Polymerization of 4‐Methyl‐1,3‐pentadiene with Catalysts Based on Cyclopentadienyl Titanium Chlorides: Effect of anti/syn Isomerism of the Allylic Group on the Chemoselectivity and the Role of Backbiting Coordination in 1,3‐Diene Polymerization

Summary: Homopolymerization of 4‐methyl‐1,3‐pentadiene (MP) and copolymerization of 4‐methyl‐1,3‐pentadiene with alkenes (ethylene, 1‐pentene, 4‐methyl‐1‐pentene) were performed to investigate the effect of the so‐called backbiting coordination on the chemoselectivity of 1,3‐diene polymerization. Three homogeneous catalyst systems were used: CpTiCl₃‐MAO, Cp₂TiCl₂‐MAO and Cp₂TiCl‐MAO. Backbiting coordination is possible with the first catalyst, but not with the other two. The three catalysts gave similar results, which indicates that backbiting has no effect on the polymerization chemoselectivity, contrary to what has been reported in recent literature. An interpretation is presented for the formation of 1,4 units in MP/alkene copolymers. This interpretation is based on the fact that allyl groups have predominantly a syn configuration in MP homopolymerization, whereas allyl groups of anti configuration are formed in MP/alkene copolymerization. The role of backbiting in diene polymerization is discussed.

The effect of anti/syn isomerism on the chemoselectivity in the different polymerizations.     

Polymerization of Butadiene with V(acac)3‐Methylaluminoxane Catalyst

Full Paper: Polymerization of butadiene (Bd) with V(acac)₃‐methylaluminoxane (MAO) catalyst was investigated. The microstructure of the polymer was found to depend strongly on the polymerization conditions. High polymerization temperature and high MAO/V(acac)₃ mole ratios were preferential to produce cis‐1,4 structure. Two kinds of active species were presumed from the GPC measurement of the polymers; one gives a high molecular weight polymer bearing high cis‐1,4 content, another affords a low molecular weight polymer with high trans‐1,4 content. A Solvent effect was observed. A polymer consisting of mainly trans‐1,4 unit was produced in CH₂Cl₂ in contrast to toluene and n‐heptane in which polymers consisting of mainly cis‐1,4 structure were observed. The addition of ethyl benzoate, triethylamine, chloranil, and tetracyanoquinodimethane to the V(acac)₃‐MAO catalyst gave an influence to not only the activity for the polymerization but also the microstructure of the polymer.     

Copolymerization of styrene and butadiene with Co(acac)3‐methylaluminoxane catalyst

The copolymerization of Styrene (St) and butadiene (Bd) with Co(acac)₃‐MAO catalyst was investigated. The copolymers consisting of St and Bd with highly cis‐1,4‐structure could be synthesized with Co‐(acac)₃‐MAO catalyst without formation of homopolymer, although the St contents in the copolymer were not high. The copolymer composition curve for copolymerization of St and Bd with the Co(acac)₃‐MAO is different from that obtained with the Ni(acac)₂‐MAO catalyst. The additive effects of triphenylphosphine (TPP) and trifluoroacetic acid (TFA) on the copolymerization of St and Bd with the Co(acac)₃‐MAO catalyst were also investigated. The copolymer yields increased by adding TPP to the Co‐(acac)₃‐MAO catalyst, although the St contents in the copolymer did not change. In the microstructure of the Bd units in the copolymers, 1,2‐contents increased remarkably. The copolymer yields and the microstructure of the copolymer did not change by addition of TFA.     

