Poly(
N
-isopropylacrylamide) microgels have found various uses
in fundamental polymer and colloid science as well as in different
applications. They are conveniently prepared by precipitation polymerization.
In this reaction, radical polymerization and colloidal stabilization
interact with each other to produce well-defined thermosensitive particles
of narrow size distribution. However, the underlying mechanism of
precipitation polymerization has not been fully understood. In particular,
the crucial early stages of microgel formation have been poorly investigated
so far. In this contribution, we have used small-angle neutron scattering
in conjunction with a stopped-flow device to monitor the particle
growth during precipitation polymerization in situ. The average particle
volume growth is found to follow pseudo-first order kinetics, indicating
that the polymerization rate is determined by the availability of
the unreacted monomer, as the initiator concentration does not change
considerably during the reaction. This is confirmed by calorimetric
investigation of the polymerization process. Peroxide initiator-induced
self-crosslinking of
N
-isopropylacrylamide and the
use of the bifunctional crosslinker
N
,
N
′-methylenebisacrylamide are shown to decrease the particle
number density in the batch. The results of the in situ small-angle
neutron scattering measurements indicate that the particles form at
an early stage in the reaction and their number density remains approximately
the same thereafter. The overall reaction rate is found to be sensitive
to monomer and initiator concentration in accordance with a radical
solution polymerization mechanism, supporting the results from our
earlier studies.