Polyethylene terephthalate polymerisation

Fig. 14. Gel permeation chromatogram (g-p.c.) of cyclics (CO C6H4 - CO O CH2 CH2 0)x with x = 3-9 obtained by extracting melt polymerised samples of polyethylene terephthalate) with 1-me-thyl naphthalene...

The melt stability of step-growth polymers is affected by water absorption prior to processing, which can reverse the polymerisation equilibrium reaction in the melt. For example, polyethylene terephthalate (PET) has an... 

The production and applications of polymers have gradually developed, gaining ground in many fields. The main classes of polymers, namely polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene and polyethylene terephthalate are produced in millions of tonnes annually [1]. There are many methods of polymer synthesis free-radical polymerisation (bulk, solution, emulsion and suspension), condensation polymerisation, ethoxylation, polymer compounding and formulations involving solvents, fillers, pigments and so on. Besides the high volume consumption of these common plastics, the demand for polymers with specific end-use properties has increased. 

Polymer preparation is based on chemical reactions (polymerisation) of gas (e.g. ethylene) or liquid (e.g. styrene) substances with low molecular weight. These molecules (monomers) are bi- or poly-functional because they show two or more reactive moieties [Nicholson and Royal Society of Chemistry (Great Britain), 2006]. As an example, the reaction between ethylene glycol and terephthalic acid promotes the formation of polyethylene terephthalate (PET, a polyester), (Fig. 2.5) (Liangbin et al., 2001). 

Amongst the polymers based on crude oil, seven groups of polymers - polyolefins (PE and PP), polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate (PET), emulsion polymerised styrene butadiene rubber (ESBR), polyamides (PA) and unsaturated polyester resins (UP) constitute approximately 80 % of the total consumption of polymers. 

This document focuses on the main products of the European polymer industry both in production figures and in environmental impact, mainly produced in dedicated installations for the production of one specific polymer. The list of products covered is not conclusive but includes polyolefins, polystyrene, polyvinyl chloride, unsatuiated polyesters, emulsion polymerised styrene butadiene rubbers, solution polymerised rubbers containing butadiene, polyamides, polyethylene terephthalate fibres and viscose fibres. 

There are a few examples in the literature of the application of thallium(i) compounds as acid catalysts, thallium(i) acetate and thallium(i) carbonate were used to produce polyethylene terephthalate (PET) by initial transesterification of dimethyl terephthalate with ethylene glycol to form diethylene glycol terephthalate, which then polymerised to give a spinnable material (Scheme 20.21). According to the authors, the use of thallium compounds as catalyst improves the thermal stability and photochemical resistance of the formed polymer. 

In polycondensation maeromoleeules are formed from monomers under the splitting aetion of another substance, usually water. If polymerisation takes plaee at a temperature of over 100 °C, steam is formed. The maeromoleeules generated by polyeondensation ean be made up of one type of monomer, (e.g., PA 6), or 2 different monomers, (e.g., PA 66), depending on the shape of the monomers. Further examples of polycondensates are polyearbonate (PC) and linear polyesters such as polyethylene terephthalate (PET). 

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