Weakly Entangled Ultrahigh Molecular Weight Polyethylene Prepared

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Weakly Entangled Ultra-high Molecular Weight Polyethylene Prepared via Ethylene Extrusion Polymerization Peng Chen, Huaqin Yang, Tao Chen, and Wei Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03059 • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on October 31, 2015

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Industrial & Engineering Chemistry Research

Weakly Entangled Ultra-high Molecular Weight Polyethylene Prepared via Ethylene Extrusion Polymerization

Peng Chen,† Huaqin Yang†, Tao Chen†, Wei Li *†

†

Department of Polymer Science and Engineering, School of Material Science and Chemical Engineering, Ningbo University, Ningbo 315211, Zhejiang, People’s Republic of China

*To whom should be correspondence: Wei Li, [email protected]

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ACS Paragon Plus Environment


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ABSTRACT Fe(acac)3/2,6-bis[1-(2-isopropylanilinoethyl)] pyridine catalyst was immobilized on MCM-41 zeolite to synthesize ultra-high molecular weight polyethylene (UHMWPE) with weakly entangled state. Microstructure, morphology, crystallization and rheological properties of the nascent polyethylene were investigated by gel permeation chromatography, scanning electron microscope, differential scanning calorimetry and rheometer, respectively. The polymer showed broad molecular weight distribution. We found that the synthesized UHMWPE had a weakly entangled state. Evolution of weakly entangled state of polymer was studied. However, it was found the polymer synthesized in the initial stage of polymerization could be more entangled. Thus, poly[styrene-co-(acrylic acid)] (PSA) was coated on the surface of Cat-Fe/M to control the diffusion rate of ethylene, especially during the initial stage of polymerization. The decay rate of catalyst activity was decreased. The polymer synthesized at this stage then became more weakly entangled. The mechanism for achieving the weakly entangled state during polymerization was further discussed. KEYWORDS: UHMWPE; weakly entangled; extrusion polymerization; core-shell support; diffusion

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1. INTRODUCTION

Ultra-high molecular weight polyethylene (UHMWPE), with molar weight exceeding 1 million g/mol, is widely used for high demanding application because of its ultra toughness, low moisture uptake, and excellent chemical stability.1 However, commercial available UHMWPE usually can not be processed by conventional methods due to the extremely high melt viscosity of the polymer, resulting from large numbers of entanglements between chains.2 Usually, 1 mg particle of a polymer with one million g/mol molecular weight contains 1014 entangled chains .3 Synthesis of weakly entangled UHMWPE is indeed very important for easier processing. Otherwise processing aids have to be used when classical processing equipments are employed 4,5. Smith et al.6 reported the synthesis of UHMWPE with low entanglements by using VCl4 catalyst supported on a glass slide and performing the polymerization at -40 °C. However, due to the poor catalytic activity of VCl4 system at this temperature, the synthetic route is no longer pursued. Rastogi et al.7-12 developed a new method to reduce the number of entanglements in nascent UHMWPE by using a homogeneous single-site catalyst (FI catalyst). Low catalyst concentration would ensure spatial distance between the growing polymer chains during ethylene polymerization. In addition, low synthetic temperature was favorable to chain crystallization compared with chain propagation. Therefore, the number of chain entanglements is greatly reduced in the synthesized UHMWPE. The synthesized “weakly entangled” UHMWPE provided a unique, solvent-free route to obtain high modulus, high strength tapes where compressed films could be processed in a broad temperature window, ranging from 125 °C to 145 °C 9. Our group13,14 immobilized the FI catalyst on polyhedral oligomeric silsesquioxane (POSS) to prepare UHMWPE/POSS nanocomposites. The synthesized nanocomposites showed a 3



