*Article* **Controlled Synthesis of Polyphosphazenes with Chain-Capping Agents**

**Robert A. Montague † and Krzysztof Matyjaszewski \***

> Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA; rmontague.ky@roadrunner.com

**\*** Correspondence: matyjaszewski@cmu.edu or km3b@andrew.cmu.edu

† Current address: 2217 Forest Avenue, Ashland, KY 41101, USA.

**Abstract:** *N*-alkyl phosphoranimines were synthesized via the Staudinger reaction of four different alkyl azides with tris(2,2,2-trifluoroethyl) phosphite. *N*-adamantyl, *N*-benzyl, *<sup>N</sup>*-*<sup>t</sup>*-butyl, and *N*-trityl phosphoranimines were thoroughly characterized and evaluated as chain-capping compounds in the anionic polymerization of *P*-tris(2,2,2-trifluoroethoxy)-*N*-trimethylsilyl phosphoranimine monomer. All four compounds reacted with the active chain ends in a bulk polymerization, and the alkyl end groups were identified by 1H-NMR spectroscopy. These compounds effectively controlled the molecular weight of the resulting polyphosphazenes. The chain transfer constants for the monomer and *N*-benzyl phosphoranimine were determined using Mayo equation.

**Keywords:** *N*-alkyl phosphoranimines; polyphosphazenes; phosphoramidate esters; fluoride; trifluoroethoxide; *N*-methylimidazole initiators; anionic; chain-capping agents

**Citation:** Montague, R.A.; Matyjaszewski, K. Controlled Synthesis of Polyphosphazenes with Chain-Capping Agents. *Molecules* **2021**, *26*, 322. https://doi.org/ 10.3390/molecules26020322

Academic Editors: Sławomir Rubinsztajn, Marek Cypryk and Wlodzimierz Stanczyk Received: 20 December 2020 Accepted: 7 January 2021 Published: 10 January 2021

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