Zirconocene Propylene Polymerisation: Controlling Termination Reactions

Summary: Propylene was polymerised at varying trimethylaluminium (TMA) concentration with two metallocenes activated by methylaluminoxane (MAO) in an attempt to better understand the effect of TMA on the activation process, catalyst activity and termination reactions. A chemical treatment of MAO solution with 2,6‐di‐tert‐butyl‐p‐cresol was used to obtain TMA‐free polymerisation conditions. The metallocene precursors under investigation were diphenylmethyl(cyclopentadienyl)(9‐fluorenyl)zirconium dichloride ( 1 ) and rac‐dimethylsilylbis(4‐tert‐butyl‐2‐methyl‐cyclopentadienyl)zirconium dichloride ( 2 ). Chain transfer to aluminium was the dominating termination route for 1 /MAO accompanied with β‐H/β‐CH₃ transfer to Zr and β‐H transfer to monomer. It was found that β‐H/β‐CH₃ transfer to Zr was favoured over the β‐H transfer to monomer at elevated temperatures, and that polymerisation and β‐H transfer to monomer depended on the same critical reaction. For 2 /MAO the detected termination routes were β‐H transfer to Zr and chain transfer to aluminium. A strong activity dependency on TMA concentration was observed. With 1 /MAO high TMA concentration decreased and stabilised the activity, whereas TMA free polymerisation conditions at 40 °C increased markedly the activity, indicating that TMA coordinated to the active site of 1 /MAO. Surprisingly, with more sterically hindered 2 /MAO, high TMA concentration increased the activity and nearly complete activity loss occurred at TMA‐free polymerisation conditions at 40 °C.

Metallocene precursors for investigation of propylene polymerisation.     

On the Homogeneity of Metallocene Ethylene–Propylene Copolymers as Investigated by Multiple Fractionation Techniques

The chemical heterogeneity of ethylene–propylene copolymers by multiple fractionation techniques is addressed. Three metallocene copolymer samples with different ethylene contents, ranging from 30 to 50 mol% are analyzed using bulk methods to confirm their molecular heterogeneity. In a second step, the samples are fractionated by temperature rising elution fractionation (TREF) to obtain fractions at 30, 60, 90, and 130 °C. These fractions are subsequently analyzed regarding their thermal and molecular properties. Differential scanning calorimetry, crystallization analysis fractionation, and high‐temperature high performance liquid chromatography (HT‐HPLC) results reveal that the TREF fractions collected at 130 °C are mainly due to polypropylene homopolymer, which is rather unexpected considering the high ethylene contents of the bulk samples. Most importantly, HT‐HPLC reveals a remarkably high chemical heterogeneity of the fractions and thus the bulk samples. Solution ¹³C NMR provides the comonomer contents and sequence distributions of the fractions. These indicate that the same TREF fractions from different samples have distinctively different chemical compositions.

     

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.

     

Early Stages of Propylene Bulk Phase Polymerization with Supported Metallocene Catalysts

For the initial steps of propylene bulk phase polymerization with a silica‐supported metallocene/MAO‐catalyst the processes of polymer growth, particle expansion and carrier fragmentation are investigated. Gravimetric analysis of the kinetics is given. Light optical examination of the particles shows a significant particle expansion during the first minutes of polymerization, which is not comparable to known observation from the slurry process. Electron microscopic investigation on cross sections of the polymer particles allows a detailed insight into the process of polymer growth, carrier fragmentation and particle expansion. The results are compared with the polymer growth and particle expansion model developed with slurry polymerizations under mild model conditions. In bulk phase polymerization particle expansion and carrier fragmentation are much faster due to more drastic reaction conditions. Nevertheless, it is still possible to observe a shell‐by‐shell degradation of the carrier material. Varying start up behaviors of single catalyst grains are observed. We are able to correlate the initial polymerization activity of individual grains with the cocatalyst loading and distribution on the catalyst carrier by means of microanalytical techniques. The results explain the varying start up behaviors of single grains. It becomes clear why different states of fragmentation are mixed up in the observed polymer samples.

Insight into catalyst grains after 0.5 min propylene bulk phase polymerization.     

