Thomas Chenal

Chlorolanthanocene-dialkylmagnesium systems for styrene bulk polymerization and styrene-ethylene block copolymerization

The bulk polymerization of styrene has been investigated at 105°C in the presence of exclusively dialkylmagnesium or combination of chlorolanthanocene and dialkylmagnesium. In the presence of butylethylmagnesium or n,s-dibutylmagnesium, styrene polymerization proceeds via thermal self-initiation, but is accompanied by a reversible transfer to dialkylmagnesiums to yield in turn oligostyrylmagnesium species; the latter are finally hydrolysed to oligostyrenes with M n = 300-1500 and M w / M n = 2.0-2.8. The analysis of the oligostyrenes by MALDI-TOF mass spectrometry establishes the presence of ethyl and butyl headgroups. consistent with the transfer process. When the dialkylmagnesium is combined with a lanthanocene such as (C 5 Me 5 ) 2 NdCl 2 Li(OEt 2 ) 2 (1), an increase in activity is obtained which is ascribed to addinonal styrene polymerization initiated by in situ generated alkylthydride)lanthanocene species. The influence of varions reaction parameters on the performance of this system has been investigated. The oligostyrenes (M m = 500-9000) produced under optimum conditions have a relatively narrow molar mass distribution (M w /M n = 1.20-1.40) which can be explained in terms of an efficient transfer between the chain-growing lanthanide and the oligostyrylmagnesium spectes. The MALDI-TOF mass spectra of the oligostyrenes produced with various dialkyimagnesium-lanthanocene combinations gives an insight into the initiation mechanism. Fanally, the combination of butylethylmagnesium and Cp * 2 NdCl 2 Li(OEt 2 ) 2 has been used to achieve (styrene-co-athylene) block copolymers.

A Joint Experimental/Theoretical Investigation of the Statistical Olefin/Conjugated Diene Copolymerization Catalyzed by a Hemi-Lanthanidocene [(Cp*)(BH4)LnR]

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 catalysts 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.

Neodymium(III) tert‐Butoxide‐Dialkylmagnesium, a New Initiator System for Syndiotactic Polymerization of Methyl Methacrylate

The polymerization of methyl methacrylate (MMA) in toluene at 0°C using combinations of dihexyl-magnesium (Hex 2 Mg) with various metal tert-butoxides as initiators has been studied. Purely anionic initiator systems based on alkali metal salts allow complete conversion of MMA within 1 h to yield either highly isotactic PMMA (LiOt-Bu/MgR 2 = 5, M n = 61 000, M w /M n = 1.95, 84% mm) or narrow polydispersity atactic PMMA (NaOt-Bu/MgR 2 = 20, M n = 39 000 and M w /M n = 1.18). The combination of Hex 2 Mg with Nd 3 (Ot-Bu) 9 (THF) 2 (Nd/Mg = 5-10) affords a novel initiator system that gives in 40-63% yield syndiotactic-rich PMMA (M n = 50-90 000, M w /M n = 1.06-1.12, 76-79% rr) with low initiation efficiency. In this case, characteristics of the polymers and polymerization reactivity data are consistent with the in situ generation of an alkylneodymium species.

Synthesis of 1,4:3,6-Dianhydrohexitols Diesters from the Palladium-Catalyzed Hydroesterification Reaction

The hydroesterification of alpha olefins has been used to synthesize diesters from bio-based secondary diols: isosorbide, isomannide, and isoidide. The reaction was promoted by 0.2% palladium catalyst generated in situ from palladium acetate/triphenylphosphine/para-toluene sulfonic acid. Optimized reaction conditions allowed the selective synthesis of the diesters with high yields and the reaction conditions could be scaled up to the synthesis of hundred grams of diesters from isosorbide and 1-octene with solvent-free conditions.

Rare earth alkoxides as inorganic precursors for olefin polymerization: an alternative to traditional lanthanocene chemistry

The combination of the neodymium tert-butoxide [Nd3(μ3-OBut)2(μ-OBu t)3(OBut)4(THF)2] 1 with 1 equivalent of a dialkylmagnesium reagent affords an efficient catalyst for the pseudo-living polymerization of ethylene.

Development of Catalytic Systems Based on Lanthanoïd Complexes for Olefin Polymerization

The use of the alkylation technique of Cp*2LnCl2Li(OEt2)2 complexes for the synthesis of lanthanocene active species which are useful for olefin polymerization, is described. Both butyl lithium and butyl-ethyl magnesium (BEM) are shown to be suitable as alkylating reagents for ethylenepolymerization. Using BEM in excess allows the synthesis of higher dialkylmagnesium compounds via a chain transfer reaction between magnesium and lanthanoide atoms following the insertion reaction of the monomer into the Ln-alkyl bond. For methyl methacrylate polymerization, using a butyl lithium/Ln ratio of 2 gives the best results, leading to the production of syndiotactic (>80%) PMMA with low molecular weight distributions (<2), in high yields and molecular weights (up to 2*105).

Bimetallic Catalytic Systems Based on Sb, Ge and Ti for the Synthesis of Poly(ethylene terephthalate-co-isosorbide terephthalate)

The insertion of rigid monomers such as isosorbide into poly(ethylene terephthalate) (PET) allows for the access of polymers with improved properties, notably in terms of thermal stability. This biobased monomer is however poorly reactive, and harsh reaction conditions lead to color concerns regarding the resulting polymer. This has motivated the development of catalytic systems enabling an increase of the reaction rate and a good coloration. In this study, we have assessed bimetallic catalytic systems based on the main metals used for PET catalysis, i.e., antimony, germanium and titanium, for the synthesis of poly(ethylene terephthalate-co-isosorbide terephthalate) (PEIT). The Sb2O3/Ti(OiPr)4 combination leads to a high reaction rate while maintaining an acceptable coloration. On the other hand, combining Sb2O3 with GeO2 affords the formation of poly(ethylene terephthalate-co-isosorbide terephthalate) without coloration concerns and a reaction rate higher than that observed using the single metal catalysts. Molecular weights and microstructure including diethyleneglycol (DEG) and isosorbide contents are also discussed, together with the thermal properties of the resulting PEIT. The GeO2/Ti(OiPr)4 is also assessed, and leads to average performances.