Kinetic parameters of the polymerization
process are important from both practical and theoretical
point of view. The knowledge of the reaction mechanisms let
us improve the conditions of a technical process in order
to obtain the final products with the best possible properties.
Also, the studies on the basic steps involved in the polymerization
are important in recent investigations concerning the kinetics
of diffusion-controlled reactions. Pulse radiolysis with optical
and light scattering detection can be used as a convenient
tool in this kind of research.
The water soluble monomers studied in our laboratory were
vinylpyrrolidone [20,21,52], cationic 2-[(methacryloyloxy)ethyl]trimethylammonium
chloride (MADQUAT) [60], and anionic sodium 2-(acrylamido)-2-methylpropanesulfonate
(AMPSNa) (Kujawa and Rosiak, unpublished results). These monomers
are widely used in preparation of ionic hydrogels., which
may be useful especially as the media with various adhesion
efficiency for cell growth and proliferation. Some experiments
have also been done with butyl acrylate, hydrophobic monomer
used as a sensitizer for the vulcanisation of natural rubber
latex with g-irradiation [61].
Similar studies concerning polymerization of the monomers
used in the production of biomaterials were also carried on
by Takacs et al. [62,63].
The mechanism of the radiation-induced changes was studied
by optical detection method and was similar in all cases.
It consists of several stages:
1. Addition of hydroxyl radicals (and hydrogen atoms) to carbon-carbon
double bond of monomer with subsequent formation of monomeric
radicals. The rate constant of this process is diffusion controlled.
2. Addition of hydrated electrons to carbonyl groups and formation
of radical anion. The rate constant of this reaction is also
very high (~ 1010 dm3mol-1s-1).
3. The decay of radicals with parallel addition of monomer
molecules to the growing chain. In case of hydrated electron
adducts the decay is complex and depends on the acidity of
reaction medium. At low pH radical anion undergoes fast, reversible
protonation at the carbonyl oxygen. In basic solution, the
reaction takes place at the b-carbon
atom with subsequent formation of a-centered
carbon radical. The a-centered
carbon radicals of similar structure are formed upon the addition
of hydroxyl radicals. Their decay depends on monomer concentration
and dose per pulse value (Fig. 4).
Fig. 4. Decay profiles of the radical
absorption in pulsed vinylpyrrolidone solution at two different
concentrations of monomer (0.94 and 0.094 M). Dose per pulse
ca. 500 Gy, pulse length – 1 ms,
solution saturated with nitrous oxide. Lines are calculated
based on the second-order decay equation with single value
of the rate constant, calculated during the initial stage
of the decay. Deviation from classical behavior is easily
observed, especially at high monomer concentration.
The kinetics of the radical decay reflects the termination
process, taking place in the reaction medium. It cannot be
described by single value of the second-order rate constant.
In the course of the reaction radicals grow, as the effect
of propagation reaction. These growing macroradicals decay
with lower rate constant, because the termination is diffusion
controlled. Thus, the kinetic constant of the decay decreases
with the reaction progress. The slowing down of the rate constant
is more apparent when concentrated solutions are pulsed and/or
the dose per pulse is low. This effect can be also easily
explained if we assume that the radical decay is diffusion
controlled. Thus, at low dose/pulse values and high monomer
concentration, chain growth events are favored, which leads
to the formation of bigger macroradicals that decay slower.
The light scattering (LSI) detection should give the direct
information on the macroradicals growing in the solution.
This process can proceed in two ways: propagation of the chain
(by the addition of monomer molecules) or termination by recombination
(biradical reaction). These two processes occur simultaneously
in the solution, participating in the observed LSI signal
increase. The other possibilities of LSI signal changes are
diffusion of two approaching radicals and conformation rearrangement
of newly formed macromolecules. While the termination reaction
may be described by second-order kinetics, both propagation
and diffusion processes should be the first-order. Although
kinetic treatment of the recorded curves needs further refinement,
a few general trends can be observed. With the increasing
dose per pulse, the decrease in the limiting value of LSI
signal was observed (Fig. 5). Also, the amplitude of LSI changes
decreases, when the monomer concentration increases. This
can be attributed to the smaller size of the formed oligomer
or polymer molecules, in accordance with the data obtained
by spectroscopic detection. Further experiments connected
with this topic are carried out in our laboratory. It is worth
mentioning that, to the best of our knowledge, this is the
first application of LSI detection to the direct observation
of the macromolecular growth in the real time.
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