Polymer compositions may include a polymer matrix containing a polyolefin, one or more polymer particles dispersed in the polymer matrix, wherein the one or more polymer particles include a polar polymer selectively crosslinked with a crosslinking agent, and wherein the one or more polymer particles has an average particle size of up to 200 μm. Processes of preparing a polymer composition may include mixing a polyolefin, a polar polymer, and a crosslinking agent; and selectively crosslinking the polar polymer with the crosslinking agent in the presence of the polyolefin. Methods may include increasing stress cracking resistance of a polyolefin by mixing a polar polymer with the polyolefin; and selectively crosslinking the polar polymer in the presence of the polyolefin with a crosslinking agent to form crosslinked polar polymer particles dispersed in the polyolefin.