The copper-based metal-organic framework (MOF) HKUST-1 adsorbs organic molecules into its pores. When loaded with electron-rich oligothiophenes, the resulting system reacts under heat to initiate oxidative polymerization without the use of any other oxidant or catalyst. This reaction is not observed in the non-redox-active MOF MIL-100(Al). We have characterized the composites by optical and nanoscale microscopy, vibrational and UV-vis spectroscopy, X-ray photoelectron spectroscopy, N-2 sorption analysis, and thermogravimetric analysis/residual gas analysis. Unsubstituted oligothiophenes polymerize within MOF pores, while 3,4-ethylenedioxythiophene forms a coating on the MOF surface. MOF composites with conjugated polymer dopants trapped inside their pores undergo profound shifts in the composite electronic structure. Reasoning from time-dependent density functional theory calculations of an HKUST-1 model system bound to monomers, we rationalize the observed reactivity and propose an initiation mechanism based on a ligand-to-metal charge-transfer state.