Ester-interchange polymerisation

Figure 6.1 Examples of important condensation (a, b, c and d) and non-condensation (e) step-growth polymerisations, (a) Polyesterification, (b) Ester-interchange polymerisation, (c) Polyamidation. (d) Self-condensation of an A-B monomer.

Polymerisation of a diol with a dicarboxybe acid is exemplified by the production of a polyester from ethylene glycol and terephthabc acid either by direct esterification or by a catalysed ester-interchange reaction. The resulting polyester (Terylene) is used for the manufacture of fibres and fabrics, and has high tensile strength and resibency its structure is probably ... 

The carboxyl end is known to catalyse both the polymerisation and hydrolysis reactions. The level of each is process dependant, with the ester interchange process tending to give the lowest carboxyl level in the final product. Some of the possible end groups in PET, along with their mechanisms of formation, are shown below in Figure 11. 

The earliest work on polyester synthesis used no catalyst or a simple acid catalyst such as p-toluenesulfonic acid, but use of weakly basic metallic salt catalysts is now almost universal. Many salts have been claimed to be useful in this context, but the best known examples are alkaline earth and transition metal acetates, tin compounds and titanium alkoxides [21-23]. Care must be exercised in selecting ester-interchange catalysts because some may cause degradation/ discoloration in the polymer during the subsequent polymerisation reaction [24], especially for PET and PEN. To prevent this occurrence, catalysts are often sequestered/complexed at the end of the ester-interchange phase by addition of phosphorus compounds such as phosphites, phosphates or polyphosphoric acid [25]. Titanium and tin compounds operate as catalysts for ester-interchange and polymerisation reactions, and in general do not require such procedures. 

The effect of initiation of epsilon-caprolactone polymerisation with mono- and polyfunctional alcohols was investigated. The resulting linear and star-shaped polymers were characterised by measurement of the molec.wt. and MWD. The polymerisations were characterised by rapid initiation, by invariance of the number of growing chains corresponding to the amount of initiator, and by the dominant role played by ester interchange reactions. 19 refs. 

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