Polymerization of 1,3‐Butadiene Initiated by Neodymium Versatate/Diisobutylaluminium Hydride/Ethylaluminium Sesquichloride: Kinetics and Conclusions About the Reaction Mechanism

The kinetics of the polymerization of 1,3‐butadiene initiated by the ternary Ziegler–Natta catalyst system comprising neodymium versatate (NdV), diisobutylaluminium hydride (DIBAH) and ethylaluminium sesquichloride (EASC) have been studied in order to quantify the impact of the catalyst components EASC and DIBAH on the polymerization rate, the control of molar masses, the molar mass distributions as well as on the microstructure of the resulting polymer (cis‐1,4, trans‐1,4 and 1,2 content). A further focus of the work was on the elucidation of the living nature of the polymerization. It has been found that the catalyst component EASC influences the reaction rate and the microstructure of the obtained polybutadiene. The main effect of the variation of DIBAH is on the molar mass, but the polymerization rate and the microstructure are also influenced. Straight lines are obtained for the dependence of the molar masses on monomer conversion revealing the living nature of the polymerization. The theoretically predicted molar masses are significantly higher than those experimentally found. This discrepancy is explained by the existence of dormant species resulting from the reversible transfer of living polymer chains from Nd onto Al. Only one third of the DIBAH which is not consumed by the processes of scavenging of impurities and activation of the Nd catalyst, is involved in this transfer reaction. This makes DIBAH an inefficient chain control agent for the experimental conditions applied (namely 60°C). A reaction scheme is put forward which accounts for the features observed.     

cis‐Specific Living Polymerization of 1,3‐Butadiene with CoCl2 and Methylaluminoxane

The polymerization of 1,3‐butadiene was conducted by CoCl₂ combined with methylaluminoxane (MAO) as a cocatalyst at 0 and 18°C. The uni‐modal molecular weight distribution curves of the resulting polymers shifted toward higher molecular weight regions and became narrower when increasing the polymerization time. The number‐average molecular weight increased linearly with polymerization time, while the polymer yield increased exponentially in the initial stage. As a consequence, the number of polymer chains, calculated from the polymer yield and M̄_n, increased gradually with polymerization time to reach a plateau value. These phenomena was interpreted based on a slow initiation system without any termination and chain transfer reaction. The microstructure of the polymer was determined by ¹H NMR and ¹³C NMR spectroscopy to be a cis‐1,4 structure in a 98–99% purity.     

Zur Katalyse der stereospezifischen Butadienpolymerisation mit den Katalysatorsystemen aus Nd(η3-C3H5)3 · dioxan und methylaluminoxan (MAO) sowie Hexaisobutylaluminoxan (HIBAO)

The activation of the tris(allyl)neodymium complex Nd(η³-C₃H₅)₃ · dioxane with alkylaluminoxanes (MAO or HIBAO) results in highly selective catalysts for the 1,4-cis-polymerization of butadiene (cis-selectivity up to 80%). Under standard conditions (50°C, toluene), the turnover frequency (TOF) of the catalyst/MAO system amounts to 10–15000 mol butadiene/(mol Nd · h). Molecular weight determinations indicate the formation of only one polymer chain per neodymium center as in a living polymerization reaction, and for the catalyst/HIBAO system the rate law r_p = k_p [Nd][C₄H₆] with k_p = 8,7 · 10^?2 mol/(L · s) (at 25°C) has been derived. As the catalytically active species, a cationic monobutenyl neodymium(III) complex is discussed, which is stabilized through coordinative interaction with the counter anion as well as the growing polybutadiene chain. This cationic complex reacts under insertion with butadiene in a bimolecular fashion.     

Highly Stereoregular Polymerization of 1,3‐Cyclohexadiene in the Presence of Cp2Ni‐MAO Catalyst

Polymerizations of 1,3‐Cyclohexadiene, in the presence of Cp₂Ni (Cp = cyclopentadienyl) activated by methylaluminoxane (MAO), were carried out at different temperatures and Ni‐MAO mole ratios. The polymers obtained are stereoregular with a 1–4 structure, and are highly crystalline with a melting point close to 315°C. Copolymerizations of 1,3‐cyclohexadiene and butadiene with the title catalyst give polymers with a nearly random distribution of the comonomeric units.     

