Copolymerization with phase separation, model

A detailed description of AA, BB, CC step-growth copolymerization with phase separation is an involved task. Generally, the system we are attempting to model is a polymerization which proceeds homogeneously until some critical point when phase separation occurs into what we will call hard and soft domains. Each chemical species present is assumed to distribute itself between the two phases at the instant of phase separation as dictated by equilibrium thermodynamics. The polymerization proceeds now in the separate domains, perhaps at differen-rates. The monomers continue to distribute themselves between the phases, according to thermodynamic dictates, insofar as the time scales of diffusion and reaction will allow. Newly-formed polymer goes to one or the other phase, also dictated by the thermodynamic preference of its built-in chain micro — architecture. 

The formation mechanism of structure of the crosslinked copolymer in the presence of solvents described on the basis of the Flory-Huggins theory of polymer solutions has been considered by Dusek [1,2]. In accordance with the proposed thermodynamic model [3], the main factors affecting phase separation in the course of heterophase crosslinking polymerization are the thermodynamic quality of the solvent determined by Huggins constant x for the polymer-solvent system and the quantity of the crosslinking agent introduced (polyvinyl comonomers). The theory makes it possible to determine the critical degree of copolymerization at which phase separation takes place. The study of this phenomenon is complex also because the comonomers act as diluents. 

In the model system, mercaptoethanol is required to prevent production of very high MW PLMA—a MW higher than can be reasonably assumed to occur during copolymerization with AAm. If the model of micellar polymerization of LMA into AAm discussed in the beginning of this chapter is correct, this need for a CTA can be explained as follows. From the point of view of LMA polymerization, AAm acts to end the propagation of the PLMA radical in one micelle and, after radical propagation in the aqueous phase (during which time AAm is added) to initiate polymerization of the LMA in another micelle. If it is desired to keep the LMA from different micelles as separate polymer molecules, then a CTA is necessary terminate the PLMA radical in each micelle. Without such termination the PLMA radical appears to transfer between micelles. 

Crosslinking polymerization of comonomers in a diluent diiiers from the above model in that the monomers themselves represent a good solvent for the emerging polymer, but their concentration, as well as that of the polymer and of the crosslinks, steadily changes with monomer conversion and remains unknown. These factors dramatically limit the practical usefulness of Eq. [3.6]. Still, it is worthwhile to analyze which of these four parameters are really critical for the phase separation to occur during cross-linking copolymerization. 

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