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weakly entangled state. However, the incorporation of POSS molecules could serve as a nucleating agent which would enhance the interaction between POSS and propagating chains, resulting in an increment of entangled density. However, homogeouse polymerization always results in reactor fouling. Meanwhile, the procedures for synthesizing FI catalyst are very complicated and expensive. It need much amounts of methylaluminoxane (MAO) to maintain its high activity. These greatly increase the production cost. Thus, it is diffucult to realize the drop-in technology for preparing weakly entangled UHMWPE with FI catalyst in an industrial reactor. Mesoporous molecular sieves (MMS) are promising support materials, which have uniform pore sizes, parallel pores to the axis, high surface areas, and big pore volumes. Therefore, they were natural micro-reactors for catalytic reaction.15-19 Aida et al.20 prepared nano-polyethylene fibres with mesoporous silica fibre (MSF) supported titanocene catalyst. The polymer chains growing in the mesoporous molecular sieves could not fold effectively in such restrained spaces. Therefore, the growing chains were extruded from the mesopores to form nanofibrous morphology. This ethylene extrusion polymerization may offer a possible way to prepare polymer with weakly entangled state. However, most of studies15-22 are mainly concentrated on morphology, mechanical property and crystallization of the synthesized polyethlene. So far, there is less study focusing on evolution of entangled state of nascent polyethylene during ethylene extrusion polymerization. In this article, Fe(acac)3/2,6-bis[1-(2-isopro pylanilinoethyl)] pyridine catalyst was immobilized on MCM-41 supports for preparing weakly entangled UHMWPE. Due to the mesoporous environment of MCM-41, the growth of polyethylene chains may crystallize directional along the limited nano-channal. It would greatly reduce probability of chain overlap during polymerization. Derivative mechanism of 4


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UHMWPE chain entanglements is studied. Furthermore, poly[styrene-co-(acrylic acid)] (PSA) is coated on the outside of the MCM-41 support in order to regulate chain entanglements densiy of the nascent particles. Monomer transfer resistance genereted by the PSA layer is used to adjust the entagled state of nascent UHMWPE, escepically during the intial stage of polymerization. The structures and properties of synthesized UHMWPE are further discussed.

2. EXPERIMENTALS 2.1 Materials All manipulations of air-sensitive and moisture-sensitive compounds were carried out under a nitrogen or argon atmosphere using standard high-vacuum Schlenk techniques or in a glove box. MCM-41 was purchased from Catalyst Plant of Nankai University (Tianjin, China). It was treated at 200 °C for 10 h under protection of ultra-high purity nitrogen. The average pore size of MCM-41 is around 30 nm. And the surface area of MCM-41 is 1140 g/cm2. Polymerization-grade ethylene and nitrogen were purchased from Fangxin Ningbo Corp. (Ningbo, China), and purified by filtering through Mn molecular sieves and subsequent molecular sieves of 5 Å. Methylaluminoxane (MAO, 1.5 mol/L in toluene) was purchased from Albermarle Chemical Inc. (USA). Toluene and n-heptane (Ningbo Chemical Reagents Co., China) were purified over sodium/benzophenone ketyl and distilled prior to use. PSA was provided by Changchun Institute of Applied Chemistry, Chinese Academy of Science, dried at 70 °C under nitrogen flow for 24 h before used. The weight average molecular weight (Mw) of PSA was 1.9 × 104 g/mol, and the molar ratio between styrene and acrylic acid was 1.5. Commercial UHMWPE (PE-Com) was purchased from Sigma-Alderich. The Mw was 1.96 × 106 g/mol. 5



