Metal-Free Addition/Head-to-Tail Polymerization of Transient Phosphinoboranes, RPH-BH2: A Route to Poly(alkylphosphinoboranes)

Mild thermolysis of Lewis base stabilized phosphinoborane monomers R1R2P–BH2⋅NMe3 (R1,R2=H, Ph, or tBu/H) at room temperature to 100 °C provides a convenient new route to oligo- and polyphosphinoboranes [R1R2P-BH2]n. The polymerization appears to proceed via the addition/head-to-tail polymerization of short-lived free phosphinoborane monomers, R1R2P-BH2. This method offers access to high molar mass materials, as exemplified by poly(tert-butylphosphinoborane), that are currently inaccessible using other routes (e.g. catalytic dehydrocoupling).

Polymers based on main-group elements other than carbon represent attractive materials as ar esult of their uses as elastomers,lithographic resists,biomaterials,polyelectrolytes, ceramic precursors,and in optoelectronics. [1,2] Current routes to main-group-element macromolecules generally involve either polycondensation or ring-opening polymerization pathways.M etal-catalyzed polycondensation processes,s uch as cross-coupling and dehydrocoupling,h ave also attracted recent attention. [1p, 3] In contrast to the situation with organic polymer synthesis,t he use of addition polymerization methods is rare,p artly due to challenges associated with the generation of suitable multiply bonded monomers.N ever-theless,G ates and co-workers have shown that kinetically stable phosphaalkenes (MesP = C(Ar)Ph;Ar= Ph, C 6 H 4 OMe) undergo an addition-rearrangement polymerization in the presence of radical or anionic initiators. [1q, 4] Furthermore, Baines and co-workers have utilized anion-initiated addition polymerization of germenes and silenes (Mes 2 E=CHCH 2 tBu; E = Ge,S i) to form polygermenes and poly(silylenemethylenes), respectively, [5] demonstrating the use of addition polymerization as ap romising approach for the synthesis of main-group-element polymers. [6,7] Compounds with bonds between elements of Groups 13 and 15 are formally isoelectronic to their carbon analogues. However,d ue to electronegativity differences,t he bonds are polar and lead to different physical and chemical properties. [8][9][10] Theanalogy has nevertheless stimulated the synthesis of ar ange of new molecules and materials such as BN analogues of pyrene, [11] carbon nanotubes, [12] and fullerenelike BN hollow spheres. [13] Counterparts of organic macromolecules have also attracted much attention and polymers based on poly(p-phenylene)-like cyclolinear structures involving borazines (polyborazylenes) have been studied in detail and, more recently,a nalogues of polyolefins,p olyaminoboranes [RNH-BH 2 ] n ,have been isolated. [14] Poly(phosphinoboranes) [RPH-BH 2 ] n have been prepared over the past decade as high-molar-mass materials by the rhodium-and iron-catalyzed dehydrogenation of primary phosphine-boranes RPH 2 ·BH 3 . [15] Studies of the coordination chemistry of phosphine-borane ligands at d-block metal centers have allowed the elucidation of the fundamental PÀBb ond-formation processes leading to dehydrogenative oligomerization and polymerization. [16] These have revealed atwofold role for PÀHbonds:activation of the PÀHbond by the metal centers to form metal-phosphidoborane intermediates,and promotion of the dehydrogenative coupling of P À H (protic H) with B À Hb onds (hydridic H) to release H 2 and form aP ÀBb ond. [15g, 16] However,a sP ÀHb onds are effectively nonpolar (electronegativity: P= 2.19, H = 2.20), [9] catalytic dehydrocoupling routes have relied on the electronwithdrawing effect of aryl groups on phosphorus to promote the reaction. This has resulted in relatively limited substrate scope.T hus,t he only examples of poly(alkylphosphinoboranes) are of modest molar mass and have been prepared by the slow dehydrocoupling of iBuPH 2 ·BH 3 [15c] and FcCH 2 PH 2 ·BH 3 [15e] at 110-120 8 8Cover 13-18 hinthe presence of Rh catalysts in reactions that generally lead to appreciable chain branching and cross-linking,r esulting in av ery high polydispersity index (PDI) value (e.g.PDI > 5). [15c] Ap otential avenue to broaden the substrate scope and circumvent the shortcomings of metal-catalyzed dehydropolymerization routes to polyphosphinoboranes would be the implementation of an addition-polymerization strategy.T his would require access to suitable monomeric precursors. Significantly,r ecent progress by Scheer and co-workers has allowed the facile,gram-scale preparation of H 2 P-BH 2 ·NMe 3 (1a), aL ewis base stabilized monomeric phosphinoborane. [17,18] Elimination of the Lewis base should yield areactive monomeric phosphinoborane [H 2 P-BH 2 ]t hat might be expected to oligomerize and/or polymerize.I no rder to explore the potential of this new polymerization strategy in detail we also targeted the aryl-substituted analogue Ph 2 P-BH 2 ·NMe 3 (1b)a nd the alkyl-substituted tBuPH-BH 2 ·NMe 3 (1c). We therefore developed as alt metathesis route as an ovel and convenient method for the generation of substituted phosphanylboranes stabilized only by aL ewis base (Scheme 1). Deprotonation of the corresponding phosphines and subsequent reaction with IBH 2 ·NMe 3 afforded the desired phosphanylboranes in good yield and with high purity. Adducts 1b and 1c were obtained as white solids that are soluble in THF,toluene,Et 2 O, and MeCN and, in the case of 1c,a lso n-hexane.C haracterization was achieved by multinuclear NMR spectroscopy and single-crystal X-ray diffraction studies ( Figure 1).
