*4.1. Materials*

Tris(2,2,2 trifluoroethyl) phosphite and trimethylsilyl azide were obtained from Aldrich Chemical Co., Milwaukee, WI, USA and purified by distillation at reduced pressure from 4 Angstrom molecular sieves to molecular sieves. Tetra-*n*-butyl ammonium fluoride TBAF in THF, 1.0M and TBAF-silica gel were obtained from Aldrich and were used as received. Benzyl azide was obtained from Johnson-Mathey Co., London, England and purified by recrystallization from dry benzene-hexane solution. 1-Azidoadamantane and sodium azide from Aldrich were used as received. Trityl bromide and *t*-butyl alcohol (Fisher Scientific-Fisher Scientific Co., LLC Hampton, NH, USA), and BF3.ET2O from Aldrich were used as received.

#### *4.2. Monomer Synthesis*

A clean 100 mL three-neck round bottom flask with magnetic stir-bar, water jacketed condenser, glass or Teflon stopcock, and Claisen tube with 250 mL pressure-equalizing addition funnel was oven-dried at 150 ◦C overnight, assembled hot with natural rubber septa and gas inlet and outlet, and cooled under a dry nitrogen purge. All ground-glass joints were sealed with a light application of stopcock grease and wrapped with Teflon tape. The flask was covered with aluminum foil to prevent photolysis of the azide. In accordance with a literature procedure [51], distilled tris(2,2,2-trifluoroethyl) phosphite (0.4 mol) was combined with equimolar distilled trimethylsilyl azide and refluxed for 24 h at 120 ◦C, followed by two successive additions of equimolar azide under agitation at 24 h intervals, for a final mole ratio of 1:3 phosphite:azide. During each azide addition, the reaction flask was cooled to 0 ◦C, then the temperature was slowly raised to 120 ◦C. At the end of the 72-h period, an orange/amber liquid was observed in the flask. Upon distillation at reduced pressure, a first fraction of unreacted clear, colorless azide was obtained at 40–50 ◦C/85 mm, and a second fraction of clear, colorless liquid monomer distilled at B.P. 50–60 ◦C/0.5 mm. Yield of *P*-tris (2,2,2 trifluoroethoxy)- *N*-trimethylsilyl phosphoranimine monomer: 95% based on the phosphite.

#### *4.3. Synthesis of N-Alkyl Phosphoranimines*

The adamantyl azide (21 mmol) and an equimolar amount of the tris(2,2,2-trifluoroethyl) phosphite were combined in a three-neck round bottom 100 mL flask with condenser over an electrically heated and thermostated oil bath. The oil temperature was increased gradually from 50 ◦C to 140 ◦C over a 1.75 h period, in order to moderate the pressure increase in the system as the nitrogen outgassed. The *N*-adamantyl phosphoranimine distilled under reduced pressure (93 ◦C/2.5 torr) as a clear, colorless liquid which crystallized as colorless needles with a melting point near 0 ◦C as shown in Table 1.

In a similar fashion, 136 mmol benzyl azide and equimolar amount of phosphite were combined with stirring at 40–80 ◦C for 22 h. *CAUTION: very vigorous reaction with rapid outgassing and pressure build-up in apparatus***!** *N*-benzyl phosphoranimine distilled as a clear colorless liquid at 95 ◦C/2.5 torr. The *t*-butyl azide was prepared by the reaction of 100 mmol *t*-butyl alcohol and 120 mmol trimethylsilylazide in the presence of 120 mmol BF3.Et2O, following a published procedure [52]. Due to difficulty in isolating the azide by distillation, 100 mmol phosphite was added to this reaction at room temperature (RT) for the Staudinger coupling in situ, at 90 ◦C for 18 h. *<sup>N</sup>*-*<sup>t</sup>*-butyl phosphoranimine distilled from the reaction as a clear, colorless liquid. Trityl azide was prepared by the reaction of sodium azide suspended in MeCN with trityl bromide in benzene at RT over several days, in a manner similar to published methods [53,54]. The crystallized azide (13.5 mmol) was combined with an equimolar amount of phosphite at 60–140 ◦C over a 2-h period. *N*-trityl phosphoranimine crystallized as an amber, fibrous solid in high purity.

