Handbook of Solvents - George Wypych - ChemTech - Ventech!

790 Michelle L. Coote and Thomas P. Davis
study, Golubev et al., 108 used ESR to show that for dimethylbutadiene/MAH the cross-propagation
of the free monomers dominated. However, Barton et al. 109 has questioned the assignments
of ESR signals in the previous studies and suggested that the ESR evidence was
inconclusive. Furthermore, the predominance of one type of ESR signal may also be explained
without invoking the MCP model. Assuming that cross-propagation is the dominant
reaction, and that one of the radicals is much less stable than the other, it might reasonably
be expected that the less stable radical would undergo fast cross-propagation into the more
stable radical, resulting in an ESR signal dominated by the more stable radical. Hence it appears
that ESR is not able to discriminate between this and the MCP mechanism.
13.2.3.3.3 Monomer-monomer complex dissociation model
Tsuchida and Tomono 84 suggested that the monomer-monomer complex described in the
MCP model may dissociate upon addition to the chain, with only one unit adding. The concept
was developed by Karad and Schneider 110 who argued that the dissociation of the complex
is likely since its heat of formation is typically less than the heat of propagation. As an
example, they measured the heat of formation for a STY/fumaronitrile complex, and found
that it was only 1.6 kcal/mol, significantly less than the heat of propagation (15-20
kcal/mol). Under a complex-dissociation mechanism, the role of the complex is merely to
modify the reactivity of the reactant monomers.
A model based on the complex-dissociation mechanism was first formulated by Karad
and Schneider 110 and generalized by Hill et al. 111 Again, eight rate constants and two equilibrium
constants are required to describe the system.
k ij
RMi ⋅+ Mj ⎯⎯→RMiMj ⋅ where:, i j = 1or 2
k ijc
RMi ⋅+ MjC ⎯⎯→RMiMj ⋅ where:, i j = 1or 2
K i Mi + C ←⎯ →MiC
where:, i j = 1or 2
As for the previous models, expressions for k p and composition can be derived in
terms of these parameters by first calculating k ii and r i and then using them in place of k ii and
r i in the terminal model equations. The relevant formulae are:
k = k
ii ii
[ ]
[ ]
1+
sicKi Ci
1+
K C
i i
and r = r
i i
1+
sicKi[ Ci]
1 + ( r / r ) s K [ C ]
i ci ic
where: r i =k ii/k ij; r ic =k iic/k ijc; s ic =k iic/k ii; i,j=1or2andi≠j
Efforts to compare this model with the MCP model have been hindered by the fact that
similar composition curves for a given system are predicted by both models. Hill et al. 111
showed that the composition data of Dodgson and Ebdon 87 for STY/MAH at 60°C could be
equally well described by the MCP, MCD or penultimate unit models. They suggested that
sequence distribution would be a more sensitive tool for discriminating between these models.
One study which lends some support to this model over the MCP model for describing
this system was published by Rätzsch and Steinert. 105 Using Giese’s 112 ‘mercury method’ to
study the addition of monomers to primary radicals, they found that in mixtures of MAH
and STY, only reaction products from the addition of free monomers, and not the EDA
i i