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Kinetics, Catalysis, and Reaction Engineering

Synthesis and Characterization of Isotactic Poly(1hexene)/Branched Polyethylene Multiblock Copolymer via Chain Shuttling Polymerization Technique Mohammad hossein jandaghian, Ahmad Soleimannezhad, Saeid Ahmadjo, Seyed Mohammad Mahdi Mortazavi, and Mostafa Ahmadi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05339 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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Industrial & Engineering Chemistry Research

Synthesis and Characterization of Isotactic Poly(1-hexene)/Branched Polyethylene Multiblock Copolymer via Chain Shuttling Polymerization Technique Mohammad Hossein Jandaghian1, Ahmad Soleimannezhad2, Saeid Ahmadjo2, Seyed Mohammad Mahdi Mortazavi2*, Mostafa Ahmadi1,3*

1

Department of Polymer Engineering and Color Technology, Amirkabir University of

Technology, Tehran, Iran. 2

Engineering Department, Catalyst Group, Iran Polymer and Petrochemical Institute, Tehran,

Iran. 3

Institute of Physical Chemistry, Johannes Gutenberg-Universität Mainz, Duesbergweg 10-14,

D-55128 Mainz, Germany.

Correspondence to: Mostafa Ahmadi ([email protected]) and Mohammad Mahdi Mortazavi ([email protected])

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract: Understanding from the underlying mechanism of chain shuttling polymerization (CSP) is limited due to scarceness of successful reports and incompetence of traditional characterization techniques to distinguish blocky structures. Here a simple synthesis approach for production of isotactic poly(1-hexene)/branched polyethylene multiblock copolymer from 1hexene monomer is presented. Resulting copolymers can be easily characterized thanks to their solubility in most organic solvents. This novel blocky architecture is synthesized using ansaethylenebis(1-η5-indenyl)zirconium dichloride and α-diimine nickel(II) bromide catalysts. While the former participates in 1,2-enchainment of monomers and produces amorphous segments, the latter forms methylene sequences through chain walking reaction. A quasi-living polymerization is established by reversible transfer of growing chains between catalyst components, as manifested by narrowing molecular weight distribution.

13

C NMR analysis

confirms that the blocky structure can be tuned by adjusting polymerization conditions. Decrease of the crystallizable methylene sequence in presence of CSA leads to a significant transparency of the product. Keywords: Poly(1-hexene); Branched Polyethylene; Chain Shuttling Polymerization; Late Transition Metal Catalyst; Metallocene Catalyst


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1. Introduction Synthesis and characterization of olefinic polymers with new architectures and as a consequences, novel properties, has been one of the most active fields of research in polyolefin science and technology1. For a very long time, random incorporation of α-olefin comonomers in polyethylene backbone has been used to tune the physical and mechanical properties of polyethylenes2,3. To this end, crystallinity and the melting temperature, density and even material stiffness and modulus could be effectively controlled 4,5. However, new polymerization methods and versatile catalytic systems are introduced that open new horizons to novel architectures like blocky incorporation of comonomers, which do not follow the traditional structure-properties relationships for random copolymers and can therefore offer new properties. Although examples on synthesis of multiblock polyolefins through living polymerization catalysts and degenerative chain transfer reactions were reported before6, chain shuttling technique as introduced by Arriola et al. could deliver the synthesis of olefin block copolymers (OBCs) in industrial scale4. They used a metal alkyl complex as chain shuttling agent (CSA) for reversible transfer of growing chains between two catalytic sites with high and extremely low comonomer affinities, which resulted in formation of soft and hard blocks, respectively. Invention of “chain shuttling polymerization” (CSP) technique beside development of postmetallocene catalytic systems have even led to synthesis of multiblock polyolefin using one monomer alone. In the following, a linear/branched multiblock polyethylene was produced using ethylene monomer alone using a metallocene/post-metallocene catalytic system7. Linear



polyethylene blocks were made by metallocene catalyst through common insertion of ethylene, while branched blocks were made by post-metallocene catalyst through “chain walking” mechanism as introduced by Brookhart8. CSA could facilitate sequential cross-shuttling of growing polymer chains between catalytic sites at a rate lower than monomer insertion but higher than chain formation. As traditional characterization methods are still unable to reveal the blocky structure of OBCs, theoretical modeling techniques could draw useful guidelines for accessing the desirable architectures9. Tailored microstructures can be obtained by changing operational variables of the polymerization process, i.e. catalyst ratio, CSA concentration and monomer composition can altered the length, number and chemical composition of the resultant hard and soft blocks, respectively10. Theoretical models could also determine possible horizons for the synthesis of potentially accessible microstructures

