Chain-end Control Research Articles

Synthesis and MMA polymerization of chiral ansa-zirconocene ester enolate complexes with C2- and Cs-ligation

A series of chiral ansa-zirconocene ester enolate complexes incorporating C 2- or C s-symmetric ligands, including neutral rac-(EBI)ZrCl[OC(O i Pr) CMe 2] ( 1), rac-(EBI)Zr(OTf)[OC(O i Pr) CMe 2] ( 2), rac-(EBI)Zr(OTf)[OC(OMe) C(Me)CH 2C(Me 2)C(O i Pr) O] ( 3), [Me 2C(Cp)(Flu)]ZrMe[OC(O i Pr) CMe 2] ( 4), and cationic [Me 2C(Cp)(Flu)]Zr +(THF)[OC(O i Pr) CMe 2][MeB(C 6F 5) 3] − ( 5), have been synthesized. Within the neutral C 2-ligated zirconocene ester enolate series, the chloride derivative 1 is inactive toward any methyl methacrylate (MMA) additions, the methyl derivative rac-(EBI)ZrMe[OC(O i Pr) CMe 2] adds cleanly only 1 equiv. of MMA, and the triflate derivative 2 can add either 1 equiv. of MMA to form the single-MMA-addition product 3 or multiple equivalents of MMA to form P(MMA). Unlike the C s-ligated methyl cation [Me 2C(Cp)(Flu)ZrMe] +, which is inactive for MMA polymerization under various conditions, the C s-ligated ester enolate cation 5 is moderately active for polymerization of MMA and N, N-dimethylacrylamide at ambient temperature; the resulting P(MMA) has a high molecular weight of M n = 388 000 Da but a low syndiotacticity of [ rr] = 64%, and the polymerization conforms to a chain-end control mechanism.

Copolymerization of ethene with norbornene using palladium(II) α-diimine catalysts: Influence of feed composition, polymerization temperature, and ligand structure on copolymer properties and microstructure

Four cationic palladium(II) α-diimine complexes, [{ArNC(R)–C(R)NAr}Pd(Me)(CH3CN)]BAr4f (1, R=H, Ar=2,6-Me2C6H3, BAr4f=B[3,5-C6H3(CF3)2]4; 2, R=CH3, Ar=2,6-iPr2C6H3; 3, R=CH3, Ar=2-tBuC6H4; 4, R,R=An, Ar=2,6-iPr2C6H3) were used for the copolymerization of ethene with norbornene. The copolymerization behavior of the catalysts and the influence of the polymerization temperature were investigated. The copolymers were characterized using 13C NMR spectroscopy, differential scanning calorimetry, and gel permeation chromatography techniques. Sterically demanding ortho -N-aryl substituents and rigid bulky bridge units increase the copolymer molar masses, while the incorporation level of norbornene is decreased. Microstructures with isolated norbornene units and alternating sequences are predominant. Less bulky substituted catalysts yield copolymers with higher norbornene contents and lower molar masses. Norbornene diblock sequences are dominant which are exclusively racemic connected, indicating that the insertion proceeds under chain end control. Optimal polymerization results are achieved at temperatures between 10 and 30°C, while temperatures below 0°C result in lower polymerization rates and molar masses. Above 30°C, activities, molar masses, and norbornene incorporation decreases due to catalyst decomposition.

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.

Propene Polymerization with Bis(phenoxy–imine) Group 4 Transition Metal Complexes

Abstract Olefin polymerizations with group 4 transition metal complexes having two phenoxy–imine ligands (FI Catalysts) are reviewed with an emphasis on the characteristics and mechanisms of propene polymerization. The structures and properties of FI Catalysts can be easily modified by changing phenoxy–imine ligand structures. Such catalysts can be synthesized straightforwardly from readily available chemicals. An enormous library of FI Catalysts can be built up by combining a group 4 metal center and a variety of cocatalysts. The diversified library makes FI Catalysts so versatile that polyolefins with desired properties (molecular weight, molecular weight distribution, chain-end structure, tacticity, and so on) can be obtained sometimes predictably, by selecting the appropriate combination of ligand, metal, and cocatalyst. In propene polymerization, it remains a challenge to control the stereo- and regiochemistry of monomer enchainment in order to obtain commercially valuable products. FI Catalysts can produce syndiotactic and isotactic polypropene by Ti–FI catalysts/MAO and Zr– and Hf–FI catalysts/iBu3Al/Ph3CB(C6F5)4, respectively. Because of the well-defined and tunable nature of the catalysts, the coordination environment around the metal center can be controlled by the ligand structures to achieve extremely high stereoregularities, comparable to those of heterogeneous Ziegler–Natta catalysts and metallocene catalysts. These syndio- and isospecific FI Catalysts have contrasting reaction mechanisms, i.e., the syndiospecific polymerization is mediated via 2,1-insertion under chain-end control, while isospecificity arises from 1,2-insertion and a site control mechanism. The observed syndiospecificity can originate from the inherent fluxionality of FI Catalysts between configurational isomers.

