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Understanding free radical addition to multiple bonds is important to elucidating the mechanistic details of addition polymerization reactions, albeit the fleeting radical intermediates are very difficult to detect by conventional methodologies. Muon spin spectroscopy (μSR) is a highly sensitive method that can detect radical species at 106 spins (cf. EPR: 1012 spins, NMR: 1018 spins). Herein, we employ μSR to detect the radical‐addition products from three 1‐phosphabutadiene monomers, P‐analogues of isoprene. We show that muonium (Mu), a "light" H‐atom surrogate, adds predominantly at the C4 position of the P1=C2−C3=C4 moiety to give unprecedented 1‐phosphaallyl radicals as the major products. Our structural assignments are supported by assignment of muon, phosphorus and proton hyperfine coupling constants using DFT‐calculations. A minor radical product is also detected that is tentatively assigned to an PC3‐heterocyclic free radical. On the basis of DFT‐predictions, we speculate that its formation may involve initial addition of Mu+ at the C3 position followed by electron capture. These studies provide rare insights into the prospective radical (or cationic) polymerization of 1‐phosphabutadienes, which have previously been polymerized using anionic initiation. [ABSTRACT FROM AUTHOR]

The reactions of the N -heterocyclic carbenes (NHCs) IDipp and I t Bu and the cyclic(alkyl)amino carbene (CAAC) CAAC Me with polyaminoborane [MeNH-BH 2 ] n were investigated. Stoichiometric quantities of each carbene were found to cause rapid and complete depolymerization, with the major B-N-containing product identified as the NHC-aminoborane adduct, IDipp-BH 2 NMeH ( 1 ), cyclic borazane [MeNH-BH 2 ] 3 , or borazine [MeNBH] 3 with IDipp, I t Bu, and CAAC Me , respectively. With substoichiometric quantities of IDipp and I t Bu (down to 10 and 2.5 mol %, respectively), complete loss of high molar mass material was also detected, indicating that the depolymerization is catalytic. The main products of the reaction with substoichiometric IDipp were IDipp-BH 2 NMeH ( 1 ) and [MeNH-BH 2 ] 3 and with substoichiometric I t Bu, [MeNH-BH 2 ] 3 , and [MeNBH] 3 with product ratios dependent on the quantity of NHC used. Under analogous conditions with CAAC Me , high molar mass material persisted alongside the formation of [MeNBH] 3 . Further reactivity studies with cyclic borazane [MeNH-BH 2 ] 3 and MeNH 2 ·BH 3 provided insights into depolymerization pathways. IDipp showed no reactivity toward [MeNH-BH 2 ] 3 , whereas with 3 equiv of I t Bu and CAAC Me , the dehydrogenation product [MeNBH] 3, was formed. With MeNH 2 ·BH 3 , 2 equiv of carbene were used as the first acts to accept dihydrogen; the major products with IDipp, I t Bu, and CAAC Me were IDipp-BH 2 NMeH ( 1 ), [MeNBH] 3 , and (CAAC Me H)HB═NMeH ( 2 ), respectively. The double E-H (E = B, N) bond activation product (CAAC Me H)HB═NMe(HCAAC Me ) ( 3 ) was isolated from the reaction between 3 equiv of CAAC Me and MeNH 2 ·BH 3 . A unified mechanism for donor-mediated depolymerization of [MeHN-BH 2 ] n is proposed.