Polymerization in the confinement of molecular sieves: Facile preparation of high performance polyethylene

European Polymer Journal xxx (2014) xxx–xxx

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Macromolecular Nanotechnology

Polymerization in the confinement of molecular sieves: Facile preparation of high performance polyethylene Hong Xu a, Cun-Yue Guo b,⇑ b

Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan 030024, PR China School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, PR China

a r t i c l e

i n f o

Article history: Received 17 September 2014 Received in revised form 14 November 2014 Accepted 23 November 2014 Available online xxxx Keywords: Ethylene Confinement (Co)polymerization Molecular sieves High performance

a b s t r a c t Polyethylene (PE) and ethylene copolymers (ECP) play significant roles in modern society. Molecular sieves (MS) act as both the catalyst support and nanoreactor for ethylene (co)polymerization. Mesoporous molecular sieves (MMS, typically MCM-41 and SBA-15) offer suitable pore sizes for ethylene (co)polymerizations to proceed in an extrusion mode in which the formation of polymeric nano-fibers composed of extended polymer chains from the nanopores of MMS is completed in one step. Additionally, the PE and ECP are compounded with MMS particles to form polymer nanocomposites when the MMS framework collapses due to polymeric chain growth and/or enormous polymerization exotherms. Such a unique methodology integrates numerous merits into one and endows the resultant ethylene (co)polymers with significant advantages, such as increased molecular weights and their distribution, elevated strength and modulus, and improved processability. This review addresses the progress in ethylene (co)polymerization catalyzed by organometallic complexes (pre-catalysts) immobilized onto various molecular sieves and property investigation on the resultant polymers over the past 16 years. This article comprises three major parts which focus on pre-catalysts immobilization, ethylene (co)polymerization in different molecular sieves, and the structure and properties of the as-prepared ethylene (co)polymer nanocomposites. Finally, the outlook for future research and development trends is proposed. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Polyethylene (PE) including ethylene copolymers with other comonomers (ECP) claims the highest production volumes among all of the synthetic plastics in the world. Having been an important and active field in both academy and industry for nearly sixty years [1], ethylene (co)polymerization catalyzed by highly efficient catalytic system comprising organometallic complexes and activators is still the predominant method in commercial production of PE and ECP of varying brands. From 1970 [2] to 2002 ⇑ Corresponding author. Tel.: +86 10 88256978; fax: +86 10 88256092. E-mail address: [email protected] (C.-Y. Guo).

[3], the quota of polyethylene in the market of low cost and high performance polymeric materials jumps from 30% to 60% due to concerns over health and environment safety. Although conventional PE has the advantages of being light in weight, insulating, weather-proof, superior in mechanical properties, etc., there remains great room for further improving PE’s comprehensive performance, such as low molecular weights, low strength and modulus, and weak interfacial interaction with polar fillers. Despite the virtues of being free of de-ashing and granulation of the polymers due to the development of generations of Ziegler–Natta catalysts [4], their multiple active sites hardly guarantee polyethylenes of homogeneous properties. Since the invention of methylaluminoxane

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Please cite this article in press as: Xu H, Guo C-Y. Polymerization in the confinement of molecular sieves: Facile preparation of high performance polyethylene. Eur Polym J (2014), http://dx.doi.org/10.1016/j.eurpolymj.2014.11.037

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(MAO) [5], metallocenes [6], post metallocenes and nonmetallocene sprang up like mushrooms after rains [7– 15]. These single site catalysts, particularly late transition metal (LTM) catalysts, even polymerized polar monomers [16–20] due to less oxophilicity. Moreover, such LTM complexes catalyzed polymerizations could be carried out in aqueous emulsion [21–25] or supercritical CO2 [26,27]. The great success in developing novel ethylene polymerization catalysts themselves does not exclude the adoption of other means to comprehensively improve the polymerization process and polymer properties. Most industrialized single site catalysts for ethylene polymerization necessitate the immobilization of catalysts on certain supports [28,29]. To carry out ethylene polymerization at temperatures above the melting temperatures (Tm) of the resulting polyethylenes requires that the catalysts have to be thermally stable and the polymers are of low crystallinity and molecular weights, such as various low density PEs. To produce polyethylene of high density or crystallinity, gas-phase or slurry polymerization process is employed [28] in which the morphology of the catalysts is the key issue. Presently, most successfully commercialized single-site catalysts are generally supported on SiO2, Al2O3 and MgCl2, etc. to resolve the morphology issue [29–31]. To further investigate new support/pre-catalyst/activator system, varying supports have to be attempted in order to continually push forward the supported catalysts and polyethylene industries. Compared with conventional zeolites, mesoporous molecular sieves (MMS, pore diameter = 2– 50 nm) which combine favorable properties of crystalline microporous materials (for example, structural variety) with those of amorphous silica and possess large BET surface areas, high porosities, controllable and narrow pore size distribution and ordered pore arrangements allow the insertion of certain monomer and the growth of polymeric nano-fibers, which are composed of extended PE crystals, out of pores of the molecular sieves at varying activities (from generally low [32–43] to moderate [32,44–57] and even very high values [58–74]) depending on the structure and pore size of the molecular sieves. Due to the confinement effect of nanopores in molecular sieves, the insertion mode and chain propagation during the polymerization differ greatly from those in free space. The constrained geometries, to certain extent, inhibit the bimolecular termination endowing the polymerization of polar monomers with a ‘‘living’’ nature [17–20]. Details about the progress in organometallic a-olefin polymerization catalysts supported on zeolites or mesoporous silicas, including the role of these nanostructured materials in establishing new catalytic behaviors and polymer properties, have been well reviewed in literature [75] although it focuses mainly on catalyst deposition and the resulted polymer characteristics but less on the polymerization itself. To immobilize catalysts onto molecular sieves affords the following strong points in ethylene polymerization [71,72,75]: (i) the artificially synthesized molecular sieves avoid the occurrence of impurities that are liable to cause polymer degradation, hence improving polyethylenes’ anti-aging performance; (ii) the nanopores of molecular

sieves act as both the support of catalysts and the reactor for ethylene polymerization, thus realizing highly efficient catalyst fixation and easily controllable polymerization process which ensure high polymerization activities; (iii) the nanopores exert stereo-selective effects on monomer insertion and polymerization and are beneficial for increased molecular weights and melting points of the polyethylenes. As mentioned above, temporal development is gradually steering its way towards the nature of ethylene (co)polymerization upon the introduction of the confinement effect instead of investigating the roles of molecular sieves mainly as catalyst supports. Therefore, this review article will present systematic knowledge and critical comments in the following aspects: (i) catalyst immobilization on molecular sieves; (ii) kinetics of ethylene oligomerization, polymerization, and copolymerization with other olefins in the confinement of molecular sieves; (iii) structure and properties of the resulting polyethylene; (iv) morphology evolution and enhancement in physical properties of the resulting polyethylene; (v) outlook for ethylene (co)polymerization in fabricating high performance polyethylene materials via the employment of molecular sieves. 2. Preparation of molecular sieve supported catalysts To realize ethylene polymerization in the porous channels of molecular sieves, the catalytically active sites have to be introduced into the inner walls of molecular sieves by mainly chemical fixation. It’s well known that MMS is a very important member of the family of periodic mesoporous silicas (PMS) materials [76] and suitable for catalyst immobilization. Table 1 lists the classification and properties of major mesoporous molecular sieves used herein [77–84] to better understand their application in catalyst supporting. With regard to Si-containing MMS, the surface silanol groups (Si–OH) are capable of reacting with sequestering agents such as organometallic chlorides and alkoxides, with the elimination of one or more of the original ligands. The immobilization modes of catalysts influence greatly the polymerization activity and polyethylenes’ properties. In most cases, the structures of the catalysts remain to be the same after they are immobilized into the confinement of nanopores. This section will present prevalent routes to the immobilization of ethylene polymerization catalysts. There has been, until now, four major methods for the fixation of ethylene polymerization catalysts. The first one is the direct impregnation method in which dry MCM-41 is contacted with a solution of precatalysts followed by calcination at high temperature (500 °C) to ensure as much as possible active metal sites are Table 1 Classification of mesoporous molecular sieves. Type

Largest pore (dp/nm)

Refs.

