Edward L. Marshall

Study of ligand substituent effects on the rate and stereoselectivity of lactide polymerization using aluminum salen-type initiators

A series of aluminum salen-type complexes [where salen is N,N′-bis(salicylaldimine)-1,2-ethylenediamine] bearing ligands that differ in their steric and electronic properties have been synthesized and investigated for the polymerization of rac-lactide. X-ray crystal structures on key precatalysts reveal metal coordination geometries intermediate between trigonal bipyramidal and square-based pyramidal. Both the phenoxy substituents and the backbone linker have a significant influence over the polymerization. Electron-withdrawing groups attached to the phenoxy donor generally gave an increased polymerization rate, whereas large ortho substituents generally slowed down the polymerization. The vast majority of the initiators afforded polylactide with an isotactic bias; only one exhibited a bias toward heteroselectivity. Isoselectivity generally increases with increased flexibility of the backbone linker, which is presumed to be better able to accommodate any potential steric clashes between the propagating polymer chain, the inserting monomer unit, and the substituents on the phenoxy donor.

Magnesium and zinc complexes of a potentially tridentate β-diketiminate ligand

The synthesis of the unsymmetrically substituted β-diketimine, 2-(2-methoxyphenylimino)-4-(2,6-diisopropylphenylamido)pent-2-ene, (BDI-2)H, is described and its complexation chemistry with magnesium and zinc is explored. Emphasis is placed on the preparation of alkoxide and amide derivatives for the ring-opening polymerisation of lactide; their behaviour as polymerisation initiators is compared to analogous compounds supported by the N,N′-bis(2,6-diisopropylphenyl) β-diketiminate ligand, BDI-1. (BDI-2)H reacts with Me2Mg to give the bis(chelate) complex, (BDI-2)2Mg, 3. Magnesium alkyls supported by BDI-2 may be prepared by increasing the size of the alkyl group. Hence, lithiation of (BDI-2)H affords [(BDI-2)Li]2, 4; its subsequent treatment with iPrMgCl produces (BDI-2)MgiPr, 5. Aminolysis of complex 5 using iPr2NH yields the amide complex, (BDI-2)MgNiPr2, 6. Zn(NTMS2)2 and ZnEt2 react with (BDI-2)H to give (BDI-2)Zn(NTMS2), 7, and (BDI-2)ZnEt, 8, respectively. The former is converted into the siloxide complex,(BDI-2)Zn(OSiPh3), 9, upon reaction with Ph3SiOH. The chloride derivative, (BDI-2)ZnCl, 10, has also been prepared via the reaction of ZnCl2 with 4. Crystallographic analysis of compounds 3, 4, 7 and 8 reveals that the potential for (BDI-2) to bind in a tridentate manner is only realised with the more electrophilic metals Li and Mg. Compared to their (BDI-1) counterparts, complexes 6, 7 and 9 are more active, but less well-controlled, initiators for the ring-opening polymerisation of rac-lactide, a consequence of the diminished steric protection afforded by (BDI-2) relative to (BDI-1).

