Stereoselective Polymerization Research Articles

Single site catalysts for stereoselective ring-opening polymerization of lactides

Poly(lactide) (PLA) is the most well known biodegradable and biocompatible material among the aliphatic polyesters nowadays explored for biomedical, pharmaceutical and environmental applications. Different poly(lactide)s are distinguished, namely stereoregular PLLA and PDLA, atactic, heterotactic, syndiotactic and stereoblock PLAs. Because the stereochemistry of the monomeric units in the polymer chains plays a decisive role in the mechanical, physical and degradation properties of PLA materials, stereospecific catalysts to prepare different polylactide architectures are a major topic. In this review, after a general introduction on metal catalyzed ring opening polymerization, we mainly focus on single site catalyst systems inducing stereoselective polymerization of lactides.

Controlled stereoselective polymerization of lactide monomers by group 4 metal initiators that contain an (OSSO)-type tetradentate bis(phenolate) ligand

Group 4 metal complexes Zr-1 to Zr-4, Ti-4 and Ti-4a that contain an (OSSO)-type tetradentate bis(phenolate) ligand were found to initiate the ring-opening polymerization of meso-, rac-, and L-lactides in toluene solution. The polymerizations were controlled with initiator efficiency values around one and gave polymers with low polydispersity (Mw/Mn < 1.2). The polylactides were atactic when rac-lactide was used as the monomer and heterotactic biased when meso-lactide was used. Kinetic measurements showed that meso-lactide was polymerized generally faster than rac- and L-lactide.

Stereoselective Polymerization of meso-Lactide: Syndiotactic Polylactide by Heteroselective Initiators Based on Trivalent Metals

Stereoselective Polymerization of <i>meso</i>-Lactide: Syndiotactic Polylactide by Heteroselective Initiators Based on Trivalent Metals

Radical polymerization of styrene derivatives bearing N-free amino acid side chains, synergic effect of chirality, and hydrogen bonding for stereoselective polymerization

Radical polymerization of styrene derivatives having a series of amino acid, alanine, glycine, leucine, valine, Boc-leucine, and Boc-valine, in the side chain bound at the C-terminal was conducted to regulate the stereoinduction system in the propagation step. Isotacticity increased in the polymer main chain, especially in the polymerization of monomers bearing N-free L-leucyl and L-valyl esters in THF or DMF at 50 °C, by the synergic stereoregulation with chirality control and hydrogen bonding between the radical polymer terminal and the monomer. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Efficient Colorimetric Anion Detection Based on Positive Allosteric System of Urea-Functionalized Poly(phenylacetylene) Receptor

Poly(phenylacetylene) with rationally designed urea groups was demonstrated to be a superior anion receptor possessing both a colorimetric response ability and a positive homotropic allosteric binding mode. The target polymer, poly(phenylacetylene) with [bis(trifluoromethyl)phenyl]urea pendants (poly-1), was synthesized by the stereoselective polymerization with Rh(nbd)BPh4 (nbd = norbornadiene) in high yield. The anion sensing ability of poly-1 was evaluated using the tetra-n-butylammonium (TBA) salts of a series of anions in THF. Upon the addition of anions such as CH3CO2−, C6H5CO2−, F−, Cl−, Br−, NO3−, N3−, and HSO4−, the yellow color of the THF solution of poly-1 immediately turned to a different color depending on the types of anions, indicating the anion recognition capability of poly-1. The 1H NMR titrations of poly-1 by using increasing amounts of CH3CO2− revealed that the colorimetric response of poly-1 was considered to be the direct consequence of the hydrogen-bonding complex formation between ...

