Propene polymerization using homogeneous MAO-activated metallocene catalysts: Me2Si(Benz[e]Indenyl)2ZrCl2/MAO vs. Me2Si(2-Me-Benz[e]Indenyl)2ZrCl2/MAO

Propene was polymerized at 40°C and 2-bar propene in toluene using methylalumoxane (MAO) activated rac-Me₂Si(Benz[e]Indenyl)₂ZrCl₂ ( BI ) and rac-Me₂Si(2-Me-Benz[e]Indenyl)₂ZrCl₂ ( MBI ). Catalyst BI /MAO polymerizes propene with high activity to afford low molecular weight polypropylene, whereas MBI /MAO is less active and produces high molecular weight polypropylene. Variation of reaction conditions such as propene concentration, temperature, concentration of catalyst components, and addition of hydrogen reveals that the lower molecular weight polypropylene produced with BI /MAO results from chain transfer to propene monomer following a 2,1-insertion. A large fraction of both metallocene catalyst systems is deactivated upon 2,1-insertion. Such dormant sites can be reactivated by H₂-addition, which affords active metallocene hydrides. This effect of H₂-addition is reflected by a decreasing content of head-to-head enchainment and the formation of polypropylene with n-butyl end groups. Both catalysts show a strong dependence of activity on propene concentration that indicates a formal reaction order of 1.7 with respect to propene. MBI /MAO shows a much higher dependence of the activity on temperature than BI /MAO. At elevated temperatures, MBI /MAO polymerizes propene faster than BI /MAO. © 1995 John Wiley & Sons, Inc.     

Random propene/4‐methyl‐1‐pentene copolymers synthesized with C2 symmetric highly isospecific metallocenes

Propene (P)/4‐methyl‐1‐pentene (Y) copolymers in a wide range of composition were prepared with isospecific single center catalysts, rac‐Et(IndH₄)₂ZrCl₂ ( EBTHI ), rac‐Me₂Si(2‐Me‐BenzInd)₂ZrCl₂ ( MBI ), and rac‐CH₂(3‐^tBuInd)₂ZrCl₂ ( TBI ). ¹³C NMR analysis of copolymers and statistical elaboration of microstructural data at triad level were performed. Unprecedented and surprising results are here reported. Random P/Y copolymers were prepared with the most isospecific catalyst, TBI , that is known to prepare ethene/propene and ethene/4‐methyl‐1‐pentene copolymers with long homosequences of both comonomers, whereas longer homosequences of both comonomers were observed in copolymers from the less enantioselective metallocenes EBTHI and MBI . These findings, which are against what is acknowledged in the field, can pave the way for the preparation on a large scale of random propene‐based copolymers. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2575–2585     

Toward block copolymers from nonliving isospecific single‐site catalytic systems

Ethene/1‐olefin blocky copolymers were obtained through nonliving insertion copolymerizations promoted by an isospecific single site catalyst. Propene or 4‐methyl‐1‐pentene were copolymerized with ethene with metallocenes endowed with different stereospecificity in propene polymerization: (i) aspecific “constrained geometry” half‐sandwich complex, {η¹:η⁵‐([tert‐butyl‐amido)dimethylsilyl](2,3,4,5‐tetramethyl‐1‐cyclopentadienyl)}titanium dichloride [Me₂Si(Me₄Cp)(N‐^tBu)TiCl₂] ( CG ), (ii) moderately isospecific rac‐ethylenebis(indenyl)zirconium dichloride [rac‐(EBI)ZrCl₂] ( EBI ), (iii) slightly more isospecific hydrogenated homologue, rac‐ethylenebis(tetrahydroindenyl)zirconium dichloride [rac‐(EBTHI)ZrCl₂] ( EBTHI ), (iv) highly iso‐specific rac‐[methylenebis(3‐tert‐butyl‐1‐indenyl)]zirconium dichloride [rac‐H₂C‐(3‐^tBuInd)₂ZrCl₂] ( TBI ), (v) most isospecific rac‐[isopropylidene‐bis(3‐tert‐butyl‐cyclopentadienyl)]zirconium dichloride [rac‐Me₂C‐(3‐^tBuCp)₂ZrCl₂] ( TBC ). Copolymerizations were described by a 2^nd order Markovian copolymerization model and data are proposed to correlate the formation of 1‐olefin sequences with catalytic site isospecificity, made by the cooperation of organometallic complex and growing chain. Blocky copolymers were prepared over wide ranges of compositions: with any of the isospecific metallocenes when 4‐methyl‐1‐pentene was the 1‐olefin and only with the highly isospecific ones ( TBI , TBC ) when propene was the comonomer. A penultimate unit effect was observed with TBI as the metallocene, whereas a 1^st order Markov model described the ethene/propene copolymerization from TBC . A moderately isospecific metallocene, such as EBI , is shown to be able to prepare blocky ethene copolymers with 4‐methyl‐1‐pentene. These results pave the way for the synthesis of new ethene based materials. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2063–2075, 2010     

