THERMOCHEMICAL ASPECTS OF POLYMERIZATION . EQUILIBRIUM COPOLYMERIZATION

An expression is derived for the free energy of formation of an infinite molecular weight binary copolymer from the corresponding homopolymers where only nearest neighbor intrachain interactions are significant. Thermodynamic properties and the sequence length distribution of an equilibrium copolymer are then calculated. When there is no excess energy of interaction between unlike monomer units (∆εab) the randomness and the entropy of sequence length distribution (aSd) are independent of temperature and, for a given composition, maximized. If ∆εAB is negative and the copolymer contains unequal numbers of the two monomers there is a residual ∆Sd at 0°K. Positive values of ∆εab in this one-dimensional model never lead to the simultaneous generation of two independent sequence length distributions. The average sequence length of a unit increases as its mole fraction increases and as ∆εab becomes more positive; it tends toward randomness as the temperature increases. Two parameters, B and Q, which characterize copolymer randomness are compared. Both depend on ∆εAB and the temperature; B also depends on composition. Q is analogous to the reactivity ratio product of kinetically controlled copolymerizations and is the reciprocal of the equilibrium constant for equilibrium copolymerization. © 1969, American Chemical Society. All rights reserved.