Higher-molecular-weight polymers

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that form tertiary carbocations In some cases reaction conditions can be developed that favor the formation of higher molecular weight polymers Because these reactions proceed by way of carbocation intermediates the process is referred to as cationic polymerization We made special mention m Section 5 1 of the enormous volume of ethylene and propene production in the petrochemical industry The accompanying box summarizes the principal uses of these alkenes Most of the ethylene is converted to polyethylene, a high molecular weight polymer of ethylene Polyethylene cannot be prepared by cationic polymerization but is the simplest example of a polymer that is produced on a large scale by free radical polymerization... 

Would you expect solutions C and E or B and D to contain the higher molecular weight polymer Briefly explain. Outline a strategy for nylon fractionation based on steps (a)-(c). Some steps may be repeated as needed. 

Eor some uses, higher molecular weight polymer consisting of 150—200 repeat units is required. Such polymer usually is prepared by soHd-state polymerization in which pellets are heated under an inert atmosphere to 200—240°C. The 2G is removed continuously. The rate of polymerization depends on particle size, end group composition, and crystallinity (65). 

The lower molecular weight oils, waxes, and greases of PCTFE can be prepared direcdy by telomerization of the monomer or by pyrolysis of the higher molecular weight polymer (45—54). 

Petroleum resins are low molecular weight thermoplastic hydrocarbon resins synthesized from steam cracked petroleum distillates. These resins are differentiated from higher molecular weight polymers such as polyethylene and polypropylene, which are produced from essentially pure monomers. Petroleum resin feedstocks are composed of various reactive and nonreactive aliphatic and aromatic components. The resins are usually classified as C-5... 

Film or fibers derived from low molecular weight polymer tend to embrittle on immersion ia acetone those based on higher molecular weight polymer (>0.60 dL/g) become opaque, dilated, and elastomeric. When a dilated sample is stretched and dried, it retains orientation and is crystalline, exhibiting enhanced tensile strength. The tensile heat-distortion temperature of the crystalline film is iacreased by about 20°C, and the gas permeabiUty and resistance to solvent attack is iacreased. 

The optimum flow rate for most SEC separations using conventional PLgel column dimensions (internal diameter 7.5 mm) is 1.0 ml/min. It may be of some benefit to work with lower flow rates, particularly for the analysis of higher molecular weight polymers where the reduced flow rate improves resolution through enhanced mass transfer and further reduces the risk of shear... 

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that fonn tertiary carbocations. In some cases reaction conditions can be developed that favor the formation of higher molecular-weight polymers. Because these reactions proceed by way of carbocation intermediates, the process is refened to as cationic polymerization. 

In 1957, Bamford et reported that the addition of small amounts of lithium chloride brought about a significant (up to two-fold) enhancement in the rate of polymerization of AN in DMF and led to a higher molecular weight polymer. Subsequent studies have shown this to be a more general phenomenon for polymerizations involving, in particular, acrylic and vinyl heteroaromatic monomers in the presence of a variety of Lewis acids.71... 

RAFT polymerizations under very high pressure (5 kbar) have been reported.509-512-513 At high pressures, radical-radical termination is slowed and this allows the formation of much higher molecular weight polymers and higher rates of polymerization than are achievable at ambient pressure. 

Like other step-growth polymerization methods, factors such as the monomer purity, ratio of the monomers, conversion, temperature, and concentration will greatly influence the transition metal coupling polymerization. These factors have to be taken into account when higher molecular weight polymers need to be prepared.33... 

The syrup discharging from the tower enters an enclosure where it is subjected to milling under vacuum and/or an inert atmosphere. Vacuum as high as 29.8 in. Hg is suggested. The action in this chamber accomplishes three purposes a) blends the polymer melt, b) removes most of the unreacted volatile materials in the melt, and c) degrades the higher molecular weight polymer fraction = 150,000) without... 

For example, the same dimer complex which contains two molar equivalents of ethanol, underwent photoreaction to give a higher molecular weight polymer (M = 12 000). The formation of such inclusion complexes with the solvent is rather generally observed with similar types of dimers formed with alcohols and some other solvents and, consequently, this enhances photopolymerizability. Such complex formation with a solvent may be one of the promising techniques that can be used for diolehn compounds in order to obtain polymers with high molecular weights. 

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