Enantiomorphic Site Control Research Articles

A highly active and site selective indium catalyst for lactide polymerization

Chiral indium salen complexes are highly active, isoselective catalysts for the ring opening polymerization of racemic lactide. The polymerizations are well controlled and polymers with high molecular weights and low molecular weight distributions are obtained. Preliminary kinetic investigations with the enantiopure complex confirm enantiomorphic site control as the dominant contributor to selectivity and formation of block copolymers.

Propylene polymerization byC1-symmetric {ONNO′}-type salan zirconium complexes

The activities of C1-symmetric dibenzyl zirconium complexes of Salan ligands that bear a halo-substituted phenolate ring and an alkyl-substituted phenolate ring in propylene polymerization with methylaluminoxane as cocatalyst were studied. These {ONNO′}ZrBn2-type catalysts exhibited moderate-to-high activities and yielded polypropylene of low molecular weight. The degree of tacticity was found to depend on the steric bulk of the substituents on both phenolate rings and ranged from practically atactic to substantially isotactic (74–78% [mmmm] for polymerizations at room temperature by Lig5ZrBn2). Hemi-isotactic polypropylene was not obtained, despite the diastereotopicity of the two positions. The pattern of stereo errors was consistent with the enantiomorphic site control of propylene insertion typically observed for C2-symmetric catalysts and implied a facile site-averaging mechanism. A regular 1,2-insertion and a β-H transfer to an incoming monomer correspond to the main propagation and termination processes, respectively. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Propylene Polymerization with α-Keto-β-Diimine Initiators Proceeds via Enantiomorphic Site Control

13C NMR spectroscopy is used to characterize the microstructure of polypropylene (PP) obtained using a nickel α-keto-β-diimine initiator activated with methylaluminoxane (MAO) at different reaction temperatures (Trxn). The product prepared at −20 °C has structural features that resemble an ethylene-propylene copolymer. We find that the main sequences present in this sample arise as a result of normal 1,2-insertion and that propylene sequences are predominantly isotactic (m = 88%, mmmm = 63%). There is also evidence of ethylene-like sequences as a result of 3,1-enchainment, and from 2,1-insertion. A reduction in Trxn to −60 °C results in a polymer that has undetectable regioirregularities and is highly isotactic (m = 93%, mmmm = 85%). Moreover, the application of stereo propagation models gives indication that the isospecificity is a result of enantiomorphic site control.

Ring-Opening Polymerization of Cyclic Esters and Trimethylene Carbonate Catalyzed by Aluminum Half-Salen Complexes

A series of ONO-tridentate Schiff base ligands derived from chiral and achiral amino alcohols and amino acids were synthesized and reacted with AlEt(3) to provide dimeric aluminum complexes. These complexes were tested for the ring-opening polymerization (ROP) of rac-lactide at 70 °C in toluene, producing poly(lactide) with up to 82% isotacticity. The most active of these aluminum complexes was chosen to perform ring-opening homopolymerizations of rac-lactide, trimethylene carbonate (TMC), rac-β-butyrolactone (rac-β-BL), δ-valerolactone (δ-VL), and ε-caprolactone (ε-CL). Kinetic parameters were investigated, and each polymerization was found to be first order with respect to monomer concentration. Fractional orders were observed with respect to catalyst concentration, indicating catalyst aggregation during the polymerization processes. Activation parameters were determined for all monomers, with their ΔG(‡) values at 90 °C being in the order rac-lactide ≈ rac-β-BL > δ-VL > TMC ≈ ε-CL. Fineman-Ross and kinetic studies of the copolymerization of rac-lactide and δ-VL both indicate that the rate of rac-lactide enchainment is higher than that of δ-VL, resulting in a tapered copolymer. In addition, single crystals of one of these aluminum complexes were grown in the presence of rac-lactide and characterized using X-ray crystallography. The unit cell contains two lactide monomers, one D- and one L-lactide, adding further proof that polymerization takes place via an enantiomorphic site control mechanism.

