This work demonstrates that threshold voltage (V-T) of organic thin-film transistors (OTFTs) can be controlled systematically by introducing new copoly mer dielectrics with electropositive functionality. A series of homogeneous copolymer dielectrics are polymerized from two monomers, 1,3,5-trimethyl- 1,3,5-trivinyl cyclotrisiloxane (V3D3) and 1-vinylimidazole (VI), via initiated chemical vapor deposition. The chemical composition of the copolymer dielectrics is exquisitely controlled to tune the V-T of C-60 OTFTs. In particular, all the copolymer dielectrics demonstrated in this work exhibit extremely low leakage current densities (lower than 2.5 x 10(-8) A cm(-2) at +/- 3 MV cm(-1)) even with a thickness less than 23 nm. Furthermore, by introducing an ultrathin pV3D3 interfacial layer (about 3 nm) between the copolymer dielectrics and C-60 semiconductor, the high mobility of the C-60 OTFTs (about 1 cm 2 V-1 s(-1)) remains unperturbed, showing that V-T can be controlled independently by tuning the composition of the copolymer dielectrics. Coupled with the ultralow dielectric thickness, the independent V-T controllability allows the V-T to be aligned near 0 V with sub-3 V operating voltage, which enables a substantial decrease of device power consumption. The suggested method can be employed widely to enhance device performance and reduce power consumption in various organic integrated circuit applications.