Processes and systems for preparing copolymers of ethylene and a polar comonomer include contacting an initial feed comprising ethylene and a polar comonomer with a first polymerization initiator in a continuous stirred tank reactor to form a first polymer, which may then be contacted with second polymerization initiator in a plug flow reactor to form a second polymer. The process yields a second polymer having a lower melt flow index (and higher molecular weight) with high polar comonomer content. The polar comonomer may be, for example, vinyl acetate to prepare a poly(ethylene-vinyl acetate) copolymer having a vinyl acetate content of up to about 33% while maintaining the melt flow rate less than about 10 g/10 minutes without the use of fillers.