Tandem Action of SNS–Cr and CGC–Ti in Preparation of Ethylene–1‐Hexene Copolymers from Ethylene Feedstock

Ethylene–1‐hexene copolymer materials, ranging from semicrystalline linear low‐density poly­ethylene (LLDPE) to completely amorphous polyethylenes (PEs), are prepared from ethylene alone in a single reactor by the tandem polymerization of bis(2‐dodecylsulfanyl‐ethyl)amine–CrCl₃ (SNS–Cr) and (N‐tert‐butylamido) (tetramethylcyclopentadienyl)–titanium dichloride (CGC–Ti) at 75 °C and under atmospheric pressure. The polymerization activities are on the order of 10⁵–10⁶ g (mol Ti)^?1 h^?1. 1‐Hexene incorporation in the resulting copolymers can be adjusted by varying the Cr–Ti molar ratio and/or applying a short period of pre‐trimerization. Copolymers with high 1‐hexene incorporation up to 15 mol% are obtained. Few vinyl and vinylene chain ends are detected by ¹³C NMR, suggesting that 1‐hexene does not act as a chain‐transfer agent. Narrow molecular‐weight distributions with polydispersities from 1.9 to 2.6 are obtained, characteristic of a single‐active‐site nature of the catalyst system.

     

Propylene Polymerization with rac‐SiMe2(2‐Me‐4‐PhInd)2ZrMe2/MAO: Polymer Characterization and Kinetic Models

Poly(propylene)s were prepared with metallocene catalyst rac‐SiMe₂(2‐Me‐4‐PhInd)₂ZrMe₂/MAO (rac‐dimethylsilylbis(2‐methyl‐4‐phenylindenyl)dimethylzirconium/methylaluminoxane) in heptane solution at temperatures from 50 to 80 °C with varying concentrations of monomer, hydrogen, triisobutylaluminium (TIBA) and MAO. Polymer molar mass depended on the monomer, MAO, TIBA, and hydrogen concentrations and on polymerization temperature. The isotacticity was very high (mmmm > 95%), and only a slight decrease was detected at high temperatures. Regio selectivity was also high; the total amount of 2,1‐ and 3,1‐insertions was less than 0.4 mol‐%. Lowering the monomer concentration and raising the temperature increased the amount of 3,1 defects over the amount of 2,1 defects. End‐group analysis by ¹³C NMR spectroscopy revealed isobutyl and allyl end‐groups. Chain transfer to aluminium and β‐CH₃ elimination were concluded to be the dominating chain‐termination mechanisms. The importance of β‐CH₃ elimination increased with temperature. Hydrogen addition changed both the initiation and termination mechanisms as indicated by the presence of propyl, butyl and 2,3‐dimethylbutyl end‐groups. According to modeling studies, the molar mass follows a first‐order relationship with propylene and hydrogen concentrations, and a half‐order relationship with MAO concentration. Arrhenius‐type activation energy coefficients were 125 kJ · mol^?1 for β‐CH₃ elimination, 66 kJ · mol^?1 for chain transfer to aluminium, and 53 kJ · mol^?1 for chain transfer to hydrogen. A value of 45 kJ · mol^?1 was used for the propagation.

     

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 .     

Stereoblock Copolymerization of Propylene and Methyl Methacrylate with Single‐Site Metallocene Catalysts

Sequential stereoblock copolymerization of propylene (P) and methyl methacrylate (MMA) using Group IV single‐site metallocene catalysts efficiently produces PP‐b‐PMMA stereodiblock copolymers. When activated with B(C₆F₅)₃, C₂‐symmetric rac‐Et(Ind)₂ZrMe₂ yields isotactic‐PP‐b‐isotactic‐PMMA diblock copolymer, whereas C_s‐symmetric Me₂Si(C₅Me₄)(tBuN)TiMe₂ affords atactic‐PP‐b‐syndiotactic‐PMMA diblock copolymer. In the copolymerization catalyzed by the C₂‐symmetric catalyst, a very small amount of PMMA homopolymer formed can be removed from the copolymer by extracting the bulk polymer product with boiling methylene chloride. However, separation of isotactic PP formed if any from the copolymer product approves very difficult, due to very similar solubility between the diblock copolymer and isotactic PP homopolymer in various high‐boiling chlorinated solvents. On the other hand, in the copolymerization catalyzed by the C_s‐symmetric catalyst, both PMMA and atactic PP homopolymers formed in small weight fractions during the copolymerization can be successfully removed from the predominant copolymer product by solvent extraction using boiling heptane. After successful removal of both homopolymers, for example, an atactic‐PP‐b‐syndiotactic‐PMMA diblock copolymer has high molecular weight (M _n = 21 100), narrow molecular weight distribution (PDI = 1.08), high PMMA incorporation (33.8 mol‐% of PMMA), and moderate syndiotacticity for the PMMA block ([rr] ≈ 80%). Furthermore, the comonomer composition in the copolymer can be controlled by the time for propylene polymerization and the conversion of MMA. A pronounced activator effect is observed; when the same C_s‐symmetric catalyst is activated with Ph₃CB(C₆F₅)₄, formation of homopolymers is predominated.

