High Molar Mass Poly(alkylphosphinoboranes) via Iron-Catalyzed Dehydropolymerization

Abstract

High molar mass polyphosphinoboranes substituted with an alkyl group at phosphorus [RPH−BH 2 ] n (R = tBu, 1-Ad, iPr, Cy, nHex, and Me) have been successfully prepared via dehydropolymerization of the phosphine−boranes RPH 2 •BH 3 using an iron precatalyst, [CpFe(CO) 2 (OTf)] (100 °C, toluene, 2.0 M, 10−100 h). Substrate purity and the reaction conditions were found to be crucial in obtaining a high molar mass material as the major product (M n = 18,200−57,200 g mol −1 and D̵ = 1.24− 3.40). For example, the addition of small quantities of primary phosphines, a potential monomer contaminant, was found to lead to a lower molar mass oligomeric material [RPH−BH 2 ] x . Our experiments indicated that the added Lewis basic primary phosphine does not induce main-chain scission post-polymerization. In contrast, a phosphine-mediated termination process during the catalytic cycle appears to compete with the polymerization of the phosphine−borane monomer. The resulting poly(alkylphosphinoboranes) were characterized by multinuclear NMR spectroscopy, gel permeation chromatography, and electrospray ionization mass spectrometry. The thermal properties were also investigated by thermogravimetric analysis, which showed the materials to be stable to weight loss up to 100−120 °C, and differential scanning calorimetry, which revealed strongly side group-dependent T g values that ranged from −76 to 87 °C.

Polyphosphinoborane Block Copolymer Synthesis Using Catalytic Reversible Chain‐Transfer Dehydropolymerization

“…[18] Here, the atacticity of the polymer backbone is revealed in the 31 P NMR spectrum by a statistical distribution of proposed rr, [rm, mr] or mm triads, δ( 31 P{ 1 H}) À 45.6, À 46.9, À 48.3 (1 : 2 : 1 respectively), as noted previously. [18,23] n-Hexyl phosphine-borane is also dehydropolymerized using 1 to form [H 2 BP(nhexyl)H] n as a colorless, hexane-soluble, oil, M n = 33 000 g mol À 1 , Ð = 1.3; [17,34] but in this case a higher catalyst loading (3 mol%) and longer reaction times (66 hours) are required, consistent with the lower reactivity of alkyl phosphines. [16] Purification of this hexane-soluble polymer was by passage through a short silica plug using CH 2 Cl 2 , although this does not remove a trace of (H 3 B) 2 • dppe (see later).…”

“…In the 31 P NMR spectrum a broad signal at δ À63.0 is observed. [17,34] GPC analysis also showed a small amount of lower molecular weight oligomers (M n � 2 500 g mol À1 ).…”

“…Additional smaller signals are observed in the 31 P NMR spectrum at δ À 55.8 as an apparent triplet, that simplifies to a singlet on decoupling 1 H, and δ À 48 (partially obscured). These could reflect connecting units between block segments, P(nhexyl)H 2 end groups [17,34] or hexane insoluble small oligomers. GPC analysis using MALS detection showed polymer with an increased degree of polymerization M n = 15 000 g mol À 1 (Ð = 1.3; CCC, M n = 46 000 g mol À 1 ), Figure 1B.…”

“…arenes, 100 °C) have been reported, [28][29][30][31] exemplified by [Cp*RhCH 3 (PMe 3 )(ClCH 2 Cl)][BAr F 4 ], I, [32] and versatile CpFe(CO) 2 OTf, II. [18,33,34] A step-growth-like mechanism is suggested to operate using catalyst I. [32] In contrast, for II a non-living chain-growth mechanism is proposed.…”

“…While initial reports used melt conditions at temperatures of up to 130 °C with catalysts such as [Rh(COD)Cl] 2 , [27] more recently catalyst systems that operate in solution at lower temperatures (e.g. arenes, 100 °C) have been reported, [28–31] exemplified by [Cp*RhCH 3 (PMe 3 )(ClCH 2 Cl)][BAr F 4 ], I , [32] and versatile CpFe(CO) 2 OTf, II [18, 33, 34] . A step‐growth‐like mechanism is suggested to operate using catalyst I [32] .…”