13C NMR Study of Copolymers of Propene with Higher 1‐Olefins with New Microstructures by ansa‐Zirconocene Catalysts

Copolymers of propene and higher 1‐olefins (1‐butene, 1‐pentene, 1‐hexene, 1‐heptene, 1‐octene, and 4‐methyl‐1‐pentene (4M1P)), prepared with rac‐Et(Ind)₂ZrCl₂ ( EI ) and rac‐Me₂Si(2‐MeBenz‐[e]Ind)₂ZrCl₂ (MBI), were comprehensively analyzed by ¹³C NMR spectroscopy. These catalysts known to produce isotactic polypropene lead to two vastly different families of copolymers under the chosen experimental conditions. All copolymers produced with EI possessed comparable isotacticities as well as similar stereosequences. Furthermore, these copolymers did not significantly differ from those of the reference propene homopolymer. However, the copolymers produced with MBI significantly differed both from the reference propene homopolymer and among themselves. In fact, in contrast to the corresponding highly isotactic homopolymer, the whole series of copolymers was characterized by rather low isotacticities. The degree of tacticity was strongly influenced by the comonomer type. A detailed ¹³C NMR analysis revealed that these copolymers are constituted by highly isotactic sequences and by atactic sequences of various lengths. It is proposed that the formation of atactic sequences is triggered by the insertion of a comonomer. It seems likely that MBI can shift from a stereoselective to a non‐stereoselective state due to an interaction with higher linear or branched comonomer.

¹³C NMR spectra of propene/1‐pentene (a) and propene/4‐methyl‐1‐pentene (b) prepared with MBI .     

Unexpected Formation of Atactic Blocks in Propylene/1‐Pentene Copolymers from rac‐Me2Si(2‐MeBenz[e]Ind)2ZrCl2

Summary: Supplementary experimental findings are reported in addition to those presented in a previous paper dealing with the microstructure of propylene/1‐pentene copolymers prepared with rac‐Me₂Si(2‐MeBenz[e]Ind)₂ZrCl₂. The fractionation results which demonstrate that the atactic part of the copolymer can not be separated from the isotactic one confute the hypothesis suggested by other authors according to which the catalyst used could contain a percent of impurities of the meso (aspecific) form. This evidence is in agreement with the observed narrow polydispersities and the almost random distribution of the comonomers (r_P · r_Cm ≈ 1) which are typical of single site catalysts.

¹³C NMR methyl region of pentane‐soluble (a) and pentane‐insoluble (b) fractions of copolymer prepared with MBI .     

13C NMR Study of the Effect of Coordinating Solvents on Zirconocene‐Catalyzed Propene/1‐Hexene Copolymerization

The solvent effect observed in propene/1‐hexene copolymerizations performed with the isospecific catalyst rac‐Et(Ind)₂ZrCl₂/MAO is studied. A range of solvents with increasing donor character and steric hindrance has been tested, and their effect on copolymer yield, composition, and microstructure has been thoroughly analyzed. Our results demonstrate that the solvent can have a significant influence on the comonomer reactivities, even though the solvent polarity is not the relevant factor. At the same comonomer compositions in solution, polymerizations carried out in coordinating solvents (e. g., aromatic solvents), lead to the formation of products with considerably decreased content of 1‐hexene. The reduced incorporation of the higher α‐olefin is explained in terms of competition between the nucleophilic medium and the olefin monomer for coordination to the active polymerization site. These results give us valuable information regarding the mechanism of polymerization at the active centers.     

Analyses of Propene and 1‐Hexene Oligomers from Zirconocene/MAO Catalysts – Mechanistic Implications by NMR,SEC, and MALDI‐TOF MS