Effect of co-catalyst system on α-olefin polymerization with rac- and meso-[dimethylsilylenebis(2,3,5-trimethyl-cyclopentadienyl)]zirconium dichloride

Propene, 1-butene and 1-hexene polymerization was conducted with a mixture of rac- and meso-[dimethylsilylenebis((2,3,5-tetramethyl-cyclopentadienyl))]zirconium dichloride (Me₂Si(2,3,5-Me₃Cp)₂ZrCl₂) ( 1 ) combined with methylaluminoxane (MAO), triethylaluminium (AlEt₃)/triphenylcarbenium tetrakis(pentafluorophenyl)borate (Ph₃CB(C₆F₅)₄) ( 2 ) and triisobutylaluminium (AliBu₃)/Ph₃CB(C₆F₅)₄, respectively, as co-catalyst systems. The ratios of polymerization rates R_p(rac)/R_p(meso) were changed with the combined cocatalysts. It was found that in the case of using trialkylaluminium/ 2 as co-catalyst R_p(rac)/R_p(meso) is lower than when using MAO in any kind of α-olefin polymerization.     

Crystal structure of ansa-Me2Si-(2Me-4-p-Tol-cyclopenta[b]indol-3-yl)2ZrCl2 and its catalytic properties in the polymerization of propylene

A new zirconocene ansa-Me₂Si-(2Me-4-p-Tol-cyclopenta[b]indol-3-yl)₂ZrCl₂ complex (I), in which the Cp ligand is fused with the indole ring, has been synthesized and studied by X-ray diffraction analysis. Light brown crystals are triclinic, space group P [`1]\bar 1; M = 734.92, a = 9.252(2) ?, b = 12.914(3) ?, c = 15.619(3) ?, α = 111.83(3)°, β = 81.03(3)°, γ = 117.77(3)°, V = 1569(3) ?³, Z = 2, ρ_calc = 1.525 g/cm³. The structural parameters of complex I are compared with the known bis-indenyl zirconium complexes: rac-Me₂Si(Ind)₂ZrCl₂ (II) and rac-Me₂Si(2Me-2Ph-1-Ind)₂ZrCl₂ (III) and analogous substituted rac-Me₂Si(2,5-Me₂-3Ph-6-Cp[b]Tp)₂ZrCl₂ (IV) and rac-Me₂Si(2,5-Me₂-1Ph-4-Cp[b]Py)₂ZrCl₂ (V). Complex I alkylated by the Grignard reagent (MgMe₂) in the presence of the Al-iso-Bu₃ cocatalyst is an efficient catalyst for the polymerization of propylene into isotactic polypropylene.     

Co- and terpolymerizations of ethene and α-olefins with metallocenes

The suitability of the (n-butCp)₂ZrCl₂/methylaluminoxane (MAO) catalyst system for the copolymerization of ethene with propene, hexene, and hexadecene was studied and Ind₂ZrCl₂/MAO was tested as a catalyst for ethene/propene and ethene/hexene copolymerizations. The synergistic effect of longer α-olefin on propene incorporation in ethene/propene/hexene and ethene/propene/hexadecene terpolymerizations was investigated with Et(Ind)₂ZrCl₂MAO and (n-butCp)₂ZrCl₂/MAO catalyst systems. The molar masses, molar mass distributions, melting points, and densities of the products were measured. The incorporation of comonomer in the chain was further studied by segregation fractionation techniques (SFT), by differential scanning calorimetry (DSC), studying the β relaxations by dynamic mechanical analysis (DMA) and by studying the microstructure of some copolymers by ¹³C-NMR. In this study (n-butCp)₂ZrCl₂ and Ind₂ZrCl₂ exhibited equal response in copolymerization of ethene and propene and both catalysts were more active towards propene than longer α-olefins. A nearly identical incorporation of propene in the chain was found for the two catalysts when a higher propene feed was used. A lower hexene feed gave a more homogeneous comonomer distribution curve than a higher hexene feed and also showed the presence of branching. In terpolymerizations catalyzed with (n-butCp)₂ZrCl₂, the hexadecene concentrations of the ethene/propene/hexadecene terpolymers were always very low, and only traces of hexene were detected in ethene/propene/hexene terpolymers. With hexene no clear synergistic effect on the propene incorporation in the terpolymer was detected and with hexadecene the effect of the longer α-olefin was even slightly negative. With an Et(Ind)₂ZrCl₂/MAO catalyst system both hexene and hexadecene were incorporated in the chain in the terpolymerizations. © 1997 John Wiley & Sons, Inc.     

