Synthesis and characterization of polyaryloxyphosphazenes

The synthesis, dilute solution characterization, and thermal analysis of seven polyaryloxyphosphazenes are described. Synthesis is accomplished by the ring-opening polymerization of hexachlorocyclotriphosphazene at 245°C, followed by reaction of polydichlorophosphazene with sodium aryloxide salts in solution at 115°C. Polymers prepared and characterized have the general structure [(ArO)₂PN]_n, with Ar = C₆H₅, m- and p-CH₃C₆H₄, m- and p-ClC₆H₄, p-C₂H₅C₆H₄, or p-CH₃OC₆H₄. Elemental and infrared analyses show these polymers are essentially free of reactive chlorine sites. All the polymers displayed high intrinsic viscosities [η] > 1 dl/g, in tetrahydrofuran or chloroform. Closer examination of the dilute solution properties of two polyaryloxyphosphazenes revealed high molecular weights (M?_w> 6 × 10⁵) and broad molecular weight distributions (M?_w/M?_n > 4.7). The experimental values for the Z-average radii of gyration, 〈S²〉_z^1/2, characterized at near theta conditions, are larger than the calculated values for a freely rotating chain, which suggests that these polymers are relatively linear and not highly branched. Thermal analysis revealed second-order glass transitions between ?37 and +13°C and first-order endothermic transitions between 43 and 160°C for the different polymers. Although crystalline structure can persist above this first-order transition, this temperature can be regarded as a melting temperature or softening temperature at which films can be molded. Decomposition temperatures, measured in argon and oxygen, ranged from 250°C to 400°C.     

Polydichlorophosphazene Polymerization Studies

Polydichlorophosphazene (NPC1₂)_X is unique as a synthetic precursor for poly(organo)phosphazenes [1–6] and presents special problems in polymerization studies because of branching, crosslinking, and cyclization reactions and in characterization due to the polymer's hydrolytic instability. This paper reviews the literature and discusses problem areas and recent advances relating to polydichlorophosphazene polymerization. Melt, solution, and irradiation polymerization reactions and mechanisms are discussed. Previously unpublished results are presented, and current research efforts and areas for future investigation are considered.     

Formation of Silver and Gold Dendrimer Nanocomposites

Structural types of dendrimer nanocomposites have been studied and the respective formation mechanisms have been described, with illustration of nanocomposites formed from poly(amidoamine) PAMAM dendrimers and zerovalent metals, such as gold and silver. Structure of {(Au(0))_n–PAMAM} and {(Ag(0))_n–PAMAM} gold and silver dendrimer nanocomposites was found to be the function of the dendrimer structure and surface groups as well as the formation mechanism and the chemistry involved. Three different types of single nanocomposite architectures have been identified, such as internal (I), external (E) and mixed (M) type nanocomposites. Both the organic and inorganic phase could form nanosized pseudo-continuous phases while the other components are dispersed at the molecular or atomic level either in the interior or on the surface of the template/container. Single units of these nanocomposites may be used as building blocks in the synthesis of nanostructured materials.     

Dilute solution characterization of polyfluoroalkoxyphosphazenes

Results from the dilute solution characterization of polyfluoroalkoxyphosphazenes in Freon ether (E2) solutions are reported. Anomalous viscosity data suggest that polymer aggregation sometimes occurs in E2 and may be caused by the presence of relatively few anomalous polar groups on the polymer backbone. Since small amounts of acetone added to the E2 solutions inhibit aggregate formation, samples are also characterized in an E2-acetone mixed solvent. Light scattering and osmometry indicate that E2 and E2-acetone (9.09% by volume) are theta solvents for the polymers. High molecular weights (M?_w < 3 × 10⁶) and unusually broad molecular weight distributions (M?_w/M?_n < 16) are found. One polymer is fractionated by extracting solutions in 1,1,2-trichloro-1,2,2-trifluoroethane with acetone. Although the samples are highly polydisperse, intrinsic viscosities correlate with number-average molecular weights satisfying the Mark-Houwink relation with the exponent a ≈ 1/2. The z-average mean-square radius of gyration increases linearly with molecular weight so that 〈S²〉_g/M?_w ≈ 0.098. Because of the large polydispersity and unknown form of the distribution function, a quantitative interpretation of characterization results relating dilute solution parameters to polymer skeletal structure is not possible. The tentative conclusion is that the fluoroalkoxy-substituted phosphazene polymers are relatively linear and therefore that the broad distribution of molecular weights must be due to some polymerization mechanism other than branching.     

Helium states II. Perturbation treatment of the ground state using the coordinates r< and r>

In an attempt to improve upon the convergence properties of the Hylleraas-Scherr-Knight-Midtdal perturbation expansion for the ground-state energies and eigenfunctions of the helium isoelectronic sequence, the term r is included in the zeroth-order Hamil-tonian. This term dominates the usual perturbation r for the ground state of these systems, and by removing it from H⁽¹⁾ we substantially reduce, in some sense, its size. In order to find the exact eigenfunction of the resulting zeroth-order Hamiltonian it was found necessary to include in H⁽⁰⁾ two additional terms involving the delta function δ(r₁ ? r₂) = δ(r_< ? r_>) and one such term in H⁽¹⁾. Approximate first- and second-order eigenfunctions are calculated variationally giving the energies to fifth order. The results are disappointing. The errors in the energies to fifth order for He, Li⁺, and Be²⁺, although quite small, are significantly larger than the corresponding errors in the more conventional perturbation treatment. Reasons for the failure to improve upon the earlier results are discussed. A “paradox” noted some time ago by Snyder and Parr is examined in an Appendix.