A series of Cp′(C₅H₅)ZrCl₂ and Cp′₂ZrCl₂ pre‐catalysts (Cp′ = C₅HMe₄, C₄Me₄P, C₅Me₅, C₅H₄^tBu, C₅H₃‐1,3‐^tBu₂, C₅H₂‐1,2,4‐^tBu₃) together with (C₅H₅)₂ZrCl₂ was used for the directed oligomerization of propene and 1‐hexene in comparative experiments. Oligomer characterization was carried out by ¹H NMR, SEC (GPC), MALDI‐TOF MS, cryoscopy and Raman spectroscopy. From ¹H NMR the nature and relative ratio of the double‐bond end group is determined together with M̄_n if every oligomer molecule contains such a double‐bond end group. Normally vinylidene double bonds (from β‐hydrogen elimination) are found. From ¹H NMR and MALDI‐TOF MS also vinyl end groups (from β‐methyl elimination) are observed in the case of oligopropenes with (C₄Me₄P)‐ (C₅H₅)ZrCl₂ and with the symmetrical methyl containing Cp′₂ZrCl₂ pre‐catalysts. The vinylidene/vinyl ratio depends on the ligand and increases from C₅HMe₄ (65/35) over C₄Me₄P (61/39) to C₅Me₅ (9/91). A comparison of M̄_n from ¹H NMR and SEC together with MALDI‐TOF MS shows that the phospholyl‐zirconocenes and (C₅HMe₄)₂ZrCl₂ also exhibit chain transfer to aluminium, thereby giving saturated oligomers.     

The Development of Stereoerrors During the Homo‐ and Copolymerization of Propylene Using C2‐Symmetric Metallocene Catalysts – A Cautionary Note

Summary: A recent paper described the development of atactic sequences during the copolymerization of propylene and 1‐pentene using the C₂‐symmetric metallocene catalyst rac‐dimethylsilyl‐bis(2‐methylbenz(e)indenyl) zirconium dichloride. We have found similar stereoerrors, and showed that atactic material could in fact be extracted from the material, indicating that the catalyst in question had probably isomerized in solution, leading to a mixture of isotactic and atactic polymers.

     

13C NMR Studies of Ethene/Norbornene Copolymers using 13C‐Enriched Monomers: Signal Assignments of Copolymers Containing Norbornene Microblocks of up to a Length of Three Norbornene Units

Ethene and norbornene were copolymerized at 0.8 bar ethene absolute pressure using metallocene catalysts that produce microstructures containing norbornene blocks, particularly rac‐Me₂Si[Ind]₂ZrCl₂/MAO, iPr[CpFlu]ZrCl₂/MAO and iPr[CpInd]ZrCl₂/MAO. Intensive ¹³C NMR spectroscopic investigations of the obtained copolymers were performed and norbornene di‐ and triblock sequences detected clearly. Several copolymerization experiments were carried out by using ¹³C₁‐enriched ethane, or ¹³C_5/6‐enriched norbornene, respectively. Thus, resonances of the norbornene carbon atoms C5/C6 and the ethene carbon atoms Ca/Cb, which overlap extensively in the ¹³C NMR spectra, could clearly be differentiated. In addition, further copolymerizations with natural monomers were performed at 2 bar ethene excess pressure to investigate the influence of the monomer concentration on the microstructure. By comparing the ¹³C NMR spectra of copolymers obtained under different conditions, assignments of most resonances were achieved. These ¹³C NMR studies made it possible to assign norbornene triblock resonances with differing stereochemical connections such as meso,meso and rac,meso which have not been mentioned in previous works. The results led to new insights about the copolymer microstructure and allow development of a scheme showing the ethene/norbornene microstructures depending on the various catalyst systems utilized, including the metallocenes iPr[(3‐iPr‐Cp)Flu]ZrCl₂ and iPr[(3‐tert‐But‐Cp)Ind]ZrCl₂ which have been applied in previous publications.     

Syndiospecific Homopolymerisation of Higher 1‐Alkenes with Two Different Bridged [(RPh)2C(Cp)(2,7‐tert‐BuFlu)]ZrCl2 Catalysts

Summary: Syndiotactic polymers of 1‐pentene, 1‐hexene and 1‐octene were obtained with the C_S‐symmetric metallocene catalysts [Ph₂C(Cp)(2,7‐tert‐Bu₂Flu]ZrCl₂ and [(4‐MePh)₂C(Cp)(2,7‐tert‐Bu₂Flu)]ZrCl₂, which have already been proven to give high molar masses and excellent tacticities in propene polymerisation. The monomers were polymerised in bulk and in solution processes at temperatures between 0 and 60 °C using methylaluminoxane as a cocatalyst. The effects on catalyst activity as well as on polymer microstructure and molar mass were determined. Polymers with high syndiotacticities and molar masses (up to 550 000 g · mol^?1 in the 1‐octene polymerisation) compared to other metallocenes were obtained.