Synthesis of oxazoline functionalized polypropene using metallocene catalysts

Using two different zirconocene/MAO catalyst systems, propene was copolymerized with the comonomers 2‐(9‐decene‐1‐yl)‐1,3‐oxazoline and 2‐(4‐(10‐undecene‐1‐oxo)phenyl)‐1,3‐oxazoline, respectively. The catalysts used were rac‐Et[Ind]₂ZrCl₂ and rac‐Me₂Si[2‐Me‐4, 5‐BenzInd]₂ZrCl₂. Up to 0.53 mol‐% oxazoline could be incorporated into polypropene. Oxazoline content, molecular weight, degree of isotacticity and melting behavior were dependent on the catalyst system, comonomer structure and comonomer concentration in the feed.     

SYNTHESIS AND PROPERTIES OF HOMOPOLYMER AND COPOLYMERS OBTAINED FROM 1-OCTADECENE WITH ZIRCONOCENE CATALYSTS

Studies related to the behavior of different metallocene catalysts for the homopolymerization of 1-octadecene andits copolymerization with ethylene will be presented. The metallocenes: rac-Et(Ind)_2ZrCl_2, rac-Me_2Si(Ind)_2ZrCl_2 andPh_2C(Flu)(Cp)ZrCl_2 were chosen for the homopolymerization study. They show important differences in catalytic activity athigh temperatures (≥70℃), with rac-Et(Ind)_2ZrCl_2 showing the highest activity. At lower temperatures (≤30℃) thedifferences are negligible. For the copolymerization of ethylene with 1-octadecene only the catalysts rac-Et(Ind)_2ZrCl_2 andrac-Me_2Si(Ind)_2ZrCl_2 were studied. The results show that their catalytic activity is just like that for the homopolymerizationof 1-octadecene, with higher activity for the metallocene with the Et-bridged catalyst. ~(13)C-NMR analysis shows that thecomposition of the copolymerization products depends on the catalytic systems. Copolymers obtained with rac-Me_2Si(Ind)_2ZrCl_2 have greater comonomer incorporation. Thermal analysis shows that poly-1-octadecene synthesized withthe catalyst rac-Et(Ind)_2ZrCl_2 is very dependent on the polymerization temperature. The homopolymer obtained at 70℃presents two endothermal peaks at 41℃ and 53℃, as compared with the one obtained at 30℃ which presents one wider peakwith a maximum at 67℃. For the catalyst rac-Me_2Si(Ind)_2ZrCl_2 this trend is not observed. The type of metallocene and thereaction time do not significantly change the intrinsic viscosity, but the polymerization temperature changes it drastically,giving higher values at lower temperature. Viscosity measurements on the copolymers show that an increase of comonomerconcentration in the feed reduces the molecular weight of the copolymer, and it was also found that for homopolymer, themolecular weight is independent of the catalytic systems.     

Copolymerization of vinylcyclohexane with ethene and propene using zirconocene catalysts

Vinylcyclohexane (VCH) was copolymerized with ethene and propene using methylaluminoxane‐activated metallocene catalysts. The catalyst precursor for the ethene copolymerization was rac‐ethylenebis(indenyl)ZrCl₂ ( 1 ). Propene copolymerizations were further studied with C_s‐symmetric isopropylidene(cyclopentadienyl)(fluorenyl)ZrCl₂ ( 2 ), C₁‐symmetric ethylene(1‐indenyl‐2‐phenyl‐2‐fluorenyl)ZrCl₂ ( 3 ), and “meso”‐dimethylsilyl[3‐benzylindenyl)(2‐methylbenz[e]indenyl)]ZrCl₂ ( 4 ). Catalyst 1 produced a random ethene–VCH copolymer with very high activity and moderate VCH incorporation. The highest comonomer content in the copolymer was 3.5 mol %. Catalysts 1 and 4 produced poly(propene‐co‐vinylcyclohexane) with moderate to good activities [up to 4900 and 15,400 kg of polymer/(mol of catalyst × h) for 1 and 4 , respectively] under similar reaction conditions but with fairly low comonomer contents (up to 1.0 and 2.0% for 1 and 4 , respectively). Catalysts 2 and 3 , both bearing a fluorenyl moiety, gave propene–VCH copolymers with only negligible amounts of the comonomer. The homopolymerization of VCH was performed with 1 as a reference, and low‐molar‐mass isotactic polyvinylcyclohexane with a low activity was obtained. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6569–6574, 2006     

