In this work, a proportional/integral (PI) material-balance scheme to control continuous free-radical solution homopolymer (possibly open-loop unstable) reactors with level, temperature, and flow measurements is presented. First, the combination of inventory and constructive control ideas yields (i) a nonlinear feedforward state-feedback static passive controller whose closed-loop dynamics are the limiting behavior attainable by robust feedback control and (ii) the related solvability conditions with physical meaning. Then, such a limiting behavior is recovered via a measurement-driven dynamic control system with (i) linear PI-type decentralized volume and cascade temperature controllers and (ii) material-balance monomer and molecular weight (MW) controllers. The temperature controller requires two approximate static parameters, the monomer controller requires the heat capacity function, the MW controller requires initiation-transfer parameters, and the automatic-to-manual switching of the MW control does not affect the functioning of the controllers that perform the stabilization task. The design methodology has a systematic control construction and conventional-like tuning guidelines coupled with a nonlocal stability assessment. The proposed approach is put into perspective with previous polymer reactor model predictive control and geometric control studies and tested with an industrial-size reactor through numerical simulations.