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2.2 Preparation of catalyst. 2.2.1 Preparation of ligand and Cat-Fe Double amino pyridine ligand was synthesized according to the literature.23 The route of synthesizing ligand was shown in Figure 1. Ligand was mixed with acetylacetone iron according to the molar ratio 1:1 (n (acetylacetone iron) : n (the ligand) = 1:1), then was dissolved in an amount of toluene. The mixture was dried in vacuum at 60 °C to get the solid powder, termed as Cat-Fe. 2.2.2 Preparation of MCM-41 supported Cat-Fe (Cat-Fe/M) MAO was supported on the MCM-41 following the procedures used by Srijumnong 24 with minor modifications. In a Schlenk tube equipped with a magnetic stirrer, 1.4 g of MCM-41 was added to 50 ml of toluene and the suspension was sonicated for 10 min to break up particles agglomerates. Then, 5 ml of MAO was added under vigorous stirring. The slurry was stirred for 10 h at room temperature. The solid was filtered, washed three times with 30 ml of toluene. Subsequently, the treated MCM-41 was mixed with 50 ml of Cat-Fe (0.25 g) toluene solution. The solid part was washed three times by 30 ml of toluene and dried in vacuum at 60 °C after the reaction reached 6 h with slowly stirring at room temperature. This catalyst was termed as Cat-Fe/M. 2.2.3 Preparation of MCM-41/PSA supported Cat-Fe (Cat-Fe/M/PSA) 1.0 g of PSA was suspended in 20 ml of toluene. 3 ml of PSA/toluene solution was stirred with 1.5 g of Cat-Fe/M particles at 0 °C. 50 ml of n-hexane was introduced into the mixture of Cat-Fe /PSA/toluene through vapor phase. In the presence of n-hexane, a phase separation of the polymer solution was involved. Then precipitation of PSA took place. The volume of the n-hexane was more than 3 times of that of toluene. After all the n-hexane was added into the mixture, the residual solids 6


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were separated, washed several times by n-hexane at room temperature, and then dried at 50 °C under nitrogen flow for 2 h. Cat-Fe/M/PSA catalysts were obtained with 10 wt% PSA, termed as Cat-Fe/M/ PSA-10. As the same method, Cat-Fe/M/PSA-20 and Cat-Fe/M/PSA-30 catalysts were synthesized. 2.3 Ethylene solution polymerization Normal pressure ethylene polymerization was carried out in a 100 ml of glass reactor equipped with a mechanical stirrer at atmospheric pressure. The reactor was set to polymerization temperature. 50 ml of toluene was added into the reactor. 30 mg of catalyst was introduced into the reactor under nitrogen purging after injection of appropriate MAO as co-catalyst. Polymerization took place under a continuous ethylene flow to meet 1.0 bar at a stirring rate of 100 rpm. The polymerization was proceed until to given time. High pressure solution ethylene polymerization was carried out in a 100 ml PARR high pressure vessel (USA). Procedure of polymerization was the same with normal pressure polymerization. The reactions were terminated at fixed time by the addition of acidified ethanol. The obtained polymer was precipitated and washed with ethanol, filtered, and dried at 50 °C under vacuum for 12 h. 2.4 Characterization measurements 2.4.1 Inductively Coupled Plasma Mass (ICP-MS) measurements Measurement of iron contents in the catalyst was conducted using inductively coupled plasma optical emission spectrometry (730-ES, Varian, Palo Alto, CA). The catalyst was firstly put into oven at 600 °C under nitrogen flow, in order to decompose PSA layer. The amounts of iron in the catalysts were listed here: 1.04 wt% in Cat-Fe/M, 0.89 wt% in Cat-Fe/M/PSA-10, 0.77 wt% in Cat-Fe/M/PSA-20, and 0.58 wt% in Cat-Fe/M/PSA-30. 7



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2.4.2 Gel permeation chromatography (GPC) measurements Weight average molar mass (Mw) and molecular weight distribution (MWD) were determined using GPC at 150 °C with a PLG-PC-220 instrument (Polymer Laboratories, UK), with 1,2,6-trichlorobenzene as solvent. The calibration was made by polystyrene. 2.4.3 Differential scanning calorimetry (DSC) measurements DSC was performed with a DSC-7 instrument (Perkin Elmer Corp., USA) to measure the melting point and the crystallinity of polyethylene. Samples (8 mg) were first heated to 160 °C at a rate of 10 °C /min. They were held at 160° C for 3 min to eliminate their thermal history. Subgsequently, they were cooled down to 40 °C at the same rate and held at that temperature for 1 min before they were again heated to 160 °C. Melting temperature was taken at the peak of the endotherm. Crystallinity was calculated by comparison with the heat of fusion of a perfectly crystalline polyethylene, i.e. 289 J/g. 2.4.4 Scanning electron microscope (SEM) characterization Morphology of catalysts was monitored by SEM (Hitachi S-4700, Japan). The samples were sputter-coated with Au. The characterizations were made at room temperature. 2.4.5 Rheological measurements Rheological studies were performed on a strain-controlled rheometer, HAAKE III instruments. The nascent powder obtained from reactor was mixed with 0.7 wt% antioxidant (Irganox 1010), to prevent any degradation over the long rheological experiments. To have homogeneous mixing of the antioxidant with the powders, the antioxidant was first dissolved in acetone and subsequently mixed with the nascent UHMWPE powder. The powder was dried overnight in vacuum oven at 40 °C after mixing.7,25 The dried powder was compressed into a plate of diameter 8 mm and thickness 2 mm at 135 °C, under 8