11,12

. However, due to practical limitations in the

synthesis and characterization of OBCs, theoretical models are not truly validated in different reaction conditions, and our understanding from the underlying rules of chain shuttling reaction is limited to the inadequate pool of reported experimental data. Simplifying polymerization process by using commercially available liquid monomer for production of commodity polymers has stimulated series of researches. Accordingly, versatile polyethylene like structures from highly linear to hyperbranched, are effectively synthesized using 1-hexene13, 1-octene14, 1-dodecene15, 1-octane/1-decene16, 1-decene/ethylene17 and mixture of linear α-olefin monomers 18 by means of post-metallocene catalysts. In this manner, polyethylene can be made without expensive handling equipment for gaseous ethylene monomer and polymerization reaction can take place in liquid phase and environmental 4 ACS Paragon Plus Environment

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pressure. Control of chain walking mechanism by adjusting polymerization conditions can provide control over final architecture and properties. Towards combination of the mentioned goals, i.e. synthesis of new simply characterizable OBCs, we report here the synthesis and characterization of multiblock isotactic poly(1hexene)/branched polyethylene polymers with 1-hexene monomer alone. The ansaethylenebis(1-η5-indenyl)zirconium dichloride (CAT1, Scheme 1) was used for formation of isotactic poly (1-hexene) blocks and α-diimine nickel(II) bromide complex (CAT2, Scheme 1) for branched polyethylene segments. Based on previous literatures19–22, ZnEt2 is one of best chain transfer agents in CSP of olefinic monomers. As the corresponding polymer shows a good solubility in organic solvents at room temperature, the efficiency of chain shuttling between two catalytic systems could be confirmed by GPC and full characterization of all polymer microstructures by 13C NMR spectroscopy. Such simple synthesis and efficient characterization could be used in validation of current theoretical models.

2. Experimental section 2.1. General Consideration All manipulations involving air and/or moisture sensitive materials were carried out under a dried and purified nitrogen atmosphere using the standard vacuum-line and in atmosphere controlled glove box.



2.2. Materials 1-hexene monomer (Sigma-Aldrich, 97%) and solvents (toluene (Arak Petrochemical Co.) and dichloromethane (Merck)) were dried by refluxing over CaH2 for 72 hours and were freshly distillated and stored over molecular sieves under nitrogen atmosphere. The ansaethylenebis(1-η5-indenyl)zirconiumdichloride catalyst (CAT1, Scheme 1), methylaluminoxane (MAO, 10 wt% Al in toluene) and diethyl zinc (1M in hexane) were supplied from Sigma-Aldrich (Steinheim, Germany) and used as received. The α-diimine nickel complex (CAT2, Scheme 1) was prepared according to pervious literature procedure8,23. Deuterated benzene and 1,3,5trichlorobenzene as solvent and co-solvent for NMR measurements (both from Sigma-Aldrich) were used without further purification. 2.3. Polymerization of 1-hexene In a typical procedure, three-necked round-bottom schleck flask with stirring bar was heated for an hour at 130 ˚C and then deoxygenated by several vacuum-nitrogen purging cycles. A desired amount of freshly distilled 1-hexene and toluene was introduced into flask, followed by the required amount of methylaluminoxane activator and chain shuttling agent, under nitrogen atmosphere (specific amounts for each run can be found in Table 1). Polymerization reaction was started by injecting the catalysts solution mixture into the flask. After the desired reaction time, polymerization was stopped by pouring the reaction mixture in a 500 mL of acidic methanol. The precipitated polymer was filtered and washed by methanol (3200 mL). The purification process was carried out by dissolving the resultant polymer in 50 mL of toluene and


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re-precipitating in methanol (200 mL). The final polymer was dried in reduced pressure condition at 80 ˚C for 48 hours. 2.4. Characterization NMR measurements were carried out by dissolving about 80 mg of final polymer in 0.5 mL of benzene-D6 and 1,3,5-trichlorobenzene (20% v/v). Spectra were recorded using Bruker NMR AVANCE400 spectrometer operating at room temperature and methylene sequences chemical shift in 30.00 ppm was used as internal chemical shift reference. The measurement applied conditions were as the following: 5 mm probe, 90˚ pulse angle, acquisition time 1.5 s, delay time 4.0 s, about 17000 scan. Differential scanning calorimetry (DSC) measurements were performed on a PerkinElmer DSC-Q100 system with a heating or cooling rate of 5 ˚C/min. The second heating curves were recorded from -100 to 100 ˚C. The molecular weight and molecular weight distribution of the samples were determined on WATERS440 GPC at 30 ˚C. Toluene was used as eluent at a flow rate of 1.0 mL/min, and relative molecular weight data were obtained using a calibration curve based on narrow polystyrene standards.