MCM-41 MCM-48 SBA-15

2.0–10.0 2.0–4.0 5.0–30.0

[77,78] [79–83] [84]

Please cite this article in press as: Xu H, Guo C-Y. Polymerization in the confinement of molecular sieves: Facile preparation of high performance polyethylene. Eur Polym J (2014), http://dx.doi.org/10.1016/j.eurpolymj.2014.11.037

immobilized [35]. The second is the activation of molecular sieves by reactive agents (e.g., MAO or AlR3 and its chlorinated compounds) followed by catalyst immobilization (Scheme 1) [46,48,62,66,68]. The third method is similar to the second one except that the MAO is introduced in the last step (Scheme 2) [70]. N2 adsorption–desorption isotherms disclose that decreased BET surface area, decreased total pore volume, lowered pore diameter, and increased wall thickness are resulted after the introduction of zirconocene molecules and subsequent MAO. The last method employs functionalized mesoporous molecular sieves to react with the pre-catalyst and MAO and form supported catalyst [40]. Using this method, the Zr content of catalysts was proportional to N content in amine-functionalized SBA-15 and more anchoring of (n-BuCp)2ZrCl2 occurs with larger number of amine groups (nitrogen atoms) in organosilane on the SBA-15. For all the porous material supported catalysts, there was a reduction in the specific surface area of the support after metallocene grafting [58]. Besides, the AFM images indicate that the shape of grains is maintained for SBA-15 and MCM-22 after metallocene grafting. 3. Ethylene (co)polymerization kinetics with molecular sieve supported catalysts As for highly active ethylene (co)polymerization in the nanopores of molecular sieves, it’s essential for the access of ethylene and/or bulkier comonomer molecules to the

Scheme 1. Representation of the interaction between Cp2ZrCl2 and the inner surface of MAO-modified zeolite [Ref. 46].

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porous channels where catalytic sites are located. Either in a solution or a gas process, monomer delivery has to be performed by a driving force of monomer concentration gradient and fresh ethylene and/or its comonomer molecules are transferred to the bound active sites on molecular sieves because monomers therein are quickly consumed and a lack of monomers around the active sites is resulted as the polymerization proceeds. So, the shape and size or kinetic diameter of the monomers surely influence monomer diffusion. Moreover, the catalyst amount inside the pores and the type of molecular sieves also affect the catalyst activity. The confinement of molecular sieves’ pores, after catalyst immobilization, surely influences the diffusion of ethylene and/or its comonomers and finally leads to decreased polymerization activity compared to that of homogeneous catalyst system [70]. Pioneering study has carried out the copolymerization of ethylene with higher a-olefins, such as 4-methyl-1-pentene (4MP), 1-hexene, 1-octene and 1dodecene, with both aspecific and isospecific bridged metallocene complexes supported on MAO-pretreated HY zeolite (/ = 0.72 nm) although it still suffers from a lack of detailed systematic investigation [46]. Subsequent work of the same group finds that the pretreatment of HY zeolite by MAO increases ethylene polymerization activity of Cp2 ZrCl2@HY by more than four times than the case of AlMe3 pretreated support [47]. Mesoporous molecular sieves (MMS, pore diameter: 2– 50 nm), such as MCM-41, SBA-15 and MSF have ordered mesopores, uniform and adjustable pore diameter, and stable framework structure. Their wall thickness, easily doped amorphous framework, big surface area, and modifiable interior pore surfaces [32] afford mesoporous molecular sieves the confinement towards ethylene (co)polymerization and room big enough for relatively bulky catalyst to be supported, thereby ensuring desired catalytic activities compared to catalysts supported onto zeolites. The porous channels, to certain extent, inhibit the occurrence of chain transfer and termination, thus producing polyethylene with significantly increased molecular weights through an extrusion polymerization mode [33]. The increased acidity from the introduction of other metal element in the MMS support plays an important role in activating the metallocene and accordingly improving the polymerization activity [54]. The polymerization kinetics data show that active sites inside molecular sieves existed stably and their activation rate markedly exceeded the deactivation rate, exhibiting relatively low polymerization rate at initial stage followed by gradually increased activity until a stable

Scheme 2. The potentially active alkyl cationic surface species [Ref. 70].

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platform is reached. This is quite different from its homogeneous catalytic system and, moreover, the amount of activator is greatly reduced. Additionally, the polymerization activity relates to not only the immobilization mode, but also the pore diameter and thermal treatment temperature of the molecular sieves prior to the polymerization. The following part will describe in detail the kinetic behavior of molecular sieve supported catalysts in ethylene polymerization and ethylene copolymerization with a-olefins.

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3.1. Ethylene oligomerization with molecular sieve supported catalysts Ethylene oligomerization is an economical way to produce a-olefins. There are very limited reports about ethylene oligomerization within the micropores or mesopores of inorganic materials. Bis(imino)pyridine iron complexes supported on MAO-treated MCM-41 and SBA-15 oligomerize ethylene to linear a-olefins at high reaction temperatures in which the fraction of lower molar mass a-olefins is realized in the case of MCM-41 support due to more remarkable confinement effects [71]. The greatly improved reaction temperature (70 °C) and significant increase in the selectivity for lower a-olefins are beneficial for industrial application and subsequent in situ copolymerization with ethylene over zirconocene catalyst [72]. Most recently, nickel supported on plug-containing Al-SBA-15 is used as the catalyst for ethylene oligomerization, exhibiting a higher yield of C10+ olefins in ethylene oligomerization; more importantly, after steaming and acid post-treatments, the as-prepared catalyst afford shape selectivity for the C16 olefin [43] although its catalytic activities are very low compared to the case of employing bis(imino)pyridine iron complexes supported on MCM-41 and SBA-15 in ethylene oligomerization [71,72].

A computational study adopting grand canonical Monte Carlo simulations (GCMC) discloses that a Ti-based catalyst [(g5-C5H4CMe2C6H5)TiCl3/MAO] capable of selectively trimerizing ethylene into 1-hexene changes its selectivity when zeolite (host) frameworks displaying different pore sizes (mazzite (MAZ), AIPO-8 (AET), UTD-1F (DON), faujasite (FAU), and VPI-5 (VFI)) participate in the oligomerization [85]. The study after kinetic modeling of the effect of confinement on oligomerization activity and selectivity finds that microporous structures help large-size intermediates, which eventually lead to 1-octene or even 1-decene, stabilize as a result of confinement than smaller-sized ones, thereby shifting the selectivity from 1-hexene to higher order olefins at varying activities (Fig. 1). This study predicts the parameters for obtaining maximal 1-octene production are: aH in the range 0.15–0.18 (like FAU, DON, AET) and relatively low temperatures between 23 and 25 °C. Although this model neglects numerous factors existing in real ethylene oligomerization which include the presence of guest/host specific interactions and the effect of dispersive forces on the guest geometries, limited oligomers production rates in reality by mass transfer of reactant ethylene or produced oligomers in microporous hosts, existence of electrostatic interactions, etc., it’s still a useful theoretical tool in making quick and dependable match between the pore size and the activity and selectivity regarding certain ethylene oligomerization. 3.2. Ethylene polymerization over molecular sieve supported catalysts Molecular sieve supported metallocene catalysts, chromium catalysts, and late transition metal (LTM) catalysts could be employed in ethylene homopolymerization under slurry and gas phase conditions. When metallocene complexes (such as Cp2ZrCl2, Cp2ZrMe2) were fixed onto zeolite molecular sieves (pore diameter

Fig. 1. Effect of confinement described by aH,j on selectivity in 1-octene at 1 bar ethylene pressure and T = 250 K (black), 300 K (dark gray), and 350 K (light gray). Symbols correspond to the predicted values. Interrupted lines are mere guides for the eye. HP stands for ‘‘homogenous phase’’ (aH = 0). (Error bars are placed according to the results of error calculations available in Supporting Information.) (Left). Effect of confinement described by aH,j on overall oligomerization rates at 1 bar ethylene pressure and T = 250 K (black), 300 K (dark gray), and 350 K (light gray). Symbols correspond to the predicted values. Interrupted lines are mere guides for the eye. HP stands for ‘‘homogenous phase’’ (aH = 0). (Error bars are placed according to the results of error calculations available in Supporting Information.) (Right) [Ref. 85].