Synthesis and characterization of ethylene‐propylene‐1‐pentene terpolymers

Ethylene (E), propylene (P), and 1‐pentene (A) terpolymers differing in monomer composition ratio were produced, using the metallocenes rac‐ethylene bis(indenyl) zirconium dichloride/methylaluminoxane (rac‐Et(Ind)₂ZrCl₂/MAO), isopropyl bis(cyclopentadienyl)fluorenyl zirconium dichloride/methylaluminoxane (Me₂C(Cp)(Flu)ZrCl₂/MAO, and bis(cyclopentadienyl)zirconium dichloride, supported on silica impregnated with MAO (Cp₂ZrCl₂/MAO/SiO₂/MAO) as catalytic systems. The catalytic activities at 25 °C and normal pressure were compared. The best result was obtained with the first catalyst. A detailed study of ¹³C NMR chemical shifts, triad sequences distributions, monomer‐average sequence lengths, and reactivity ratios for the terpolymers is presented. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 947–957, 2008     

Evidence of the Quasi‐Living Character of the ansa‐Zirconocene/MAO‐Catalyzed Copolymerization of Ethylene and Norbornene

The kinetics of the ethylene‐norbornene copolymerization, catalyzed by rac‐Et(Ind)₂ZrCl₂/MAO, 90%rac/10%meso‐Et(4,7‐Me₂Ind)₂ZrCl₂/MAO and rac‐H₂C(3‐tert‐BuInd)₂ZrCl₂/MAO was followed by sampling from the reaction mixture at fixed time intervals. Catalyst activity, copolymer composition and molar mass were studied as a function of time. The polymers showed an unusually low polydispersity and a significant increase in their molar mass with time, suggesting a quasi‐living polymerization.     

Copolymerization of propene and 1-hexene using metallocene amide compounds

Copolymerization of propene and 1-hexene has been carried out at 30°C in toluene under atmospheric pressure by using three isospecific metallocene amide compounds, rac-(EBI)Zr(NMe₂₎₂ (EBI = ethylenebis(1-indenyl), rac- 1 ), rac-(EBI)Zr(NC₄H₈₎₂ (rac- 2 ), and rac-Me₂Si(1-C₅H₂-2-Me-4-t-Bu)₂Zr(NMe₂₎₂ (rac- 3 ), in the presence of methylaluminoxane (MAO) or [Ph₃C][B(C₆F₅)₄]. The rate enhancements in the presence of 1-hexene were recorded as a function of the catalytic systems. The incorporation of 1-hexene decreases in the following order: rac- 2 /MAO > rac- 3 /Al(i-Bu)₃/[Ph₃C][B(C₆F₅)₄] > rac- 1 /MAO. All copolymers investigated in this study have a nearly random sequence distribution.     

Low-temperature isospecific polymerization of propylene catalyzed by alkylzirconocene-type ‘cations’

The rac-ethylenebis(indenyl)methylzirconium ‘cation’ (1), generated from rac-Et(Ind)₂ZrMe₂ and Ph₃CB(C₆F₅)₄, has recently been shown to be exceedingly active and stereoselective in propylene polymerization. The ethyl analog (2) can be produced by an alternate, efficient route involving a reaction between rac-Et(Ind)₂ZrCl₂ and AlEt₃ (TEA), followed by addition of Ph₃CB(C₆F₅)₄. The use of excess AlEt₃ serves both to alkylate the zirconium complex as well as to scavenge the system. The propylene polymerization activity of the ‘cation’ 2 is about 7000 times greater than the activity of rac-Et(Ind)₂ZrCl₂/methylaluminoxane (MAO) at T_p=?20°C. The related catalyst system rac-Me₂Si(Ind)₂ZrCl₂/TEA/Ph₃CB(C₆F₅)₄ (3) was found to produce 98.3% i-PP with T_m 156.3°C and an activity of 1.8 × 10⁹ g PP {(mol Zr) [C₃H₆]h}^?1.     