Scheme 1. Structures of the utilized catalysts.



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3. Results and discussion Eight samples were prepared using single and dual catalytic systems, in presence and absence of CSA, for subsequent detailed examinations. Specific details of the polymerization reactions and molecular characteristics are depicted in Table 1. Table 1. Polymerization conditions and molecular characteristics of the polymers made by the individual and dual catalytic systems in presence and absence of CSA.

Run

CAT1 / CAT2

CSA

Yield

Mna

code

[μmol / μmol]

[mmol]

[g]

[kg/mol]

1

0/4

0

0.70

381

2

0/4

2

1.53

3

4/0

0

4

4/0

5

PDIa

Tgb

Tmb

[˚C]

[˚C]

1.9

-57

-23 to - 32

171

1.2

-56

-20 to 30

0.41

199

2.3

-65

-

2

1.75

25

2.6

-63

-

1/3

0

1.00

51c

13.7

-63, -54

-30 to 18

6

1/3

2

0.43

21

1.7

-61, -56

-29 to -4

7

3/1

0

1.20

24c

12.4

-63, -57

- 18 to 27

8

3/1

2

0.76

54

1.9

-61, -52

-

Polymerization conditions: 1-hexene = 80 mmol, [Al]/[CAT1+CAT2]= 1000, [DEZ]/[CAT1+CAT2]= 500, Temperature= 25 ˚C, time= 24 h, toluene= 6cc, 2cc dichloromethane used as solvent for CAT 1, a

Calculated by GPC,

b

Calculated by DSC,

c

Bimodal curve


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By comparing the GPC data for individual catalytic systems (Runs 1 – 4 in Table 1 and Figure 1) a tangible decrease in molecular weight and molecular weight distribution can be seen by adding CSA, indicating chains are reversibly transferred from each catalytic center to zinc during the polymerization reaction process. Furthermore, chain transfer reaction is more effective in the Zr based catalyst system in comparison with Ni catalyst as revealed by sharper reduction of molecular weight of polymers made by Zr catalyst in the presence of CSA. Wang and co-worker7 have attributed this feature to simpler transfer of linear polyethylene chain made by Zr catalyst with less steric hindrance compared to the branched polyethylene chain made by Ni catalyst with larger spatial interference. In addition, the fully amorphous isotactic poly(1-hexene) should be more soluble in the reaction media, and therefore more accessible for transfer reaction, compared to the Ni-derived chains which are expected to contain crystallizable methylene sequences.

Figure 1. Molecular weight distribution of the resultant polymers.



Synthesis of multiblock isotactic poly(1-hexene)/branched polyethylene was carried out in presence of both catalyst components. Polymers made in the absence of CSA in Runs 5 and 7, show broad and bimodal molecular weight distributions (Table 1 and Figure 1) as catalysts create individual chains with dissimilar molecular weights. In fact, polymer samples produced in Run 5 and 7 should be considered as a blend of two polymers made in Runs 1 and 3. However, the molecular weight of the peak at lower molecular weight region is significantly lower than both of these references. From the kinetics point of view, as the other reaction components are available in excess amount, there is no competition in either in activation or chain transfer to metal alkyls or monomer insertion. Therefore, the simultaneous presence of both catalyst components may somehow alter the active nature of each other. Such interference of catalytic components in a CSP reaction has been already reported as well24,25. Nevertheless, by adding a specified amount of CSA in Runs 6 and 8, a unimodal GPC curve could be obtained, which indicates that diethylzinc has established the reversible chain transfer reaction between two catalysts. If catalysts performances are not affected by the simultaneous function of the other one, the weight percent of each block in a single polymer chain and also in each polymer sample should correspond to their individual activities in presence of CSA (Runs 2 and 4). Accordingly, the weight percent of isotactic poly(1-hexene) blocks of the resultant polymers in Run 6 and 8 should be about %27 and %77 respectively. Accordingly, the multiblock isotactic poly(1-hexene)/branched polyethylene with tunable content of isotactic and branched blocks, can be synthesized by adjusting the relative catalyst ratios. 13

