Solid‐state polymerization in poly(butylene terephthalate): The Equilibrium constant

Poly(butylene terephthalate) PBT or Poly‐1, 4‐butanediyl 1, 4‐diearboxylate. polycondenses readily in the presence of small amounts of organo‐titanium compounds to form progressively higher molecular weight polymers, The reaction is reversible in the presence of the condensation byproduct, 1,4‐butanediol. Usually this reaction is carried out in the melt phase, but the viscosity is soon limiting. In the current work the polymerization is carried out in the solid phase just below the melting point of 227°C. The equilibrium constant for this reaction is measured by subjecting the polymer to a nitrogen stream saturated with a controlled quantity of butanediol. The reaction is followed by measuring the change in the number of alcoholic end‐groups by infrared spectroscopy. At about 220°C the equilibrium constant is 0.31 (σ = 0.08).