(73c) Automatic, Simultaneous Control of Polymer Composition and Molecular Weight during Copolymer Synthesis

Fully automatic, simultaneous control of polymer composition and molecular weight trajectories during copolymer synthesis was achieved by coupling the continuous data stream from ACOMP into a feedback based controller which operated pumps feeding comonomers into the reactor. The controller operates without recourse to a detailed kinetic model or reactivity ratios. Rather, it makes use of the fact that the instantaneous composition of copolymer chains is simply related to the instantaneous rate of consumption of each comonomer, and the rate for each of the latter is described by a proportionality constant times the comonomer concentration, a₁ and a_­2 for comonomers 1 and 2, respectively. The instantaneous weight average molecular weight M_w,inst is related to the sum of the comonomer concentrations via a proportionality constant p. Hence, target trajectories for the instantaneous fraction of comonomer 1 in copolymer form, F_1,inst,t (and then F_2,inst,t=1-F_1,inst,t) and M_w,inst,t can be independently prescribed and followed. Since a₁, a_­2 and p are directly measurable from the ACOMP data stream, their values can be constantly updated by the automatic controller during the reaction process, in order to insure the target trajectories F_inst,t and M_w,inst,t are followed, even when reaction conditions such as radical concentrations, temperature, and initiator are varying in unknown ways. The controller is demonstrated using free radical copolymerization of acrylamide and styrene sulfonate. The combination of UV-Vis and NMR spectroscopy allows for monomer concentration measurements allows the controller to be applicable to a variety of comonomer and termonomer free radical polymerizations. The composition portion of the controller can also be used for ‘living’ type gradient copolymerization, such as can be carried out by RAFT, NMP, ATRP, and other mechanisms.