C NMR measurements were performed to evaluate the microstructural characteristics of the

polymers obtained from 1-hexene homopolymerization. Peak assignments were based on 10 ACS Paragon Plus Environment

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DEPT135 NMR spectra(CH2 ↓) and theoreUcal chemical shiVs were calculated according to Lindemann and Adams26 and pervious reports7,27–36. To the best of our knowledge, such a complete assignment has not been done previously for this type of branched polyethylene. 13C and DEPT135 NMR spectra of polymers obtained from Runs 2 and 4 are shown in Figure 2 and 3, respectively, and the corresponding peak assignments are described in Supporting Information. First, second and third columns of Table 1 in Supporting Information, provide experimental, previously assigned and calculated chemical shifts, respectively, while fourth and fifth columns show assignments for atoms and triad sequences.

Figure 2. DEPT135 and 13C NMR spectra of Run 2.



Figure 3. DEPT135 and 13C NMR spectra of Run 4 13

C NMR results reveal that a regioirregular hyperbranch polyethylene-like polymer, with

backbone methylene sequences and side chains with various lengths is made by the polymerization of 1-hexene in the presence of post-metallocene catalyst (Run 2). In a common chain growth reaction by successive 1,2-insertion of 1-hexene to metal-carbon bonds, ideally, only butyl branches are formed (as seen in Run 4, Figure 3, made with metallocene catalyst). Resonance of methyl branches (peak no 5 in Figure 2) that arise from 1,2-insertion of 1-hexene monomer followed by 2,6-enchainment, is an another important kind of side chain that can be found in this type of polymer. Existence of ethyl, propyl and long chain branches resonances, as realized through peaks number 1, 3 and 8 respectively, specifies a unique architecture. This microstructure is created by the insertion of monomer into the Ni-carbon bond, and complete or partially migration of the metal center along the growing polymer chain via continuous β-


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hydride elimination and reinsertion steps, known as chain walking mechanism38. Due to chain walking mechanism, poly(1-hexene) homopolymer produced by the post-metallocene catalyst component in this work, is a type of branched polyethylene. For further quantitative examination, the numbers of each branch type and sequences lengths are calculated according to the corresponding equations derived in Supporting Information. The resonance integrals, Ix, where x is the peak number, are related to the different structures found in the chains. Whenever possible the combinations of peaks are used for further accuracy. Results of the application of the derived equations on

13

C NMR spectra of selected samples (Figure 4), is

presented in Table 2, where the type and number of each branches per 1000 carbon atoms and percentage of each sequences in polymer backbone is determined.

Figure 4. 13C NMR spectra of the selected runs. 13 ACS Paragon Plus Environment


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Table 2 . Type and amount of branches and monomer sequences.

Run

Number of branches per 1000 carbons

Monomer sequences (mol %)

code

Me

Eth

Pro

Bu

Lon

total

[E]

[P]

[B]

[A]

[H]

[L]

2

56

3

2

70

6

137

54.7

18.5

1.0

0.6

23.1

2.1

4

0

0

0

166

0

166

0

0

0

0

100

0

5

29

2

1

116

3

151

36.5

12.4

0.7

0.3

48.8

1.3

7

8

0

0

152

1

161

12.6

4.3

0.2

0.1

82.1

0.7

8

10

0

0

149

1

160

16.4

5.5

0.3

0.2

76.9

0.7

Comparing the results presented in Table 2 for individual catalytic systems (Runs 2 and 4), it can be concluded that both catalytic systems extensively make butyl branches, while methyl branches can be ranked as second in post-metallocene catalytic system. Besides, comparing the 13

C NMR chemical shifts of butyl branch carbons (peaks no. 2, 9, 15, 25, 28, 37 of Run 2 and 4 in