Isoselective Styrene Polymerization Catalyzed by ansa-Bis(indenyl) Allyl Rare Earth Complexes. Stereochemical and Mechanistic Aspects

Mechanistic features of the isoselective polymerization of styrene promoted by racemic allyl ansa-lanthanidocene complexes rac-{CMe2(Ind)2}Ln(allyl)(THF)n (Ind =5-indenyl; allyl = C3H5, n = 1, Ln = Y (1), Nd (2); allyl =1,3-(SiMe3)2C3H3, n = 0, Ln = Y (3), Nd (4)), which act as unique single-component catalysts, have been investigated by experimental and theoretical methods. Combined 1H NMR and MALDI−ToF−MS analyses conducted on low molecular weights isotactic polystyrene (iPS) produced by compounds 1 or 2 established the presence of allyl end-groups and therefore that polymerization is initiated by insertion of styrene into Ln−C(allyl). 1H, 2D, and 13C NMR analyses performed on H(D)-iPS-nBu polymers, selectively produced by coordinative chain transfer polymerization (CCTP) of styrene with the binary system 3/Mg(nBu)2 (1:10), evidenced that the polymerization is highly regioselective and proceeds in a secondary insertion mode, both in the initiation and propagation steps. Microstructural analyses of the backbone of crude (unfractionated) polymers produced with 1−4 evidenced a virtually perfect isoselective (mmmmmm > 0.99) polymerization up to 120 °C. Stereoerrors observed in the 13C{1H} NMR spectra of the iPS produced at 130 °C with 1−4 are consistent with an enantiomorphic site control operative during the polymerization. DFT computations of the first and second insertions of styrene in 3 confirmed the above features and the overall mechanistic scenario.

Synthesis of stereoregular ROMP polymers using molybdenum and tungsten imido alkylidene initiators

Three of the four possible structures for polymers formed from an achiral monomer through a single ROMP polymerization step have been prepared for a small collection of monomers. Trans,syndiotactic structures have been prepared through chain end control, cis,isotactic polymers have been prepared through enantiomorphic site control, and cis,syndiotactic polymers have been prepared through stereogenic metal control. Stereogenic metal control at the metal center as a means of forming syndiotactic polymers is virtually unknown. Synthesis of ROMP polymers with a regular structure that contain alternating enantiomers from a racemic mixture of monomers is a natural consequence of stereogenic metal control. Ruthenium catalysts do not display ROMP specificities analogous to those described here, perhaps since alkylidene isomers have not been observed for Ru catalysts and the barrier to rotation of the carbene in a generic NHC dichloride Ru catalyst has been calculated to be relatively low.

Isomerization in bent phosphametallocenes: Combining rotational barriers and the intramolecular slip-inversion-slip mechanism to control stereo-conformation

Four new phosphazirconocenes, [(4,5,6-trihydro-1-phosphapentalenyl) 2ZrCl 2] ( 8), [(4,5,6,7-tetrahydro-1-phosphindolyl) 2ZrCl 2] ( 9), [(4,5,6,7,8-pentahydro-1-phosphazulenyl) 2ZrCl 2] ( 10), (2,4-dimethylphospholyl) 2ZrCl 2 ( 11), were prepared and characterized. A crystal structure of [(4,5,6,7-tetrahydro-1-phosphindolyl) 2ZrCl 2] ( 9), was obtained that revealed disorder ( rac and meso isomers in the same crystal) and demonstrates unambiguously the meso configuration in the bent early transition metal phosphametallocenes. Lacking a spectroscopic probe of the rotational dynamics in phosphametallocenes, propylene polymerization was chosen to provide a record of the stereoselective insertions and by inference insight into the phosphametallocene dynamics. Phosphazirconocenes 8– 11 were evaluated, and the rac/ meso mixture of [(4,5,6,7-tetrahydro-1-phosphindolyl) 2ZrCl 2] ( 9) was found to produce a mixture of isotactic (iPP) and atactic (aPP) polypropylenes. 13C NMR spectral analysis of the stereo-errors contained in the iPP fraction indicated enantiomorphic site control as the source of the stereoselectivity. The rotational dynamics of the η 5-phospholyl ligand of the parent phosphazirconocene, [(C 4H 4P) 2ZrCl 2], was examined as a first order surrogate of reasonable computing complexity for the catalytically active species. The phosphazirconocene was examined using a restricted Hartree–Fock method with a split basis set of 6-311+G(d,p) for C, H, Cl, P and of LANL2DZ for Zr. The rotational potential energy surface was found to be unsymmetrical with a maximum barrier of 8.8 kcal mol −1, which is about an order of magnitude greater than that reported for Cp 2ZrCl 2. The projection of the calculated rotational dynamics onto a 1-phosphatetrahydroindenyl system is illustrated. This model leads to an interpretation of the polymerization results and the rotational dynamics in phosphametallocenes.