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<2 nm), the amount of cocatalyst is much lower than the case of using homogeneous catalyst. MCM-41 after Almodification possesses less ordered hexagonal pore structure [60]. Also slightly diminishing is the pore size with increasing Al content. Such Al-MCM-41 is more suitable than Grace silica and MCM-41 as the support for Cp2ZrCl2 (the highest amount of added zirconium is 56 wt%) [60]. Taking the Zr content of the catalyst into consideration, the use of Si-MCM-41 as a support results in the lowest activity (6.10  106 g PE (mol Zr)1 h1, while the highest activity (1.58  107 g PE (mol Zr)1 h1 is observed for the Al-MCM-41 (Si/Al = 32) supported metallocene. The structure of cocatalysts played great roles in regulating ethylene polymerization. MMS has been so far the most extensively studied mesoporous support in ethylene polymerization in which the pore size plays significant role in regulating ethylene polymerization activities [39,41,51,61,63,64]. MCM-41 (pore diameter = 2.90 nm) and SBA-15 (pore diameter = 5.46 nm) supported Cp2ZrCl2 behaved quite differently towards ethylene polymerization. The temperature matters remarkably in the case of smaller pore-sized MCM-41 supported catalyst [51]. The polymerization activities increase drastically with the increasing reaction temperature and the activities with Cp2ZrCl2@MCM-41 exceed those of Cp2ZrCl2@SBA-15 after certain polymerization time (ca. 9 min at 50 °C and 4 min at 70 °C) due to the different pore diameters and surface areas. (n-BuCp)2ZrCl2 supported on MAO-treated mesoporous molecular sieves of varying pore diameters (2.6–25 nm) [63] and pore diameters (2.5–20 nm) [64] have been attempted to examine the effect of pore size on gas-phase ethylene polymerization. These results further confirm that the effects of polymerization temperature and pressure on the activities depend on the pore size of the molecular sieve support. Supports with large-pores afford catalysts of lower activities and the catalysts made with the small-pore silicalite have the lowest homopolymerization activities at all temperatures. The dependence of the activity on the pore size of the MMS supports appears to decrease with rising polymerization temperature (Fig. 2) [63] and the fastest polymerization rates correspond to

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pore diameters of 2–6 nm. The supported catalyst formed multiple active sites during the polymerization process, the type and concentration of the active sites correlate with the pore dimension of the supports. All activity profiles show a period of activation followed by deactivation, but the rates of activation and deactivation are functions not only of the temperature but also of the pore diameter. Ethylene slurry polymerization with three ansa-zirconocenes supported on MCM-41 and silica gel (SG) shows that MCM-41 carries lower Zr loading than SG because zirconocene already entrapped inside pores of MCM-41 might restrict further grafting of metallocene molecules [70]. This situation results in decreased polymerization activities and improved Mw for MCM-41 supported metallocenes while the PDI and Tm values are almost the same for MCM-41 and SG supports. Although the zirconocene catalysts supported directly on silicious MCM-41 (Cp2ZrCl2/MCM-41) did not show any activity in ethylene polymerization, the substitution of Al3+ for partial Si4+ in MCM-41 promoted the formation of active Cp2ZrCl2 sites whose activities are comparable to those of homogeneous zirconocenes and much higher than the activities of the zirconocenes supported on MAO modified Al-MCM-41 (Cp2ZrCl2/MAO/ Al-MCM-41). Particularly at lower Al/Zr molar ratios, Cp2ZrCl2@Al-MCM-41exhibited higher activities than its homogeneous analogue [61]. This indicates that the Lewis acidity in Al-MCM-41 seems to play an important role for anchoring the zirconocenes, the stronger the Lewis acidity in MCM-41, the higher the catalytic activity (i.e., low value of Si/Al ratio in MCM-41 results in high ethylene polymerization activity). p-Aminophenylsilyl-functionalized mesoporous silica is used to tether titanium catalyst (CGC) to the interior pore surfaces to form heterogenized catalyst (C5(CH3)4Si(CH3)2 NR)TiCl2@mesoporous silica [41]. The structure of the support matrix becomes the ‘‘second sphere of influence’’ (the ligand environment provides the primary influence) on catalyst activity and product properties. With regard to different pore sizes (2.5–7.0 nm), higher degree of crystallinity for high-density polyethylene (HDPE) produced by the heterogenized CGC corresponds to smaller pore mesoporous silica. Moreover, ethylene polymerization activities

Fig. 2. Effect of the support pore diameter on the average homopolymerization activity (Left). Activity profile as a function of the reaction temperature for CAT2.6-2 (Right) [Ref. 63].

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vary in the range of 5.5–43.7 kg PE (mol Ti)1 h1 for the above mentioned pore sizes. Cp2ZrCl2@SBA-15 catalyzed ethylene polymerization with the activity decreasing from 0.86  106 to 0.35  106 g PE (mol Zr)1 h1 atm1 as the polymerization time extends from 10 to 60 min whereas Tm of the resulted PE is around 130 °C [50]. (n-BuCp)2ZrCl2 supported on AlSBA-15 (Si/Al = 4.8, 15, 30, 60, and 1) by incipient wetness impregnation affords better ethylene polymerization activities than the zirconocene supported on amorphous silica and silica–alumina [52]. As depicted in Fig. 3, lower support Si/Al ratios enhance the metallocene catalytic activity due to increased surface acidity of the support and the increase of the support pore size decrease the polymerization activity. This activity trend is similar to the case of gas-phase polymerization of ethylene over (n-BuCp)2ZrCl2@MCM-41 [63]. The electron-donating effect of the amine or nitrile group on the surface of functionalized SBA-15 supported (n-BuCp)2ZrCl2 is a significant factor in acquiring higher activity of ethylene homopolymerization [40]. For all these supported catalysts, their activities are 2–3 times higher than that of SBA-15/(n-BuCp)2ZrCl2 without functionalization of SBA-15. [4-Me-Ph-C(NSiMe3)2]2ZrMe2 is supported on MAO-pretreated MCM-41 and mesoporous HMS (hexagonal molecular silica having a worm-like or sponge structure) to form heterogeneous catalysts [38]. The supported catalyst polymerizes ethylene 40 times slower than the homogeneous catalyst while the Mw of PE (701,900 g mol1) is 4.33 times that of using homogeneous benzamidinate zirconium complex. Besides, ordered mesoporous materials can also be employed to support chromium and LTM catalysts. Cr(acac)3@MCM-41 [36] or Cr(acac)n-Al-MCM-41 (with n = 2 or 3) [35] catalyzed gas phase ethylene polymerization with moderate activities. The activities experience a rise with the loading of chromium in the supported catalysts before a peak value (7.3  105 g PE (mol Zr)1 h1) is reached at Cr3+ content of 1 wt%, thereafter, the activities gradually drop [36]. Employing a single step method, Crcontaining catalysts prepared by chemical vapor deposition (CVD) of CrO2Cl2 vapor produce HDPE in the following activity order: Cr/MCM-41 > Cr/SiO2  Cr/MCM-48 due to the higher Cr content of Cr/MCM-41 [53]. In slurry

