We report the synthesis and characterization of a new linear polyphosphazene architecture in which rigid, bulky side units provide the possibility of interdigitation with their counterparts on neighboring chains to generate noncovalent cross-links and distinct elastomeric properties. The bulky side groups are cyclotriphosphazene rings substituted with trifluoroethoxy groups connected to the main chain via aryloxy spacers. These bulky units are distributed along the polymer backbone and separated from each other by trifluoroethoxy units linked directly to the main chain. Compared to the well-known poly[(bis-2,2,2-trifluoroethoxy)phosphazene], [NP(OCH2CF3)2] n , which is a microcrystalline film- and fiber-forming polymer, several of the new materials are elastomers with properties that arise partly from interactions of the protruding cyclotriphosphazene side units with those on nearby polymer chains. Specific elastomers are capable of regaining up to 89% of their original shape when elongated to high strain (up to 1000%) over four elongation cycles and show even longer elongations at break (>1600%). The overall physical properties depend on the ratios of the cyclic trimeric side units to main chain linked trifluoroethoxy side groups. The polymers were characterized using 1H, 31P NMR, DSC, TGA, X-ray diffraction, GPC, and stress–strain techniques.

New polymers with a phosphazene backbone and both 2,2,2-trifluoroethoxy- and phenoxy-functionalized cyclotriphosphazene substituents exist in three phases depending on the side group ratios. At low concentrations of the bulky substituents (up to ∼7 mol %), the polymers are semicrystalline thermoplastics, with properties that are minor variations of poly­[bis­(2,2,2-trifluoroethoxy)­phosphazene]. However, after the incorporation of between ∼7 mol % and ∼20 mol % of the bulky cyclic trimeric side groups, the polymers lose their semicrystalline properties and become amorphous elastomers. At still higher trimer loadings (>20 mol %) the materials develop gum-like behavior. The elastomeric phase appears to be generated by interdigitation or agglomeration of the bulky aryloxy-cyclotriphosphazene side groups, which act as quasi-physical cross-links between the polymer chains. The presence of these interactions allows the materials to experience high strain values before rupture (up to 1000%), and elastic recovery of more than 85% of the original dimensions when stressed up to 60% of the break elongation over four cycles. In addition, the chemical and physical nature of the substituents on the cyclic trimeric side groups alters the physical characteristics of the polymer in a way that provides a facile method to tune the properties.

The range of polyphosphazene-based elastomers has been expanded through the use of phenoxy or oligo-pphenyleneoxy minor cosubstituent side groups with majority 2,2,2-trifluoroethoxy side groups. Specifically, polymers with both trifluoroethoxy and low ratios of phenoxy, p-phenylphenoxy, p-diphenylphenoxy, or p-triphenylphenoxy cosubstituents, can generate noncrystalline, noncovalently cross-linked elastomers. These are formed through the steric interactions of the oligo-p-phenyleneoxy side groups. Small-angle X-ray scattering (SAXS) analysis of polymers containing p-diphenylphenoxy or p-triphenylphenoxy cosubstituents suggests that these macromolecules contain microdomains caused by the phase separation of the trifluoroethoxy and aryloxy groups, through stacking or agglomeration of the aryloxy units, and that those serve as noncovalent cross-linking points. Moreover, annealing of the polymers at elevated temperatures (150°C) causes a decrease in the average spacing between the aryloxy aggregates and has a direct effect on the mechanical properties, similar to the toughening caused by increases in the cross-link density in conventional elastomers.

Reagents. Tetrahydrofuran and triethylamine (EMD) were dried using solvent purification columns, and the final water content was monitored by Karl Fisher titration. 26 Ethanol and acetonitrile (EMD) were distilled from sodium metal and stored over molecular sieves (4Å) before use. Dichloromethane (EMD), methanol (EMD), hexanes (EMD), ethyl acetate (EMD), alanine ethyl ester hydrochloride (Chem Impex), phenylalanine ethyl ester hydrochloride (Chem Impex), sodium (Sigma), tyramine (TCI), ditert butyl dicarbonate (Sigma), sodium bicarbonate (Sigma), diethyl vinylphosphonate (TCI), ferulic acid (Sigma), trifluoroacetic acid (Sigma), sodium iodide (Aldrich), and trimethylsilyl chloride (Fluka) were used as received. Spectra/Por molecular porous cellulose dialysis membranes with a molecular weight cutoff of 12,000-14,000 were used for purification of the polymers.Equipment. 31 P and 1 H NMR spectra were obtained with a Bruker 360 WM instrument operated at 145 MHz and 360 MHz, respectively. 31 P shifts are reported in ppm relative to 85% H 3 PO 4 at 0 ppm. Glass transition temperatures were measured with a TA Instruments Q10 differential scanning calorimetry apparatus with a heating rate of 10 °C/min and a sample size of ca. 10 mg. Gel permeation chromatography was performed using a Hewlett-Packard 1047A refractive index detector and two Phenomenex Phenogel linear 10 columns. The samples were eluted at 1.0 mL/min with a 10 mM solution of tetra-n-butylammonium nitrate in THF. The elution times were calibrated with polystyrene standards. Mass spectrometric analyses were performed using a Waters LCT Premier time-of-flight (TOF) mass spectrometer in positive or negative ion mode, using electrospray ionization (ESI). Ultraviolet spectra were obtained with a double-beam spectrophotometer (Varian, Cary 500). Measurements were conducted in de-ionized water or in a 10 mM phosphate buffered saline solution (PBS). Synthesis of bis-(diethyl phosphonate)-N,N-diethyltyramine (DPT) and (diethyl phosphonate)-Nethyltyramine (MPT).Tyramine (1.73 g, 12.6 mmol) and diethylvinylphosphonate (3.60 g, 21.9 mmol) were dissolved in de-ionized water (160 mL) and heated at 50 °C for 48 h. The sample was cooled and extracted with DCM. The organic layers were combined and dried over magnesium sulfate. Once dry, the solvent was removed under reduced pressure and dried further under vacuum to afford the product as a crude oil. The