The postpolymerization of various vinyl monomers was applied for mechanistic studies of the template polymerization of methacrylic acid (MAA) in porous isotactic (it-) poly(methyl methacrylate) (RMMA) thin films on quartz crystal microbalance (QCM) substrates and on silica gels, which were prepared by the alternative layer-by-layer (LbL) assembly of it-PMMA/syndiotactic- (st-) poly(methacrylic acid) (PMAA). In this study, QCM analyses, FT-IR/ATR spectra, H-1 NMR spectra, and SEC were employed to investigate the polymerization mechanism, and they highlighted the following points: (1) the livingness of polymer radicals by postpolymerization on a QCM, (2) substituent effects on the polymerizability in porous it-PMMA thin films, and (3) partial stereocomplex formations of polymerized MMA with host it-PMMAs. Stabilities of porous it-PMMA thin films on QCM substrates and on silica gels are also discussed herein.