Summary: A review of the processes underlying the solid state postcondensation of poly(ethylene terephthalate) (PET) is presented. Fundamental aspects of the reactions are treated, and it is shown that the rate of polycondensation in the solid state depends on the relative rates of two types of diffusion. On the one hand, the diffusion of reaction by‐products (physical diffusion) controls the rate of the forward reactions. And on the other hand the diffusion of end‐groups (chemical diffusion) allows the reaction to proceed.The transesterification of BHET to form PET.magnified imageThe transesterification of BHET to form PET.

Summary: A process for the solid state polycondensation of PET is proposed. It is shown that by correctly choosing the prepolymerisation conditions it is possible to crystallise the product and to directly polymerise it in a dispersed phase. This process is significantly faster than the “standard” PET processes, and allows one to obtain high molecular weights directly from a prepolymer without the need to use an intermediate solution polymerisation step.Reactor set‐up for precursor preparation and dispersed phase prepolymerisation.magnified imageReactor set‐up for precursor preparation and dispersed phase prepolymerisation.

Summary: It was demonstrated that it is possible to produce prepolymers with a number‐average degree of polymerisation on the order of 5–40 directly in a liquid‐liquid dispersion in less than three hours. It was also shown that prepolymers made via this route and rapidly crystallised by the addition of a dispersant at ambient temperature are more porous than prepolymers made in an industrial liquid melt process.SEM micrograph of prepolymers pLL‐PTA with \overline {DP} _{\rm n} = 28, dp ∈ 63–125 μm.magnified imageSEM micrograph of prepolymers pLL‐PTA with \overline {DP} _{\rm n} = 28, dp ∈ 63–125 μm.

Summary: A new method of polymerising PET in the solid state is proposed in either a gas phase reactor, or in hydrocarbon dispersion. It is shown that the reaction can be carried out efficiently at temperatures on the order of 200–240 °C directly from a prepolymer without the need for a melt phase step. It is shown that the crystal structure of the prepolymer plays a determining role in the kinetics of the SSP reaction.Schema of the reactor used for gas phase SSP.magnified imageSchema of the reactor used for gas phase SSP.