Distribution narrow molecular weight

We shall see that, as polymers go, this is a relatively narrow molecular weight distribution. 

Three polystyrene samples of narrow molecular weight distribution were investigatedf for their retention in GPC columns in which the average particle size of the packing was varied. In all instances the peaks were well resolved. The following results were obtained ... 

A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

AlCl efficiency (based on g resin/g catalyst) can be markedly improved by polymerizing dry feeds (<10 ppm H2O) with an AlCl /anhydrous HCl system. Proceeding from 250 ppm H2O down to 10 ppm H2O, catalyst efficiency improves from 30.6 to 83.0 (26). Low levels of tertiary hydrocarbyl chlorides have been shown to gready enhance the activity of AlCl, while yielding resins with narrow molecular weight distributions relative to systems employing water or HCl (27). 

AlkyUithium compounds are primarily used as initiators for polymerizations of styrenes and dienes (52). These initiators are too reactive for alkyl methacrylates and vinylpyridines. / -ButyUithium [109-72-8] is used commercially to initiate anionic homopolymerization and copolymerization of butadiene, isoprene, and styrene with linear and branched stmctures. Because of the high degree of association (hexameric), -butyIUthium-initiated polymerizations are often effected at elevated temperatures (>50° C) to increase the rate of initiation relative to propagation and thus to obtain polymers with narrower molecular weight distributions (53). Hydrocarbon solutions of this initiator are quite stable at room temperature for extended periods of time the rate of decomposition per month is 0.06% at 20°C (39). 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (eqs. 6,7) (3,20,64). However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization, since the unique abiUty of lithium to provide high 1,4-polydiene microstmcture is lost in polar media (1,33,34,57,63,64). Consequentiy, a significant research challenge has been to discover a hydrocarbon-soluble dilithium initiator which would initiate the polymerization of styrene and diene monomers to form monomodal a, CO-dianionic polymers at rates which are faster or comparable to the rates of polymerization, ie, to form narrow molecular weight distribution polymers (61,65,66). 

Low Temperature Brittleness. Brittleness temperature is the temperature at which polyethylene becomes sufficiently brittle to break when subjected to a sudden blow. Because some polyethylene end products are used under particularly cold climates, they must be made of a polymer that has good impact resistance at low temperatures namely, polymers with high viscosity, lower density, and narrow molecular weight distribution. ASTM D746 is used for this test. 

A number of papers and patents describe polymerization processes to poly(tetramethylene ether) glycols having a narrow molecular weight distribution = 1.2—1.4). In principle, this can be achieved by having all chains grow quickly at one time, either by high temperature initiation (33)... 

Fatty Acid Process. When free fatty acids are used instead of oil as the starting component, the alcoholysis step is avoided. AH of the ingredients can therefore be charged into the reactor to start a batch. The reactants are heated together, under agitation and an inert gas blanket, until the desired endpoint is reached. Alkyds prepared by the fatty acid process have narrower molecular weight distribution and give films with better dynamic mechanical properties (34). 

The search for new, high performance materials requites the synthesis of weU-defined, narrow molecular weight distribution, cycHc-free, homo- and copolymers. Synthesis of these polymers can be accompHshed by the kinetically controUed polymerization of the strained monomer. 

Geresin wax [8001 -75-0] originally was a refined and bleached o2okerite wax, but now is a paraffin wax of very narrow molecular weight distribution or blend of petroleum waxes. 

Some by-product polyethylene waxes have been recently introduced. The feedstock for these materials are mixtures of low molecular weight polyethylene fractions and solvent, generaHy hexane, produced in making polyethylene plastic resin. The solvent is stripped from the mixture, and the residual material offered as polyethylene wax. The products generaHy have a wider molecular weight distribution than the polyethylene waxes synthesised directly, and are offered to markets able to tolerate that characteristic. Some of the by-product polyethylene waxes are distHled under vacuum to obtain a narrower molecular weight distribution. 

Cocatalysts, such as diethylzinc and triethylboron, can be used to alter the molecular-weight distribution of the polymer (89). The same effect can also be had by varying the transition metal in the catalyst chromium-based catalyst systems produce polyethylenes with intermediate or broad molecular-weight distributions, but titanium catalysts tend to give rather narrow molecular-weight distributions. 

These values are given for polymers of narrow molecular-weight distribution, with number-average molecular weights (M ) of about 20,000 prior to chlorination. Chlorination reactions are carried out under homogeneous conditions in CCl solutions at temperatures between 90 and 110°C with viscosities at about 5 Pa (50 P). 

Ah these polymerizations proceed only in the absence of oxygen or water, which react with the highly reactive propagating species. Polymerization is usuahy carried out in an inert, hydrocarbon solvent and under a nitrogen blanket. Under these conditions, polymers with narrow molecular-weight distributions and precise molecular weights can be produced in stoichiometric amounts. 

Figure 8.7. The blend W of high and low molecular weigh polymers (curves 1 and 2) becomes non-Newtonian at a lower shear rate than a polymer of narrow molecular weight distribution...

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