     

Reaction Calorimetric Investigation of the Propylene Slurry Phase Polymerization with a Silica‐Supported Metallocene/MAO Catalyst

The slurry phase polymerization of propylene with a silica‐supported metallocene/MAO catalyst was kinetically investigated by two different methods at several polymerization conditions. Heat flow calorimetry on the one hand as an innovative method and flowmeter‐technique on the other hand as a well established classical method led to consistent kinetic profiles. Thus, the principle applicability of the heat flow calorimetry for the kinetic investigation of the described polymerization process was demonstrated, contrary to some expectations in literature. Furthermore, the sensitivity of the calorimetric method is shown and polymerization heats for the slurry process have been determined.     

Polyethylene Made with In Situ Supported Ni–Diimine/SMAO: Replication Phenomenon and Effect of Polymerization Conditions on Polymer Microstructure and Morphology

The catalyst prepared by supporting in situ 1,4‐bis(2,6‐diisopropylphenyl)acenaphthenediiminenickel(II) dichloride ( 1 ) on silica–methylaluminoxane (SMAO) has high activity for ethylene polymerization, although it is less active than the unsupported system 1 /MAO. No external alkylaluminum cocatalyst needs to be added to the in situ supported system. Short chain branches are produced without addition of α‐olefin comonomer, due to the chain‐walking mechanism. Ethylene pressure and polymerization temperature affect the degree of short chain branch formation. At constant ethylene pressure, when the polymerization temperature rises, the short chain branch content also increases. For a given ethylene pressure, there is a threshold polymerization temperature above which polymer particle agglomeration starts to occur. For the conditions investigated herein, from 30 to 50°C the morphology of the polymer particles replicates that of the support. At 55°C, polyethylene starts to become soluble in the reaction medium, and therefore morphological control is gradually lost. Above 70°C, only soluble polymer is produced. Similar results are observed by keeping the temperature constant and changing the ethylene pressure.     

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.     

Characterization of Metallocene Epdm Terpolymers with High Diene and Propylene Content Crosslinked by Dicumyl Peroxide and β‐Radiation

Summary: Two metallocene EPDMs with the same weight fraction of ethylene but differing in diene content were crosslinked, either by dicumyl peroxide (DCP) or β‐radiation. The effect of different diene and propylene content on the molecular structure and the mechanical properties once the materials were crosslinked was studied. The final gel content was very high due to the large level of unsaturations. The crosslinking process was monitored by FTIR spectroscopy by following the decay of unsaturations and the variation of the carbonyl groups that are related to the oxidation grade. It was found that β‐radiation crosslinked samples exhibited a lower oxidation grade than those crosslinked by DCP. An oscillant disc rheometer was employed to follow the evolution of the rheological properties, the scorch time, and the time corresponding to full cure during the crosslinking reaction with DCP. In addition, in order to characterize the state of cure we have studied the rheological properties in shear employing a dynamic parallel plate geometry. These results were correlated with those obtained from the molecular characterization of the soluble fraction by size exclusion chromatography. The experiments indicate that, at low irradiation doses, there is a high rate of chain scission reactions that cause an important decrease in storage modulus. Whereas, at high irradiation doses the rate of chain scission reactions diminishes, thus the storage modulus increases but it still remains at lower levels than those corresponding to the original terpolymers. The tensile properties, hardness (Shore A) and compression set tests also suggest the presence of chain scission reactions.

Storage modulus (G′) versus frequency for a β‐irradiated sample.     