ChemInform Abstract: Stereoselective Synthesis with Zirconium Complexes

Two types of electrophilic zirconium based catalyst systems are described in several variants. The organometallic Lewis acid CpZrCl is easily available. It catalyzes a variety of organic reactions quite selectively. Starting from &e camphor enantiomers the chiral Lewis acid (diborna-Cp)ZrClg was prepared. It catalyzes the enantioselective ortho-hydroxylation of a- naphthol. In separate experiments the enantiomeric a-naphthoVethylpyruvate addition products were obtained on a preparative scale with 285% ee using 1 to 5 mol% of the optically pure (+) or (-)(diborna-Cp)zirconium mchloride catalyst, respectively. Bis(cyclopentadieny1)zirconium dihalides (R*Cp)2ZrCl bearing chiral substituents at the Cp-rings were used for generating homogeneous group 3 metallocene/alumoxane Ziegler catalysts for the stereoselective polymerization of propene. Systematic variation of substituents at the chiral Cp-side chains revealed a substituent additivity effect of the stereoselective CC-coupling process. Non-bridged bis( 1-alkylindeny1)zirconium complexes exhibiting planar chirality can very easily be prepared optically pure in an asymmemc organometallic synthesis using terpene- or steroid-derived indenyl substituents. Some of these complexes produce homogeneous Ziegler catalysts that are nearly as stereoselective propene polymerization catalysts as the commonly used ansa- metallocene complexes. Bis(isopropylcyclopentadieny1)titanium dichloride/alumoxane is an achiral organometallic catalyst system that produces isotactic block polypropylene at -50°C by means of chain end control. At +lO°C this is changed to slightly syndiotactic polymer formation. A reaction scheme is proposed that may account for the observed linear lg o/(l-o) vs l/r relationship observed for this specific catalyst system.

Mechanistic Study of the Stereoselective Polymerization of d,l-Lactide Using Indium(III) Halides

We report the results of a comprehensive investigation of the recently discovered stereoselective and controlled polymerization of racemic lactide (D,L-LA) using an initiator prepared in situ from indium(III) chloride (InCl(3)), benzyl alcohol (BnOH), and triethylamine (NEt(3)). Linear relationships between number-average molecular weight (M(n)) and both monomer to alcohol concentration ratio and monomer conversion are consistent with a well-controlled polymerization. Studies on polymerization kinetics show the process to be first-order in [InCl(3)](0) and zero-order in both [BnOH](0) and [NEt(3)](0). The rate of D,L-LA conversion is also dependent on the indium(III) halide (i.e., t(1/2)(InCl(3)) approximately = 43 min versus t(1/2)(InBr(3)) approximately = 7.5 h, 21 degrees C, CD(2)Cl(2), [D,L-LA](0)/[BnOH](0) approximately = 100, [D,L-LA](0) = 0.84 M, [InX(3)](0)/[BnOH](0) = 1) and lactide stereoisomer (i.e., k(obs)(D,L-LA) approximately = k(obs)(meso-LA) > k(obs)(L-LA)). A model system that polymerizes D,L-LA with the same high degree of stereoselectivity was developed using 3-diethylamino-1-propanol (deapH) in lieu of BnOH and NEt(3). The product of the reaction of deapH with InCl(3) was identified as [InCl(3)(deapH)(H(2)O)](2) by elemental analysis, X-ray crystallography, and NMR and FTIR spectroscopies. An anhydrous version of the complex was also isolated when care was taken to avoid adventitious water, and was shown by pulsed gradient spin-echo (PGSE) NMR experiments to adopt a dinuclear structure in CD(2)Cl(2) solution under conditions identical to those used in its stereoselective polymerization of D,L-LA. The combined data suggest that the initiating species for the InCl(3)/BnOH/NEt(3) system is similar to [InCl(3)(deapH)(H(2)O)](2) and of the type [InCl((3-n))(OBn)(n)](m). With this information we propose a mechanism that rationalizes the observed stereocontrol in D,L-LA polymerizations. Finally, in an exploration of the scope of the InCl(3)/BnOH/NEt(3) system, we found this system to be effective for the polymerization of other cyclic esters, including epsilon-caprolactone and several substituted derivatives.