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the force of 20 tons for average time of 20 min. A disk of 8 mm diameter was obtained and used in rheological study. During the rheological study, the disk between the parallel plates of the rheometer was heated to 160 °C under a nitrogen environment. The sample was compressed with a compressive force of 1 N. The rheological experiment was started after waiting for thermal stabilization at 160 °C for 5 min. Dynamic time sweep test was performed to follow the entanglement formation at a fixed frequency of 10 rad/s and strains 0.5 % in linear viscoelastic regime of the polymer, where ultimately the maximum stograge modulus (G′max) was achieved.25

3. RESULTS AND DISCUSSIONS 3.1 Ethylene polymerization with Cat-Fe/M Table 1 summarizes the results of ethylene polymerization with Cat-Fe/M at different conditions. Figure 2 shows that the catalyst activity firstly increases with increasing polymerization temperature, then decreases after reaching the maximum value at 30 °C. An increase in polymerization temperature is expected to result in an overall enhanced propagation rate and therefore increased activity. However, higher temperature will decrease ethylene solubility and make the catalyst unstable, leading to a reduction of the activity. Moreover, the catalyst system shows the maximum activity at 1260 of [Al]/[Fe] molar ratio. Generally, more active species are formed when more MAO is added. However, superfluous MAO can deactivate the active centers.25 Therefore, the catalyst can reach the maximum activity (1.08 × 105 g/(mol Fe.h.bar) ) at 30 °C and 1260 of [Al]/[Fe] molar ratio. Following experiments are proceed at this condition. Table 2 summarizes the results of ethylene polymerization with Cat-Fe/M. 9



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Figure 3 shows the activity of Cat-Fe/M decreases upon increasing polymerization time. The supported catalyst maintains high activity in the initial 10 min of polymerization. The activity decreases to half of the maximum value after 20 min of polymerization. It indicates that the catalyst activity is mainly released during the initial 20 min of polymerization. 26 Figure 4 shows Mw and MWD of the synthesized polymers. The Mw and MWD increase with the increment of polymerization time. The Mw of PE-20min can reach one million g/mol, which suggests UHMWPE can be synthesized by the Cat-Fe/M. More synthesized polymer can surround the active catalyst centers at long time of polymerization. These semi-crystallized polymer can greatly increase the monomer transfer resistance, resulting in a hindrance of chain termination.

8, 14

Thus, higher Mw of

polymers can be synthesized at long time of polymerization. Moreover, the synthesized polymer has broad MWD due to the multi kinds of active center in the Cat-Fe. 26, 27 This broad MWD may be helpful in improving processability of the synthesized polymer. 28-30 Figure 5 depicts DSC curves of polyethylene. The products show high melting points (141 °C) and high crystallinity (65%) during the first heating scan. Such high melting temperature is normally found in ‘chain-extended’ polyethylene crystals, which are extremely thick (>1 µm). However, the high melting temperature of 141 °C is lost on the second heating. In addition, width of melting curves during the second heating scan is broad than that of the first heating scan. This may caused by the formation of entangled chains during the first heating scan. These materials, when crystallized from the melt, form crystals where a larger number of chains are shared between different crystallites and more entanglements will be formed, thus having a lower melting temperature of 134 °C and crystallinity.7 More entangled chain structure will also enlarge the difference of crystal size, enlarging the width of 10


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melting curves.