Dual‐Active‐Site Nature of Magnesium Dichloride‐Supported Cyclopentadienyl Titanium Chloride Catalysts Switched by an Activator in Propylene Polymerization

MgCl₂‐supported metallocene catalysts often show peculiar catalytic aspects, which are partially similar to both non‐supported complexes and traditional Ziegler–Natta catalysts. In this study, the active‐site natures of various MgCl₂‐supported cyclopentadienyl Ti chloride catalysts are systematically investigated and compared with traditional catalysts in propylene polymerization. The supported titanocene catalysts offer both metallocene‐type and Ziegler–Natta‐type active‐site natures according to the details of the preparation and the activation procedures. The observed dual‐active‐site natures are plausibly correlated with the valence and charge states of the Ti center.

     

(Co)polymerisation Behaviour of Supported Metallocene Catalysts: Carrier Effect

Summary: The polymerisation and copolymerisation of ethylene with 1‐hexene over metallocene catalysts L₂ZrCl₂/MAO (L = Cp, n‐BuCp, t‐BuCp, i‐PrCp, Me₅Cp) supported on different types of carriers (MgCl₂(MeOH)₆ or silica with CH₃ surface groups obtained in the sol‐gel process) were studied. It was demonstrated that both the metallocene structure and the type of inorganic support affected catalyst activity and polymer properties such as melting point, molecular weight and molecular weight distribution. The metallocene structure also determined comonomer incorporation, both for homogeneous and supported catalytic systems. When a catalyst is anchored on a support, it becomes less effective at incorporating a comonomer into the polyethylene chain, and the type of the support material has no influence on that process. The type of carrier used does, however, have an influence on the molecular weight and molecular weight distribution of the product obtained. Most polymers and copolymers obtained with catalysts supported on an oxide carrier have higher molecular weights and broader molecular weight distributions than those produced by catalysts anchored on a magnesium compound.

Melting curves (obtained using the SSA method) of ethylene/1‐hexene copolymers synthesised with Cp₂ZrCl₂/MAO (A) and with the same catalyst supported on various carriers (B,C,D).     

Synthesis and Properties of Syndiotactic Poly(propylene)/Carbon Nanofiber and Nanotube Composites Prepared by in situ Polymerization with Metallocene/MAO Catalysts

Summary: The preparation of syndiotactic poly(propylene) (sPP) nanocomposites with multi‐walled carbon nanotubes (MWNTs), carbon nanofibers (CNFs), and carbon black (CB) as fillers was accomplished by the in situ polymerization of propylene with a metallocene/methylalumoxane (MAO) catalyst. Different pre‐treatments were applied to achieve a homogeneous dispersion of the fillers in the matrix. The resulting nanocomposites were investigated with respect to their properties, which were then compared to those of the pure polymer and among each other. The thermal stability of the nanocomposites was slightly enhanced compared to the pure polymer. In addition, the yield strength of the nanocomposites could be slightly raised in comparison to the neat sPP. The most significant influence of the nanofillers was detected on the crystallization behavior. The crystallization temperature was increased with rising filler content in all cases. Moreover, the half‐time of crystallization was significantly reduced in the nanocomposites. The rate constant of crystallization was also greatly enhanced for all nanocomposites as compared to the neat sPP. The nanofillers acted, therefore, as nucleating agents. This effect was most pronounced in the case of MWNTs as fillers.

Influence of the type of filler on the degradation temperature (temperature of maximum weight loss).     

Chemistry and Mechanism of Alkene Polymerization Reactions with Metallocene Catalysts

The structures of oligomers formed in ethylene polymerization reactions catalyzed by metallocene complexes are described. Ethylene was homopolymerized and copolymerized with four 1‐alkenes—propene, hex‐1‐ene, hept‐1‐ene, and 3,3‐dimethylbut‐1‐ene—using as catalysts (n‐Bu‐Cp)₂ZrCl₂ and (Me‐Cp)₂ZrCl₂ complexes activated with MAO, both in the absence and in the presence of H₂. Oligomers generated in these reactions are the low molecular weight “tails” of the molecular weight distributions of the respective homopolymers and copolymers. GC analysis affords structural identification of each polymer molecule, albeit a very short one, individually. Analysis of the co‐oligomer structures provides explicit confirmation of the standard mechanisms and kinetics of chain growth and chain transfer reactions. The kinetic features include primary and secondary insertion reactions of 1‐alkene molecules into Cp₂Zr? C and Cp₂Zr? H bonds, chain transfer reactions to AlMe₃, independence of chain growth rate constants on the size of polymer chains attached to active centers, etc. GC analysis also yields detailed information on an unusual feature of metallocene catalysis, namely the formation of saturated macromolecules in ethylene polymerization reactions performed in the absence of H₂.

