Molecular weight, significance

However, the synthesis process, depicted in scheme 5, is rather idealized. In reality, the chemistry appears to be quite complex, resulting in a partially cross-linked rubber and the evolution of gaseous species other than chloromethane. Dietrich et al.17 reported that the progress of the polymerization at 116°C, as measured by gas evolution and polymer molecular weight, significantly slowed at around 50% conversion. The reaction could, however, be driven further forward by increasing the temperature to > 150°C. 

Data for examples of polymerization of PBT cyclic oligomers are shown in Table 3.2 [14], Polymerization under mild conditions (0.2-0.3mol% titanate initiator at 190 °C) for 6min led to Mws of 95000-115 000 (entries 2, 4 and 5). These results indicate that either the PBT cyclic polymerization is far more facile than observed for PET cyclics, or that the cyclics had higher purity, since the rates were faster and the final molecular weights significantly higher. As... 

Micropowders, PTFE homopolymers with molecular weight significantly lower than normal PTFE, are commonly used as additives in a large number of applications where they provide nonstick and sliding properties. They are added to plastics, inks lubricants, and lacquers.14... 

PTFE micropowders (also referred to as fluoroadditives) are homopolymers of tet-rafluoroethylene with molecular weight significantly lower than normal PTFE (see Table 7.6). They are prepared mainly by irradiation of PTFE (often of PTFE scrap) by high-energy electron beam [104]. The effect of radiation dose is illustrated by... 

This type of initiation is limited by the nucleophilicity of the anion A -derived from the acid. For acids other than the very strong acids, such as fluorosulfonic and triflic acids, the anion is sufficiently nucleophilic to compete with monomer for either the proton or secondary and tertiary oxonium ions, and consequently, only very low-molecular-weight products are possible. Water, often present as impurity, can also reduce the molecular weight significantly since its nucleophilicity allows it to compete with monomer for the oxonium ions. 

The vapor pressure of a polymer solution deviates downwards largely from the Raoult s law. Figure 2.6 plots the pressure of the solvent vapor in equilibrium with a solution of uncrosslinked rubber in benzene. The depression becomes larger with the molecular weight of the polymer. Thus, polymers of high molecular weights significantly suppress the solvent activity. 

These studies suggest that (a) the biological toxicity of polyanions increases with molecular weight (significant toxicity increases occur above 15,000 MW and becomes serious over 50,000 MW) (b) antitumor activity does not appear to be molecular weight dependent (c) antiviral activity is related to molecular weight (it is diminished below 50,000 MW) and (d) sensitization to bacterial endotoxin is related to molecular weight in the case of pyran and seems to be affected by polydispersity of the polyanionic polymer. 

Figure 7. Emission maxima of solutions containing dansyl labeled PAA and POE of various molecular weights. Significance of symbols as in Figure 6.

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