7,12

Furthermore, the polymer synthesized at long time of polymerization presents

slightly increased width of melting curves. This may be caused by the gradually increased MWD which can enlarges the difference of crystal size. 12, 31 Figure 6 shows SEM morphology of products synthesized by Cat-Fe/M at different polymerization time. The synthesized polymers show similar cauliflower morphology with nanofibrous structure. The cauliflower structures are caused by obvious monomer transfer resistance, generated by confined environment of MCM-41 27. Many fibrous can be found in the products at larger magnification (Figure 7). These tiny nanofibrous morphology may be generated by the extrusion polymerization.17-20 It indicates that the polymer chains may achieve weakly entangled state in the nascent polymers. 3.2 Entanglement structures of polymer synthesized by Cat-Fe/M Generally, the storage modulus for a thermodynamically stable polymer melt is related to average molecular weight between entanglement by the equation 10-12: G 0N = g N ρR T M e

Where G 0N is the plateau modulus (in the rubbery regime), g N is the numerical factor (1 or 4/5 depending on the convention), ρ is the melt density, R is the gas constant, T is the absolute temperature, and Me is the molecular weight between entanglements. Therefore, an increased storage modulus is related to an increment of entanglement density in the melt state at a certain temperature. Thus, the formation process of chain entanglements can be followed by rheometer.10 Figure 8a shows storage modulus (G′) buildup curves of UHMWPE synthesized by Cat-Fe/M with different polymerization time. The corresponding curves of commercial UHMWPE is presented for comparison. The synthesized UHMWPE exhibits low starting modulus values which is a reflection of 11



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less number of entanglement per chain.10-12 Here, it is apparent that the initial modulus of PE-60min is 8.0×105 Pa. It then reaches a maximum value of 1.8 × 106 Pa. With the formation of entanglements, where the driving force is toward the thermodynamically stable melt state, the storage modulus increases with time. The increase in the modulus with time suggests an weakly entangled state of the synthesized polymers.9-12 The time required for a weakly entangled polymer to reach 98% of its maximum plateau modulus is termed as the total buildup time (tm). The tm of synthesized UHMWPE is longer than that of commercial UHMWPE (Table 2), which indicates that the synthesized UHMWPE requires more time to reach the thermodynamic melt state.

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The tm increases as the polymerization time increased suggests

that the polymer chains gradually turn to weakly entangled state at longer polymerization time. Figure 8b plots the normalized storage modulus GtN buildup curves. Generally, chains acquire the maximum number of entanglement at G′max in a thermodynamically stable melt. Therefore, GtN indicates the present number of entanglement. It is a fraction of the total number of entanglements in the thermodynamically stable melt.10 In Figure 8b, all the synthesized UHMWPE have a lower G 0N compared to the commercial UHMWPE (0.82 for PE-Com and 0.53 for PE-60min), which indicates the synthesized polymer has lower entanglement density and higher molecular weight between entanglement. These may be due to the extrusion polymerization generated by the confined structure of MCM-41. Durring the polymerization, molecule chains growing in MCM-41 can not be fold effectively in such restrained spaces. The chain overlap procedure is hindered.

19,20

Thus, most of molecule chains

were extruded from the nano- channel of the MCM-41, forming the less number of entanglements. Moreover, the synthesized polymer needs more time to achieve the thermodynamically stable state 12