     

Novel Dinuclear Half‐Titanocene‐Producing Styrene/Ethylene Copolymers Containing Syndiotactic Styrene/Styrene Sequences

The synthesis and characterization of a novel titanium complex, {[C₅(CH₃)₅]ClTi}₂O₂C₂(CH₃)₄(µ‐O) ( 2 ), which has two titanium metal centers linked by heteroleptic bridges, oxo and pinacolato, is described. The compound shows good stability under air. The 2/modified methylalumoxane (MMAO) catalytic system produces ethylene/styrene interpolymers (ESIs) containing syndiotactic styrene/styrene sequences (syndiotactic polystyrene blocks) with a high styrene content. To the best of our knowledge, this is the first example of a group‐4‐based catalytic system producing an ESI with syndiotactic polystyrene blocks.

     

New Group IV Metallocene Systems Active in the Copolymerization of α‐Olefins and Conjugated Dienes

Summary: Rac‐[CH₂(2,4‐di‐tert‐butyl‐cyclopentadienyl)₂]ZrCl₂ ( 1 ), rac‐[CH₂(3‐tert‐butyl‐4,5,6,7‐tetrahydroindenyl)₂]ZrCl₂ ( 2 ), rac‐[CH₂(3‐isopropyl‐4,5,6,7‐tetrahydroindenyl)₂]ZrCl₂ ( 3 ), and rac‐[CH₂(3‐methyl‐4,5,6,7‐tetrahydroindenyl)₂]ZrCl₂ ( 4 ) were synthesized and characterized by ¹H NMR analysis. These C₂ symmetric ansa‐zirconocenes, activated with methylaluminoxane (MAO), were able to promote copolymerization of ethene and 1,3‐butadiene. The structures of the copolymers are strongly influenced by zirconocene utilized as catalytic precursor. Depending on the reaction conditions polyethenes containing cyclopropane and cyclopentane rings, or 1,1‐ 1,3‐butadiene inserted constitutional units, were obtained.

Structures of the synthesized catalytic precursors.     

Stereochemical Pseudohexad 13C NMR Resonances and Regioregular Propylene/Ethylene‐1‐13C Copolymers

This paper reports the assignment of the ¹³C NMR resonances of the S_ββ and S_αγ methylenes observed in the spectra of copolymers of propylene with trace amounts of enriched ethylene‐1‐¹³C. The assignment is achieved by comparing the chemical shifts calculated by an empirical method (at the CH₂ stereochemical pseudohexad level), with the experimental spectra of regioregular copolymers with different stereochemical structure. The implications of the observed S_ββ and S_αγ resonances on the stereochemical polymerization mechanisms are also discussed.

     

Ethylene/1‐Hexene Copolymers Produced with MAO/(nBuCp)2ZrCl2 Supported on SBA‐15 Materials with Different Pore Sizes

The influence of the support architecture (structure and pore size) on the activity of MAO/(nBuCp)₂ZrCl₂ heterogeneous catalysts in the polymerization of ethylene and 1‐hexene is studied through the characterization of the polymers' microstructure. SBA‐15 silica‐based mesostructured materials are synthesized with different pore sizes and compared with commercial silica. All SBA‐15 supported catalysts lead to higher catalytic activities than amorphous silica and give rise to ethylene/1‐hexene copolymers with bimodal CCD, which suggests that the carrier structure plays an important role not only in the global rate of the process but also in the copolymer composition. The structures and pore sizes of the support show a marked influence on the polymer CCD shape.

     

Complex Branched Polyolefin Generated from Quasi‐living Polymerization of 4‐Methyl‐1‐pentene Catalyzed by α‐Diimine Palladium Catalyst

Branched α‐olefin 4‐methyl‐1‐pentene (4MP) is polymerized with a cationic α‐diimine palladium catalyst. The influences of polymerization parameters including reaction temperature and monomer concentration on polymer architecture are evaluated in detail. Increasing temperature and lowering monomer concentration can lead to the formation of more complex branched polyolefin because of chain walking. At 0 °C, complex branched polyolefins are generated from quasi‐living polymerization of 4MP with α‐diimine palladium catalyst.

