Handbook of Solvents - George Wypych - ChemTech - Ventech!

778 Michelle L. Coote and Thomas P. Davis
initiation reactions with t-butoxy radical where the primary radical rarely initiates a chain
but instead abstracts a hydrogen atom from the solvent medium, which subsequently initiates
the chain. 1 The consequence of this is that the polymer chains contain fragments of solvent.
As the stability of the chains to thermal and photochemical degradation is governed, in
part, by the nature of the chain ends then the solvent moieties within the chain can have a
substantial impact on the material performance of the polymer. The efficiency factor, f, decreases
as the viscosity of the reaction medium increases. 2 This is caused by an increase in
the radical lifetime within the solvent cage, leading to an increased possibility of radical-radical
termination. In this regard the diffusion rates of the small radicals becomes an
important consideration and Terazima and co-workers 3,4 have published results indicating
that many small radicals diffuse slower than expected. They have attributed this to specific
interactions between radical and solvent molecules.
13.2.2.2 Propagation
The ability of solvents to affect the homopropagation rate of many common monomers has
been widely documented. For example, Bamford and Brumby 5 showed that the propagation
rate (kp) of methyl methacrylate (MMA) at 25°C was sensitive to a range of aromatic solvents.
Burnett et al. 6 found that the kp of styrene (STY) was depressed by increasing concentrations
of benzonitrile, bromobenzene, diethyl phthalate, dinonyl phthalate and diethyl
malonate, while in other studies 7,8 they found that the kp for MMA was enhanced by
halobenzenes and naphthalene. More recent work by Zammit et al. 9 has shown that solvents
capable of hydrogen-bonding, such as, benzyl alcohol and N-methyl pyrrolidone have a
small influence on both the activation energy (Ea) and pre-exponential factor (A) in STY
and MMA homopropagation reactions. These are but a few of the many instances of solvent
effects in the homopolymerization reactions of two typical monomers, STY and MMA. For
these monomers, solvent effects are relatively small, and this is indicative of the majority of
homopropagation reactions. However, in some instances much larger effects are observed,
especially in cases where specific interactions such as H-bonding or ionization occur. Examples
of this type include the polymerization of N-vinyl-2-pyrrolidone (where water has
been found to dramatically increase kp) 10 and the polymerization of acrylamide (where pH
plays a strong role). 11 There is only limited data on the Arrhenius parameters for
homopropagation reactions in different solvents and this indicates that both the activation
energy and pre-exponential factor are affected. 9,12 In some cases the solvent effect is not on
the elementary rate constant kp but on the local monomer concentration (sometimes referred
to as the ‘Bootstrap" effect). This effect can originate in the preferential solvation of either
the monomer (which is always present as a solvent) or the added solvent. It has also been
suggested that in some instances the growing polymer coil can ‘shield’ the radical chain-end
resulting in a low monomer concentration. This shielding effect would be expected to be
greatest in poor solvents (hence a tighter coil). 13 For methyl methacrylate and styrene the
largest solvent effects on propagation seem to be in the order of a 40% change in kp. 14,15 In
some solvents there seems to be reasonably strong evidence that the solvent does cause
changes to the geometry of the transition state (e.g., dimethyl formamide and acetonitrile in
styrene polymerization) 14 and in liquid carbon dioxide it appears that the 40% change in kp for methyl methacrylate can be ascribed to the poor solvent medium. 16 Recent work has
found that some fluoro-alcohols 17 can influence the tacticity of free radical polymerization
lending further credence to the concept of solvent-induced changes to the transition state of