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with the increment of polymerization time. It means that it is more easier to form entanglements during the intial stage of polymerization. This is similar to the results of Rastogi’s publication.10 It is because the catalyst activity of iron series is high in the initial stage of polymerization. While, the catalyst activity obviously decreases at long time of polymerization. The high catalyst activity will leads to a fast chain propagation rate in the initial stage of polymerization. This is not fit for the formation of weakly entangled state in the nascent polymer 10,12. 3.3 Ethylene polymerization with Cat-Fe/M/PSA According to the above results, it is necessary to adjust the catalyst activity, especially in the intial stage of polymerization, in order to decrease the entangled degree of nascent polymer. In our previous work, incorporation of a PSA layer on sillica supported Ziegler-Natta catalyst could make the catalyst activity decayed slowly. The half-life period of catalyst was increased at least two times.28 Thus, the PSA is coated on the surface of MCM-41 supported catalyst to control the diffusion rate of monomer, especially in the initial stage of polymerization. Catalyst mophorlogy and surface area of the Cat-Fe/M/PSA are shown in supporting information. Table 3 summarizes the results of ethylene polymerization with Cat-Fe/M/PSA. The Cat-Fe/M/PSA shows much higher polymerization activity compared to that of the Cat-Fe/M. For instance, the activity of Cat-Fe/M/PSA-30 is nearly four times of the activity of Cat-Fe/M. The acrylic acid groups of PSA can react with trimethylaluminium (TMA) of the MAO. This PSA layer can serve as a barrier for monomer and TMA diffusion. This will decrease the deactivation of actived centers, resulting in an increment of catalyst activity. 28-32 Figure 9 shows the GPC curves of products synthesized by Cat-Fe/M, Cat-Fe/M/PSA-10, 13



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Cat-Fe/M/PSA-20 and Cat-Fe/M/PSA-30. The Mw and MWD are increased when slighly incorporation of PSA on the Cat-Fe/M surface. It shall be metioned that PSA is mainly deposited on the surface of MCM-41 due to the confined pore structure of MCM-41. Thus, the physcial enviroment of active centers, located inisde the pore of MCM-41, are almost unchanged. Slightly incorporation of the PSA layer can effciently adsorp the TMA through the reaction with acrylic acid groups of PSA. increase the molecular weight of synthesized polymer

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This can

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. However, the acrylic acid groups may

influence the stablility of active centers due to the stong electron density of oxygen atoms when more PSA are introduced. It is negtative to the chain propagation procedure.

28, 34

This influence may be

enlaged when the growing polymer chains contact with the PSA, especilally during the fragmentation of support 28. Thus, the Mw and MWD are decreased. Figure 10 depicts DSC curves of the polymers prepared by Cat-Fe/M/PSA, the products show high melting points and high crystallinity during the first heating scan. The high melting points are also lost after the samples are melt. This is the typical crystallization behavior of UHMWPE.

10-12

Furthermore,

the melting point and crystallinity are less changed when the PSA loading of the catalyst is increased. This may indicate simliar chain structures of polyethylene are synthesized by these catalyts. Interestingly, the width of melting curves turns to narrow when the PSA loading of the catalyst is increased. This may be caused by the decrement of MWD. Figure 11 shows the SEM morphology of products synthesized by Cat-Fe/M/PSA. In the SEM images, some nanofibrous morphology are clearly seen in PE-0% and PE-20%, while the nanofibrous morphology can not be seen in the PE-10% and PE-30%. The growing polymer chains may be interacted with the PSA layer when the chains are grow out from the nanopores during the 14


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polymerization. This will increase the chain overlap procedure.

13,14

Thus, the nanofibours structure is

hardly to be found in the PE-10% and PE-30%. However, the nanofier structure can still be found in the PE-20%, indicating a more weakly entangled state. 3.4 Entanglement Structures of Polymer synthesized by Cat-Fe/M/PSA Figure 12 plots the normalized storage modulus GNt buildup curves of commercial UHMWPE and UHMWPE synthesized with different PSA content. All the UHMWPE synthesized shows higher tm and lower GN0 (Table 3) compared to the PE-Com, which demonstrates the weakly entangled state in the synthesized UHMWPE 10-12. However, the PE-10% and PE-30% have lower tm and higher GN0 compared with that of the PE-20%. It suggests the PE-20% can achieve the most weakly entanled state. This may be generated by the balance between the monomer diffusion resistance and catalyst activity: in the Cat-Fe/M/PSA-20% system, the incorporation of PSA present diffusion resistance to the monomer. This diffusion resistance can retard the decay rate of catalyst activity, especially in the intial stage of polymerization. In addition, crystallization rate is fast at 30 °C of polymerization temperature. Thus, the synthseized UHMWPE gradually turns to weakly entangled. However, in the Cat-Fe/M/PSA-10% and Cat-Fe/M/PSA-30% system, the catalyst activity is increased so much although the diffusion resistance is enhanced. The chain propergation rate is still faster than the crystallization rate. Thus, the synthesized polymer turns to entangled.