Additive Effect of Triphenylphosphine on the Living Polymerization of 1,3‐Butadiene with a Cobalt Dichloride‐Methylaluminoxane Catalytic System

The additive effect of triphenylphosphine (Ph₃P) was investigated in 1,3‐butadiene polymerization with cobalt dichloride (CoCl₂) activated by methylaluminoxane (MAO); the catalytic system resulted in slowly‐initiated living polymerization with a high cis‐1,4 content (about 99%) at 0 °C. The polymerization behavior, such as the relationship of polymer yield and number‐average molecular weight ( ) with polymerization time, was dependent on the amount of Ph₃P added. In the presence of a small amount of Ph₃P (Ph₃P/Co = 0.025), the kinetic behavior qualitatively resembled that observed in the absence of Ph₃P, which indicated that the catalytic system conducted slowly‐initiated living polymerization. An increase in the amount of Ph₃P (Ph₃P/Co > 0.025), however, caused chain transfer reaction. The ¹H NMR analysis of the produced polymers showed that Ph₃P changed the microtacticity from 99% of cis‐1,4 to 88% of 1,2‐unit, depending on the amount of Ph₃P added. At the maximum amount of Ph₃P (Ph₃P/Co = 0.025) necessary for maintaining the livingness of the propagating species, 14% of the 1,2‐inserted unit was incorporated at the expense of the cis‐1,4‐inserted unit. The addition of Ph₃P (Ph₃P/Co = 0.025) during the polymerization did not affect the livingness and gave a regio‐block copolymer of 1,3‐butadiene.

Number‐average molecular weight as a function of polymerization time.     

On the 1,4‐cis‐Polymerization of Butadiene with the Highly Active Catalyst Systems Nd(C3H5)2Cl·1.5 THF/Hexaisobutylaluminoxane (HIBAO), Nd(C3H5)Cl2· 2 THF/HIBAO and Nd(C3H5)Cl2·2 THF/Methyl‐aluminoxane (MAO) – Degree of Polymerization,Polydispersity, Kinetics and Catalyst Formation

The allylneodymium chloride complexes Nd(C₃H₅)₂Cl·1.5 THF and Nd(C₃H₅)Cl₂·2 THF can be activated by adding hexaisobutylaluminoxane (HIBAO) or methylaluminoxane (MAO) in a ratio of Al/Nd = 30 for the catalysis of butadiene 1,4‐cis‐polymerization. A turnover frequency (TOF) of about 20 000 mol butadiene/(mol Nd·h) and cis‐selectivity of 95–97% are achieved under standard conditions ([BD]₀ = 2 m, 35°C, toluene). Molecular weight determinations indicate a low polydispersity (M̄_w (LS)/M̄_n (LS) = 1–1.5), the formation of only one polymer chain per neodymium and the linear increase of the degree of polymerization (DP) with the butadiene conversion, as observed for living polymerizations. First indications of chain‐transfer reaction occur only at the highest conversion or degree of polymerization. The rate law r_P = k_P[Nd][C₄H₆]^1.8 is derived for the catalyst system Nd(C₃H₅)₂Cl·1.5 THF/HIBAO and for the system Nd(C₃H₅)Cl₂·2 THF/MAO the rate law r_P = k_P[Nd] [C₄H₆]² with k_P = 3.24 L²/(mol²·s) (at 35°C). Taking into account the Lewis acidity of the alkylaluminoxanes and the characteristic coordination number of 8 for Nd(III) in allyl complexes the formation of an η³‐butenyl‐bis(η⁴‐butadiene)neodymium(III) complex of the composition [Nd(η³‐RC₃H₄)(η⁴‐C₄H₆)₂(X‐{AlOR}_n)₂] is assumed to be a single‐site catalyst for the chain propagation by reaction of the coordinated butadiene via the π‐allyl insertion mechanism and the anti‐cis and syn‐trans correlation to explain the experimental results.     

