Molecular weight polymer processing

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... 

Engineering problems involved in the production of TEE seem simple compared with those associated with polymeriza tion and processing of PTEE resins. The monomer must be polymerized to an extremely high molecular weight in order to achieve the desired properties. The low molecular weight polymer does not have the strength needed in end use appHcations. 

High molecular weight polymers or gums are made from cyclotrisdoxane monomer and base catalyst. In order to achieve a good peroxide-curable gum, vinyl groups are added at 0.1 to 0.6% by copolymerization with methylvinylcyclosiloxanes. Gum polymers have a degree of polymerization (DP) of about 5000 and are useful for manufacture of fluorosiUcone mbber. In order to achieve the gum state, the polymerization must be conducted in a kineticaHy controlled manner because of the rapid depolymerization rate of fluorosiUcone. The expected thermodynamic end point of such a process is the conversion of cyclotrisdoxane to polymer and then rapid reversion of the polymer to cyclotetrasdoxane [429-67 ]. Careful control of the monomer purity, reaction time, reaction temperature, and method for quenching the base catalyst are essential for rehable gum production. 

In addition to these mechanical problems there are two aspects of the compression process which relate specifically to ethylene. Eirst, there is a tendency for small amounts of low molecular weight polymer to be formed and, second, the gas may decompose into carbon, hydrogen, and methane if it becomes overheated during compression. Cavities in which the gas can collect and form polymer, which hardens with time or in which the gas can become hot, need to be avoided. 

Plastics and Resins. Plastics and resin materials are high molecular weight polymers which at some stage in their manufacture can be shaped or otherwise processed by appHcation of heat and pressure. Some 40—50 basic types of plastics and resins are available commercially, but HteraHy thousands of different mixtures (compounds) are made by the addition of plasticizers, fillers, extenders, stabilizers, coloring agents, etc. 

Nylon-4,6. This nylon is produced from diaminobutane and adipic acid. The process is similar to that for nylon-6,6, but the amine has a high tendency to cyclize and the temperatures are therefore kept low. This results ia a low molecular weight polymer, which is subsequently iacreased ia viscosity by sohd-state polymerisation. 

Polyetherification is similar to a polycondensation process formation of high molecular weight polymer requires precise adjustment of composition to approximately 1 1 ratio of bisphenol to dihalosulfone. Trace amounts of water gready reduce the molecular weight attainable owing to side reactions that unbalance the stoichiometry (76). The reactivity of the halosulfone is in the order expected for two-step nucleophilic aromatic displacement reactions ... 

Miscible blends of high molecular weight polymers often exhibit LOST behavior (3) blends that are miscible only because of relatively low molecular weights may show UCST behavior (11). The cloud-point temperatures associated with Hquid—Hquid phase separation can often be adequately determined by simple visual observations (39) nevertheless, instmmented light transmission or scattering measurements frequendy are used (49). The cloud point observed maybe a sensitive function of the rate of temperature change used, owing to the kinetics of the phase-separation process (39). 

Bulk Polymerizations. In the bulk polymerization of vinyl acetate the viscosity increases significantly as the polymer forms making it difficult to remove heat from the process. Low molecular weight polymers have been made in this fashion. Continuous processes are known to be used for bulk polymerizations (68). 

The olefins that undergo metathesis include most simple and substituted olefins cycHc olefins give linear high molecular-weight polymers. The mechanism of the reaction is beheved to involve formation of carbene complexes that react via cycHc intermediates, ie, metaHacycles. Industrial olefin metathesis processes are carried out with soHd catalysts (30). 

---