5736 Macromolecules 1999, 32, 5736-5743 Synthesis of Telechelic Polyphosphazenes via the Ambient Temperature Living Cationic Polymerization of Amino Phosphoranimines Harry R. Allcock,* James M. Nelson,† Robbyn Prange, Chester A. Crane, and Christine R. de Denus Department of Chemistry, The Pennsylvania State University, 152 Davey Laboratory, University Park, Pennsylvania 16802 Received March 2, 1999; Revised Manuscript Received June 23, 1999 ABSTRACT: A method for the synthesis of well-defined mono-, di-, and mixed-telechelic polyphosphazenes produced via a living cationic polymerization of phosphoranimines is described. Amino phosphoranimines R-NH(CF3CH2O)2PdNSiMe3 (R ) Ph-, p-BrPh-, p-H3CPh-,CH2dCHCH2-, and CH2dCHPh-) were synthesized via a reaction between a bromophosphoranimine and the appropriate organic amine. Ditele- chelic polymers [R-NH]2-[Cl2PdN]n were prepared by quenching living poly(dichlorophosphazene) chains, + - [Cl3PdN(Cl2PdN)n-PCl3] [PCl6] with small quantities of the amino phosphoranimines. Cationic initiators of the amino phosphoranimines were also generated using PCl5 and were used to polymerize Cl3PdNSiMe3, to give monotelechelic poly(dichlorophosphazenes). In addition, a mixed telechelic system was produced by the termination of an allylamino monotelechelic poly(dichlorophosphazene) chain with a bromoanilino phosphoranimine. In all cases, displacement of the chlorine atoms with sodium trifluoroethoxide yielded hydrolytically stable telechelic polymers with controlled molecular weights and low polydispersities. Introduction Scheme 1 Polyphosphazenes form one of the largest and most diverse classes of inorganic backbone polymer systems. To date, several hundred different polymers of this type have been synthesized, with a range of physical and chemical properties that rival those of synthetic organic and inorganic macromolecules.1-4 The fundamental precursor to most polyphosphazenes is poly(dichloro- phosphazene), [Cl2PdN]n (2), which undergoes macro- thesized via the stepwise polymerization of monomers 13 molecular substitution with a wide variety of nucleo- such as 1 and PhCl2PdNSiMe3. Replacement of the philes to yield the corresponding derivatized polymers.5-8 chlorine atoms with the use of an appropriate nu- This allows for facile changes to the side group structure cleophile yields new polymers with tailored properties. which, in turn, permits the modification of bulk and Until recently, the preparation of block copolymers surface properties such as solubility, refractive index, containing polyphosphazenes has been limited to species hydrophobicity or hydrophilicity, electrical conductivity, with two phosphazene components. A more important nonlinear optical activity, and glass transition temper- objective is the synthesis of macromolecules that contain atures.4 Although several synthetic routes exist for phosphazene and organic polymer blocks. However, the obtaining 2, two general methods are commonly used synthesis of telechelic polyphosphazenes which may be in our laboratory. The classical approach is a high- used to couple with preformed organic polymers has temperature ring-opening polymerization of hexachlo- proved to be difficult. Nevertheless, the recent use of rocyclotriphosphazene which gives a high molecular commercially available organic functionalized polymers weight polymer with a broad polydispersity.1,2 The such as MeO-[CH2CH2O]nCH2CH2NH2 has provided an second approach is an ambient temperature living alternative route to organic-phosphazene block copoly- cationic induced polymerization of trichlorophosphora- mers.15 nimine (1) in the presence of a suitable initiator, such The growing demand for block copolymers with + - as PCl5 or [Cl3PdN-PCl3] [PCl6] (Scheme 1). In special properties has stimulated a great deal of interest - contrast to the classical approach, this second route in recent years.16 20 Access to hybrid copolymers of provides molecular weight control with narrow polydis- polyphosphazenes with organic or other inorganic poly- persities.9-13 An extension of this method includes the mers offers a number of advantages relative to their recent use of the trifunctional cationic species N-{CH2- respective homopolymers. The physical, mechanical, and + - CH2NH[(CF3CH2O)2PdN-PCl3] [PCl6] }3 to polymer- electronic properties of the copolymer can be adjusted ize 1, thus producing the first star-branched polyphos- in accordance with those of the individual monomeric phazenes.14 Furthermore, the utility of the cationic components. Thus, many of the valuable properties of living polymerization of Cl3PdNSiMe3 has allowed the the respective phosphazene homopolymers, such as production of block copolymers, unattainable via ring- thermal and oxidative stability or fire