Stimuli-responsive poly[(N-isopropylacrylamide-co-maleic anhydride)-g-poly(ethylene oxide)]/poly(ethylene imine) macrobranched macrocomplexes were synthesized by (1) the radical copolymerization of N-isopropylacrylamide and maleic anhydride with alpha,alpha'-azobisisobutyronitrile as an initiator in 1,4-dioxane at 65 degreesC under a nitrogen atmosphere, (2) the polyesterification (grafting) of prepared poly(N-isopropylacrylamide-co-maleic anhydride) containing less than 20 mol % anhydride units with alpha-hydroxy-omega-methoxy-poly(ethylene oxide)s having different number-average molecular weights (M-n = 4000, 10,000, or 20,000), and (3) the incorporation of macrobranched copolymers with poly(ethylene imine) (Mu = 60,000). The composition and structure of the synthesized copolymer systems were determined by Fourier transform infrared, H-1 and C-13 NMR spectroscopy, and chemical and elemental analyses. The important properties of the copolymer systems (e.g., the viscosity, thermal and pH sensitivities, and lower critical solution temperature behavior) changed with increases in the molecular weight, composition, and length of the macrobranched hydrophobic domains. These copolymers with reactive anhydride and carboxylic groups were used for the stabilization of penicillin G acylase (PGA). The conjugation of the enzyme with the copolymers significantly increased the thermal stability of PGA (three times at 45 degreesC and two times at 65 degreesC). (C) 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1580-1593, 2003.