Provided is a process for forming a polymer composition, wherein the process comprises (a) providing a slurry in an aqueous medium, wherein the slurry comprises (i) pellets comprising a polyolefin block copolymer, wherein the block copolymer comprises a crystalline block and an amorphous block, (ii) one or more vinyl monomers, (iii) one or more initiator, wherein the slurry comprises, by weight based on the total weight of the slurry, 0 to 0.01% of all surfactants, and 0 to 0.01% of all water-soluble polymers; and (b) polymerizing the one or more vinyl monomers.