Separation of a diiminopyridine iron(ii) complex into rac- and meso- diastereoisomers provides evidence for a dual stereoregulation mechanism in propene polymerization

Separation of a diiminopyridine iron(II) complex into its rac- and meso- diastereoisomers provides for first time the opportunity of observing the enantiomorphic site control competing with the chain-end control mechanism in a non-metallocene catalyst system.

Kinetic resolution of racemic α-olefins with ansa -zirconocene polymerization catalysts: Enantiomorphic site vs. chain end control

Copolymerization of racemic alpha-olefins with ethylene and propylene was carried out in the presence of enantiopure C1-symmetric ansa metallocene, {1,2-(SiMe2)2(eta5-C5H-3,5-(CHMe2)2)(eta5-C5H3)}ZrCl2 to probe the effect of the polymer chain end on enantioselection for the R- or S-alpha-olefin during the kinetic resolution by polymerization catalysis. Copolymerizations with ethylene revealed that the polymer chain end is an important factor in the enantioselection of the reaction and that for homopolymerization, chain end control generally works cooperatively with enantiomorphic site control. Results from propylene copolymerizations suggested that chain end control arising from a methyl group at the beta carbon along the main chain can drastically affect selectivity, but its importance as a stereo-directing element depends on the identity of the olefin.

Copolymerization of propylene and disubstituted diallylsilanes involving intramolecular cyclization with stereoselective zirconocene catalysts

AbstractThe copolymerization of propylene and disubstituted diallylsilanes [(CH2 CHCH2)2R2Si (R = CH3 or C6H5)] was investigated with isoselective and syndioselective zirconocene catalysts with methylaluminoxane as a cocatalyst. The syndioselective catalyst showed a higher reactivity for disubstituted diallylsilanes than the isoselective catalysts. Diallyldimethylsilane was incorporated into the polymer chain via cyclization insertion preferentially and formed 3,5‐disubstituted dimethylsilacyclohexane units in the polypropylene main chain. In the copolymerization with diallyldiphenylsilane, diallyldiphenylsilane was copolymerized via both cyclization insertion and 1,2‐insertion, which formed a pendant allyl group. The structures of isolated silacyclohexane units, determined by 13C NMR and distortionless enhancement by polarization transfer spectroscopy, proved that the 1,2‐insertion of diallylsilanes proceeded with enantiomorphic site control; however, the diastereoselectivity of the cyclization reaction was independent of the stereoselectivity of the catalysts used, and cis‐silacyclohexane units were mainly formed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6083–6093, 2006

Concerning the relative importance of enantiomorphic site vs. chain end control in the stereoselective polymerization of lactides: reactions of (R,R-salen)- and (S,S-salen)–aluminium alkoxides LAlOCH2R complexes (R = CH3and S-CHMeCl)

The preparations and structures of LAlOCH(2)C(S)HMeCl, where L = (R,R) or (S,S)-N,N'-bis(3,5-di-tert-butyl-salicylidene)-1,2-cyclohexenediamino, are reported together with the respective LAlOEt compounds, and their reactivities toward L- and rac-lactides in various solvents reveal the surprising complexity of the stereopreference for the ring-opening event.

Ansa-Zirconocene ester enolates: synthesis, structure, reaction with organo-lewis acids, and application to polymerization of methacrylates.

