Primary Alkylphosphine–Borane Polymers: Synthesis, Low Glass Transition Temperature, and a Predictive Capability Thereof

Abstract

With a multitude of potential applications, poly­(phosphine–borane)­s are an interesting class of polymer comprising main-group elements within the inorganic polymer backbone. A new family of primary alkylphosphine–borane polymers was synthesized by a solvent-free rhodium-catalyzed dehydrocoupling reaction and characterized by conventional chemicophysical techniques. The thermal stability of the polymers is strongly affected by the size and shape of the alkyl side chain with longer substituents imparting greater stability. The polymers show substantial stability toward UV illumination and immersion in water; however, they undergo a loss of alkylphosphine units during thermal degradation. The polymers exhibit glass transition temperatures (T g) as low as −70 °C. A group interaction model (GIM) framework was developed to allow the semiquantitative prediction of T g values, and the properties of the materials in this study were used to validate the model.

Phosphinoborane interception at magnesium by borane-assisted phosphine-borane dehydrogenation

“…[2][3][4] Although their synthesis, via the catalytic dehydrogenation-polymerisation of phosphine-borane complexes, was first achieved some 20 years ago, catalysts remain almost exclusively based on iron, ruthenium, rhodium or iridium. [2][3][4][5][6][7][8][9][10][11] A metal-free stoichiometric approach was described by Manners, Scheer, and co-workers, who demonstrated the metal-free head-to-tail polymerisation of tert-butylphosphinoborane, which was generated in situ from the trimethylamine-stabilised monomer (Scheme 1a). 12 More recently, Manners and co-workers have reported that treatment with stoichiometric quantities of cyclic alkyl amino carbenes (CAACs) induces the dehydrogenative coupling of phosphine-boranes to provide access to primary-and, previously unprecedented, secondary polyphosphinoboranes (Scheme 1b).…”

Metal-free dehydropolymerisation of phosphine-boranes using cyclic (alkyl)(amino)carbenes as hydrogen acceptors

“…However, attempts to dehydrocouple phosphine-borane adducts under thermal conditions yielded either low molecular weight or poorly soluble materials, which lacked convincing structural characterisation by modern standards 2 , 24 , 25 . Since 1999 a rhodium-catalysed dehydrocoupling approach to prepare soluble, high-molecular-weight (P-monosubstituted)polyphosphinoboranes has been available 26 – 28 . Examples of iron and iridium-catalysed dehydrocouplings have also been reported as routes to high-molecular-weight poly(arylphosphinoboranes) (Fig.…”

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

“…In contrast, much less is known about poly­(alkylphosphinoboranes). The synthesis of P -alkyl-substituted polymers would be expected to be significantly more challenging through catalytic dehydrocoupling routes because of the lower acidity of the P–H bond in the precursor due to the inductive effect of the alkyl group attached to phosphorus. , Consistent with this, dehydropolymerization of i BuPH 2 ·BH 3 gave a moderate molar mass polymer ( M w = 10,000–20,000 g mol –1 ) [ i BuPH–BH 2 ] n under forcing conditions (melt, 120 °C, 13 h) using [Rh­(μ-Cl)­(1,5-COD)] 2 (COD = cyclooctadiene) as a precatalyst (Scheme A) . More recent studies of the dehydropolymerization of alkyl phosphine–boranes using the same Rh precatalyst demonstrated that the dehydrocoupling of RPH 2 ·BH 3 (R = FcCH 2 , n Bu, n Hex, (2-Et)­Hex), in general, has given lower molar mass ( M n < 10,000 g mol –1 ) , and relatively branched materials with varied polydispersity index values ( D̵ = 1.2–5.0) under forcing thermal conditions (90–130 °C) and in the melt.…”