Prepolymerization Equilibria

In all the cases considered above, it is assumed that the growing chain end (free radical or ion) reacts directly with the monomer. Instead, however, the monomer can also form an intermediate product in an equilibrium reaction with the ion pair, and this intermediate can then convert into the true addition product in a rate-determining step  

If the two types of reactive chain ends are now capable of forming such an intermediate compound with the two monomers, then the changes in monomer concentrations are given by 

If the two equations are combined under the assumption that 1 12 = then this again gives the copolymerization equation (22-14). The reactivity ratios r = k i/k 2 and r = i2/k i that occur in this derivation, however, in contrast to those that are otherwise applicable, do not relate to the reactivities of the growing chain end. 

on the other hand, only one of the growing chain ends (for example, that of monomer Mj) can form intermediate products with both monomers, then the corresponding equations for the consumption of monomers are 

In this equation, the reactivity ratio t2 contains the reactivity of the m ions, while r does not bear any relation to the reactivity of the m ions. 

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Graft Copolymers. In graft copolymerization, a preformed polymer with residual double bonds or active hydrogens is either dispersed or dissolved in the monomer in the absence or presence of a solvent. On this backbone, the monomer is grafted in free-radical reaction. Impact polystyrene is made commercially in three steps first, solid polybutadiene rubber is cut and dispersed as small particles in styrene monomer. Secondly, bulk prepolymerization and thirdly, completion of the polymerization in either bulk or aqueous suspension is made. During the prepolymerization step, styrene starts to polymerize by itself forming droplets of polystyrene with phase separation. When equal phase volumes are attained, phase inversion occurs. The droplets of polystyrene become the continuous phase in which the rubber particles are dispersed. R. L. Kruse has determined the solubility parameter for the phase equilibrium. 

Continuous prepolymerization has also been described in a number of patents, for example, in stirred tanks in series or in evaporator-type equipment [68, 76, 77]. Usually patents describe prepolymers with a DP of 7-20 as feed to the lactide synthesis. Using modern HPLC methods, it has been shown that in oligomeric systems up to DP 10, an equilibrium is present with constant equilibrium constants between the oligomers [6, 72]. 

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