The synthesis, structural characterization, and abstraction chemistry of ansa-zirconocene ester enolate complexes relevant to the isospecific polymerization of methacrylates are reported. Reactions of rac-(EBI)ZrMe(OTf) and rac-(EBI)Zr(OTf)(2) [EBI = C(2)H(4)(Ind)(2)] with 1 and 2 equiv of lithium isopropylisobutyrate in toluene produce the first examples of ansa-zirconocene mono- and diester enolate complexes: rac-(EBI)ZrMe[OC(O(i)Pr)=CMe(2)] (1) and rac-(EBI)Zr[OC(O(i)Pr)=CMe(2)](2) (2) in 89% and 50% isolated yields, respectively. The reaction of 1 with B(C(6)F(5))(3) was investigated in six different organic solvents; in THF at ambient temperature, this reaction cleanly produces the isolable cationic ansa-zirconocene ester enolate complex rac-(EBI)Zr(+)(THF)[OC(O(i)Pr)=CMe(2)][MeB(C(6)F(5))(3)](-) (3) in quantitative yield. The analogous reaction of 1 with Al(C(6)F(5))(3) in toluene, however, proceeds through a proposed novel, intramolecular proton transfer process in which propylene is eliminated from the isopropoxy group, subsequently producing a carboxylate-bridged tight ion pair rac-(EBI)Zr(+)(Me)OC((i)Pr)OAl(C(6)F(5))(3)(-) (4). In addition to standard spectroscopic and analytical characterizations for the isolated complexes 1-4, complexes 2 and 4 have also been structurally characterized by X-ray diffraction studies. Polymerization of methyl methacrylate (MMA) and n-butyl methacrylate (BMA) has been investigated using complexes 1, 3, and 4. Both the isolated cationic 3 and neutral 1 (the latter combined with B(C(6)F(5))(3) in situ) are highly active (10 min for quantitative MMA conversion) and highly isospecific ([mm] > 95% for PMMA; [mm] > 99% for PBMA) via enantiomorphic-site control, producing polymethacrylates with extremely narrow molecular weight distributions (M(w)/M(n) = 1.03). The aluminate complex 4, however, produces syndiotactic PMMA predominantly via chain-end control.

Microstructure of Metallocene‐Catalyzed Propene/1‐Pentene Copolymers

AbstractThe microstructure of propene/1‐pentene copolymers prepared with the metallocene catalysts rac‐Et(Ind)2ZrCl2 and rac‐Me2Si(2‐MeBenz‐[e]Ind)2ZrCl2 was studied by 13C NMR spectroscopy. Both catalysts lead to the formation of random copolymers although rac‐Me2Si(2‐MeBenz‐[e]Ind)2ZrCl2 favors a somewhat higher incorporation of 1‐pentene than rac‐Et(Ind)2ZrCl2. Surprisingly, the presence of 1‐pentene has a significant influence on the stereoregularity of the copolymers formed with rac‐Me2Si(2‐MeBenz‐[e]Ind)2ZrCl2. Propene/1‐pentene copolymers produced with rac‐Et(Ind)2ZrCl2 retained the isotacticity of polypropene and featured only the stereoerror pentads mmmr, mmrr and mrrm typical for enantiomorphic site control. However, propene/1‐pentene copolymers obtained with rac‐Me2Si(2‐MeBenz‐[e]Ind)2ZrCl2 are characterized by a gradual loss of tacticity which is concurrent with an increase in the amount of all irregular pentads. It is proposed that the presence of 1‐pentene can lead to a reversible loss of stereocontrol over a whole sequence of inserted monomer units. Subsequently, more or less atactic blocks are formed besides isotactic blocks.Percentage of 1‐pentene in the copolymer as function of the feed composition for 1‐pentene/propene copolymers prepared with EI and MBI.magnified imagePercentage of 1‐pentene in the copolymer as function of the feed composition for 1‐pentene/propene copolymers prepared with EI and MBI.

Indenyl-amido titanium and zirconium dimethyl complexes: improved synthesis and use in propylene polymerization

The synthesis of a series of indenyl amido titanium dimethyl complexes, by means of the direct synthesis from the ligand, a 2-fold excess of MeLi, and TiCl 4 is reported. The 1H NMR spectra of the complexes show a quartet structure for the metal-bound methyl groups, due to through-metal proton–proton coupling. Coupling of Ti-methyl protons with protons on the Cp ring is also revealed by COSY 2D-NMR. The performance of the Ti complexes in propylene polymerization, including [Me 2Si(Me 4C 5)( t-BuN)]TiMe 2 ( 1-TiMe 2 ), [Me 2Si(Ind)( t-BuN)]TiMe 2 ( 2-TiMe 2 ) and six other methyl titanium complexes bearing substituted indenyl ligands, has been investigated with different cocatalysts and at different polymerization temperatures and propylene concentrations. All complexes produce amorphous polypropylene ( am-PP). The catalytic activity and molecular weight strongly depend on the substitution of the Cp ring: 2-TiMe 2 gives polymers of lower molecular weight, while the presence of a methyl group in position 2 (as in 3-TiMe 2 ) determines up to 4-fold increase in molecular weight. The type of cocatalyst influences mainly the catalytic activity, the borates being better activators than MAO, but also molecular weight, with again the borates giving higher molecular weights than MAO. 5-TiMe 2 –Ph 3CB(C 6F 5) 4 shows an overall activation energy of polymerization of 7.35 kcal mol −1. The rate of chain release is first order in monomer. The following activation energies for overall chain release have been calculated: ΔΔ E ‡ 2-TiMe 2 =3.4 kcal mol −1, ΔΔE ‡ 5-TiMe 2 =3.8 kcal mol −1, ΔΔ E ‡ 3-TiMe 2 =6.3 kcal mol −1. Even if all the polymers produced are amorphous, 2-TiMe 2 and 5-TiMe 2 show a microstructure unbalanced towards isotacticity, while 3-TiMe 2 , 6-TiMe 2 and 8-TiMe 2 are syndiotactic-enriched. Chiral induction comes mainly from a weak enantiomorphic site control.

