Study of conformational stability and chain flexibility in phosphorus based inorganic polymers using density functional theory method

Raja, Amruthavalli (1994) Study of conformational stability and chain flexibility in phosphorus based inorganic polymers using density functional theory method. Masters thesis, Memorial University of Newfoundland.

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Abstract

The structure and conformational stability of poly(thionylphosphazenes) and classical poly(phosphazenes) is investigated by modeling single polymer chains with small mimics. The model compounds are composed of repeat units of the corresponding poly(thionylphosphazenes) and classical poly(phosphazenes). Two of the model compounds of poly(thionylphosphazenes) have hydrogens and two have chlorines as substituents on phosphorus atoms. All model compounds of classical poly(phosphazenes) have only chlorine substituents on phosphorus atoms. In poly(thionylphosphazenes) the substituents on sulfur may be either fluorine or chlorine. Fully geometry optimized structures and energies of the stable conformations involving rotations around P-N bond are obtained using the density functional theory method. The objective of this work is to investigate the flexibility of the chain backbones of the corresponding poly(thionylphosphazenes) and classical poly(phosphazenes). It has been found that for all model compounds the non-planar trans-cis conformations have the lowest total energies. The structural and conformational analyses indicate that the rotation around N-P bond leads to variations in the bond lengths, the SNP, PNP bond angle openings as well as couplings between dihedral angles in different conformations in all model compounds. It was found that the values of the conformational energy differences for poly(thionylphosphazenes) range between 0.6 to 5 kcal/mole and for classical poly(phosphazenes) 0.3 to 1 kcal/mole. The effects of the sulfur and different substituents of sulfur, on conformational stability are discussed. The plots of radial density distribution function, D(r)=Aπr²p(r) computed from the spherically averaged numerical density are presented and analyzed. Using the spherically averaged total electron densities it is shown that the charge along the backbones of the model compounds is partially demoralized and accumulates primarily on the nitrogen atoms. The conformational analysis results will he correlated with the experimentally obtained glass transition temperatures.

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