Polymerizations of butadiene with Ni(acac)2-methylaluminoxane catalysts

Polymerizations of butadiene (BD) with transition metals (MtX) and methylaluminoxane (MAO) catalysts were studied. Among the catalysts examined, nicke(II) acetylacetonate [Ni(acac)₂] in combination with MAO revealed the highest catalytic activities for the polymerization of BD, giving high molecular weight polymers consisting of mainly cis-1,4-units in the microstructure. With Ni(acac)₂-MAO catalyst, the overall activation energy for the polymerization of BD between 0°C and 60°C was estimated to be 18 kJ/mol. From a kinetic study, the rate equation for the polymerization of BD with the Ni(acac)₂-MAO catalyst at 30°C can be expressed as follows: R_p = [BD]^1.8 [Ni(acac)₂-MAO]^0.9. The polymerization mechanism of the polymerization of BD with the Ni(acac)₂-MAO catalyst is discussed.     

Novel Methylaluminoxane‐Activated Neodymium Isopropoxide Catalysts for 1,3‐Butadiene Polymerization and 1,3‐Butadiene/Isoprene Copolymerization

The homo‐ and copolymerizations of 1,3‐butadiene and isoprene are examined by using neodymium isopropoxide [Nd(Oi‐Pr)₃] as a catalyst, in combination with a methylaluminoxane (MAO) cocatalyst. In the homopolymerization of 1,3‐butadiene, the binary Nd(Oi‐Pr)₃/MAO catalyst works quite effectively, to afford polymers with high molecular weight ( ≈ 10⁵ g mol^‐1), narrow molecular‐weight distribution (MWD) (/ = 1.4–1.6), and cis‐1,4‐rich structure (87–96%). Ternary catalysts that further contain chlorine sources enhance both catalytic activity and cis‐1,4 selectivity. In the copolymerization of 1,3‐butadiene and isoprene, the copolymers feature high , unimodal gel‐permeation chromatography (GPC) profiles, and narrow MWD. Most importantly, the ternary Nd(Oi‐Pr)₃/MAO/t‐BuCl catalyst affords a copolymer with high cis‐1,4 content in both monomer units (>95%).

     

Optimization of the Polymerization Parameters for Syndiotactic Polymerization of Styrene Using Supported Catalysts

The two catalytic systems Ti(OBu)₄ and CpTiCl₃ in combination with methylaluminoxane (MAO) were tested and compared in syndiotactic polymerization of styrene. In homogeneous conditions, CpTiCl₃ gives higher productivities than Ti(OBu)₄. The productivity of CpTiCl₃ increases continuously with the Al/Ti ratio. On the contrary, confirming previous results, the productivity of Ti(OBu)₄ presents an optimum for an Al/Ti ratio of about 230. To optimize the syndiotactic polymerization of styrene using these supported catalysts, different parameters were examined: we show that a high concentration of MAO for heterogeneous conditions is necessary to ensure a good reproducibility of the polymerization, the role of transfer agent of the MAO was then demonstrated. Polymerizations carried out at different temperatures show that 50°C is a good compromise: when the temperature is raised the productivity increases but the syndiotacticity drops. At 50°C, the productivity is high enough and the syndiotacticity is above 90%, at 60°C, the productivity is a little higher but the syndiotacticity drops below 90%. Increasing the temperature does not influence the molecular weights showing that supporting Ti(OBu)₄ or CpTiCl₃ will stabilize the active species. As hydrogen behaves as a transfer agent for the conventional Ziegler catalysts, we carried out some experiments adding hydrogen during the polymerization. Indeed, the molecular weights are lowered. The melting points of the polymers depend as expected on the syndiotacticity and in a lower extent on the molecular weights.     

Blends of Syndiotactic Polystyrene and Isotactic Polypropene Prepared Using a Ziegler‐Natta Catalyst Containing a Novel Donor