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3.5 Proposed mechanism of the evolution of chain entanglement during extrusion polymerization

Figure 13 shows the proposed mechanism for achieving weakly entangled state of UHMWPE via extrusion polymerization. Firstly, polymerization is conducted at 30 °C. At this low temperature, the growing chains experience a “cold” environment and crystallize individually into folded-chain crystals, ultimately forming “monomolecular crystals”. 8,12 In fact, this is an easy and direct way to form weakly entangled chains. In adition, the extended chain crystals can be achieved by ethylene extrusion polymerization due to the structured nano-confined channels of the MCM-41. This will further decrease the possibility of chain overlaps after the chains are growing out from the nanopores.

19

Thus, the

synthesized polymer presents an obvious weakly entangled state. However, the activity of Cat-Fe/M is high during the initial stage of polymerization. There is no enough time for the growing chains to be crystallized. As a result, an entangled state is synthesized at this stage. The growing chains are mainly confined in the nano-channels with further increment of polymerization time. More extended chains will be achieved, resulting in a decrement of entangled density in the nascent polymer. Interestingly, the incorporation of PSA on the MCM-41 can present transfer resistance to monomer. Thus, the concerntration of ethylene surounded active centers is decreaed during the initial stage of polymerization. This will slow down the chain propagation rate at this stage. Polymer chains will have enough time to be crystallized as soon as they are growing out. As a result, the synthesized polymer can be more weakly entagled. 4. CONCLUSIONS Fe(acac)3/2,6-bis[1-(2-isopro pylanilinoethyl)] pyridine catalyst is immobilized on the MCM-41 zeolite. The synthesized polymer shows ultra high molecular weight, high melting point and crystallinity. 16


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Some fibrous morphology can be found in polymer matrix, due to the ethylene extrusion polymerization. It is shown that the synthesized UHMWPE presents lower GN0 and higher tm compared to commercial UHMWPE. It indicates that the prepared polyethylene has a weakly entangled state. Furthermore, it is more easier to form entanglements during the early stage of polymerization, due to the fast decay of catalyst activity. Therefore, PSA is coated on the surface of MCM-41 supported catalyst to control the diffusion rate of ethylene, especially in the initial stage of polymerization. However, the incorporation of PSA can increase catalyst activity. The weakly entangled state of polymer chains is thus modulated by the balance between the catalyst activity and the monomer transfer resistance. The synthesized polymer can achieve the most weakly entangled state when 20 wt% of PSA is coated on the MCM-41 supported catalyst. ACKNOWLEDGMENTS Funding from the Project of Natural Science Foundation of China (No. 21206078), the Natural Science Foundation of Zhejiang Province (LQ12B06003), the Key Innovation Team of Zhejiang Province (2011R50001-5,-11), sponsored by the K. C. Wong Magna Fund in Ningbo University, Ningbo Key Laboratory of Specialty Polymers 2014A22001, is gratefully acknowledged. SUPPORTING INFORMATION AVAILABLE SEM morphology of the Cat-Fe/M/PSA is shown in Figure S1. Surface area of the Cat-Fe/M/PSA is shwon in Figure S2.

