Poly(carbyne)s Via Reductive C1 Polymerization

Abstract

A C1 polymerization is a polymerization in which the backbone of the polymer is grown by one carbon atom for each monomer added to the chain. By polymerizing substituted C1 monomers, persubstituted polymers can be synthesized which contain functionalized pendant groups on each atom of the polymer’s backbone. Often times, these persubstituted polymers are not easily synthesized through conventional polymerizations, and can show enhanced physical and/or chemical properties over conventional analogues. One class of C1 polymerizations utilizes carbyne precursors as monomers, which possess a geminal trifunctionalized carbon atom. When polymerized, all three geminal functional groups are replaced by new carbon-carbon bonds formed between monomers. Two of these new carbon-carbon bonds form the backbone of the polymer, with the third bond determining the polymers structure. The final bond can either be a sigma bond to a third monomer, creating a sp3 network polymer, or a pi bond to one of the other adjacent monomers, forming a highly unsaturated backbone. To date, only four geminal trihalides, bromoform, chloroform, 1,1,1-trichloroethane, and trichlorotoluene, have been utilized as monomers in C1 carbyne polymerizations. Our work has greatly expand the scope of these polymerizations by varying both the reactive geminal functionalities as well as the pendant groups. Two new functionalities, fluorine and methyl ether, have been utilized as leaving groups in reductive poly(carbyne) synthesis, with trifluorotoluene and trimethly orthobenzoate both polymerizing into poly(phenyl carbyne). Additionally, a number of ester and ketone containing monomers have successfully been polymerized into the first poly(carbynes) containing polar carbonyl groups. While these new monomers readily polymerized in the presence of Li, higher yields and molecular weights were obtained when an electron transfer agent, such as naphthalene, was also present in the reaction. Interestingly, when different carbyne precursors are polymerized, we find a dichotomy in both the bonding structure of the polymer backbone as well as the polymerization mechanism. Monomers containing phenyl groups adjacent to the trifunctionalized atom undergo a chain-growth mechanism to give polymers with a branching sp3 backbone. Conversely, carbyne precursors containing carbonyl groups in the pendant chain exclusively polymerize via a step-growth mechanism to give polymers with unsaturated, linear backbones. In the case of polymers made from trichlorotoluene and pentyl trichloroacetate, the mechanism and final polymer architecture do not depend on reaction conditions, such as the use of an electron transfer agent, and are based solely on the monomer employed. While the step-growth mechanism of poly(ester carbyne) is expected for a condensation polymerization, the chain condensation seen in the synthesis of poly(phenyl carbyne) is quite uncommon. The presence of a chain-growth mechanism suggests that growing chains of poly(phenyl carbyne) become activated towards further polymerization in comparison to their monomers. On the other hand, poly(ester carbyne)s experience the opposite effect upon polymerizing, becoming less reactive than their monomers. The changes in reactivity are likely caused by the steric and electronic differences which arise from replacing a C-Cl bond with another structural unit. The polymerization of poly(phenyl carbyne) is the first example of a chain condensation polymerization caused by “change of substituent” effects on a single carbon atom, and if the polymerization can be finely tuned, could lead to a controlled synthesis of poly(carbyne)s.