Blends of syndiotactic polystyrene (sPS) and isotactic polypropene (iPP) have been prepared using TiCl₄/MgCl₂/β‐diketone activated with methylaluminoxane (MAO). The influences of the Al/Ti ratio and the polymerization temperature on the catalyst activity and the blend composition have been investigated. It is shown that the polymerization temperature is an important factor in determining catalyst activity and the blend composition. Through controlling the polymerization conditions, blends containing a wide composition range of styrene/propene molar fractions can be prepared. The supported catalyst shows a good activity of about 1.5×10⁵ (g blend)·(mol Ti)^–1·h^–1. ¹³C NMR analysis allows the composition of the blends to be calculated. Dynamic mechanical analysis (DMA) reveals that the blends of sPS/iPP prepared this way are partially compatible; the two glass transition temperatures (T_g's) of the components shift towards each other. The values of T_g, as well as the peaks of sPS intensity, increase with an increasing molar fraction of styrene in the blend. When the sPS content is in the range of 59–80 wt.‐%, dual phase continuity occurs in the blends. The morphology found by microscopic observation is consistent with the DMA results.     

Syndiotactic styrene‐butadiene block copolymers synthesized with CpTiX3/MAO (Cp = C5H5, X = Cl,F; Cp = C5Me5, X = Me) and TiXn/MAO (n = 3, X = acac; n = 4, X = O‐tert‐Bu)

Copolymers sPS‐B consisting of blocks of syndiotactic polystyrene (sPS) and polybutadiene (B) have been prepared using CpTiX₃ (Cp = C₅H₅, X = Cl, F; Cp = C₅Me₅, X = Me) and TiX_n (n = 3, X = acetylacetonate (acac); n = 4, X = O‐tert‐Bu) activated with methylaluminoxane (MAO). If proper conditions are used, copolymers containing a range of styrene and butadiene molar fractions can be prepared. Structural analysis of these copolymers by means of ¹³C NMR spectroscopy allowed the assignment of different monomer diads (SS, SB, BB; S = styrene, B = butadiene) and the calculation of reactivity ratio products r₁·r₂. Differential scanning calorimetry (DSC) analysis further confirmed the block‐like structure of these copolymers. The melting points (T_m) of syndiotactic styrene sequences decrease as the styrene molar fraction decreases, whereas the glass transition temperature (Tg) increases with decreasing butadiene molar fraction in the copolymer. The polydispersity values (M_w/M_n) determined by GPC suggest that these copolymers are produced by a single site catalyst.     

Retarded Anionic Polymerization: Copolymerization of Butadiene and Styrene in the Presence of Alkyllithium and n,s‐Dibutylmagnesium or Triisobutylaluminium Derivatives

Summary: The influence of Lewis acid additives on the anionic butadiene polymerization using lithium as a counter ion in non‐polar solvent is investigated. A decrease of the polymerization rate was always observed. In the presence of n,s‐Bu₂Mg the percentage of 1,2‐vinyl units increases with the [Mg]/[Li] ratio. This evolution can be explained by further complexation of lithium with free dialkylmagnesium modifying the nature of the propagating species and/or possible 1,4 to 1,2 isomerization during chain exchanges between lithium and magnesium derivatives. As a result of the transmetallation process, 0.5 to 1 supplementary chains are formed by magnesium depending on the [Mg]/[Li] ratio. In contrast, the ⁱBu₃Al/RLi system does not yield any modification of polybutadiene microstructure, which remains close to the one observed with s‐BuLi alone. The determination of the reactivity ratios in styrene‐butadiene copolymerization shows that tapered‐like copolymers are obtained in the presence of both n,s‐Bu₂Mg and ⁱBu₃Al. However the magnesium derivative induces a lesser increase of styrene incorporation in the copolymer than the aluminium additive. The observed microstructure and reactivity ratios support that the monomer insertion proceeds directly into heterocomplexes.

     

Comparison of the Polymerization of Propene by Homogeneous and Heterogeneous Metallocene/MAO‐Catalysts under Different Polymerization Conditions

Propene was polymerized using rac‐dimethylsilylbis(2‐methyl‐4‐(1‐naphtyl)indenyl)zirconium dichloride (rac‐[Me₂Si(2‐Me‐4‐(1‐Naphtyl)Ind)₂]ZrCl₂) under six sets of conditions: in toluene solution, bulk, toluene slurry, bulk with the supported metallocene, a stirred bed with polyethene and a stirred bed with NaCl. The first two procedures were carried out with methylaluminoxane (MAO) as cocatalyst, the latter four were performed with methylaluminoxane supported on silicagel (MAO/SiO₂) as activator. The differences between the procedures employed were examined by comparison of the polymer properties of the resulting products and of the activities at different temperatures. The polymerization procedure has a significant influence on the products. The polypropenes obtained with the homogeneous catalyst systems have generally high melting points and molar masses and exhibit very high activities. The heterogeneous analogues, on the other hand, gain enhanced stability by supporting, even at higher temperatures.     

