The invention provides a process to form a “first composition comprising a first ethylene/α-olefin interpolymer and a second ethylene/α-olefin interpolymer,” said process comprising polymerizing a first mixture comprising ethylene, an α-olefin, and optionally a polyene, in a stirred tank reactor to form a first ethylene/α-olefin interpolymer, and transferring at least some of the first ethylene/α-olefin interpolymer to a loop reactor, and polymerizing, therein, a second mixture comprising ethylene, an alpha-olefin, and optionally a polyene, in the presence of the first ethylene/α-olefin interpolymer, to form the “first composition comprising the first ethylene/α-olefin interpolymer and the second ethylene/α-olefin interpolymer.” The invention also provides a polymerization reactor configuration comprising at least the following: a stirred tank reactor followed by a loop reactor.