Attempts to thermally induce oligomerization and polymerization (Scheme 2) were initially made for 1a and involved reactions at 80 8 8Cb oth in the presence and absence of solvent. However,irrespective of the conditions,inthe case of this precursor the major fraction of the product (3a)w as insoluble in common solvents and the soluble fraction appeared to consist of low-molar-mass,p otentially branched oligomers with multiple phosphorus and boron environments.
As aresult of the insolubility of the polyphosphinoborane 3a formed from heating 1a,w en ext turned our attention to the analogous thermally induced polymerization of phosphanylboranes with organic substituents at phosphorus (1b,c) (Scheme 2). Thermolysis of phosphanylborane 1b was conducted in toluene solution at 100 8 8Cfor 18 h. The 1 H, 31 P, and 11 BNMR resonances of the isolated product 3b were consistent with the formation of oligomeric species [Ph 2 P-BH 2 ] x and occurred at chemical shifts similar to those reported for [Ph 2 P-BH 2 ] 3 and [Ph 2 P-BH 2 ] 4 . [15b] ESI MS analysis of 3b indicated the presence of linear NMe 3 -capped oligomers with am aximum detectable mass of up to 1200 gmol À1 corresponding to about six repeat units (Figure S15), slightly greater than that in the reports of Rh Icatalyzed dehydrocoupling of Ph 2 PH·BH 3 . [15b] In addition, the Scheme 1. Synthesis of Lewis base stabilizedo rganosubstituted phosphanylboranes 1b,c.

Angewandte
Chemie ESI mass spectrum of 3b displayed several peaks corresponding to small, NMe 3 -capped oligomeric units,[ Me 3 N·BH 2 -Ph 2 P-BH 2 ·NMe 3 ] + and [Me 3 N·BH 2 -Ph 2 P-BH 2 -Ph 2 P-BH 2 ·NMe 3 ] + .These represent aclass of highly stable cationic phosphinoborane chains,w hose preparation has recently been reported. [18f] Analysis by DLS was also consistent with the formation of oligomeric products that undergo facile aggregation (see the Supporting Information for details).
Finally,weexplored the thermolysis of the tBu-substituted phosphanylborane 1c using three methods:h eating 1c at 40 8 8Cf or 48 hi nt he absence of solvent, stirring at oluene solution of 1c at room temperature (22 8 8C), and performing the latter experiment at 40 8 8Cf or 48 h. After complete consumption of the starting material (and removal of the solvent for reactions conducted in toluene), the crude product was dissolved in n-hexane and precipitated by adding the resulting solution slowly to vigorously stirred acetonitrile.All three methods led to the isolation of the product 3c as afine white powder (Figure 3, inset) with similar NMR spectra. The 11 B{ 1 H} NMR spectrum featured asingle very broad signal at d = À38 ppm. The 31 P{ 1 H} NMR spectrum featured as et of three broad signals at d = À19, À21, and À24 ppm. Further broadening and splitting into poorly defined doublets was observed in the 1 H-coupled 31 PNMR spectrum. We attribute the overlapping resonances to tacticity;t he tentative assignment of rm, mr, rr, and mm triads is based on statistical probability ( Figure 2). Similar features have been observed in poly(methylenephosphine) polymers. [4a] Overall, the observed NMR spectra for 3c were similar to those for [RHP-BH 2 ] n (R = Ph, iBu, p-nBuC 6 H 4 , p-dodecylC 6 H 4 ). [15a-c,g] TheE SI mass spectra of acetonitrile solutions of 3c (reaction in toluene,228 8C, 48 h) showed patterns corresponding to the successive loss of D(m/z) = 102, characteristic of asingle unit of [tBuPH-BH 2 ]( Figure S19). Samples obtained from the three methods were analyzed by DLS at optimized concentrations in CH 2 Cl 2 .The range of values obtained for R h of 4.4-5.5 nm correspond to molar masses of 26 800-39 900 gmol À1 for monodisperse polystyrene samples in THF ( Figure S20). [21] GPC analysis of the samples with CHCl 3 as eluent, also using polystyrene standards,w as consistent with these results within experimental error and showed the presence of polymer with molar masses (M n )o f 27 800-35 000 gmol À1 with polydispersity indices of 1.6-1.9 (Figures 3a nd S22).