Concerning the ring-opening polymerization of lactide and cyclic esters by coordination metal catalysts

A summary of the reactions involved in the ring-opening polymerization (ROP) of lactide (LA) to give polylactides (PLAs) is presented along with competing reactions. Particular attention is given to the stereoselective polymerization of rac-LA to give heterotactic PLA and meso-LA to give syndiotactic PLA by aluminum Schiff-based catalysts and to the development of highly active group 2 metal single-site catalysts. Melt or solvent-free polymerization is also described along with reactions that lead exclusively to cyclic-polylactides.

Ligand Design in Rh(diene)-Mediated “Carbene” Polymerization; Efficient Synthesis of High-Mass, Highly Stereoregular, and Fully Functionalized Carbon-Chain Polymers

Stereoselective polymerization of “carbenes” from diazo esters mediated by [(L-prolinate)RhI(Me2cod)] allows the formation of highly syndiotactic, fully functionalized carbon-chain polymers with the highest reported molecular weights in high yields, thus providing the best currently available catalyst for this new reaction.

Stereoselective Polymerization of Styrene with Cationic Scandium Precursors Bearing Quinolyl Aniline Ligands

The novel N-R-quinolinyl-8-amino ligands HL1−5 (R = 2,6-Me2C6H3 (HL1), 2,4,6-Me3C6H2 (HL2), 2,6-Et2C6H3 (HL3), 2,6-iPr2C6H3 (HL4), C6H5 (HL5)) reacted with Sc(CH2SiMe3)3(THF)2 to afford the well-defined complexes (L1−5)Sc(CH2SiMe3)2(THF) (1−5), which were fully characterized by NMR spectral and X-ray diffraction analyses. Complexes 1−3 combined with organoborates to establish binary systems that exhibited high activity for the polymerization of styrene, while 4 was less active and 5 was almost inert. The cationic complex [L1Sc(CH2SiMe3)(DME)2][B(C6F5)4] (6) was successfully isolated by treatment of 1 with [PhMe2NH][B(C6F5)4], and represents probably the structural model of the initiation active species. Remarkably, upon addition of aluminum trialkyls to the binary systems, distinguished improvement in catalytic performances was achieved, among which the ternary system 1/5AliBu3/[Ph3C][B(C6F5)4] displayed the highest activity (1.56 × 106 g mol−1 h−1) and syndioselectivity (r = 0.94) via a chain-end control mechanism governed by the concerted steric effect of the ligand and the aluminum alkyls. This represents the first non-cyclopentadienyl stabilized rare-earth metal based catalyst showing both high activity and specific selectivity for the polymerization of styrene, which might shed new light on designing more efficient precursors and further investigation of the mechanism for this polymerization.

Reaching Out to Zirconium

Chemistry When alkyl chains bind to transition metals, the metal center can sometimes loosely attract electrons from the hydrogen bonds two carbons away. This attraction, termed an agostic interaction, plays an influential role in stereoselective olefin polymerizations at zirconocene centers, but it

Book Review of Stereoselective Polymerization with Single-Site Catalysts

Book Review of Stereoselective Polymerization with Single-Site Catalysts

Stereoselective Polymerization with Single-Site Catalysts. Edited by Lisa S. Baugh and Jo Ann M. Canich.

Stereoselective Polymerization with Single-Site Catalysts. Edited by Lisa S. Baugh and Jo Ann M. Canich.

Stereoselective Polymerization with Single-Site Catalysts

Stereoselective Polymerization with Single-Site Catalysts

C3‐Symmetric Lanthanide Tris(alkoxide) Complexes Formed by Preferential Complexation and Their Stereoselective Polymerization of rac‐Lactide

In Ordnung gebracht: YIII-, EuIII- und ErIII-Tris(ligand)-Komplexe des neuen chiralen Alkoxidliganden tBu2P(O)CH2CH(tBu)OH (HL) entstehen bevorzugt als C3-symmetrische Diastereomere. Somit liefert racemisches HL (RRR)- und (SSS)-[LnL3]-Komplexe, die die stereoselektive Polymerisation von rac-Lactid zu hoch isotaktischem Polylactid effektiv katalysieren.