Structure and Physical Properties of Syndiotactic Polypropylene from Living Polymerization with Bis(phenoxyimine)-Based Titanium Catalysts

A study of the structure and mechanical properties of syndiotactic polypropylene prepared with non-metallocene catalysts, based on titanium chlorides bearing phenoxyimine ligands, is reported. This catalyst promotes living polymerization of propylene, producing high molecular weight syndiotactic polypropylene, through a secondary (2,1) mechanism of enchainment of monomer and chain-end control of stereoselectivity. The effect of the presence of m dyads defects of stereoregularity, consistent with the chain-end mechanism, and defects of regioregularity, due to (1,2) misinsertions, on the crystallization behavior and mechanical properties is analyzed. As-prepared samples crystallize in conformationally disordered modifications of the isochiral helical form II containing kink-bands defects. This is probably due to the presence of regiodefects, producing pairs of vicinal methylene groups along the polymer chains, which favors, locally, the formation of trans-planar sequences that, in turn, induces the crystallization of the sample in the kink-bands disordered modification of form II. This also explains the observed stability of the trans-planar form III obtained in stretched fibers, which does not transform completely in helical forms by releasing the tension or by annealing, as instead occurs in metallocene-made syndiotactic polypropylene. The kink-bands disordered modifications are metastable and transform into the most stable antichiral helical form I by crystallization from the melt. However, disordered modifications of form I are obtained, even at high crystallization temperatures. The presence of m dyads defects of stereoregularity prevents the crystallization of the fully ordered antichiral helical form I. These defects may induce inversions of the helical chirality along the chains so that correlations at long distances between the chirality of the helices along a and b axes of the unit cell are destroyed. The particular microstructure also influences the mechanical properties. The analyzed sample presents values of elastic modulus lower than that of metallocene-made syndiotactic polypropylene of similar stereoregularity, due to higher flexibility of chains and the presence of higher degrees of disorder in crystals induced by the microstructural defects. Unoriented compression-molded samples show a remarkable elastic recovery even though the sample is crystalline and experiences plastic deformation during stretching. As a consequence, oriented fibers present good elastic properties in a large deformation range. The elasticity is associated with polymorphic transitions occurring in the crystals during the mechanical stress−relaxation cycles.

Stereokontrolowana polimeryzacja chiralnych monomerow heterocyklicznych

A review with 80 references deals with stereoselective (race-mate-forming chirogenic) and stereoelective (asymmetric enantiomer-differentiating) ring-opening polymerizations of chiral oxiranes, thiiranes, lactones, dilactides, and N-carboxyanhydrides of alpha-aminoacids. These enantiocontrolled polymerizations are analyzed in terms of the chain-end control or site control mechanisms (CEM or SCM, respectively). Possibility of formation of highly ordered structures by the resulting izotactic and optically active macromolecules is also discussed.

Isotactic Block Length Distribution in Polypropylene: Bernoullian vs Hemiisotactic

Derivation of the isotactic block length distribution is made for polypropylenes having Bernoullian (chain-end control) or hemiisotactic microstructures. For each of these tacticities, the isotactic block length distribution depends on a single stereochemical parameter and Mn, the number-average molecular weight. For polypropylene with Bernoullian statistics, the number of isotactic blocks of length n is given by Nn = (1 − σ)2(σ)(n-1)(Mn/42.08). For hemiisotactic polypropylene, the number of isotactic blocks of length one is given by N1 = [(0.5)(1 − α)2 + (0.5)(1 − α)](Mn/42.08) and the number of isotactic blocks of length n (where n is an odd integer greater than one) is given by Nn = (0.5)(1 − α)2(α)(n-1)/2(Mn/42.08). The distributions are compared and contrasted for hypothetical polypropylenes and real polypropylenes for which σ, α (13C NMR) and Mn (GPC) have been determined. For polypropylene with Mn = 100 000 an elastomeric morphology is calculated to exist for σ = 0.752−0.827 (Bernoullian) and for α...