polymerization conditions, Cr/SBA-15 produces PE with higher activities than those of Cr/SiO2 [55]. MAO pretreated mesoporous particles (MCM-41 and MSF) were used to fix 1,4-bis(2,6-diisopropylphenyl) acenaphthene diimine nickel(II) dibromide (DMN) to perform ethylene polymerization [39]. The as-supported catalysts show much higher catalyst loading and higher catalytic activity than those of the direct impregnation of DMN onto the supports although, compared to the homogeneous catalyst counterparts, significant reductions in the catalyst activity (about 10–30% of that of the corresponding homogeneous system) are observed for all the supported systems under the same reaction conditions. The MCM-41 supported catalyst is more active than the MSF supported catalyst because ethylene diffusion limitations in the much longer MSF nanotubes. For SBA-15 supported diimine nickel complexes, their polymerization activities go up firstly and decrease subsequently with increasing polymerization temperature or pressure [44]. MAO treated MCM-41 is highly efficient for supporting diimine iron complexes or bis(phenoxyketimine) zirconium complexes [66,67]. These supported catalysts exhibit smooth polymerization kinetics and much higher activities at atmospheric pressure of ethylene (above 106 g PE (mol metal)1 h1. For Ga-MCM-41s, the very low Ga loadings proved to be enough to immobilize at least four times more metallocene than Si-MCM-41 did, implying gallium seems to be as effective as aluminum (if not better) in creating a surface able to capture the metallocene [54]. The activity values obtained with Cp2ZrCl2/Ga-MCM-41 systems represent ca. 30% of the activity obtained in solution. The polymerization activity increases with bulk Si/Ga ratio up to a maximum value and then decreases. However, at identical bulk Si/Ga ratios, impregnated supports lead to lower polymerization activities compared to direct synthesis ones. So, the polymerization activity observed with these Ga-MCM-41 is expected to increase with the fraction of ‘‘strong’’ Lewis sites present therein. Grafting the mixture of Cp2ZrCl2 and (nBuCp)2ZrCl2 (1:3 ratio) onto alumino-silicates (MCM-41, SBA-15, MCM-22, ITQ-2), alumina and chrysotiles (native and leached) affords hybrid supported catalysts [58]. With MAO

Fig. 3. Average ethylene polymerization activity using (nBuCp)2ZrCl2/MAO supported catalysts. (A) Influence of Si/Al ratio of SBA-15 supports. (B) Influence of support pore size (j) Si/Al = 4.8; (h) Si/Al = 1: (a) CAT-SiSBA, (b) CAT-SwSBA, (c) CAT-SiO2, (d) CAT-4.8AlSBA, (e) CAT-SiO2–Al2O3 [Ref. 52].

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the cocatalyst, these catalysts show high activities towards ethylene polymerization (up to ca. 3.2  106 g PE (mol Zr)1 h1). Such hybrid metallocene mixture is also supported onto microporous and mesoporous carriers including SBA-15, ITQ-2, MCM-22, Al2O3, commercial MAO-modified silica (SMAO), SiO2–ZrO2, and natural chrysotile for ethylene polymerization and the activity, with the lowest value of 4.0  105 g PE (mol Zr)1 h1 for the homogeneous catalyst system, is found to decrease in the following order of support: SMAO (6.56  106 g PE (mol Zr)1 h1) > SBA-15 > MCM-22 > ITQ-2 > Al2O3 > SiO2–ZrO2 [73]. The increase in the fraction of micropores causes a reduction in catalyst activity but no clear trend between support pore diameter and catalyst activity could be observed. Moreover, the same research group fixes Cp2ZrCl2 on silica–magnesia bisupports prepared by the sol–gel method, different Si/Mg ratios corresponding to varying crystallinities afford the supported metallocene higher ethylene polymerization activities than those obtained from bare silica immobilized catalyst due to increased porosity of the supports [59]. Furthermore, the use of silica–magnesia bisupports allows a combined activation of TEA and MAO to the metallocene and less Al/Zr ratio (500) is sufficient, implying a possible partial role of cocatalyst of the bisupport. Ethylene polymerization catalyzed by zirconocene/MAO in the presence of MAO modified with undecenoic acid (UA) and tri(isobutyl)aluminum (TIBA) produces polyethylene hybrid with significantly improved thermal stability [57]. The as prepared polyethylene hybrid is beneficial for the fabrication of high performance mixed matrix membranes (MMM) for which higher permeability, selectivity or both magnitudes relative to the existing polymeric PE membranes is expected. 3.3. Copolymerization of ethylene and a-olefins over molecular sieve supported catalysts Zeolite supported catalysts similar to those as described above also catalyzed the copolymerization of ethylene and a-olefins (such as propylene, 1-hexene, 1-octene)

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[46,48,62]. As demonstrated in Fig. 4, the increase of carbon atoms in the a-olefin weakens the ‘‘comonomer effect’’. In the case of Cp2ZrCl2@NaY/MAO catalyzed ethylene-propylene copolymerization, elevating feed ratio of propylene to ethylene markedly increases the polymerization activities (Fig. 4A). When the comonomer is 1-hexene, the activities change only slightly (Fig. 4B). To further increase the size of comonomer, 1-octene, however, significantly lowers the polymerization activities compared to that of ethylene homopolymerization. In addition, Cp2ZrCl2 confined inside supercages does not complex with isobutylene and no formation of ethylene/isobutylene copolymer is observed over such Cp2ZrCl2@NaY/MAO, suggesting a shape-selective copolymerization. By comparison of these results, it’s found that the comonomers’ structure greatly influences the copolymerization activities [48,62]. This, from other side, indicates the effects of the confinement of molecular sieves on the kinetics of ethylene-a-olefin copolymerization reactions, which subsequently result in decreased comonomer incorporation rate in the copolymers [46]. As to MCM-41 supported metallocenes (rac-Et(Ind)2ZrCl2 and Et(Ind)2Zr(CH3)2), they also exhibit ‘‘comonomer effect’’ in the copolymerization of ethylene and a-olefins and this effect weakens as the a-olefin increases in its carbon atoms, which is similar to the case of zeolite supported zirconocenes. AlMe3 can be hydrolyzed to MAO inside pores of MCM-41 and Et(Ind)2Zr(CH3)2 is supported on which to catalyze the cooligomerization of ethylene and propylene [74]. The in situ formation of MAO which is chemically bonded to pores of MCM-41 prevents MAO from aggregating, thus needing no cocatalyst in performing ethylene/propylene copolymerization. At low catalyst dosage, the as-formed catalyst possesses higher copolymerization activity compared with the homogeneous zirconocene, SiO2/MAO supported zirconocene, and silica fixed zirconocene where AlMe3 was hydrolyzed at surfaces of SiO2. This reflects that the nanosized space effectively inhibits the deactivation of active sites. Decreasing pore sizes is accompanied by lowering activities. If tetraethyl orthosilicate (TEOS) rather than silica sol was introduced

Fig. 4. (A) Profile of the monomer consumption rate in the ethylene–propylene copolymerization catalyzed by Cp2ZrCl2@NaY/MAO. Polymerization condition: 50 °C, 1.2 atm, Al/Zr = 1000, C3/C2 molar ratio in the feed = (a) 0.0, (b) 0.5, (c) 1.0. (B) Profile of the monomer consumption rate in the ethylene-1hexene copolymerization catalyzed by Cp2ZrCl2@NaY/MAO. Polymerization condition: 50 °C, 8 atm, Al/Zr = 1000, C6/C2 molar ratio in the feed = (a) 0.0, (b) 0.25, (c) 0.5, (d) 1.0 [Ref. 48].