Transition Metal Imido Catalysts for Ethylene Polymerisation

Abstract The imido complexes CpV(N-2-MeC 6 H 4 )Cl 2 ( 1 ), Cr(N- t Bu) 2 Cl 2 ( 2 ), CpNb(N-2- t BuC 6 H 4 )Cl 2 ( 3 ) and Mo(N- t Bu) 2 Cl 2 ( 4 ) have been tested as procatalysts for the polymerisation of ethylene in combination with diethylaluminium chloride or methylaluminoxane (MAO) co-catalysts. The vanadium precursors give the highest activities but are short-lived, while the chromium system gives a long-lived catalyst of moderate activity. The niobium and molybdenum derivatives gave relatively low activities under all test conditions. The polyethylene generated by the vanadium and chromium catalysts is of high molecular weight with little branching. The dialkyl complexes Cr(N- t Bu) 2 (CH 2 Ph) 2 ( 5 ), Cr(N-2,6-Pr i 2 C 6 H 3 ) 2 (CH 2 Ph) 2 ( 6 ), Cr(N-2,6-Pr i 2 C 6 H 3 ) 2 Me 2 ( 7 ), Mo(N- t Bu) 2 (CH 2 Ph) 2 ( 8 ), Mo(N-2,6-Pr i 2 C 6 H 3 ) 2 (CH 2 Ph) 2 ( 9 ), (C 5 Me 5 )Nb(N-2,6-Pr i 2 C 6 H 3 )Me 2 ( 10 ) and (C 5 Me 5 )Ta(N- t Bu)(CH 2 Ph) 2 ( 11 ) have been prepared by treatment of the dihalide precursors with appropriate alkylating reagents and investigated as precursors to well-defined cationic alkyl catalysts. Treatment of ( 5 ) with [Ph 3 C][B(C 6 F 5 ) 4 ] affords the cationic η 2 -benzyl species [Cr(N- t Bu) 2 (η 2 -CH 2 Ph)][B(C 6 F 5 ) 4 ] ( 12 ) while its reaction with [PhNMe 2 H][B(C 6 F 5 ) 4 ] liberates toluene to give a mixture of the mono and bis(dimethylaniline) adducts [Cr(N- t Bu) 2 (η 2 -CH 2 Ph)(NMe 2 Ph)][B(C 6 F 5 ) 4 ] ( 14 ) and [Cr(N- t Bu) 2 (η 2 -CH 2 Ph)(NMe 2 Ph) 2 ][B(C 6 F 5 ) 4 ] ( 15 ). Complex 12 reacts with trimethylphosphine to give the mono-phosphine adduct [Cr(N- t Bu) 2 (η 2 -CH 2 Ph)(PMe 3 )][B(C 6 F 5 ) 4 ] ( 13 ). Solutions containing cationic species ( 12 , 14 , 15 ) are active for ethylene polymerisation in the absence of co-catalyst, affording high-molecular-weight polyethylene with relatively broad molecular-weight distributions.

Low coordinate magnesium chemistry supported by a bulky β-diketiminate ligand

A series of magnesium(II) alkyl, alkoxide, carboxylate, amide and halide complexes stabilised by the bulky β-diketiminate ligand, HC(C(Me)N-2,6-iPr2C6H3)2 (BDI), have been synthesised and structurally characterised. (BDI)H reacts with MgMe2 in Et2O to give the four-coordinate complex (BDI)MgCH3(Et2O), 1, and in toluene to afford [(BDI)Mg(μ-CH3)]2, 2. Three coordinate complexes may be accessed by increasing the size of the alkyl ligand; hence, the reaction of (BDI)H with tBu2Mg yields (BDI)MgtBu, 3, while Li(BDI) reacts with iPrMgCl to afford (BDI)MgiPr, 4; a similar reaction with PhMgCl affords the diethyl ether adduct (BDI)MgPh(Et2O), 5. The etherates 1 and 5 may be converted into the base-free complexes, 2 and (BDI)MgPh, 6, respectively, upon heating in vacuo. The direct reaction of (BDI)H with RMgX (X = Cl or Br) results in relatively inert halide-bridged dimers of formula [(BDI)Mg(μ-X)]2, (X = Cl, 7; X = Br, 8). The alkylmagnesium derivatives react readily with alcohols, amines or carboxylic acids to yield alkoxide, amide and carboxylate complexes, respectively. For example, 4 reacts with iPrOH (or O2) to form [(BDI)Mg(μ-OiPr)]2, 9. Convenient one-pot synthetic procedures have been developed using commercially available Bu2Mg. Treatment of Bu2Mg with (BDI)H, followed by its reaction with MeOH, tBuOH, iPr2NH, (Me3Si)2NH, MeCO2H or PhCO2H affords [(BDI)Mg(μ-OMe)]2, 10, [(BDI)Mg(μ-OtBu)]2, 11, (BDI)Mg(NiPr2), 12, (BDI)Mg(NTMS2), 13, [(BDI)Mg(μ-O2CMe])]2, 14, and [(BDI)Mg(μ-O2CPh)]2, 15, respectively. The molecular structures of complexes 4–8 and 12–15 are reported.

The reversible amination of tin(II)-ligated imines: latent initiators for the polymerization of rac-lactide.

The 1:1 reactions of nine potentially tridentate salicylaldimines with tin(II) diamides, Sn(NR2)2 (R = Me, Et, iPr, SiMe3) have been investigated. With Sn(NiPr2)2 and Sn(NTMS2)2, the anticipated products of amine elimination, iminophenoxy tin(II) mono(amide)s, are formed. However, for R = Me and R = Et, nucleophilic attack of the amide at the imino carbon occurs to generate tin(II) complexes of tetradentate, dianionic aminoamidophenoxide ligands. The transfer of the amide is shown to be reversible, with both alcoholysis and the initiation of rac-lactide polymerization apparently mediated by the terminal amide tautomer.