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.

Polyolefin structural control using phenoxy-imine ligated group 4 transition metal complex catalysts

Abstract This paper describes the structural control of polyolefins achieved by using group 4 transition metal complex catalysts featuring a pair of phenoxy-imine chelate ligands (named FI catalysts). FI catalysts can produce very low to ultrahigh molecular weight polymers. For example, a Zr-FI catalyst bearing a cycloalkyl group on the imine-nitrogen with methylaluminoxane (MAO) activation is capable of selectively forming vinyl-terminated low molecular weight polyethylenes (Mw < 5000) whereas a Zr-FI catalyst with a triethylsilyl group ortho to the phenoxyoxygen can generate ethylene/propylene amorphous copolymers with ultra-high molecular weights (Mw > 10 000 000) when treated with iBu3Al / Ph3CB(C6F5)4. In addition, a Ti-FI catalyst, possessing an o-phenoxytrimethylsilyl group, with MAO can form highly syndiotactic polypropylenes ([rrrr] = 84%, Tm = 140°C) via a chainend control mechanism. Conversely, upon activation with iBu3Al / Ph3CB(C6F5)4, a Hf-FI catalyst with a tert-butyl group ortho to the phenoxy-oxygen is able to produce high molecular weight isotactic polypropylenes ([mm] = 69%, Tm = 124°C, Mw = 412 000) via a site control mechanism. Therefore, FI catalysts have shown the ability to create various polyolefin architectures by simple variation of the central metal, the ligand structure and the co-catalyst.

Ethylene and propylene polymerization behavior of a series of bis(phenoxy–imine)titanium complexes

Abstract This contribution reports ethylene and propylene polymerization behavior of a series of Ti complexes bearing a pair of phenoxy–imine chelate ligands. The bis(phenoxy–imine)Ti complexes in conjunction with methylalumoxane (MAO) can be active catalysts for the polymerization of ethylene. Unexpectedly, this C2 symmetric catalyst produces syndiotactic polypropylene. 13 C NMR spectroscopy has revealed that the syndiotacticity arises from a chain-end control mechanism. Substitutions on the phenoxy–imine ligands have substantial effects on both ethylene and propylene polymerization behavior of the complexes. In particular, the steric bulk of the substituent ortho to the phenoxy–oxygen is fundamental to obtaining high activity and high molecular weight for ethylene polymerization and high syndioselectivity for the chain-end controlled propylene polymerization. The highest ethylene polymerization activity, 3240 kg/mol-cat h, exhibited by a complex having a t-butyl group ortho to the phenoxy–oxygen, represents one of the highest reported to date for Ti-based non-metallocene catalysts. Additionally, the polypropylene produced exhibits a Tm, 140 °C, and syndioselectivity, rrrr 83.7% (achieved by a complex bearing a trimethylsilyl group ortho to the phenoxy–oxygen) that are among the highest for polypropylenes produced via a chain-end control mechanism. Hence, the bis(phenoxy–imine)Ti complexes are rare examples of non-metallocene catalysts that are useful for the polymerization of not only ethylene but also propylene.

Syndiospecific living propylene polymerization catalyzed by titanium complexes having fluorine-containing phenoxy-imine chelate ligands.

The propylene polymerization behavior of a series of Ti complexes featuring fluorine-containing phenoxy-imine chelate ligands is reported. The Ti complexes combined with methylalumoxane (MAO) can be catalysts for living and, at the same time, stereospecific polymerization of propylene at room temperature or above. DFT calculations suggest that the attractive interaction between a fluorine ortho to the imine nitrogen and a beta-hydrogen of a growing polymer chain is responsible for the achievement of room-temperature living propylene polymerization. Although the Ti complexes possess C(2) symmetry, they are capable of producing highly syndiotactic polypropylenes. (13)C NMR is used to demonstrate that the syndiotacticity is governed by a chain-end control mechanism and that the polymerization is initiated exclusively via 1,2-insertion followed by 2,1-insertion as the principal mode of polymerization. (13)C NMR spectroscopy also elucidated that the polypropylenes produced with the Ti complexes possess regio-block structures. Substitutions on the phenoxy-imine ligands have profound effects on catalytic behavior of the Ti complexes. The steric bulk of the substituent ortho to the phenoxy oxygen plays a decisive role in achieving high syndioselectivity for the chain-end controlled polymerization. Over a temperature range of 0-50 degrees C, Ti complex having a trimethylsilyl group ortho to the phenoxy oxygen forms highly syndiotactic, nearly monodisperse polypropylenes (94-90% rr) with extremely high peak melting temperatures (T(m) = 156-149 degrees C). The polymerization behavior of the Ti complexes can be explained well by the recently proposed site-inversion mechanism for the formation of syndiotactic polypropylene by a Ti complex having a pair of fluorine-containing phenoxy-imine ligands.

