Novel low dielectric constant (low-k) polycarbosilanes with tunable performance are reported in the present work. These well-defined linear-cyclopolycarbosilanes were prepared via the copolymerization of 1,5-hexadiene (1,5-HD) with 3,3-dimethyl-3-sila-1,5-hexadiene (DMSHD) using dimethyl (pyridylamido) hafnium/[Ph3C][ B(C6F5)(4)]/(AlBu3)-Bu-i catalytic system. They exhibited excellent dielectric properties and the dielectric constant of the film samples was all lower than 2.5. The dielectric constant decreased gradually with the increase of DMSHD content, even approaching to 1.8 for the poly( DMSHD). In addition, the thermal, surface and mechanical properties of the copolymers were improved significantly after introducing C-Si bonds and can be exquisitely modulated by tuning the incorporation of the DMSHD units. Hydrophobic polymer films with high thermal stability could be prepared by solution casting or spin-on method. It was found that there were no porous and cavity in the poly( DMSHD) film, the intrinsic low dielectric constant could be attributed to its unique structure. (C) 2017 Elsevier Ltd. All rights reserved.