Synthesis and Characterization of All-Conjugated Graft Copolymers Comprised of n-Type or p-Type Backbones and Poly(3-hexylthiophene) Side Chains

All-conjugated graft copolymers containing poly(3-hexylthiophene) (P3HT) side chains and both of p-type and n-type backbones that are connected with all pi-conjugated linkages were synthesized via a two-step method involving the Stille coupling reaction and Kumada catalyst-transfer polycondensation (KCTP). A series of naphthalene diimide copolymers with different compositions of 3-(4'-chloro-3'-tolyl)thiophene (CTT) units (PNDICTT) were designed as n-type backbones, while the poly(3-(4'-chloro-3'-tolyl)thiophene-alt-thiophene) (PCTT) was designed as a p-type backbone which were converted into n-type or p-type macroinitiators, and P3HT side chains were then in situ grafted from the macroinitiators via an externally initiated KCTP at room temperature. By using this newly developed two-step method for the synthesis of all-conjugated graft copolymers, the number of P3HT side chains in the graft copolymers can be simply controlled by varying the composition of the CTT units in PNDICTT. Meanwhile, the chain length of P3HT was controllable by varying the feed molar ratio of the thiophene monomer to CTT unit during the KCTP. The optical and electrochemical properties of the all-conjugated graft copolymers were investigated by UV-vis, cyclic voltammetry (CV), and organic field-effect transistor (OFET) measurements. Moreover, the differential scanning calorimetry (DSC) and grazing incident wide-angle X-ray scattering (GIWAXS) results revealed that there were two distinguished crystalline domains in the thin films of the graft copolymer. The morphology of the graft copolymers was first observed by transmission electron microscopy (TEM), in which there was a microphase separation between the PNDICTT and P3HT domains, and the P3HT domains showed partial nanofibril structures with a width of 10-20 nm.