#### *4.4. Bulk Polymerization of Monomer and N-Alkyl Phosphoranimines*

The bulk polymerizations of the monomer phosphoranimine with and without the *N*alkyl compounds were conducted in NMR tubes as indicated in the Results and Discussion section. Monomer in the specified amount was charged via syringe and the specified amount of TBAF initiator solution was added by microliter syringe. Heat was supplied by a mineral oil bath over an RCT Tekmar hotplate unit for the specified time period. The *N*-alkyl phosphoranimine was added neat if liquid, or as a solution by syringe as indicated. After the reaction was cooled to RT, the work-up entailed dissolving the solid polymer mass in 2–5 mL THF, and adding the solution to excess cold chloroform to precipitate the polymer. The polymer mass was re-dissolved in a 90/10 blend of chloroform and methanol and re-precipitated. The re-precipitated polymer was allowed to stand under the mother liquor at −20 ◦C overnight to maximize precipitation, and was collected on a clean, tared glass frit with thorough washing to remove any unreacted materials. The collected white polymer solid was dried in a vacuum desiccator overnight before weighing. Yield was calculated on the basis of monomer weight minus the condensate by-product as the theoretical yield.

#### *4.5. Characterization of N-Alkyl Phosphoranimines and Polymers*

NMR spectra were recorded on an IBM NR/300 MHz FT NMR spectrometer. Trimethyl phosphite in C6D6 was used as an external standard for 31P-NMR spectra, with a δ P of 141.0 ppm (85% phosphoric acid H3PO4 = 0 ppm). The 31P-NMR and 1H-NMR spectra were obtained in CDCl3 solution or in d<sup>6</sup> acetone as internal standard (1H-NMR = 7.24 ppm). Abbreviations for NMR signals: s = singlet; m = multiplet, d = doublet, t = triplet, q = quartet, p = pentet, br. = broad. Integration symbols, such as "6H", signify six protons of a particular type as shown in the table.

Mass spectra were obtained using a Hewlett Packard 5890 gas chromatograph with a silica column and equipped with a 5970 series mass spectrometer. Run time was 40 min with 2 min solvent delay; T (initial) = 100 ◦C for 10 min, heating rate of 10 ◦C/minute, and T (final) = 250 ◦C for 5 min. FTIR spectra were obtained on a Nicolet 5DXB FT-IR spectrometer. Samples were analyzed in KBr pellets, or thin translucent films between sodium chloride plates as appropriate, using polystyrene film standard. Melting points were measured with a digital melting point apparatus from Electrothermal Eng. Ltd. at a heating rate of 1 ◦C/minute. Refractive indices were measured on the Bausch and Lomb Abbe' −3L refractometer at 20 ◦C. Elemental analyses were provided by Midwest Microlab of Indianapolis, IN. Gel Permeation Chromatography (GPC) was performed on polymer samples by first dissolving the solid polymer (0.2–0.5 g) in 1 mL THF-HPLC grade, and filtering through 0.5 micron Teflon filter, 20 microliters of this solution was injected into the carrier solvent stream (THF with 0.1% tetra-*n*-butyl ammonium bromide, a literature procedure for polyphosphazenes [55]) at a flow rate of 1.0 mL/minute. Ultrastyragel columns (10,000; 1000; 100 Angstroms) and a Waters 410 differential refractometer were

used at 35 ◦C internal temperature. Data acquisition and calculations were performed with a Nelson 900 analytical interface and Samsung 286 personal computer. Calibration was based on polystyrene standards of low to high molecular weights.

**Author Contributions:** Conceptualization: R.A.M. and K.M. Methodology: R.A.M. and K.M. Validation: R.A.M. and K.M. Investigation planning: R.A.M. and K.M. Data acquisition: R.A.M. Formal analysis: R.A.M. and K.M. Writing—original draft: R.A.M. Writing—review and editing: K.M. Project administration: K.M. Funding acquisition: K.M., R.A.M. Both authors have read and agreed to the published version of the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** K.M. acknowledges support from PPG Industries, Inc., Eastman Kodak, Xerox Corp., Hoechst-Celanese. R.A.M. acknowledges support from PPG Industries, Inc.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available in this article.

**Acknowledgments:** This paper is dedicated to Professor Julian Chojnowski on the occasion of his 85th birthday. R.A.M. and K.M. acknowledge and thank Frank Burkus for his assistance with the experiments in the preliminary research study [56], and Kasi Somajajula of the University of Pittsburgh for the FAB-Mass Spectrometry measurements.

**Conflicts of Interest:** The authors declare no conflict of interest. The sponsors had no role in the design, execution, interpretation, or writing of the study.

**Sample Availability:** Samples of the compounds are not available from the authors.
