Commercialization, overcoming

The details of the commercial preparation of acetal homo- and copolymers are discussed later. One aspect of the polymerisation so pervades the chemistry of the resulting polymers that familiarity with it is a prerequisite for understanding the chemistry of the polymers, the often subde differences between homo- and copolymers, and the difficulties which had to be overcome to make the polymers commercially useful. The ionic polymerisations of formaldehyde and trioxane are equiUbrium reactions. Unless suitable measures are taken, polymer will begin to revert to monomeric formaldehyde at processing temperatures by depolymerisation (called unsipping) which begins at chain ends. 

Microemulsion Polymerization. Polyacrylamide microemulsions are low viscosity, non settling, clear, thermodynamically stable water-in-od emulsions with particle sizes less than about 100 nm (98—100). They were developed to try to overcome the inherent settling problems of the larger particle size, conventional inverse emulsion polyacrylamides. To achieve the smaller microemulsion particle size, increased surfactant levels are required, making this system more expensive than inverse emulsions. Acrylamide microemulsions form spontaneously when the correct combinations and types of oils, surfactants, and aqueous monomer solutions are combined. Consequendy, no homogenization is required. Polymerization of acrylamide microemulsions is conducted similarly to conventional acrylamide inverse emulsions. To date, polyacrylamide microemulsions have not been commercialized, although work has continued in an effort to exploit the unique features of this technology (100). 

Various ways of overcoming the PTA oxidation problem have been incorporated into commercial processes. The predominant solution is the use of high concentrations of manganese and cobalt ions (2,248—254), optionally with various cocatalysts (204,255,256), in the presence of an organic or inorganic bromide promoter in acetic acid solvent. Operational temperatures are rather high (ca 200°C). A lesser but significant alternative involves isolation of intermediate PTA, conversion to methyl/)-toluate, and recycle to the reactor. The ester is oxidized to monomethyl terephthalate, which is subsequentiy converted to DMT and purified by distillation (248,257—264). 

Methanol Synthesis. AH commercial methanol processes employ a synthesis loop, and Figure 6 shows a typical example as part of the overall process flow sheet. This configuration overcomes equiUbtium conversion limitations at typical catalyst operating conditions as shown in Figure 1. A recycle system that gives high overall conversions is feasible because product methanol and water can be removed from the loop by condensation. 

Supported aqueous phase (SAP) catalysts (16) employ an aqueous film of TPPTS or similar ligand, deposited on a soHd support, eg, controlled pore glass. Whereas these supported catalysts overcome some of the principal limitations experienced using heterogeneous catalysts, including rhodium leaching and rapid catalyst deactivation, SAP catalysts have not found commercial appHcation as of this writing. 

Methoxy-based RTV sihcones were commercialized in the late 1960s to overcome the deficiencies of their acetoxy predecessors (424). As described in Figure 8, the general chemistry of methoxy-based RTV sihcones is similar to their acetoxy analogues. 

Polymers account for about 3—4% of the total butylene consumption and about 30% of nonfuels use. Homopolymerization of butylene isomers is relatively unimportant commercially. Only stereoregular poly(l-butene) [9003-29-6] and a small volume of polyisobutylene [25038-49-7] are produced in this manner. High molecular weight polyisobutylenes have found limited use because they cannot be vulcanized. To overcome this deficiency a butyl mbber copolymer of isobutylene with isoprene has been developed. Low molecular weight viscous Hquid polymers of isobutylene are not manufactured because of the high price of purified isobutylene. Copolymerization from relatively inexpensive refinery butane—butylene fractions containing all the butylene isomers yields a range of viscous polymers that satisfy most commercial needs (see Olefin polymers Elastomers, synthetic-butylrubber). 

The earliest study describing vulcanised polymers of esters of acryUc acid was carried out in Germany by Rohm (2) before World War I. The first commercial acryUc elastomers were produced in the United States in the 1940s (3—5). They were homopolymers and copolymers of ethyl acrylate and other alkyl acrylates, with a preference for poly(ethyl acrylate) [9003-32-17, due to its superior balance of properties. The main drawback of these products was the vulcanisation. The fully saturated chemical stmcture of the polymeric backbone in fact is inactive toward the classical accelerators and curing systems. As a consequence they requited the use of aggressive and not versatile compounds such as strong bases, eg, sodium metasiUcate pentahydrate. To overcome this limitation, monomers containing a reactive moiety were incorporated in the polymer backbone by copolymerisation with the usual alkyl acrylates. 

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