Copolymerization chainwise

Initiation involves the reaction of the tertiary amine (the most widely used Lewis base) with an epoxy group, giving rise to a zwitterion that contains a quaternary nitrogen atom and an alkoxide anion. The alkoxide reacts at a very fast rate with an anhydride group, leading to a species containing a carboxylate anion as the active center. This ammonium salt can be considered as the initiator of the chainwise copolymerization. 

We will first analyze the case of the pure A2 homopolymerization that leads to a linear polymer e.g., a vinyl polymerization. This will enable the reader to get acquainted with the usual parameters that are necessary to describe a chainwise polymerization. Then, we will consider the general Af + A2 copolymerization, leading to the formation of a polymer network. 

An example of this case is a vinyl (A2 ) - divinyl (A4) polymerization. The assumption of an ideal polymerization means that we consider equal initial reactivities, absence of substitution effects, no intramolecular cycles in finite species, and no phase separation in polymer- and monomer-rich phases. These restrictions are so strong that it is almost impossible to give an actual example of a system exhibiting an ideal behavior. An A2 + A4 copolymerization with a very low concentration of A4 may exhibit a behavior that is close to the ideal one. But, in any case, the example developed in this section will show some of the characteristic features of network formation by a chainwise polymerization. 

Figure 3.22 Fraction of tetrafunctional crosslinks formed in the course of an A2 + A4 chainwise copolymerization (af = 0.01, q = 0.999), with two different termination mechanisms (E, = 0 represents the case of a termination by chain transfer or disproportionation, while , = 1 represents the case of a termination by combination).

For chainwise polymerizations, the analysis of model systems implies consideration of the homopolymerization or copolymerization of bifunctional monomers. Kinetic results cannot be directly extrapolated to the case of networks, because very important features such as intramolecular cycliza-tion reactions are not present in the case of linear polymers. However, the nature of initiation and termination reactions may be assessed. For example, using electron spin resonance (ESR), Brown and Sandreczki (1990) identified different types of radicals produced during the homopolymerization of a monomaleimide (a model compound of bismaleimides). 

Another usual family of hardeners employed to cure epoxy monomers are cyclic anhydrides, with the reaction initiated by tertiary amines or ammonium salts. The reaction proceeds through an alternating chainwise copolymerization, as shown in Figure 28.3. 

Figure 28.3 Chainwise copolymerization of epoxy monomers with cyclic anhydrides.

Figure 3.2 Network growth during the chainwise copolymerization of a vinyl...

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