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Fig. 5. (A) Profiles of monomer consumption rate in ethylene–propylene copolymerization catalyzed with MCM-41/MAO/Et(Ind)2ZrCl2. Polymerization conditions: T = 50 °C, P = 1.2 atm, Al/Zr = 1000, C3/C2 molar ratio = (a) 0.0, (b) 0.5, (c) 1.0. (B) Profiles of ethylene consumption rate in ethylene-1-octadecene copolymerization catalyzed with MCM-41/MAO/Et(Ind)2ZrCl2. Polymerization condition: T = 50 °C, P = 1.2 atm, Al/Zr = 1000, C18/C2 molar ratio = (a) 0.0, (b) 0.5, (c) 1.0 [Ref. 62].

in the synthesis of MCM-41, the coordinating action of the anion in Al-O weakens and improved oligomerization activity is realized. The activities rise with increasing Al/ Zr molar ratios [74]. Et(Ind)2ZrCl2@MCM-41/MAO catalyzed ethylene-propylene copolymerization with more remarkable ‘‘comonomer effect’’ than the homogeneous catalyst [62]. However, such effect weakens drastically to almost null when the comonomer becomes 1-octadecene, implying difficult accession of bulkier comonomer to the active metal centers to form complexes (Fig. 5) [62]. The pore size of molecular sieves to support metallocene also plays marked role in influencing the copolymerization behavior of ethylene and a-olefins. Compared to small-pore MCM-41 supported catalyst, increasing the pore size of molecular sieves to 7.2–15 nm leads to very low activities in ethylene-1-hexene copolymerization with (n-BuCp)2ZrCl2@MCM-41 although for which the reason is unknown [63]. Subsequent investigation indicates that during ethylene/1-hexene copolymerization the homopolymerization sites are also present in significant amounts at short polymerization times for the large-pored (n-BuCp)2ZrCl2@MCM-41 and the homopolymer content of products decreases with increasing pore size of the catalysts [64]. The pore size can affect the structure of MAO and the nature of the MAO-(n-BuCp)2ZrCl2 interactions and these effects will become less pronounced with increasing pore sizes. Products made with catalysts with support pore sizes >7 nm have DSC endotherms with several maxima, indicating multiple types of catalytic sites which interact differently with 1-hexene. Moreover, the ‘‘comonomer effect’’ occurs only at low concentration of 1-hexene (4–12 mol m3). There are few reports on the copolymerization of ethylene and a-olefins with SBA-15 supported catalysts. Decreasing as the activities of ethylene/1-hexene copolymerization with zirconocene fixed on the amine-functionalized SBA-15 are (possibly due to a negative ‘‘comonomer effect’’), they are higher than the case of using pristine SBA15 as the support and still in the same order with that of ethylene homopolymerization [40]. As previously mentioned, an iron-based diimine complex immobilized on

MAO-treated MMS capable of oligomerizing ethylene to a-olefins with improved selectivity to lower molar mass fraction in combination with MMS supported zirconocene can produce LLDPE through in situ ethylene copolymerization from a single ethylene monomer [72]. 3.4. Copolymerization of ethylene and dienes over molecular sieve supported catalysts There have been insofar rare reports on the copolymerization of ethylene and dienes with molecular sieves supported catalysts. MAO/MCM-41 was attempted to support four metallocenes (1: (BzCp)2ZrCl2; 2: (C18H37Cp)2 ZrCl2; 3: (CH3)2Si(Ind)2ZrCl2; 4: U2C(Cp,Flu)ZrCl2) and low activities are resulted in the copolymerization of ethylene and 5,7-dimethylocta-1,6-diene compared to those of corresponding homogeneous catalysts [42]. All the four supported metallocenes possess no ‘‘comonomer effect’’ in such copolymerization now that the diene used herein is much bulkier than common bulky a-olefins [48,62]. To sum up, the pore size of molecular sieves exerts significant influence over ethylene homopolymerization and copolymerization. The porous channels and immobilization give full play to monomer diffusion and inhibition to bimolecular termination, thus realizing smooth polymerization kinetics for the active sites. Molecular sieves supported catalysts afford easily controllable polymerization process and low dosage of cocatalyst while comparatively high activities are maintained. For the copolymerization of ethylene and a-olefins over molecular sieves supported catalysts, a mechanism of comonomer structure and diffusion controlled copolymerization is assumed [48,62]. Additionally, for metallocenes supported on zeolites, the size of metallocene and its placement in the zeolite affect the incorporation of comonomers [46]. 4. Structure and properties of polyethylene from MS supported catalysts As previously described, ethylene (co)polymerization with molecular sieves supported catalyst mainly proceeds

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in the confinement of molecular sieve pores (without excluding polymerization occurring at the outer surface of molecular sieves where active sites are physically adsorbed or chemically bonded), following an extrusion polymerization mechanism [33]. The confinement not only alters the polymerization kinetics, but also affects the molecular weights and their distribution, the melting points, and chain structures of polyethylenes, etc. Ethylene homopolymerization and copolymerization will be stated separately below due to different chain structure and properties. 4.1. Structure and properties of homo-polyethylene Aida et al. adopted fibrous mesoporous silica (MSF) (pore diameter = 2.7 nm) to support titanocene [33]. The Cp2Ti@MSF/MAO catalyzed ethylene polymerization in an extrusion mode in which polyethylene chains grow in parallel with the pore axis, forming extended-chain crystals of which the viscosity-average molecular weight (Mv) reaches 6.2  106 g mol1, a magnitude higher than that from homogeneous titanocene. More remarkable is that the density and melting point of the resultant polyethylene arrive at 1.01 g cm3 and 140 °C, respectively (Scheme 3). Cp2Ti Cl2@MCM-41 also produces PE nanofibrils (diameters 60 nm) which stack in parallel and assemble into individual microfibers with diameters of about 1–30 lm (Mw = 2.02  106 g mol1, Tm = 140 °C) following this nanofabrication technique [65]. MCM-41 supported ansa-zirconocenes hardly polymerize ethylene in the extrusion mode because all the resultant linear HDPEs have a Tm around 134 °C [70]. Polyethylenes formed with silica magnesia xerogels have broader molecular weight distributions than those obtained by the homogeneous Cp2ZrCl2 and Cp2ZrCl2/SiO2 systems [59]. The Mw of PE produced with hybrid zirconocene supported on carriers of different pore sizes falls between 254 and 477 kg mol1 and the PDI value remains around 2.2 [73]. Increasing the Zr–O distance leads to

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decreasing PE molecular weight for which the reason is that the less sterically affected metallocene favors the chain termination step, thereby causing a drop in the molecular weights. In the case of using Al-MCM-41 supported Cp2ZrCl2, the observed molecular weights of 431,000–451,000 g mol1 and the Mw/Mn ratio of around 2.2 are typical for polymers prepared with metallocene catalysts. The melting temperatures, around 135 °C, indicate the formation of HDPE [60]. These results may be explained well once the adsorption location of an active component, in or out of the pores, is confirmed by taking an alternative supporting route. (n-BuCp)2ZrCl2 fixed on MAO treated molecular sieves (pore diameter: 2.5–50 nm) polymerized ethylene to polyethylene whose molecular weights and their distribution have no relation to the pore diameters. Whereas the molecular weights decrease with rising temperature and decreasing pressure [63,64]. For PE produced with benzamidinate zirconium complex anchored to MCM-41, the PDI (2.20) is slightly lower than that from homogeneous system (2.00) [38]. However, the Tm of the former (131 °C) is 4 °C below that of PE from homogeneous catalyst. This result runs counter to general law relating to ethylene polymerization with MCM-41 supported catalysts and further investigation seems to be necessary to obtain plausible explanation. Cp2TiCl2@MCM-41/MAO catalyzed ethylene polymerization to polymers of ca. 106 g mol1 in molecular weights, which is 10 times of those obtained over Cp2TiCl2/MAO and typical of extrusion polymerization [18]. Different molecular sieves result in varying structure and properties of the polyethylene. SBA-15 supported diimine iron or nickel complex catalyzed ethylene polymerization to polymers whose molecular weights are three times of those from homogeneous catalysts [44,56]. MCM-41supported bis(phenoxyketimine) zirconium complexes polymerized ethylene to polyethylene with Mn of 1.74  105 g mol1 and molecular weight distribution (Mw/Mn or PDI) above 3 [66]. Compared to homogeneous and silica supported

Scheme 3. Conceptual scheme for the growth of crystalline fibers of polyethylene by mesoporous silica-assisted extrusion polymerization [Ref. 33].

