Thermodynamic Control in the Catalytic Insertion Polymerization of Norbornenes as Rationale for the Lack of Reactivity of Endo-Substituted Norbornenes

The catalytic insertion polymerization of substituted norbornenes (NBEs) leads to the formation of a family of polymers which combine extreme thermomechanical properties as well as unique optical and electronic properties. However, this reaction is marred by the lack of reactivity of endo substituted monomers. It has long been assumed that these monomers chelate the metallic catalyst, leading to species which are inactive in polymerization. Here we examine the polymerization of cis-5-norbornene-2,3-dicarboxylic anhydride (so-called carbic anhydride, CA) with a naked cationic Pd catalyst. Although exo-CA can be polymerized, the polymerization of endo-CA stops after a single insertion. Surprisingly, no chelate is formed between the catalyst and endo-CA. Using DFT calculation, it is shown that while the insertion of exo-NBEs is exergonic, the insertion of two endo-CA in a row is endergonic. In this latter case, the enthalpy gain corresponding to the insertion of a double bond is not sufficient to overcome the entropic penalty associated with ligand binding. Thus, the different reactivity between endo and exo NBEs is thermodynamic in nature, and it is not controlled by kinetic factors. Interestingly, thermodynamics is also the main factor controlling the stereochemistry of the chain. For CA polymerization, and even for unsubstituted NBE polymerization, the formation of r and m dyads is, respectively, exergonic and endergonic, resulting in a polymer which is essentially disyndiotactic. Thus, this study demonstrates that thermodynamics can control the chemo- and stereoselectivity of a catalytic polymerization.