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Neodymium trisborohydride, associated with MgR2 alkylating agents, affords an efficient initiator for the stereospecific polymerization of isoprene. This system presents good activity (150 kg of polyisoprene/mol of Nd/h; 95% yield in 2 h) and is trans‐stereospecific (up to 97.7%), and its polymerization shows a quasi‐living character. The molecular weights are controlled by the [Isoprene]/[Nd] ratio, with a polydispersity index lower than 1.6. A full study is presented here, including the influence of the nature of the alkylating reagent (MgR2), the solvent, the reaction temperature, the [MgR2]/[Nd] ratio, and the replacement of neodymium by other lanthanides. This work also includes a kinetic study of the system and a detailed NMR study of the microstructure of the trans‐polyisoprene obtained, providing mechanistic insights about the polymerization reaction. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2400–2409, 2007

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Cp* 2 Nd(BH 4 )(THF) (1) (Cp* = C 5 Me 5 ) has been readily obtained in one step from Nd(BH 4 ) 3 (THF) 3 and fully characterized by 1 H NMR, IR spectroscopy, and elemental analysis. Its X-Ray structure displays a neutral monomeric complex, bearing one slightly distorted terminal BH 4group. Associated to nBuEtMg, 1 affords a highly active catalyst for ethylene polymerisation, the first one prepared from a borohydrido organolanthanide pre-catalyst. In the same experimental conditions, the catalytic behaviour of 1 is similar to that of previously described Cp* 2 NdCl 2 Li(Et 2 O) 2 (2). The 1/nBuEtMg system is not deactivated even in the presence of large excesses of THF. In contrast to its chloro homologue 2, the addition of 1 equivalent of nBuEtMg to 1 affords a catalyst for a trans-selective polymerisation of isoprene.2

Statistical copolymerization of ethylene and isoprene was achieved by using a borohydrido half-lanthanidocene complex. Under copolymerization conditions, activation of [(Cp*)(BH(4))(2)Nd(thf)(2)] (Cp*=η(5)-C(5)Me(5)) by an appropriate alkylating agent affords trans-1,4-poly-isoprene-co-ethylene. Analysis of the microstructure of the copolymer revealed the presence of successive short sequences of ethylene/ethylene, trans-1,4-isoprene/ethylene, and trans-1,4-isoprene/trans-1,4-isoprene. A small amount of 1,2-insertion of isoprene was observed, and no cyclic structures within the chain were characterized. Test runs showed that these catalysts are unable to copolymerize α-olefins (such as hex-1-ene) with isoprene. The probable initial steps in the copolymerization have been computed at the DFT level of theory. Analysis of the energy profile provides insight into the catalyst's activity and selectivity. Our theoretical results highlight the key role played by the allyl intermediate, in which diene insertion, and to a lesser extent olefin insertion, is the rate-determining step of the process. These results also illustrate the coordination behavior of the allyl ligand during the insertion of an incoming monomer, which directly inserts, after pre-coordination to the metal center, into the η(3)-allyl ligand without inducing an η(3) to η(1) haptotropic shift. Finally, the inactivity of this family of catalysts towards the copolymerization of hex-1-ene was rationalized on the basis of the free-energy profile of the copolymerization.

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11 pagesInternational audienceThe combination of a neodymium borohydride, Nd(BH4)3(THF)3 (1) or Cp*Nd(BH4)2(THF)x (2), with MgnBuEt (BEM), affords an efficient and highly selective (up to 96.7% 1,4-trans) catalyst for butadiene polymerization. In the presence of excesses of Mg co-catalyst, polymer chain transfer takes place between neodymium and magnesium, and significant amounts of 1,2-units are observed. When considered for butadiene-styrene statistical copolymerization, the catalytic system based on 2 showed a good ability to produce poly[(1,4-trans-butadiene)-co-styrene)], with strong impact of the Mg/Nd ratio on the yield and on the copolymer microstructure, including the percentage of inserted styrene (up to 16.9 mol%). Whatever the co-monomers concentration the polybutadiene backbone remained 1,4-trans. The precise microstructure of the polymers and copolymers was thoroughly analyzed by means of high resolution NMR spectroscopy (900 MHz) and MALDI-ToF spectrometry

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