Chiral Ansa metallocenes with Cp ring-fused to thiophenes and pyrroles: syntheses, crystal structures, and isotactic polypropylene catalysts.

Syntheses, crystal structures, and polymerization data for new isospecific metallocenes (heterocenes) having cyclopentenyl ligands b-fused to substituted thiophenes (Tp) and pyrroles (Pyr) are reported. The C2- and C1-symmetric heterocenes are dimethylsilyl bridged, have methyl groups adjacent to the bridgehead carbon atoms, and have aryl substituents protruding in the front. rac-Me2Si(2,5-Me2-3-Ph-6-Cp[b]Tp)2ZrCl2/MAO (MAO = methyl alumoxanes) is the most active metallocene catalyst for polypropylene reported to date. rac-Me2Si(2,5-Me2-3-Ph-6-Cp[b]Tp)2ZrCl2 and rac-Me2Si(2,5-Me2-1-Ph-4-Cp[b]Pyr)2ZrCl2 have the same structure, and the former is 6 times more active, produces half the total enantiofacial errors, and is 3.5 times less regiospecific in propylene polymerizations at the same conditions. rac-Me2Si(2-Me-4-Ph-1-Ind)2ZrCl2/MAO is 3.5 times lower in activity than rac-Me2Si(2,5-Me2-3-Ph-6-Cp[b]Tp)2ZrCl2 catalyst, and while the former is the more stereospecific and the less regiospecific, the sum of these two enantioface errors is the same for both species. Fine-tuning the heterocene sterics by changing selected hydrogen atoms on the ligands to methyl groups influenced their catalyst activities, stereospecificites, regiospecificites, and isotactic polypropylene (IPP) Mw. Thus, both substituting a hydrogen atom adjacent to the phenyl ring with a methyl group on an azapentalenyl ligand system and replacing one and then two hydrogens on the phenyl ring with methyls on thiopentalenyl ligands provided increased polymer Tm and Mw with increasing ligand bulk. Polymer molecular weights are sensitive to and inversely proportional to MAO concentration, and the catalyst activities increase when hydrogen is added for molecular weight control. The polymer Tm values with the thiopentalenyls as TIBAL/[Ph3C][B(C6F5)4] systems were higher than with MAO as catalyst activator. A racemic C1, pseudo-meso complex with a hybrid dimethylsilyl-bridged 2-Me-4-Ph-1-Ind/2,5-Me2-4-Ph-1-Cp[b]Pyr ligand produced the first sample of IPP with all the steric pentad intensities fitting the enantiomorphic site control model. Speculative mechanistic considerations are offered regarding electronic effects of the heteroatoms and steric effects of the ligand structures, the preferred phenyl torsion angles, and anion effects.