We propose that polymerization of 1a-c is triggered by the initial thermolysis of Lewis base stabilized phosphanylboranes 1a-c,l eading to elimination of NMe 3 to form the unprotected monomeric phosphinoborane intermediates 2ac.T he resulting absence of the Lewis base leads to al ack of electronic stabilization for 2a-c.A sar esult, the lone pair at phosphorus together with av acant po rbital at boron, in conjunction with the aforementioned electronic destabilization, appears to promote ahead-to-tail addition oligomerization/polymerization sequence which ultimately affords 3a-c, although the full mechanistic details are not yet clear (Scheme 2). We attribute the difference in product distribution to the reactivity of 2a-c and the solubility of the polymer products 3a-c.S terically unencumbered 2a is likely to be highly reactive and forms the insoluble material, which may be of high molar mass,t ogether with soluble oligomers.I n contrast, 2b,w hich contains two bulky phenyl groups at phosphorus,appears to afford only oligomers.Presumably the steric bulk hinders polymer formation both kinetically,a nd possibly thermodynamically as well. In contrast, the tertbutyl-substituted species 3c affords soluble,h igh-molecularweight polymer.

Angewandte
Communications 31 Pand 11 BNMR analysis.Subsequent precipitation into,and washes with cold pentane afforded ad ark amber waxlike product. The 31 P{ 1 H}/ 31 Pa nd 11 B{ 1 H} NMR spectra featured multiple broad overlapping resonances (d( 11 B) = À40 ppm, d( 31 P) %À20 ppm). Although ESI MS showed peaks separated by D(m/z) = 102, attributed to units of [tBuPH-BH 2 ], masses up to only 1100 Da were detected. Moreover,G PC analysis of the products with CHCl 3 as eluent revealed no high-molar-mass component and the product appears to be an oligomer of 10 units or less.T his is in stark contrast to the high-molar-mass polymer (3c)o btained via the thermally induced polymerization of phosphanylborane 1c.
In summary,as traightforward synthesis of organosubstituted monomeric phosphanylboranes stabilized only by aL ewis base has been developed to obtain compounds 1b and 1c.S imple thermal treatment of the monomeric Lewis base stabilized phosphinoboranes 1a-c led to the formation of oligomeric and polymeric compounds 3a-c.Due to the low solubility of 3a,characterization of this polymer was limited. Polymerization of 1b led to short-chain oligomers 3b which could be characterized by multinuclear NMR spectroscopy and mass spectrometry.H owever,p olymerization of 1c afforded 3c with high molar mass (M n = 27 800-35 000 gmol À1 )a nd reasonably low PDI (1.6-1.9) characteristic of am ainly linear material. In contrast, previous work with Rh catalysts has given lower-molar-mass,b ranched materials (M n < ca. 10 000 gmol À1 )u nder forcing thermal conditions in the melt where the yields have been limited by gel formation. [15c] In addition, polyphosphinoborane 3c could not be accessed via the recently reported Fe-catalyzed catalytic dehydrocoupling route,p resumably also due to the deactivated P À Hb ond in the alkylphosphinoborane monomer.
Based on these results,the new metal-free polymerization method described offers considerable promise for the preparation of ar ange of new polyphosphinoboranes with alkyl substituents on phosphorus that are of interest as elastomers, flame-retardant materials,and ceramic precursors.Expansion of the substrate/polymer scope,o ptimization of the reaction conditions,a nd the detailed elucidation of the reaction mechanism, which appears to involve the addition/head-totail polymerization of transient phosphinoborane monomers, are topics currently under investigation.