Stereoselective ring-opening polymerization of racemic lactide using aluminum-achiral ligand complexes: exploration of a chain-end control mechanism.

Stereoselective polymerization of racemic lactide (rac-LA) was examined using Al-achiral ligand complexes. By introduction of substituents in aromatic rings of Schiff base ligands, a higher selectivity was obtained without any chiral auxiliaries in the catalyst via a chain-end control mechanism. The T(m) values (T(m) 170-192 degrees C) were comparable to or higher than that of homochiral polymer, poly(L-LA) (T(m) 162 degrees C), and a thermally more stable polylactide than poly(L-LA) was prepared from rac-LA.

Highly isotactic poly(vinyl alcohol). III: Heterogeneous cationic polymerization of tert-butyl vinyl ether

We studied the polymerization of tert-butyl vinyl ether (tBVE) and benzyl vinyl ether with heterogeneous catalysts, that is, modified Ziegler type (Vandenberg type) catalysts and metal sulfate–sulfuric acid complexes.Vandenberg type catalysts gave high molecular weight and highly isotactic poly(tBVE)s with relatively narrow molecular weight distribution at high temperature, and then the resultant poly(tBVE)s were converted into the stereoregular poly(vinyl alcohol)s (PVAs). With titanium based Vandenberg type catalyst, a relative high isotactic PVA, which has 52% triad isotacticity, was obtained from the poly(tBVE) polymerized at 30°C. It was found from NMR study that the content of the triad tacticity of PVAs derived from poly(tBVE) catalyzed by titanium based catalysts agreed with the value calculated from the chain-end control model (Bovey's model). This fact suggests that the steric structure of the adding monomer in this system is determined by same mechanism to homogeneous BF3 complexes catalysts system. On contrast to that, the metal sulfate–sulfuric acid complexes show significantly low activity to tBVE polymerization.

Novel Calcium Half-Sandwich Complexes for the Living and Stereoselective Polymerization of Styrene.

Tackling tacticity: The first well-defined heteroleptic benzylcalcium complex initiates the living polymerization of styrene. Chain-end control results in a polymer enriched in syndiotactic sequences. Stereo errors arise from fast inversion of the chiral carbanionic chain end. Increasing the styrene concentration accelerates the insertion and leads to a considerable reduction of the stereo errors.

Microstructure of Highly Syndiotactic “Living” Poly(propylene)s Produced from a Titanium Complex with Chelating Fluorine-Containing Phenoxyimine Ligands (an FI Catalyst)

Highly syndiotactic “living” poly(propylene)s were synthesized at 25°C using a bis[N-(3-tert-butylsalicylidene)-2,3,4,5,6-pentafluoroanilinato]titanium (IV) dichloride/MAO catalyst system, and microstructures of the polymer were analyzed by means of 13C NMR spectroscopy. The syndiotactic poly(propylene) contains isobutyl, isopentyl and propyl end groups, suggesting that the living polymerization of propylene was initiated via 1,2-insertion, followed by 2,1-insertion as the principal mode of polymerization. Pentad distribution analysis revealed that the syndiospecific polymerization proceeds under chain-end control.

Stereospecific Polymerization of Methylthiirane in Homogeneous Phase, 6.13C NMR Study of the Stereocontrol Mechanism: An Access to the Size of Stereoblocks

The polymerization of methylthiirane initiated by bis(isopropyl-(S)-cysteinato)cadmium is stereospecific and enantioasymmetric and occurs through a catalyst site control. Until now, the mechanism of polymerization could not be verified by NMR because the too low resolution of the spectra showed only a diad effect. 13C NMR at 125.76 MHz shows triad effects that permit the examination of the polymerization mechanism. Experimental triad contents do not fit either with the values calculated from the expected catalyst site control (Bernoullian statistics) or from a growing chain-end control (Markov 1st order statistics). This can be explained by the interconversion between active sites occurring in this homogeneous system. These interconversions lead to the formation of stereoblocks the length of which have been estimated by comparing experimental NMR results with those calculated from catalyst site control. We found about 38 interconversions for the studied polymer corresponding to DPn of stereoblocks of around 12–14.

