Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.2 Solvent effects on free radical polymerization 783
k ij
RMi ⋅+ Mj ⎯⎯→RMiMj ⋅or RMiMj ⋅ S where:, i j = 1or 2
k cij
RMi ⋅ S + Mj ⎯⎯→RMiMj ⋅orRMiMj ⋅ S where:, i j = 1or 2
There are also two equilibrium reactions for the formation of the complex:
K i
RMi ⋅+ S ←⎯→RMi ⋅ S where:, i j = 1or 2
Applying the quasi-steady-state and long-chain assumptions to the above reactions,
Kamachi derived expressions for r i and k ii, which are used in place of r i and k ii in the terminal
model equations for composition and k p:
k = k
ii ii
[ ]
[ ]
1+
sciKi Ci
1+
K C
i i
and r = r
i i
1+
sciKi[ Ci]
1 + ( r / r ) s K [ C ]
i ci ci
where: ri =kii/kij; ric =kcii/kcij; sci =kcii/kii; i,j=1or2andi≠j
Variants of this model may be derived by assuming an alternative basis model (such as
the implicit or explicit penultimate models) or by making further assumptions as to nature
of the complexation reaction or the behavior of the complexed radical. For instance, in the
special case that the complexed radicals do not propagate (that is, sci = 0 for all i), the reactivity
ratios are not affected (that is, ri =rifor all i) and the complex formation serves only removal
of radicals (and monomer, if monomer is the complexing agent) from the reaction,
resulting in a solvent effect that is analogous to a Bootstrap effect (see Section 13.2.3.4).
13.2.3.2.3 Experimental evidence
There is certainly strong experimental evidence for the existence of radical-solvent complexes.
For instance, Russell 45-47 and co-workers collected experimental evidence for radical-complex
formation in studies of the photochlorination of 2,3-dimethylbutane in various
solvents. In this work, different products were obtained in aliphatic and aromatic solvents,
and this was attributed to formation of a π-complex between the Cl atom and the aromatic
solvent. Complex formation was confirmed by flash photolysis. 48-51 Complex formation
was also proposed to explain experimental results for the addition of trichloromethane radical
to 3-phenylpropene and to 4-phenyl-1-butene 52 and for hydrogen abstraction of the
t-butoxy radical from 2,3-dimethylbutane. 53 Furthermore, complexes between nitroxide
radicals and a large number of aromatic solvents have been detected. 54-57 Evidence for complexes
between polymer radicals and solvent molecules was collected by Hatada et al., 58 in
an analysis of initiator fragments from the polymerization of MMA-d with AIBN and BPO
initiators. They discovered that the ratio of disproportionation to combination depended on
the solvent, and interpreted this as evidence for the formation of a polymer radical-solvent
complex that suppresses the disproportionation reaction.
There is also experimental evidence for the influence of radical-solvent complexes in
small radical addition reactions. For instance, Busfield and co-workers 59-61 used radical-solvent
to explain solvent effects in reactions involving small radicals, such as t-butoxyl radicals
towards various electron donor-electron acceptor monomer pairs. The observed solvent
effects were interpreted in terms of complex formation between the t-butoxyl radical and
the electron-acceptor monomer, possibly via a sharing of the lone pair on the t-butoxyl oxy-
i i