Microstructure and Reactivity Ratios

In several sections of this volume the microstructure of homo- and copolymers is discussed. The microstructure of the products can be used as a diagnostic tool to get information as to the uniformity of polymers formed, reactivities of comonomers, etc. In the absence of penultimate unit effect and subsequent segmental redistribution the microstructure of the copolymer chain is defined by the reactivity ratios and comonomer feed. Thus, the dyad and triad content may be a test of the applicability of the simple four-parameter scheme. 

The probability of 11 addition (Pn) is proportional to the concentrations of the active center [/wvwvmf], monomer [MJ, and the rate constant klt. Similar relationship is valid for P12. Therefore, their ratio is given by Eq. (15-5)  

Among these sequences there are dyads or triads. Let us analyse the proportion of dyads P(ll), P(12) = P(21) and P(22) in the system. If P(12) is the probability of finding the sequence 12 among any sequences consisting of two units, we have  

The probability of the formation of the dyad P(11) equals the product of probability of the P(l) unit and the conditional probability of addition Pn. Thus 

Since P(12) = P(21), the equality similar to that for the steady-state conditions holds 

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Determination of microstructure and reactivity ratios revealed a strong inherent tendency to form alternating copolymers. 

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