13C and 2D NMR Analysis of Propylene Polymers Made with α-Diimine Late Metal Catalysts

Late transition metal catalysts bearing α-diimine ligands allow ethylene and α-olefin homo- and copolymerizations to polyolefins with unprecedented structures. The polypropylenes made with these new late metal catalysts have very complex microstructures that include combinations of features not seen in any known polypropylenes. These unusual structures include long branches, branches on branches including isobutyl branches, and moderate highly variable levels of 1,3-enchainment leading to runs of methylenes in the backbone of many different well-defined lengths. These features vary with the nature of the catalyst used for polymerization and with the polymerization conditions. Many of the polypropylenes are made primarily by 1,2-insertions to give syndiotactic placements via chain end control. A mechanistic description of catalyst behavior has been developed to explain these observed microstructures.

High molecular weight atactic polypropene synthesized with (η5-pentamethylcyclopentadienyl)tribenzyl titanium activated with MAO

The half-titanocene (η5-pentamethylcyclopentadienyl)tribenzyl titanium (Cp*TiBz3) with methylaluminoxane (MAO) as the cocatalyst was employed to catalyze propene polymerization at ambient pressure. A novel atactic polypropene elastomer with a high molecular weight (M̄w = 2 − 8 × 105) was produced. The effects of the polymerization conditions on the catalytic activity and polymer molecular weight are discussed. 13C NMR analysis confirmed that the catalyst system Cp*TiBz3/MAO produced atactic polypropenes, and the polymerization mechanism was in agreement with the Bernoullian process. The triad sequence distribution of the polymer was measured and found to be as follows: mm = 6.15%, mr = 40.87%, and rr = 52.98% (Bernoullian factor B = 1.03); this indicated that the insertion of propene with the catalyst system followed a chain-end control model. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 411–415, 2000

High molecular weight atactic polypropene synthesized with (?5-pentamethylcyclopentadienyl)tribenzyl titanium activated with MAO

The half-titanocene (η5-pentamethylcyclopentadienyl)tribenzyl titanium (Cp*TiBz3) with methylaluminoxane (MAO) as the cocatalyst was employed to catalyze propene polymerization at ambient pressure. A novel atactic polypropene elastomer with a high molecular weight (Mw = 2 − 8 × 105) was produced. The effects of the polymerization conditions on the catalytic activity and polymer molecular weight are discussed. 13C NMR analysis confirmed that the catalyst system Cp*TiBz3/MAO produced atactic polypropenes, and the polymerization mechanism was in agreement with the Bernoullian process. The triad sequence distribution of the polymer was measured and found to be as follows: mm = 6.15%, mr = 40.87%, and rr = 52.98% (Bernoullian factor B = 1.03); this indicated that the insertion of propene with the catalyst system followed a chain-end control model. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 411–415, 2000

Structure and morphology of poly(?-hydroxybutyrate) synthesized by ring-opening polymerization of racemic (R,S)-?-butyrolactone with distannoxane derivatives

Predominantly syndiotactic poly((R,S)-β-hydroxybutyrate) (PHB) was synthesized by ring-opening polymerization of racemic β-butyrolactone with distannoxane derivatives as catalysts. We have studied the polymerization of (R,S)-β-BL using distannoxane derivatives as catalysts and the effects of polymerization time on crude yield and molecular weight of the polymers obtained. Then, a more detailed study of the characterization of polymers obtained using hydroxy- and ethoxy-distannoxanes was performed. 13C NMR spectroscopy resolved stereosequences in synthetic PHB at the diad level for the carbonyl carbon and at the triad level for the methylene carbon. These analyses show that distannoxane catalysts produce preferentially syndiotactic polyesters (syndiotactic diads fraction from 0.56 to 0.61). Triad stereosequence distribution of PHB samples agrees favourably with the Bernoullian statistical model of chain-end control, where ideally Φ = 4(mm) (rr)/(mr + rm)2 = 1 for perfect chain-end control. Polymer samples synthesized from distannoxane catalysts are composed of two distinct transition endotherm components with peak temperatures of approximately 42 °C and 75 °C. The formation of two melting endotherms may be due to the presence of two different crystalline structures. © 2000 Society of Chemical Industry