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A novel boron‐containing novolac (triphenyl borate‐formaldehyde resin, TPBF) was synthesized. The structure, thermoplasticity, molecular weight, and molecular weight distribution of TPBF have been characterized with FT‐IR, melt viscosity, 13C NMR and GPC. The thermal stability of TPBF was investigated by TGA, indicating the thermal stability of TPBF was much better than that of normal novolac (phenol‐formaldehyde resin, PF). TPBFs with different molar ratios of formaldehyde to benzene ring in triphenyl borate (TPB) were also synthesized and compared for molecular size, polydispersity and thermal stability. Further, the thermal degradation kinetics of TPBFs and PF were studied by TGA using Madhusdanan‐Krishnan‐Ninan method and the activation energies were calculated at different degradation stages. It was found that the thermal degradations of TPBFs and PF are a multistage reaction and the degradation reactions in every stage follow the first order reaction mechanism. Finally, the activation energies of thermal degradations of novolac increase with the introduction of boron, the progress of degradation reaction as well as the increase of molar ratios. [ABSTRACT FROM AUTHOR]