C3‐Symmetric Lanthanide Tris(alkoxide) Complexes Formed by Preferential Complexation and Their Stereoselective Polymerization of rac‐Lactide

Restoring order: YIII, EuIII, and ErIII tris(ligand) complexes of a new chiral alkoxide ligand, tBu2P(O)CH2CH(tBu)OH (HL), preferentially form as C3‐symmetric diastereomers. Thus racemic HL affords (RRR)‐ and (SSS)‐[LnL3] complexes, which are active catalysts for the stereoselective polymerization of rac‐lactide to afford highly isotactic polylactic acid.

Stereoselective polymerization with single-site catalysts

Stereoselective polymerization with single-site catalysts

Enolic Schiff Base Aluminum Complexes and Their Catalytic Stereoselective Polymerization of Racemic Lactide

A series of enolic Schiff base aluminum(III) complexes LAlR (where L=NNOO-tetradentate enolic Schiff base ligand) containing ligands that differ in their steric and electronic properties were synthesized. Their single crystals showed that these complexes are five-coordinated around the aluminum center. Their coordination geometries are between square pyramidal and trigonal bipyramidal. Their catalytic properties in the solution polymerization of racemic lactide (rac-LA) were examined. The modifications in the auxiliary ligand exhibited a dramatic influence on the catalytic performance. Lengthening the backbone from C(2) alkylene to C(3) alkylene resulted in remarkable enhancement of both the stereoselectivity and the polymerization rate because of the increasing flexibility of the diimine backbone. Electron-withdrawing substituents in the diketone also highly improved the activity and the stereoselectivity. Among these complexes, 4 b had the highest activity and the stereoselectivity owing to the C(3) alkylene backbone and the two gem-methyl groups on the middle carbon atom. The value of the polymerization rate constant (k(p)) catalyzed by 4 b in 70 degrees C was 1.90 L mol(-1) min(-1), the activation energy of the polymerization (35.4 kJ mol(-1)) was calculated according to the Arrhenius equation. Other factors that influenced the polymerization, such as the polymerization time, the temperature, and the monomer concentration, are also discussed in detail.

Organocatalytic Stereoselective Ring-Opening Polymerization of Lactide with Dimeric Phosphazene Bases

Highly isotactic polylactide with a high melting temperature was synthesized from rac-lactide through an organocatalytic route using dimeric phosphazene base 1-tert-butyl-2,2,4,4,4-pentakis(dimethylamino)-2Λ5,4Λ5-catenadi(phosphazene) (P2-t-Bu) catalyst at low temperature. Microstructural analysis of the prepared polymer using homodecoupled 1H NMR spectroscopy revealed the formation of a stereoblock architecture containing long isotactic sequence of R and S blocks in the main chain. A proposed mechanism involving chain-end control and stereoerror explains the stereoselective polymerization.

Stereospecific and Stereoselective Polymerization of 4‐Methyl‐1‐hexene by Enantiomeric Binaphthyl‐Bridged Salen Dichlorozirconium (IV) Complexes

AbstractTwo enantiomers of a chiral binaphthyl‐bridged salen dichlorozirconium (IV) complex (namely R‐Λ‐1 and S‐Δ‐1) were synthesized and tested as pre‐catalysts for 4‐methyl‐1‐hexene (4MH) polymerization. Their behavior was compared with that of the same complex in its racemic form. Isotactic poly(4‐methyl‐1‐hexene) was produced in both cases, but the polymerization of racemic 4MH promoted by the optically active catalysts preferentially consumed one antipode, showing some stereoselectivity for the process.magnified image