Copolymerization of propylene and 1,5-hexadiene with stereospecific metallocene/Al(i-Bu)3/[Ph3C][B(C6F5)4

Copolymerizations of propylene (P) with 1,5-hexadiene (1,5-HD) were carried out with isospecific rac-1,2-ethylenebis(1-indenyl)Zr(NMe2)2 [rac-(EBI)Zr(NMe2)2, 1] and syndiospecific isopropylidene(cyclopentadienyl)(9-fluorenyl)ZrMe2 [i-Pr(Cp)(Flu)ZrMe2, 2] compounds combined with Al(i-Bu)3/[Ph3C][B(C6F5)4] as a cocatalyst system. Microstructures of poly(propylene-co-1,5-HD) were determined by 1H NMR, 13C NMR, Raman spectroscopies and X-ray powder diffraction. The isospecific 1/Al(i-Bu)3/[Ph3C][B(C6F6)4] catalyst showed much higher polymerization rate than 2/Al(i-Bu)3/[Ph3C][B(C6F6)4] system, however, the latter system showed higher incorporation of 1,5-HD (rP = 8.85, r1,5-HD = 0.274) than the former system (rP = 16.25, r1,5-HD = 0.34). The high value of rP × r1,5-HD far above 1 demonstrated that the copolymers obtained by both catalysts are somewhat blocky. The insertion of 1,5-HD proceeded by enantiomorphic site control; however, the diastereoselectivity of the intramolecular cyclization reaction of 1,2-inserted 1,5-HD was independent of the stereospecificity of metallocene compounds, but dependent on the concentration of 1,5-HD in the feed. The insertion of the monomers by enantiomorphic site control could also be realized by Raman spectroscopy and X-ray powder diffraction of the polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1590–1598, 2000

Copolymerization of propylene and 1,5-hexadiene with stereospecific metallocene/Al(i-Bu)3/[Ph3C][B(C6F5)4

Copolymerizations of propylene (P) with 1,5-hexadiene (1,5-HD) were carried out with isospecific rac-1,2-ethylenebis(1-indenyl)Zr(NMe2)2 [rac-(EBI)Zr(NMe2)2, 1] and syndiospecific isopropylidene(cyclopentadienyl)(9-fluorenyl)ZrMe2 [i-Pr(Cp)(Flu)ZrMe2, 2] compounds combined with Al(i-Bu)3/[Ph3C][B(C6F5)4] as a cocatalyst system. Microstructures of poly(propylene-co-1,5-HD) were determined by 1H NMR, 13C NMR, Raman spectroscopies and X-ray powder diffraction. The isospecific 1/Al(i-Bu)3/[Ph3C][B(C6F6)4] catalyst showed much higher polymerization rate than 2/Al(i-Bu)3/[Ph3C][B(C6F6)4] system, however, the latter system showed higher incorporation of 1,5-HD (rP = 8.85, r1,5-HD = 0.274) than the former system (rP = 16.25, r1,5-HD = 0.34). The high value of rP × r1,5-HD far above 1 demonstrated that the copolymers obtained by both catalysts are somewhat blocky. The insertion of 1,5-HD proceeded by enantiomorphic site control; however, the diastereoselectivity of the intramolecular cyclization reaction of 1,2-inserted 1,5-HD was independent of the stereospecificity of metallocene compounds, but dependent on the concentration of 1,5-HD in the feed. The insertion of the monomers by enantiomorphic site control could also be realized by Raman spectroscopy and X-ray powder diffraction of the polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1590–1598, 2000

Direct Experimental Observation of the Stereochemistry of the First Propene Insertion Step at an Active Homogeneous Single-Component Metallocene Ziegler Catalyst

rac-[Me2Si(1-indenyl)2]ZrCl2 was treated with (butadiene)magnesium to yield the respective ansa-metallocene (s-cis-η4-butadiene) complex, 6. It reacts with the organometallic Lewis acid B(C6F5)3 to give the ansa-metallocene-(μ-C4H6)-B(C6F5)3 betaine 7 that was characterized by X-ray diffraction. Complex 7 contains a distorted (σ,π-type) allyl ligand, bonded to zirconium, that bears a syn-oriented −CH2B(C6F5)3 substituent. An ortho C−F substituent of one of the C6F5 groups at boron coordinates to the electron-deficient zirconium center. Complex 7 is an active single-component Ziegler catalyst that stereoselectively polymerizes propene to give isotactic polypropylene by enantiomorphic site control (up to ca. 90% mmmm). Treatment of 7 with propene at −15 °C in toluene-d8 leads to a regioselective but stereounselective stoichiometric monoinsertion of the prochiral 1-alkene into the Zr−CH2 bond of the betaine 7 to give a 60:40 mixture of the metallacyclic products 9A and 9B. Cleavage of both the intramolecular C4−C5 olefin coordination to zirconium and the internal C6···Zr ion pair interaction in the 9A/9B pair of diastereoisomers by the addition of THF yields a 60:40 mixture of the respective open-chain THF adducts 10A and 10B. This result shows that the first propene insertion reaction at the rac-[Me2Si(1-indenyl)2]Zr derived single-component Ziegler catalyst system is not stereoselective, whereas all subsequent propene insertion steps show a high degree of stereoselectivity. A stereochemical “relay mechanism” is proposed to account for this behavior, where an intermediately generated stereogenic center at the α-carbon atom of the growing σ-ligand hydrocarbon chain effectively serves to transfer the stereochemical information from the chiral metallocene backbone onto the growing carbon chain. The ansa-metallocene backbone alone seems to have no intrinsic ability for direct stereochemical control. The essential role of the α-carbon stereochemistry is supported by the stereoselective propene insertion into the betaine (16) derived from the regioselective B(C6F5)3 addition to (1,3-pentadiene)ZrCp2.