H-, C-NMR and ethylene polymerization studies of zirconocene/MAO catalysts: effect of the ligand structure on the formation of active intermediates and polymerization kinetics

Using ¹H- and ¹³C-NMR spectroscopies, cationic intermediates formed by activation of L₂ZrCl₂ with methylaluminoxane (MAO) in toluene were monitored at Al/Zr ratios from 50 to 1000 (L₂ are various cyclopentadienyl (Cp), indenyl (Ind) and fluorenyl (Flu) ligands). The following catalysts were studied: (Cp-R)₂ZrCl₂ (R=Me, 1,2-Me₂, 1,2,3-Me₃, 1,2,4-Me₃, Me₄, Me₅, n-Bu, t-Bu), rac-ethanediyl(Ind)₂ZrCl₂, rac-Me₂Si(Ind)₂ZrCl₂, rac-Me₂Si(1-Ind-2-Me)₂ZrCl₂, rac-ethanediyl(1-Ind-4,5,6,7-H₄)₂ZrCl₂, (Ind-2-Me)₂ZrCl₂, Me₂C(Cp)(Flu)ZrCl₂, Me₂C(Cp-3-Me)(Flu)ZrCl₂ and Me₂Si(Flu)₂ZrCl₂. Correlations between spectroscopic and ethene polymerization data for catalysts (Cp-R)₂ZrCl₂/MAO (R=H, Me, 1,2-Me₂, 1,2,3-Me₃, 1,2,4-Me₃, Me₄, Me₅, n-Bu, t-Bu) and rac-Me₂Si(Ind)₂ZrCl₂ were established. The catalysts (Cp-R)₂ZrCl₂/AlMe₃/CPh₃⁺B(C₆F₅)₄⁻ (R=Me, 1,2-Me₂, 1,2,3-Me₃, 1,2,4-Me₃, Me₄, n-Bu, t-Bu) were also studied for comparison of spectroscopic and polymerization data with MAO-based systems. Complexes of type (Cp-R)₂ZrMe⁺←Me⁻-Al≡MAO (IV) with different [Me-MAO]⁻ counteranions have been identified in the (Cp-R)₂ZrCl₂/MAO (R=n-Bu, t-Bu) systems at low Al/Zr ratios (50-200). At Al/Zr ratios of 500-1000, the complex [L₂Zr(μ-Me)₂AlMe₂]⁺[Me-MAO]⁻ (III) dominates in all MAO-based reaction systems studied. Ethene polymerization activity strongly depends on the Al/Zr ratio (Al/Zr=200-1000) for the systems (Cp-R)₂ZrCl₂/MAO (R=H, Me, n-Bu, t-Bu), while it is virtually constant in the same range of Al/Zr ratios for the catalytic systems (Cp-R)₂ZrCl₂/MAO (R=1,2-Me₂, 1,2,3-Me₃, 1,2,4-Me₃, Me₄) and rac-Me₂Si(Ind)₂ZrCl₂/MAO. The data obtained are interpreted on assumption that complex III is the main precursor of the active centers of polymerization in MAO-based systems.     

Effects of cocatalyst on structure distribution of propylene polymers catalyzed by rac-Me2Si(Ind)2ZrCl2/aluminoxane

Propylene polymerization and propylene/1-octene copolymerization were studied using rac-Me₂SiInd₂ZrCl₂(1)/MAO or rac-Me₂SiInd₂ZrCl₂/(MAO + TIBA) as catalyst (methyl aluminoxane, MAO; AliBu₃, TIBA). The structure distribution of the polymers was characterized by temperature gradient extraction fractionation or precipitation fractionation, as well as by DSC analysis of the thermal segregated samples. By comparing the structure distribution of polypropylene and propylene-1-octene copolymer synthesized by 1/MAO and 1/(MAO + TIBA), it is found that adding TIBA in the catalyst system increase the blockiness of the polymer chain, especially in the copolymerization system. It is assumed that, when iso-butyl is incorporated in the aluminoxane, ion pair of the active center and the aluminoxane counter ion may exist in different states that show different catalytic behaviors, resulting in the formation of polymers with block structure.     