Living ring-opening metathesis polymerisation of amino ester functionalised norbornenes

The living ring-opening metathesis polymerisation of norbornene monomers bearing amino ester- residues, using the molybdenum initiators Mo(=CHCMe2Ph)(=N?2,6-i-Pr2C6H3)(OR)2, {R = CMe3, IA, CMe2CF3, IB, CMe(CF3)2, IC}, is reported. The monomers are derived from reactions of exo- and endo- himic anhydrides with the amino ester hydrochlorides of glycine (1), alanine (2) and isoleucine (3). The optically pure monomers derived from 2 and 3 afford optically active polymers whose optical activities are independent of cis content and molecular weight, indicating the absence of a cooperative effect between chiral centres along the chain. The polymers derived from the exo monomers show a cis-trans vinylene dependence upon the ancillary alkoxide ligands of the initiator with IA giving high trans contents and IC giving high cis. The cis/trans content for the endo polymers show relatively little or no dependence upon the initiator. Poly(exo-1) is exceptional in its ability to incorporate various hydrocarbons in the solid state, including hexane and methane.

Ruthenium initiated ring opening metathesis polymerisation of amino-acid and -ester functionalised norbornenes and a highly selective chain-end functionalisation reaction using molecular oxygen

Abstract [Ru( CHPh)(PCy 3 ) 2 Cl 2 ] is shown to initiate the living ring opening metathesis polymerisation of amino-acid and -ester functionalised norbornenes; treatment of the living polymers with molecular oxygen gives rise to a highly selective chain-end functionalisation reaction.

Dielectric and pyroelectric properties of poly[2,3-bis(trifluoromethyl)norbornadiene]

The fluoropolymer poly[2,3-bis(trifluoromethyl)norbornadiene] (poly(BTFMND)) has been synthesized via ring-opening metathesis polymerization, using initiators based on W, Mo or Ru compounds or via variations of the Schrock-type initiator Mo(CHR)(N-2,6-C 6 H 3 -i-Pr 2 )(OR′) 2 (R = CMe 3 , CMe 2 Ph; R′ = CMe 3 , CMe(CF 3 ) 2 ), giving poly(BTFMND) with controllable proportions of trans -vinylene units. Analysis of the 13 C nuclear magnetic resonance fine structure establishes that the high- trans (> 98%) polymer is 92% tactic and the high- cis (> 98%) polymer is 75% tactic, but these methods cannot determine whether the material is predominantly isotactic or syndiotactic. Dielectric and thermally stimulated current measurements have been performed and give relaxed permittivities % strongly dependent on trans content, ranging from above 40 for the 98% trans material through 15 for the 54% trans polymer to 6 for 98% cis polymer. The significantly higher permittivity above T g for the high- trans material than for the 54% trans polymer and the significantly reduced value for the 98% cis polymer suggest that both the high- cis and high- trans materials have syndiotactic structures. The total polarization for free films of 98% trans poly(BTFMND) saturates at about 20 mC m −2 , with a pyroelectric coefficient approaching 6 μ C m −2 K −1 above poling fields of 200 MV m −1 . These usefully high values combined with low dielectric loss in its glassy state at ambient temperatures indicate an excellent material for potential use as a pyroelectric transducer, with a figure of merit comparable with or better than that of poly(vinylidene fluoride).

Bimolecular termination in living ring opening metathesis polymerization

Abstract Previously observed high molecular weight impurities in the products of living ring opening metathesis polymerizations are identified as arising from a post-polymerization bimolecular dimerization initiated by dioxygen.

Molybdenum(VI) complexes containing differing cis multiply-bonded ligands: Some structural consequences of competing π-donor groups

Abstract The mixed bis(imido) molybdenum complex Mo(N-2,6-Pr2iC6H3)N-But)Cl2(dme) ( 1 ; dme = 1,2- dimthoxyethane ) is accessible in multi-gram quantities upon treatment of Na2MoO4 with equimolar quantities of H2N-2,6-Pr2iC6H3 and ButNH2 in the presence of excess NEt3 and chlorotrimethylsilane. Treatment of Na2MoO4 with one equivalent of 1-adamantamine under analogous conditions afforded Mo(N-1-adamantyl)(O)Cl2(dme) (2). The molecular structures of 1 and 2 are reported. A rationale for the orientation of the imido aryl substituent based on electronic considerations is proposed.