     

Ethylene/4‐Methyl‐1‐pentene Copolymers by a “Constrained Geometry Catalyst”: Advances in 13C NMR Assignment

Ethylene/4‐methyl‐1‐pentene (E/4M1P) copolymers were synthesized by [Me₂Si(η₅‐Me₄C₅)‐(η₁‐N‐^tBu)TiCl₂] and methylalumoxane (MAO) catalyst ( CG ) and analyzed by ¹³C NMR. The low regio‐ and stereoregularity of CG leads to spectral multiplicities which can be ascribed not only to differences in comonomer sequences but also to the presence of regio‐ and stereoirregularities. The tendency of CG to give a nearly random comonomer distribution gives rise to so far undetectable alternating sequences. The complete set of assignments proposed allows a satisfactory characterization of variously microstructured E/4M1P copolymers and the evaluation of the amount of regioirregular sequences in addition to the full determination of dyad and triad copolymer composition.

     

Molar Mass and Microstructure Analysis of PI‐b‐PMMA Copolymers by SEC‐NMR

The online coupling of size‐exclusion chromatography and NMR is used to characterize block copolymers consisting of polyisoprene (PI) and poly(methyl methacrylate) (PMMA) regarding their distributions of molar mass (MMD) and chemical composition (CCD). Using the CCD, $\overline{M}_{\rm n}$ and $\overline{M}_{\rm w}$ are calculated on the basis of PI and PMMA homopolymer calibrations. The microstructure distribution of PMMA and the distribution of isomeric units of PI in dependence of molar mass is also demonstrated. Furthermore, a simulation analysis is presented for a bimodal eluting sample. It allows for full separation, quantification and molar mass determination of the coeluting co‐ and homopolymer fractions. The quantification of the fractions is verified by liquid chromatography at critical conditions.     

Selective Dimerization of γ‐Branched α‐Olefins in the Presence of C2v Group‐4 Metallocene‐Based Catalysts

Summary: The oligomerization of γ‐branched α‐olefins in the presence of catalytic systems based on group‐4 metallocenes with C_2v symmetry has been investigated. The highest reactivity was obtained by using dimethylsilyl‐bis(cyclopentadienyl)zirconium‐dichloride activated by methylaluminoxane. Highly regioregular dimers were selectively obtained for hindered γ‐branched monomers, while the less hindered ones produced higher molecular weight oligomers. A molecular modeling approach was used to rationalize the experimental results. In fact, a decrease in the β‐hydrogen elimination barrier and an increase in the insertion barrier with the monomer bulkiness were calculated.

General structure of the obtained dimers.     

Evidence of Intramolecular Cyclization in Copolymerization of Ethylene with 1,3‐Butadiene: Thermal Properties of the Resulting Copolymers

Summary: Ethylene and butadiene were copolymerized with a silylene‐bridged bis(fluorenyl) complex {[Me₂Si(C₁₃H₈)₂]NdCl} ( 1 ) in combination with various alkylating agents (BuLi/AlH(iBu)₂, (Bu)MgCl, Mg(Bu)(Oct)). Copolymers have a unique microstructure since they contain 1,2‐cyclohexane rings and unsaturations along the polymer chain. An intramolecular mechanism was proposed for the formation of six‐membered rings. The influence of these cyclic structures on thermal properties of the copolymers has been investigated.

Proposed mechanism for formation of 1,2‐cyclohexane rings.     

Ethene Co‐ and Terpolymerizations with TIBA‐Protected Norbornenemethanol and TIBA‐Protected Norbornenecarboxylic Acid Using Homogeneous Metallocene/MAO Catalyst Systems

Ethene copolymerizations were carried out with triisobutylaluminum (TIBA)‐protected norbornenemethanol and norbornenecarboxylic acid, respectively, using homogeneous metallocene/MAO catalyst systems. Moreover, ethene terpolymerizations with both polar norbornene derivatives were investigated for the first time. The metallocenes utilized, such as iPr[CpInd]ZrCl₂, iPr[(3‐iPr‐Cp)Ind]ZrCl₂, and iPr[(3‐tert‐But‐Cp)Ind]ZrCl₂ contain ligand frameworks of various sterical demands. The incorporation of polar monomers into the polymer chain was determined by NMR spectroscopic investigations. Kinetic and analytical results of the polymerization experiments revealed increasing activities and a decreasing comonomer incorporation into the polymer chain with an increasing sterical demand of the metallocene ligand. Additionally, the TIBA‐protected norbornenecarboxylic acid shows a lower incorporation rate into the copolymer chain in comparison with the TIBA‐protected norbornenemethanol.