Copolymerization of Propene and Buta‐1,3‐diene in the Presence of Highly Hindered C2‐Symmetric Zirconocene‐Based Catalyst

Summary: Copolymerizations of propene and buta‐1,3‐diene performed in the presence of rac‐[CH₂(3‐tert‐butyl‐1‐indenyl)₂]ZrCl₂ and methylaluminoxane (MAO) have been investigated. Buta‐1,3‐diene gives prevailingly primary coordination to the metal, producing overall 1,2 units. Cyclopropane and cyclopentane rings, although in low amounts, are also obtained. The presence of butadiene would be responsible for some regioirregular 2,1‐inserted propene units, which at high temperatures give rearrangement to 3,1 units.

     

Polymerization of olefins with a novel dinuclear ansa-zirconocene catalyst having a biphenyl bridge

Both the rac- and meso-dinuclear ansa-zirconocene catalysts (μ-C₁₂H₈{[SiPh(Ind)₂]ZrCl₂}₂) were prepared by a coupling reaction between 2 equiv of diindenylphenylchlorosilane (rac- and meso-isomers) and 1 equiv of p-dilithiobiphenyl in diethyl ether at −80°C, followed by a successive reaction with ZrCl₄ · 2THF in THF at −78°C. Polymerizations of ethene and propene were conducted in a 1 dm³ high-pressure glass reactor equipped with a mechanical stirrer at 60, 80, 100, 120, and 150°C using methylalumoxane (MAO) as cocatalyst and toluene or decahydronaphthalene as the solvent. Copolymerization of ethene and 1-octene was also checked in brief. For ethene polymerization, the meso-catalyst was found to be more active, which displayed an extremely high activity to give linear polyethene with a high molecular weight and a narrow molar mass distribution (MMD). The apparent activity increased monotonously with rising polymerization temperature from 60°C up to 150°C, indicating that the active species are stable even at a high temperature. On the other hand, both the rac- and meso-catalysts showed very poor activities for propene polymerization. However, copolymerization of ethene and 1-octene proceeded at a high speed. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2269–2274, 1998     

Gas phase polymerization of ethylene with a silica-supported metallocene catalyst: influence of temperature on deactivation

Ethylene was polymerized at 5 bar in a stirred powder bed reactor with silica supported rac-Me₂Si[Ind]₂ZrCl₂/methylaluminoxane (MAO) at temperatures between 40°C and 80°C using NaCl as support bed and triethylaluminium (TEA) as a scavenger for impurities. For this fixed recipe and a given charge of catalyst. the average catalyst activity is reproducible within 10% for low temperatures. The polymerization rate and the rate of deactivation increase with increasing temperature. The deactivation could be modeled using a first order dependence with respect to the polymerization rate.     

Stopped‐flow polymerizations of ethene and propene in the presence of the catalyst system rac‐Me2Si(2‐methyl‐4‐phenyl‐1‐indenyl)2ZrCl2/methylaluminoxane

The kinetics of ethene and propene polymerization at 20–60°C in the presence of the homogeneous catalyst system rac‐Me₂Si(2‐methyl‐4‐phenyl‐1‐indenyl)₂ZrCl₂/methylaluminoxane was investigated by means of stopped‐flow techniques. The specific rate of chain propagation, measured at the very short reaction times typical of this method, turned out to be ≈10² times higher for ethene than for propene; this suggests that diffusion limitations through the poly(ethylene) precipitating at longer reaction times may be responsible for the fact that the two monomers polymerize instead at comparable rates under “standard” conditions. It was also found that the concentration of active sites is significantly lower than the analytical Zr concentration.     

Triboluminescence of zirconium η<Superscript>5</Superscript>-complexes

This work describes a comparative study of tribo- and photoluminescence of metallocenes (Cp₂HfCl₂, Cp₂TiCl₂, Cp₂ZrCl₂, (CpMe)₂ZrCl₂, rac-Me₂C(3-Bu^t-Cp)₂ZrCl₂, Ind*₂ZrCl₂). Occurring under mechanodestruction of polycrystals, triboluminescence of zirconium bis-cyclopentadienyl complexes, Cp₂ZrCl₂, (CpMe)₂ZrCl₂, and rac-Me₂C(3-Bu^t-Cp)₂ZrCl₂ has been revealed for the first time. The triboluminescence spectrum in air is similar to the photoluminescence spectrum of zirconocene polycrystals. The triboluminescence spectrum does not show *N₂ luminescence. Ne and Ar lines are observed in the triboluminescence spectrum in the atmosphere of these gases. An increase in the number of substituents in zirconocene ligands leads to a bathochromic shift of the emission band maximum in triboluminescence spectra of the complexes. Possible mechanisms of zirconocene triboluminescence excitation are discussed.