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iron complexes, Fe@MCM-41/MAO catalyzes ethylene to PE of greatly improved Mv (up to 4.48  105 g mol1) and Tm (highest 140.9 °C) at much lower pressure [32]. A bimodal melting behavior is observed in the thermograms for polyethylenes produced with MCM-Ni-1 and MSF-Ni systems in which larger short-chain branches and fewer short-chain branches correspond to low Tm and high Tm, respectively, indicating that the polymer sample is a mixture of two chain populations with different short chain branch densities [39]. With regard to ethylene polymerization over (C5(CH3)4 Si(CH3)2NR)TiCl2 fixed on mesoporous silica, the polymerization results are affected by local environment such as pore structures, and pore sizes. Polyethylene of high degree of crystallinity and low molecular weights is obtained for small pore diameters. Therefore, polyethylenes with crystalline, mesomorphic, and amorphous structures were formed depending on varying pore sizes, indicating the existence of ‘‘second sphere of influence’’ in ethylene polymerization in confined geometries [41]. However, polyethylenes obtained over DMN supported on MAO treated MSF and various pore sized MCM-41 have molecular weights similar to those from homogeneous catalyst. This phenomenon is quite different from the fact that DMN@SiO2 produced PE of smaller molecular weights than the case employing homogeneous catalyst [39]. Aqueous Cr(NO3)3 solution reacted with MCM-41 and formed supported catalyst capable of producing crystalline and amorphous polyethylenes [30]. Cr/SBA15 produces PE of very high melting points (up to 142.9 °C) and higher crystallinity (up to 77.7%) due to the control of chain propagation direction [55]. 4.2. Structure and properties of ethylene copolymers with aolefins The high and long stability of active sites in the supercage of NaY molecular sieve endows Cp2ZrCl2@NaY/MAO the ability to form ethylene/propylene copolymer with higher molecular weights than those resulted from Cp2ZrCl2/MAO. Due to limited diffusion of propylene molecules into pores of molecular sieves, the incorporation rate of propylene unit is only 0.7 and 1.7 mol% when the feed C3/C2 molar ratio is 0.5 and 1.0, respectively. Whereas propylene incorporation rate of 3.0 and 3.9 mol% are realized at the same C3/C2 molar ratios for homogeneous catalyst. Compared to homogeneous catalyst, the supported catalyst formed ethylene/propylene copolymers with reduced content of propylene sequence and increased [EEE] content, tending to produce block structures. The confinement effect affords the copolymer a single melting enthalpy upon annealing, implying a uniform distribution of propylene units [48,62]. For Et(Ind)2ZrCl2@MCM-41/ MAO, the contents of EPP + PPE and PPP segments increase, demonstrating an improved stereoregularity owing to the porous confinement [48,62]. MCM-41 supported [{C2H4(ind)2}Zr(CH3)2] cooligomerized ethylene and propylene to form cooligomers of molecular weights varying between 790 and 5590 g mol1and PDI values around 2. For smaller pores, less amount of metallocene was introduced into pores of MCM-41, this causes increased molecular weights and the liquid

cooligomer turns to be waxy product. In this case, the incorporation rate of propylene decreases (especially at low Al/Zr molar ratios), manifesting shape-selective oligomerization behavior [74]. For a-olefins bulkier than propylene, the diffusion effects in the confinement of molecular sieves are more pronounced in their copolymerization with ethylene. The molecular weight of ethylene/1-hexene copolymer from Cp2ZrCl2@HY/MAO is eight times that of the copolymer from homogeneous catalyst although the comonomer incorporation rate is only 8 mol% and the Tm is 112 °C. As to Ind2ZrCl2@HY/MAO, the molecular weights and Tms of ethylene/1-hexene copolymer are similar to those of copolymers from homogeneous catalyst except that the comonomer content is below one fourth of that of the copolymer from homogeneous catalyst. Further varying the catalyst to be Et(Ind)2HfCl2@HY/MAO, the viscosity average molecular weights reach 1.32  105 g mol1 (8.1  104 g mol1 for homogeneous catalyst) and two melting points are observed, Tm1 = 94.7 °C and Tm2 = 125.5 °C (94.6 and 128.3 °C for homogeneous catalyst) and the comonomer incorporation rate is 16 mol%, reflecting uneven comonomer distribution. Employing Et(Ind)2ZrCl2 of much better copolymerization performance, the immobilization of the catalyst makes no difference in comonomer incorporation rate, both are 36 mol% before and after immobilization. The molecular weights get slight increase and the Tm drop by 5.4 °C (125.1 °C for homogeneous catalyst). Copolymers of ethylene/1-octene, ethylene/1-dodecene, and ethylene/4ME formed over Cp2ZrCl2@HY/MAO all possesses decreased comonomer incorporation rate, increased molecular weights by two magnitudes, and one melting points due to the bulkiness of the comonomer and the confinement of molecular sieves, this trend becomes more significant when the comonomer molecule is larger [46]. As to copolymers of ethylene/isobutylene, ethylene/1hexene, and ethylene/1-octene formed with Cp2ZrCl2@ NaY/MAO, the molecular weights of these copolymers are higher than those from homogeneous metallocene due to inhibited b-H elimination in the supported catalysts. The bulkier kinetic diameter of isobutylene contributes to a Tm of 137.5 °C for the ethylene/isobutylene copolymer, approaching that of PE (135 °C) from the same catalyst. The copolymers containing less bulky 1-hexene or 1octene possess Tm around 120 °C. The pretreatment of NaY molecular sieves with MAO results in much narrower pore size, further decreasing the comonomer incorporation rate in the copolymers [48,62]. Mesoporous molecular sieves of varying pore diameters (2.5–50 nm) were attempted to examine the effect of pore size on the copolymerization of ethylene and a-olefins [63,64]. The copolymers of ethylene and 1-hexene formed with (n-BuCp)2ZrCl2@MMS have lower molecular weights than those of PE formed with the same catalyst. The Mws changes a little with pore size variation but decrease with rising content of 1-hexene in the copolymers (PDI = 2.7– 3.1). No structure analyses are disclosed for the copolymers. The incorporation rate of 1-octene in its ethylene copolymer from Et(Ind)2ZrCl2@MCM-41/MAO is far below that from Et(Ind)2ZrCl2/MAO [48]. Similar to this result,

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the contents of 1-octadecene and EOO + OOE in ethylene/ 1-octene copolymers from this supported catalyst are also lower than the values of the copolymers from homogeneous catalyst. The reason for this phenomenon is that the coordination of bulkier 1-octadecene to active sites makes it more difficult for other comonomers to access, exhibiting random copolymerization behavior [62]. As elaborated above, molecular sieve supported catalysts possess decreased performance in copolymerizing ethylene with bulkier a-olefins, especially at smaller pore sizes [46,48,62]. The copolymerization of ethylene and enormous 5,7-Me2-1,6-octadiene over MCM-41/MAO supported non-bridged metallocenes (1: (BzCp)2ZrCl2; 2: (C18H37Cp)2ZrCl2) afforded copolymers with increased Mws and basically invariant Tms. Compared with the copolymers from unsupported catalysts, the copolymer from 1@MCM-41/MAO has narrower PDI, lower crystallinity, and hardly changeable diene content; the copolymer from 2@MCM-41/MAO has narrower PDI, lower diene content, and basically constant crystallinity. Changing the metallocenes to be bridged ones (3: (CH3)2Si(Ind)2ZrCl2; 4: /2C(Cp,Flu)-ZrCl2), the molecular sieve supported catalysts formed partially crosslinked copolymers with markedly decreased molecular weights and crystallinity besides narrower PDI. Compared with the copolymers from unsupported catalysts, the copolymer from 3@MCM-41/ MAO experiences invariable Tms whereas the copolymer from 4@MCM-41/MAO acquires decreased Tms which, alongside with the crystallinity, are the lowest among those of copolymers from the four MCM-41 supported catalysts [42]. The LLDPE made with a two catalyst system supported MMS possesses high molecular weight and broad molecular weight distribution compared to its homogeneous counterpart in which such trend is more prominent in using SBA-15 support [72]. 5. Morphology and physical properties of polyethylenes obtained with MS supported catalysts As disclosed in prior sections, ethylene (co)polymerization in the nanopores of molecular sieves also proceeds in a highly active fashion while greatly improved molecular weights are obtained in most cases. However, for a polymeric material to be useful, especially in the case of polymer production employing in situ polymerization method, reactor fouling is a significant issue to tackle. So, the morphology of polyethylene formed with catalysts supported molecular sieves is highly worthy of investigation. Moreover, greatly improved mechanical properties are also essential for the as-fabricated polyethylene to be of high performance. Therefore, this section will elaborate the morphology and mechanical properties of such polyethylenes originated from the morphology and confinement effect of molecular sieves. 5.1. Morphologies of the polyethylene formed with MS supported catalysts The morphology of polyethylene is the replication of the support for ethylene polymerization from heterogenized