Binary copolymerizations of styrene and conjugated diolefins in the presence of cyclopentadienyltitanium trichloride-methylaluminoxane

Binary copolymerizations of styrene, isoprene, butadiene and 4-methyl-1,3-pentadiene, in the presence of the catalyst cyclopentadienyltitanium trichloride-methylaluminoxane (CpTiCl₃-MAO), are investigated. The reactivity ratios as well as the reactivities of the different monomers in homopolymerization are tentatively correlated to a possible polymerization mechanism. A qualitative explanation of the reactivity of different monomers in homo- and copolymerization is achieved by considering the nucleophilicity of the monomers and the electrophilicity of the active species. The latter is affected by the nature of the bond between the active Ti and the last unit of the growing chain end. The block copolymerization of ethylene with styrene is also tentatively explained by considering that the more the active species are electrophilic the less they should be selective toward monomers of different nucleophilicity.     

Effect of Different Aluminum Alkyls on the Metallocene/Methylaluminoxane Catalyzed Polymerization of Higher α‐Olefins and Styrene

The replacement of MAO by different aluminum alkyls in the polymerization of higher α‐olefins catalyzed by rac‐EtInd₂ZrCl₂ and rac‐(CH₃)₂CInd₂ZrCl₂ and in the syndiospecific polymerization of styrene with CpTiCl₃ as a catalyst was studied. An activating effect for all catalysts investigated was found when adding TIBA, while TEA and THA have a deactivating effect. It was found that more than half of the expensive methylaluminoxane can be successfully replaced by TIBA without deleterious effects on the catalyst activity and the properties of the polymers synthesized during higher α‐olefin polymerization catalyzed by rac‐EtInd₂ZrCl₂ and rac‐(CH₃)₂CInd₂ZrCl₂.

     

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.     

NMR Characterization of Carbazole‐Substituted Norbornene Comonomer 9‐(Bicyclo[2.2.1.]hept‐5‐en‐2‐ylmethyl)‐9H‐carbazole (BHMCZ) and Poly(ethylene‐co‐BHMCZ) Copolymer

Carbazole‐substituted norbornene comonomer 9‐(bicyclo[2.2.1.]hept‐5‐en‐2‐ylmethyl)‐9H‐carbazole (BHMCZ) can be copolymerized with ethylene using the [Ph₂C(Ind)(Cp)ZrCl₂] catalyst and methylaluminoxane (MAO) cocatalyst system. The microstructures of BHMCZ comonomer and of ethylene–BHMCZ copolymer containing 4.6 mol‐% BHMCZ units in the chain were characterized by one‐dimensional ¹³C DEPT and two‐dimensional homonuclear ¹H‐¹H COSY and heteronuclear ¹H‐¹³C HXCO NMR spectroscopy. The BHMCZ comonomer appears as endo and exo stereoisomers, which were identified on the basis of chemical shifts, signal multiplicities, and coupling in the 2D NMR spectra. The NMR information on the BHMCZ isomers was used to assist in the determination of the chemical shifts and microstructure of ethylene–BHMCZ copolymer. The NMR analysis of ethylene–BHMCZ copolymer indicated that exo‐BHMCZ polymerizes with ethylene slightly more readily than does endo‐BHMCZ under the polymerization conditions employed.

Isolated segments of exo(2)‐exo(5)‐exo(6) and endo(2)‐exo(5)‐exo(6) ethylene‐BHMCZ copolymer showing carbon atom numbering.