     

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.

     

Synthesis and Characterization of Defined Branched Poly(propylene)s with Different Microstructures by Copolymerization of Propylene and Linear Ethylene Oligomers (Cn = 26–28) with Metallocenes/MAO Catalysts

Summary: Copolymers of propylene and hexacosene (C_n = 26–28) were synthesized in the presence of three different metallocene catalysts activated by methylaluminoxane. The poly(propylene) copolymers were prepared with iso‐, syndio‐, and atactic backbone microstructures by using different symmetric metallocenes such as rac‐{Me₂Si[2‐Me‐4‐(1‐Naph)Ind]₂}ZrCl₂ ( 1 ), [Ph₂C(Cp)(Flu)]ZrCl₂ ( 2 ), and [(H₃C)₂Si(9‐Flu)₂]ZrCl₂ ( 3 ) and up to 46.6 mol‐% comonomer content in the feed. The influence of the incorporated linear, ethylene‐based side chains into the poly(propylene) backbone were investigated by DSC, GPC, and ¹³C NMR. Generally, a decreasing content of comonomer in the feed enhances the activity of metallocene based catalysts. The determination of the branched microstructure by ¹³C NMR of the copolymers allows a smart identification of the amount of inserted hexacosene because of the separated backbone and side chain signals. Moreover, the relationship between the population of the side chains and the melting behavior of resulting copolymers were discussed. The melting point of the syndiotactic and isotactic poly(propylene) backbone decreases with increasing hexacosene content. When the inserted comonomer content exceeds 2 mol‐%, a second melting point of the crystallized ethylene based side chains can be observed which increases with an increasing amount of hexacosene.

Thermal behavior of isospecific hexacosene/propylene copolymers in dependence on the incorporation of hexacosene.     

Influence of Temperature on the Regioselectivity of Highly Isospecific C2‐Symmetric Zirconocenes in Propene Polymerization

The influence of polymerization temperature (T_p) on the isoselectivity of the C₂‐symmetric zirconocene catalysts rac‐Me₂Si(2‐Me‐4‐PhInd)₂ZrCl₂/MAO ( 1 /MAO), rac‐Me(CyHex)Si[2‐Me‐4‐(4′‐^tBuPh)Ind]₂ZrCl₂/MAO ( 2 /MAO), and rac‐Me₂Si(2‐Me‐4‐Ph‐5‐OMe‐6‐^tBuInd)₂ZrCl₂/MAO ( 3 /MAO) in propene polymerization, has been studied in the range 30–85 °C in liquid propene. All three catalysts show similar polymerization activity versus T_p profiles, with a maximum at about 60 °C ( 1 /MAO), 80?85 °C ( 2 /MAO), and 70?80 °C ( 3 /MAO). 3 /MAO shows the highest activity at any T_p. The isoselectivity of the three catalysts remains high and approximately constant in the whole T_p range investigated, as indicated by the roughly constant isotactic triad content (mm > 99%) of the produced isotactic poly(propylene)s (iPP). Surprisingly, the melting and crystallization temperatures of iPP increase by increasing T_P. This behavior is due to an unexpected increase in catalyst regioselectivity with T_p, as indicated by the decrease in the content of 2,1‐erythro regiodefects (2,1e) at increasing T_p. A mechanism involving a metal cation–2,1‐coordinated propene slow state is proposed to account for this so far unnoticed behavior. The correlations between T_p, T_m, and [2,1e] are defined for these three catalysts.

     

Photocatalytic Suzuki–Miyaura Coupling Reactions over Palladium Anchored on 8‐Hydroxyquinoline‐Based Polymers

Through a one‐pot Friedel–Crafts polymerization method, a series of 8‐hydroxyquinoline‐based polymers with excellent light capturing ability is synthesized to support palladium for light‐driven Suzuki–Miyaura reactions (SMRs). The results of X‐ray powder diffractometer and transmission electron microscopy analyses suggest that the 8‐hydroxyquinoline‐based polymers are amorphous in structure and irregular in morphology. According to the data of thermogravimetric analysis and Brunauer‐Emmett‐Teller gas absorption, they are thermally stable up to 340 °C and are large in specific surface area. With palladium anchored securely on the 8‐hydroxyquinoline units of the polymer skeleton, the obtained photocatalysts exhibit excellent activity and reusability in SMRs under blue‐light irradiation.