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catalysts. However, the porous structure of molecular sieves affords them more functions than simple morphology replication. The protean structures of molecular sieves endow ethylene (co)polymers from molecular supported catalysts with versatile morphology regulation abilities. Mesoporous silica film (MSF) supported Cp2TiCl2 catalyzed ethylene to unbranched, smoothfaced, crystalline, and extended-chain PE fibers whose diameters are 30–50 nm [33]. Cp2TiCl2@MCM-41 formed PE composed of myriads of parallel nanofibers (/ = 60 nm) which assemble into microfibers and finally aggregate to fiber bundles [65]. Two-dimensional wide angle X-ray diffraction (2D WAXRD) analyses reveal that some PE microfibers made with the above mentioned Cp2TiCl2@MCM-41 possesses a very high degree of chain orientation along the fiber axis. Owing to a random aggregation of anisotropic microfibers, the microfiber aggregates exhibit rather isotropic diffraction patterns [69]. Atmospheric ethylene polymerization with Cp2TiCl2@ MCM-41 forms PE nano-fibers (80–100 nm in diameter) and floccules (Fig. 6) where the amount of floccules which originate from random aggregation of PE foldedchains increase with extended polymerization time [49]. Some floccules also result from the extended-chains not aggregating into fibers immediately. In subsequent study, the same group produces PE nanofibers with Cp2ZrCl2 fixed on MCM-41 and SBA-15 where the former possesses small diameters due to low pore diameter of MCM-41 [51]. Moreover, the polymerization temperature greatly affects the polymeric morphology in addition to pore structure of the supports. At elevated Tp of 70 °C, PE from Cp2ZrCl2@SBA-15 takes on particle morphology instead of the nanofibers formed at 50 °C due to the collapse of SBA-15 channels into broken small particles at higher activities (0.9  105 g PE (mol Zr)1 h1 atm1 at 70 °C versus 0.53  105 g PE (mol Zr)1 h1 atm1 at 50 °C). CpTi(OAr)C12 (Ar = 4-Me-2,6-tBu2C6H2) supported on SBA-15 polymerizes ethylene to nano-fibers which aggregate to sheet-like morphology with rising polymerization temperature [45]. For a SBA-15 of pore diameter = 5.64 nm, Cp2ZrCl2 supported on it produces PE micro-fibers and floccules in which the single nanofibers (diameter: 120–200 nm) assemble to form fiber aggregates and bundles and, with the extension of polymerization time, the number of floccules increases [50]. With regard to polyethylene formed with ansa-zirconocene supported on MCM-41, less ordered fibrous morphology (Fig. 7c) is observed whereas sponge-like PEs produced by the homogeneous system (Fig. 7a) is and SG supported catalyst (Fig. 7b) are obtained [70]. To polymerize ethylene over titanocene-mounted mesoporous silica layer on a mica or glass plate affords polyethylene of different morphologies: PE/silica composite thin film (roughly 1.1 lm in thickness) on mica surface and a composite with an islanded morphology, which replicates the original silicate domains [37]. The polyethylenes produced by immobilizing metallocene on a crystalline bisupport (SiMg0.5) had fiber-like morphologies [59] which are less regular than the case of using Cp2TiCl2@MCM-41 probably due to the high crystallinity and low porosity of the bisupport. For the polyethylene obtained with mixture of Cp2ZrCl2 and

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Fig. 6. SEM images of the PE fiber sample: (a) parallel aggregates of PE fibers with floccules among them. (b) Magnified view of (a). (c) Magnified view of (b), showing compactly aggregated fiber bundles with floccules among them. (d) Magnified view of (c), showing parallel aggregate of single nano-fibers of PE [Ref. 49].

Fig. 7. Scanning electron microscope pictures of polyethylene from: (a) homogeneous catalyst [Zr{Me2Si(g5-C5Me4)(g5-C5H4)}Cl2]. (b) SG/[Zr{Me2Si(g5C5Me4)(g5-C5H4)}Cl2]/MAO. (c) MCM-41/[Zr{Me2Si(g5-C5Me4)(g5-C5H4)}Cl2]/MAO [Ref. 70].

(nBuCp)2ZrCl2 (1:3 ratio) supported on different porous materials takes on fiber morphology of varying amounts [73]. The PE particles made with MMS-supported catalysts all consist of agglomerates of fairly dense, platelike particles and the fiber morphology of certain MCM-41supported catalysts is not replicated into the polymer particle morphology [63]. The morphologies of the ethylene/ 1-hexene copolymer particles made with some (n-BuCp)2ZrCl2 (CAT2.6-2 and CAT5.8) are more porous than those of PE homopolymers. As for (n-BuCp)2ZrCl2 supported on AlSBA-15 of varying Si/Al ratio, round-shape

polyethylene particles are obtained with acceptable sizes to the adequate polymer processing [52]. Polyethylene obtained over iron(II) complexes bearing 2,6-bis(imino)pyridyl ligands supported in MCM-41 displays a spherical morphology (possible replication rates close to 200), reflecting that polymer particles follow the original morphology of the support [32]. At some conditions, replication of support morphology is also found in the polyethylenes produced with mesoporous particle supported nickel-diimine catalysts [39]. It is unclear whether large quantities of PE nanofibers form within the mesopores of MCM-41 since small support fragments

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remain in the larger particles of the polymer. In the case of using SBA-15 as the support, the morphology of PE produced by amine-functionalized catalysts exhibit more regular and larger particles (above 100 lm) than that (below 100 lm) of non-funtionalized catalyst (Fig. 8) [40]. The plate-like PE particles produced with Cr/MCM-41 are separated from each other, whereas those produced with Cr/SiO2 aggregates to form a bigger chunk with irregular shape [53]. Some intercrystalline fibers are observed in PE from Cr/SiO2, however, no fibrous structure is observed in PE from Cr/MCM-41. Compared to the porous morphologies of PE from Cr/SiO2 (Fig. 9a and b), PE obtained with Cr/SBA-15 mainly takes on smooth nanofiber morphologies (Fig. 9c and d) [55]. The polymerization temperature (Tp) exerts pronounced influence over the morphology evolution of the resulting HDPE: the track starts at predominant fiber and floccular morphologies (Tp = 90 °C), pauses at the appearance of porous morphology and decreasing amount of nanofibers (Tp = 104 °C), and stops at a shish-kebab morphology (Tp = 112 °C) [55]. In such a process, the nanofibers are attributed to internal Cr sites, and porous morphology is due to external Cr sites. The marked morphology difference for polyethylenes from Cr/SBA-15 and Cr/MCM-41 [53] may be, with great possibility, ascribed to the variation of support structure and polymerization parameters. 5.2. Mechanical properties of polyethylenes produced with MS supported catalysts In the field of ethylene (co)polymerization within the porous confinement of molecular sieves, the great majority research has up to now concentrated on the fabrication of