Copolymerization of Propene and Nonconjugated Diene Involving Intramolecular Cyclization with Metallocene/Methylaluminoxane

Copolymerizations of propene with 1,5-hexadiene and 1,7-octadiene were carried out with isospecific rac-dimethylsilylenebis(indenyl)zirconium dichloride (1) and syndiospecific diphenylmethylene(cyclopentadienyl)(9-fluorenyl)zirconium dichloride (2) combined with methylaluminoxane. Microstructures of the copolymers were determined by 1H NMR, 13C NMR, and DEPT (distortionless enhancement of polarization transfer) spectroscopies. Incorporation of nonconjugated dienes in the copolymer was higher with 2 than with 1. Intramolecular cycloaddition of the nonconjugated dienes proceeded regardless of the catalysts. Stereoselectivity in cycloaddition of 1,5-hexadiene was investigated based on the structures of isolated methylene-1,3-cyclopentane units. It was found that 1,5-hexadiene was inserted stereospecifically by enantiomorphic site control with both catalysts. The diastereoselectivity of the cyclization step of 1,2-inserted 1,5-hexadiene was, however, found to be independent of stereospecificity of the catalyst. The cyclization selectivity of 1,5-hexadiene copolymerization was higher than that of 1,7-octadiene copolymerization. 1 gave the copolymers with higher selective in cyclization than 2 in the propene/1,5-hexadiene copolymerization, while the opposite tendency was observed in the propene/1,7-octadiene copolymerization. Decrease of cyclization selectivity was observed with the increases of 1,5-hexadiene and 1,7-octadiene concentration in both copolymerizations. Propene insertion could disturb the cyclization of 1,2-inserted dienes, which declined the cyclization selectivity of the copolymers, except propene/1,5-hexadiene copolymerization with 2.

Synthesis, structure, and catalytic properties of ansa-zirconocenes, Me 2Si(RInd) 2ZrCl 2 (R=2- p- or 3- p-tolyl)

ansa-Ligands, Me 2Si(RInd) 2 [R=2- p ( 1) and 3- p-tolyl ( 2)], have been prepared as a mixture of rac- and meso-isomers in a ratio of 1:1 from the reactions of Me 2SiCl 2 with cyanocuprates of 1- p-tolylindene and 2- p-tolylindene in diethyl ether, respectively. Pure rac-isomers, 1r and 2r, have been isolated by fractional recrystallization of 1 and 2. Thermal diastereomerization of 1 and 2 between rac- and meso-isomers has been studied by 1H-NMR spectroscopy. These results imply that 1 and 2 undergo the diastereomerization process by migration of silicon moiety and hydrogen atom, respectively. ansa-Zirconocenes, Me 2Si(RInd) 2ZrCl 2 [R=2- p ( 3) and 3- p-tolyl ( 4)], have been prepared from the reactions of the corresponding dilithiated ligands with ZrCl 4 in diethyl ether and hexane at −78°C. The molecular structure of rac- 3 has been determined by a single crystal X-ray diffraction study. Propylene polymerization has been studied using rac- 3 and meso- 4 in the presence of methylaluminoxane (MAO). Catalyst rac- 3 produces a highly isotactic polypropylene (PP) at 0°C by the enantiomorphic site control mechanism and meso- 4 affords a syndiotactic dominant PP having a rr triad of 49% at −30°C by the chain end control mechanism.