Free-radical polymerizations using mixed initiator systems at two thermally distinct polymerization stages

A process for the free-radical polymerization of vinyl monomers, including vinyl aromatic monomers, comprises polymerizing said vinyl monomers in at least two thermally distinct polymerization stages, each between the temperatures of about 70.degree. C. and 150.degree. C. in the presence of at least two free-radical initiators, one of which initiators is particularly effective at the lowest temperature stage and has the formula ##STR3## and where R is an alkylene, alkynylene or alkenylene diradical, R.sub.1 is alkyl, alkenyl, alkynl or alkoxy and R.sub.2 is lower alkyl and R.sub.3 is hydrogen or lower alkyl and a second of which initiators have a 10 hour half-life temperature of at least about 80.degree. C., said temperature being at least about 15.degree. C. above the 10 hour half-life temperature of the compound of Formula (I) wherein the same solvent is used for measuring the half-life of all initiators, wherein the initiator of Formula (I) allows completing the polymerization in a shorter time with the resulting polymer having a higher viscosity-average molecular weight than without the initiator of Formula (I).