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polyethylenes of varying chain topology and polymeric morphology. The special porous structures and facile modification on the internal and external surfaces of molecular sieves, particularly mesoporous molecular sieves will undoubtedly endow the as-fabricated polyethylenes with pronounced changes in their physical properties. However, very few works relating to mechanical properties are reported while a vast amount of research results on PE/clay nanocomposites has come into publication which discloses valuable information on the effect of clay nano-platelets on the significant improvement in the resulting mechanical properties. As presented in the previous part of this work, the in situ formed polyethylenes through ethylene (co)polymerization with MS supported catalysts conveniently realize uniform dispersion of the molecular sieves particles throughout the polymeric matrix, thus improving the polymer’s strength, toughness, and even rigidity in certain situation. Moreover, uniform dispersion of smaller MS fillers in the polymer can increase the bulk density of the materials. In general, the addition of silica (similar to molecular sieves in chemical composition and structure in certain aspects) into polyethylene matrix will increase the strength, modulus, and thermal stability of the polymers whereas the toughness is deteriorated. However, strong physical or chemical interaction between the polymer and inorganic filler plus steady, uniform, and ordered structure of the inorganic filler enhance the polymer’s tensile modulus, strength, elongation at break, and toughness while little disturbance from the environment occurs and the nanoparticles hardly aggregate. The polyethylene composites (named so if the MS content is high enough) formed by molecular sieve supported

Fig. 8. FE-SEM images of polyethylene particles obtained with the supported catalysts: (a) SBA-15/(n-BuCp)2ZrCl2. (b) SBA-15/1NS/(n-BuCp)2ZrCl2. (c) SBA-15/2NS/(n-BuCp)2ZrCl2. (d) SBA-15/3NS/(n-BuCp)2ZrCl2. (e) SBA-15/1NCy/(n-BuCp)2ZrCl2. (f) SBA-15/2NIm/(n-BuCp)2ZrCl2. The scale bar is 100 lm [Ref. 40].

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Fig. 9. SEM micrographs of the PE produced from the chromium-supported catalysts at a polymerization temperature of 90 °C: (a) Cr/SiO2 (Aldrich). (b) Magnified view of part a. (c) Cr/SBA-15. (d) Magnified view of part c [Ref. 55].

catalysts are different from conventional PE/clay composites because obviously different structures of molecular sieves and clay. It’s the unique topology of the porous confinement of molecular sieves that force polyethylene macromolecules to grow as extended-chains along the axial direction. Special mechanical properties are expected for PE grown from MCM-41 supported catalyst due to the coexistence of nano-fibers (beneficial for the strength) and floccules (good for the elasticity) [49]. Cp2TiCl2/MMAO catalyzed ethylene polymerization to form PE microfibers with low secant tensile modulus (5% strain) of 3.0– 7.0 GPa, high tensile strength at break of 0.3–1.0 GPa, and much higher elongation at break (8.5–20%) with the help of molecular sieves [65]. The low tensile modulus is attributed to imperfect orientation and packing of the nanofibrils in the microfiber and a rather relaxed state instead of a fully extended one of the constituting nanofibrils. Besides, after one cycle with the highest strain of 3.6%, the microfiber retains a permanent strain of 1.7%, showing a significant irreversible change in the fiber dimension even after slight stretching. The fracture test shows an irregular rupture with fibril fragments of different lengths, indicating the presence of an epitaxial fibrous structure. Finally, slight tensile drawing with a drawing ratio of 1.5–1.6 improves the tested microfibers’ tensile modulus by 6.0–15.0 GPa and tensile strength by 0.6–1.6 GPa (Fig. 10). Compared to LLDPE from the two homogeneous catalysts, both MCM-41- and SBA-15-supported two-catalyst systems improve the mechanical properties of LLDPE [72]. MCM-41 (smaller pore size) seems superior to SBA15 in such improvements. Besides, the uniformly dispersed MMS particles help to acquire comparable elongation at

Fig. 10. Effect of drawing on the fiber tensile properties: (a) strain as a function of time for a PE microfiber [initial fiber diameter (deff,0) = 35.0 lm and initial fiber length = 9.81 mm] produced in run 2 under programmed stress (drawing temperature = 80 °C and final drawing ratio = 1.6). (b) A comparison of the tensile stress–strain curves for the microfiber before and after tensile drawing. The diameter of the drawn microfiber is calculated according to the diameter before drawing and the drawing ratio [deff = deff,0/(draw ratio)0.5]. The test conditions were a preload force of 0.005 N, a force ramping rate of 0.05 N/min, and a temperature of 35 °C [Ref. 65].

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break, increased tensile strength, increased Young’s modulus and drastic increase in Izod impact strength. Moreover, the shear-thinning phenomenon becomes increasingly pronounced in the following order: LLDPE from LTM-Fe + rac-Et(Ind)2ZrCl2)/MAO < SBA/Fe + SBA/Zr < MCM/Fe + MCM/ Zr (Fig. 11A). This is in agreement with the trend of storage modulus (G0 , Fig. 11B) and loss modulus (G00 , Fig. 11C), indicating that the interfacial interaction of LLDPE with the support decreases in the order: MCM/Fe + MCM/Zr > SBA/ Fe + SBA/Zr. 6. Summary and outlook As described above, ethylene (co)polymerization with molecular sieve supported catalysts not only maintain the performance of homogeneous catalysts and macroporous support immobilized catalysts, but also develop into unique function of their own. The confinement in microand mesoporous molecular sieves inhibits the deactivation of active sites albeit the polymerization activities decline. In certain cases, ‘‘living polymerization’’ of polar monomers is expected for molecular sieve supported catalysts. Ethylene (co)polymerization in confined geometries significantly affects the polymer’s properties, including the alteration of polymer chain structures and crystallization behavior, efficient control over the polymer’s morphology, elevation of the molecular weights and melting points of

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the polymer, and enhancement of polyethylene’s mechanical properties. In addition to effective control of polyethylene’s molecular weights, the predominant issues confronting R&D institutions are the morphology regulation and property improvement of the polymers. As disclosed in this review, the introduction of molecular sieves as catalyst supports make the solution to these obstacles easier to certain extent. However, the research on ethylene coordination (co)polymerization in confined geometries is still limited to the scope of activities and fundamental properties of polymers. There has been great room left as to systematic investigation on how the confinement influences the activities and the rheological properties, the uniform dispersion of molecular sieves nanoparticles in PE matrix, and the mechanism on the interaction between the two phases. So, functional supports as various molecular sieves open up a new field for ethylene polymerization with heterogeneous catalysts. With the help of the unique structures of these supports, polyethylene and its nanocomposites with nano molecular sieve fillers are highly expected with regard to nonbranched architecture, ultrahigh molecular weights, high melting points and modulus, high impact strength and elongation at break. Such polyethylene and/or its nanocomposites liberate polymers from tedious and costly gel spinning and shearing stress induction which are necessary for fiber forming and

Fig. 11. (A) Complex viscosity versus angular frequency measured at 170 °C for LLDPE from (a) LTM-Fe + rac-Et(Ind)2ZrCl2/MAO, (b) SBA/Fe + SBA/Zr/TEA and (c) MCM/Fe + MCM/Zr/TEA. Dynamic storage modulus G0 (x) (B) and dynamic loss modulus G00 (x) (C) versus angular frequency measured at 170 °C for LLDPE from (a) LTM-Fe + rac-Et(Ind)2ZrCl2/MAO, (b) SBA/Fe + SBA/Zr/TEA and (c) MCM/Fe + MCM/Zr/TEA [Ref. 72].

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blending, thereby fabricating high performance polyethylene simply by nano-extrusion polymerization over MS supported catalysts and homogeneous dispersion of MS nano-particles in the polymer matrix. The elimination of a series post processing is surely benefited from the in situ ethylene polymerization over (molecular sieve) supported catalysts. Therefore, to shed new light on rules about catalyst immobilization and ethylene polymerization in the confinement, the establishment of the relationship among the support—catalyst—polymer from the viewpoint of molecular design, quantitative elucidation of the polymer’s mechanical properties, and the production of high performance polyethylene resins has been and will still be an essential and important aspect in academic research and industrial application of polyethylene.

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