Experimental evidence is presented
showing that the direct formation
of anisotropic colloidal polymer particles via aqueous heterophase
polymerization is essentially controlled by the entropy gain of the
linear fraction in the semi-interpenetrating network of the seed particles
during swelling. Anisotropic particles are produced via photoinitiated
polymerization, allowing swelling and polymerization to take place
at the same temperature. These experiments prove that the temperature
effect on rubber elasticity is, if at all, only of minor importance
for the formation of anisotropic polymer particles. The major significance
of swelling of the seed particles for the whole process is underlined
by additional studies with optical microscopy and model simulations.