Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.2 Solvent effects on free radical polymerization 791
complex, to primary cyclohexyl radicals were found. Thus they concluded that the
STY/MAH complex in the monomer solution is disrupted during the propagation step. It is
likely that both the MCP and MCD mechanisms are valid and their validity in a specific system
will depend on the relative strength of the EDA complex concerned. The MCD model
may be useful for accounting for those systems, in which EDA complexes are known to be
present but the MCP model has been shown not to hold.
13.2.3.3.4 Specific solvent effects
Several monomers are particularly susceptible to strong solvent effects via specific interactions
such as hydrogen bonding, ionic strength and pH. The kinetic consequences of these
specific interactions will vary from system to system. In some cases the radical and/or
monomer reactivity will be altered and in other cases a Bootstrap effect will be evident. It is
worth noting that monomers which are susceptible to strong medium effects will not have
reliable Q-e values, a good example of this is 2-hydroxyethyl methacrylate (HEMA) where
there is a large variation in reported values. The reactivity ratios of HEMA with STY have
been reported to be strongly dependent on the medium, 113 similarly the copolymerization of
HEMA with lauryl methacrylate is solvent sensitive; 114 behavior which has been attributed
to non-ideal solution thermodynamics (cf Semchikov’s work in Section 13.2.3.4).
Chapiro 115 has published extensively on the formation of molecular associates in
copolymerization involving polar monomers. Other common monomers which show strong
solvent effects are N-vinyl-2-pyrrolidone, (meth)acrylic acids and vinyl pyridines.
13.2.3.4 Bootstrap model
13.2.3.4.1 Basic mechanism
In the Bootstrap model, solvent effects on kp are attributed to solvent partitioning and the resulting
difference between bulk and local monomer concentrations. In this way, a solvent
could affect the measured kp without changing the reactivity of the propagation step. Bootstrap
effects may arise from a number of different causes. As noted previously, when radical-solvent
and monomer-solvent complexes form and the complexes do not propagate, the
effect of complexation is to alter the effective radical or monomer concentrations, thereby
causing a Bootstrap effect. Alternatively, a Bootstrap effect may arise from some bulk preferential
sorption of one of the comonomers around the growing (and dead) polymer chains.
This might be expected to occur if one of monomers is a poor solvent for its resulting polymer.
A Bootstrap effect may also arise from a more localized from of preferential sorption,
in which one of the monomers preferentially solvates the active chain end, rather than the
entire polymer chain. In all cases, the result is the same: the effective free monomer and/or
radical concentrations differ from those calculated from the monomer feed ratios, leading to
a discrepancy between the predicted and actual propagation rates.
13.2.3.4.2 Copolymerization model
Copolymerization models based upon a Bootstrap effect were first proposed by Harwood 116
and Semchikov 117 (see references cited therein). Harwood suggested that the terminal
model could be extended by the incorporation of an additional equilibrium constant relating
the effective and ‘bulk’ monomer feed ratios. Different versions of this so-called Bootstrap
model may be derived depending upon the baseline model assumed (such as the terminal
model or the implicit or explicit penultimate models) and the form of equilibrium expression
used to represent the Bootstrap effect. In the simplest case, it is assumed that the magni-