreases the propagation rate, allowing each chain to spend more time around the active site, thus increasing the probability of such side reactions. Internal... 

Moisture has, in itself, usually not much effect on polymer properties, though the amount of moisture which can be absorbed by polymers varies within wide limits (between zero and a few %). Logically, the electric properties such as resistivity and dielectric losses are the most sensitive to water. As to mechanical properties, nylons show the strongest dependence on water absorption. PA-6 is able to take up a... 

Although the occurrence of constitutive isomerism can have a profound effect on polymer properties, the quantitiative characterization of such structural variations has been difficult. Recent research has shown that the chemical shifts of polymers are sensitive to the type, length, and distribution of branches as well as to positional isomerism and stereochemical isomerism (Section 4.2.2). This technique has great potential when the bands in the polymer spectra can be assigned unequivocally. 

The mechanism for polymerization of propylene using Ziegler-Natta catalysts is analogous to that discussed in section 3.7 with ethylene. However, unlike ethylene, propylene can be said to have "head" and "tail" portions and regiochem-istry can vary. More importantly, the orientation (stereochemistry) of the methyl group in the polymer has a dramatic effect on polymer properties. These factors make polymerization of propylene (and other a-olefins) more complex (17). 

PCS are the most interesting from the series of potential highmolecular modifiers. However, problems of such modifiers effect on polymer properties are not fully studied in literature. 

In none of the above examples of the structural representation of polymers the end groups have been shown. This is partly because the exact nature of the end groups is often not known and partly because end groups constitute an insignificant fraction of the mass of high molecular weight polymer and so usually have negligible effect on polymer properties of major interest. 

Our discussion above on polymer conformations in single chains and in crystals has assumed regularity of macromolecular structure. However, irregularities such as inversions of monomer placements (head-to-head instead of head-to-tail), branches, and changes in configuration may occur. These irregularities, which are considered in a later section, may Inhibit crystallization and have a profound effect on polymer properties. 

They can be sterilized by terminal gamma irradiation with minimal effect on polymer properties. 

This study paved the way to the systematic exploration of surface modifications, and their effects on polymer properties. 

In formulating polyurethane/urea systems using ultra-low monol PPG polyols, it was found that polyol MWD has a major effect on polymer properties. A broadened MWD is required to achieve a property profile similar to a PTMEG-based system. The MW-distribution effect is illustrated using TDI prepolymers cured with MBOCA, TDI moisture-cured prepolymers and aqueous polyurethane/urea dispersion coatings. 

If only one carbon atom of the type shown in Figure 1 existed in the polymer, the effect on polymer properties would be negligable and it would be hard to distinguish this polymer from one in which there were no optically active sites. Such a polymer would be one in which, for example, all carbon atoms other than the one shown in Figure 1 have X = Y. However, the common way that optically active sites show up in polymers is to have at least one site per mer. This means that most polymer molecules with optical activity contain at least 100 optically active sites and the presence of an bonded atom that is distinct fiom its mirror image will have pronounced effects on the optical, chemical, and physical properties of the polymer. 

The incorporation of a reinforcing agent such as glass fibres into a non-fire retardant or a fire retardant formulation might also have an effect on polymer properties as discussed next. 

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