retardance, may opening polymerization methods. For example, phos- be imparted into the copolymer without sacrificing the 1,2 phazene-phosphazene block copolymers [Cl2PdN]m- overall bulk properties. Potential applications of such [PhClPdN]n with different side groups have been syn- compounds may include uses as nonburning elastomers, flame-retardant foams, or solid polymer electrolytes. † Present address: 3M Corporate Process Technology Center, A viable method for the synthesis of block copolymers St. Paul, MN 55144. is through the use of telechelic polymers.19-22 Telechelic 10.1021/ma990318f CCC: $18.00 © 1999 American Chemical Society Published on Web 08/19/1999 Macromolecules, Vol. 32, No. 18, 1999 Synthesis of Telechelic Polyphosphazenes 5737 polymers have been prepared via free radical and living Scheme 2 methods. However, radical methods often yield undesir- able features such as branching or high polydispersi- ties.23 As a result, much of the research on telechelic organic polymers is now based on living anionic polym- erization methods which allow for easy end group control.16,23-28 For instance, the introduction of func- tional groups via the termination of polystyrene living chain ends has been investigated extensively. These materials can be used either as scaffolds from which monomers can be polymerized or as linking groups which can couple with other preformed polymers to give the corresponding block copolymers.17,18 The ability to prepare telechelic polymers from living anionic systems prompted our interest in the generation - of similar materials via the living cationic polymeriza- with 7b and 10b in an attempt to produce polystyrene tion of phosphoranimines. Previous work has demon- phosphazene block copolymers. Similarly, polyphos- - strated that the tris(organo)phosphoranimine species, phazene siloxane block copolymers are being investi- (CF CH O) PdNSiMe , can be used to quench living gated via hydrosilylation reactions employing 7d and 3 2 3 3 32 cationic chain ends.10 Thus, it seemed possible that the 10d. Future possibilities also exist for conducting polymer end group structure could be controlled while condensation polymerizations by the conversion of the introducing additional functionalities via the use of tris- p-tolyl end unit into a reactive carboxylic acid. (organo)phosphoranimines such as 5a-e. The approach Synthesis of Ditelechelic Polyphosphazenes. As to phosphazene block copolymers described here in- mentioned previously, telechelic polyphosphazenes are volves the preparation of end-functionalized materials attractive precursors for the preparation of block co- that are capable of undergoing further reactions. The polymers. In an attempt to produce telechelic polyphos- advantage of employing these materials stems from phazenes, the living nature of the cationic polymeriza- their versatility in the production of block copolymers, tion process was investigated. The addition of successive either A-B, A-B-A, or A-B-C, which can be phos- amounts of amino phosphoranimines 5a-e to a living phazene-organic or phosphazene-inorganic in nature. chain of poly(dichlorophosphazene) allowed an exami- In this paper we report the first telechelic polyphos- nation to be made of the effects this would have on quenching the polymerization. Figure 1a illustrates a phazenes synthesized via a living cationic polymeriza- 31 tion of phosphoranimines at ambient temperature. typical P NMR spectrum which was obtained for a living poly(dichlorophosphazene) chain. The intense Ditelechelic polymers are prepared by utilizing mono- - mers 5a-e as quenching agents for the polymerization peak at 17 ppm (Cl2P) is characteristic of the middle units of the polymer chain [NdPCl2]n while the down- process. In addition, the use of short chain cationic + + - d - + field peaks correspond to the terminal PCl3 (d, 8 ppm) initiators, such as R NH[(CF3CH2O)2P N PCl3] - + - - - as well as the switching groups PCl2PCl3 (t, 14 ppm) [PCl6] (8a d), to induce the polymerization of 1 + - permits the preparation of monotelechelic polymers. and PCl2PCl2PCl3 (t, 15 ppm). Figure 1b,c indicates Furthermore, the generation of a mixed telechelic poly- the changes that occur when successive amounts of the phosphazene using a combination of the above methods end cap are added to the reaction mixture. As il- is described. lustrated, 1 equiv of the amino phosphoranimine (Fig- ure 1b) was not sufficient to terminate the living polymer chain, as indicated by the resonance from the Results and Discussion + remaining PCl3 . Subsequent addition of 1.2 equiv Synthesis of the Functional Phosphoranimines (Figure 1c) of amino phosphoranimine was followed by - d - + R NH(CF3CH2O)2P NSiMe3 (5a e). Phosphoran- the disappearance of the PCl3 resonance, and this