Structure, Properties, and Thermodynamics of Poly(carbon dichalcogenides)

Structural studies indicate that poly(carbon diselenide) has a head-to-head structure rather than the previously assumed head-to-tail structure. Thermodynamic properties and structure are investigated by using dual approaches for poly (carbon dichalcogenides):analysis of results for model compounds and quantum chemical calculations. Calculations using model compounds indicate that the head-to-tail structure has much lower energy than the head-to-head structure for poly (carbon dioxide), the energy difference is small for poly (carbon disulfide), and the head-to-head structure has much lower energy than the head-to-tail structure for poly(carbon diselenide)-which is consistent with the observation of the head-to-head structure for the latter polymer. The derived heat and free energy of polymerization (298 K) for formation of the amorphous head-to-tail polymer dramatically decreases in going from poly(carbon dioxide), H_p= 3.0 kcal/mol and G_p= 13.3 kcal/moi for gaseous C0₂, to poly(carbon disulfide),H_p= -7.0 kcal/mol and G_p=-3.4 kcal/mol for liquid CS₂. This heat of polymerization for liquid S=C=S is in good agreement with observation (-5.6 ± 1.9 kcal/mol). These results suggest that the high pressure typically used in the synthesis of poly(carbon disulfide) is required in order to obtain favorable kinetics but not to change the sign of the free energy of polymerization. Quantumchemical calculations provide that the head-to-tail structure has 38.8 kcal/mol lower energy than the head-to-head structure for poly(carbon dioxide), which is in agreement with the 38.5 kcal/mol energy difference derived by using experimental data on model compounds, and that the head-to-head structure has a 3.9 kcal/mol lower energy than the head-to-tail structure for poly(carbon disulfide), which isin agreement with the 4.0 kcal/mol energy difference derived by using experimental data ofsulfides and disulfides.