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Figure legends Figure 1 Route of synthesizing 2,6-bis[1-(2,6-diisopropylphenylimino)ethyl] pyridine. Figure 2 Polymerization results of the Cat-Fe/M a) polymerization temperature, b) [Al]/[Fe] molar ratio. Figure 3 The dependence of catalyst (Cat-Fe/M) activity on the polymerization time. Figure 4 The GPC curves of polymers synthesized by Cat-Fe/M at different polymerization time. Figure 5 The DSC curves of polymers synthesized by Cat-Fe/M at different polymerization time. (a) First heating scan; (b) Second heating scan. Figure 6 The SEM images of PE synthesized using Cat-Fe/M with different polymerization time: (a) PE-5min; (b) PE-10min; (c) PE-20min; (d) PE-40min; (e) PE-60min. Figure 7 The SEM images of PE-10min and PE-40min synthesized by Cat-Fe/M: (a) PE-10min; (b) PE-40min. Figure 8 (a) Dynamic time sweep test for PE synthesized with different polymerization time at a constant frequency 10 rad/s and strain 1%; (b) Normalized date of (a). Figure 9 The GPC curves of polymers synthesized by Cat-Fe/M/PSA with different PSA contents. Figure 10 The DSC curves of PE synthesized by Cat-Fe/M/PSA with different PSA contents. (a) First heating scan; (b) Second heating scan. Figure 11 The SEM images of PE synthesized by Cat-Fe/M/PSA with different PSA contents. (a) PE-0%; (b) PE-0%; (c) PE-20%; (d) PE-30%. Figure 12 Dynamic time sweep test for PE synthesized by Cat-Fe/M/PSA with different PSA contents at a constant frequency 10 rad/s and strain 1%. Figure 13 Proposed idea for achieving disentangled UHMWPE via ethylene extrusion polymerization. 23



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Page 24 of 40

Tables Table 1. Polymerization results of Cat-Fe/Ma temperature

[Al]/[Fe]

PE weight Activity b

Run (°C)

molar ratio

(g)

1

10

1260

0.23

6.31

2

30

1260

0.34

10.80

3

50

1260

0.29

7.71

4

70

1260

0.17

4.55

5

30

630

0.19

5.25

6

30

930

0.25

6.83

7

30

1575

0.22

5.95

8

30

1890

0.14

3.85

a

Polymerization conditions: 40 mg of catalyst, 1 bar, 30 min of polymerization, 50 ml of n-heptane.

b

Activity in units of 104 g of PE/h·bar·(mol of [Fe]).

24


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Table 2. Polymerization results of Cat-Fe/M at different polymerization time a

sample

run

time

Activity b

(min)

a

T1m

T2m

X1c

X2c

(°C)

(°C)

(%)

(%)

Mw ×104

MWD

tm

G0N

(s)

PE-5min

8

5

15.79

140.0

135.4

61.9

48.3

/c

/

/

/

PE-10min

9

10

15.12

140.3

135.4

64.1

49.8

64.5

8.5

/

/

PE-20min

10

20

10.58

141.4

136.1

65.0

49.0

167.4

9.2

4800

0.62

PE-40min

11

40

7.90

141.7

138.8

65.6

47.8

281.2

15.9

13400

0.60

PE-60min

12

60

7.48

141.5

135.8

65.6

49.6

395.4

17.6

58600

0.53

PE-Com

-

-

-

145.8

138.2

58.4

42.2

196.5

7.1

2100

0.82

Polymerization conditions: 30 mg of catalyst, 30 °C, 10 bar, [Al]/[Fe] molar ratio 1260, 50 ml of

n-heptane. b


c

“/” means the value cannot be measured.

25



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Page 26 of 40

Table 3. Polymerization results of Cat-Fe/M/PSAa sample

Activity (wt%)

a

T2m

X1c

X2c

Mw× 104

(°C)

(°C)

(%)

(%)

(g/mol)

T1m

PSA contents b

tm (s)

G0N

PE-0%

0

10.58

141.4

136.1

65.0

49.0

167.4

9.2

4800

0.62

PE-10%

10

21.40

141.1

136.2

65.4

53.2

353.7

14.3

3500

0.79

PE-20%

20

27.30

140.7

135.9

63.8

52.8

221.7

8.3

31000

0.59

PE-30%

30

38.60

140.8

136.0

63.7

50.5

60.1

4.0

5000

0.68

PE-Com

/

/

145.8

138.2

58.4

42.2

196.5

7.1

2100

0.82

Polymerization conditions: 30 mg of catalyst, 30 °C, 10 bar, [Al]/[Fe] molar ratio=1260,

polymerization time=20 min, 50 ml of n-heptane. b

MWD


26


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Weakly Entangled Ultrahigh Molecular Weight Polyethylene Prepared

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