Disparity between rate and selectivity in the controlled synthesis of gradient conjugated copolymers

In this report, a one-pot synthesis of gradient copolymers is attempted employing Kumada catalyst transfer condensative polymerization (KCTCP). The hypothesis that a more reactive monomer is preferentially being built into the chain at the beginning, while the slower reacting monomer is only being built into the chain at the end due to it being in excess, is investigated. The monomer rate constants are influenced by the usage of differently sized sidechains (octyl versus 3-octyldodecyl) and nucleophilicity of the monomers (thiophene versus selenophene). Nonetheless, while the rate constants differ by a factor 2, no selectivity is obtained. The cause thereof is further investigated by increasing the sterical crowdedness around the catalyst-complex by employing a 4-substituted monomer, the polymerization of which also gave rise to random copolymers. Accompanied by density functional theory calculations of the geometry and Gibbs free energy, it is concluded that the distance between the nickel catalyst and the polymer chain is much smaller compared to the distance between the catalyst and the incoming monomer. This results in different polymerization rates (depending on the polymer-nickel complex), while having no selectivity for one incoming monomer over the other.