The continuous record of monomer and polymer concentrations, C-m and C-p, and cumulative weight-average mass, M-w, furnished by automatic continuous online monitoring of polymerization reactions (ACOMP) has been harnessed to provide feedback to control reactor monomer flow in order to follow a target trajectory M-w,M-t(t) during linear chain growth free radical polymerization. This was achieved without a detailed kinetic model. Two proportionality parameters to pilot the controller, alpha and p, result from (i) reaction rate = alpha C-m and (ii) M-w,M-inst = pC(m), where M-w,M-inst is instantaneous M-w. Using Ansatz values for alpha and p, the controller periodically recomputes these, based on the ACOMP data stream, in order to follow M-w,M-t(t). A histogram of concentration vs Mw,inst estimates the molecular weight distribution width. Invoking an instantaneous distribution provides polydispersities. Results are compared to GPC analysis on end products. The concept of isomorphic reaction pair is introduced: two reactions that follow the same trajectory under different reaction variables, e.g., varying T at constant [initiator] and varying [initiator] at T = constant. The controller can be used, as is, for high solids reactions, and extended to copolymerization, including for possible control of composition gradients in controlled radical polymerization.