Homogeneous nucleation and photochemical polymerization in supersaturated isoprene vapor were investigated using a diffusion cloud chamber. The mechanism of photoinitiation appears to involve the photolysis of a volatile product which results from the primary photodecomposition process. The photonucleation results from direct propagation as well as recombination of polymer molecules. At very low supersaturation ratio (e.g., 1.2), recombinative termination becomes the only possible way to achieve the tunable size required for the nucleation. A nonsteady state experiment allows us to resolve the nucleation spectrum into contributions due to polymers resulting from recombination and those due to direct propagation.
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