A method of preparing a polymer under predetermined temperature conditions comprises conducting the polymerization within a closed reaction chamber configured such that most or all of an inner surface has heat exchange capability. The chamber's dimensions are such that the polymerization mixture contacts the effective heat exchange surface sufficiently to ensure that the temperature throughout the polymerization mixture does not vary more than a few degrees from a desired temperature. In one embodiment the chamber is formed of two adjacent, parallel heat exchange plates, wherein at least one plate has a peripheral lip which, in contact with the opposing plate, forms the closed chamber. Multiple plates can be arrayed to form multiple chambers, and each chamber's polymerization temperature, as well as resident polymerization mixture, can be individually predetermined. This method and apparatus are particularly useful for preparing polymers used in drag reducing agents for hydrocarbon transportation pipeline applications, as well as other polymers benefiting from narrow and/or customized temperature profiles during production.