The issue arises from the radical abstraction from HCl, which is endothermic by about 5kcal/mol (energy of breaking the H-Cl bond). so the chain transfer wouldn't be as favorable, the alkyl radical would more preferentially combine with another alkyl radical than abstract a radical H from HCl. this recombination would lead to oligomers. Refer to Ansyl & Dougherty page 571 for more.
Hope this helps