Figures 2 and 3) reveals that all of butyl branch carbons have almost the same chemical shifts in both catalytic systems, except 4B4 carbon peaks (specified by asterisk in Figure 4), which is mainly affected by tacticity. As the utilized catalyst components have very different isoselectivities, in the following this feature can be used to determine the microstructural characteristics of the synthesized polymers. Galland and coworkers31 have determined 4B4 carbon chemical shifts for various poly(α-olefins). Based on this work the peak in 35.30 ppm (peak no. 28 in Figure 2) is marked as an isotactic butyl branches pentad in poly(1-hexene) polymer. The very sharp and clear resonance in 35.30 ppm of Run 4 spectra represents highly isotactic butyl branches are made by the metallocene catalyst. By calculating the ratio between isotactic pentad integral and sum of all other 4B4 14 ACS Paragon Plus Environment

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carbon integrals in Figure 2, it is found that metallocene catalyst makes a poly(1-hexene) with an about 96.9% isotactic butyl branches, while post-metallocene catalyst makes an atactic chain. Accordingly, the 13C NMR spectra of Run 2, where only post-metallocene catalyst is used, shows a very weak resonance in 35.28 ppm related to isotactic butyl branches. Most of 4B4 carbon resonance in this Run appears in higher fields as designated by peaks no. 27. Besides the non-isospesific nature of this catalyst, another major reason for this feature is the chain walking mechanism where accidental chain walking phenomena can also provide a tandem butyl branched unit. Based on the above arguments and GPC results, we can confirm the simultaneous existence of isotactic poly(1-hexene) and branched polyethylene blocks in a single OBC chain formed in the presence of CSA. The

13

C NMR spectra recorded from polymers made in the presence of CSA

(Runs 6 and 8) show a very differentiable resonance in 35.30 ppm due to isotactic blocks made by the metallocene component in addition to the multiple atactic 4B4 resonances in 34.80 ppm, that represent branched polyethylene blocks made by post-metallocene catalyst. By comparing the intensity of 35.30 ppm peak in the presence of CSA (Runs 6 and 8), one can conclude that the isotactic poly(1-hexene) blocks/branched polyethylene blocks molar ratio can be adjusted by changing the composition of the catalytic system. Although, there is no specific difference between the

13

C NMR spectra of the polymers made in Runs 7 and 8 (with or without CSA

respectively), which demonstrated non detectable difference between simple blend and block polymers in the present sensitivity limit of the 13C NMR measurement. To further explore the microstructural characteristics of the synthesized polymers, differential scanning calorimetry (DSC) analysis is conducted and resulting thermograms are shown in 15 ACS Paragon Plus Environment


Figure 5. A very clear glass transition temperature (Tg) for the polymers created in the presence of each individual catalytic systems with or without CSA (Runs 1 to 4 in Figure 5), can be found. The glass transition temperatures of the polymers made by metallocene and post-metallocene catalysts are about -63 ˚C and -56 ˚C, respectively. Besides the glass transition, a very broad and indistinct melting peak between -23 ˚C and 32 ˚C can be noticed for the polymer made by the post-metallocene catalyst which is absent in the samples made by the metallocene catalyst. The absence of any melting peak in the polymer made in Run 3 and 4, indicates that a completely amorphous poly(1-hexene) is formed by the metallocene catalyst.

Figure 5. DSC curves of the resultant polymers.

Such broad melting peak was previously reported and attributed to the irregularity in placing methyl branches along the backbone and as a consequences methylene sequences with different number of carbon atoms39. According to model branched PE systems reported by Wagener40, a PE chain with 8 methylenes between two adjacent methyl branches show melting temperature of -14 ˚C while 14 methylenes between two adjacent methyl branches show 16 ACS Paragon Plus Environment

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melting temperature of 39 ˚C. Consequently, it is reasonable to conclude that the polymers obtained by the post-metallocene catalyst system contain long methylene sequences with 8-14 carbons, which can be rationalized by one or two successive 1,6-enchainments of 1-hexene monomer. Accordingly, when both catalytic system are used in the absence of CSA (Run 5 and 7 in Figure 5), two distinct glass transition temperatures in -63 ˚C and -54 ˚C, corresponding to isotactic poly(1-hexene) and branched polyethylene, could be designated, and a broad melting peak from the methylene sequences of the branched polyethylene chains could be obtained. Oppositely, in the presence of CSA at the same conditions, besides two glass transition temperatures in almost the same temperature range, relatively narrower melting endotherms at lower temperatures could be identified. These observations also support formation of blocky structures with shorter crystallizable methylene sequence lengths. In Figure 6, one can see the effect of microstructural characteristics on the final properties, where the transparencies of solvent (toluene) casted films are compared. Branched Polyethylene made in Run 2 and the blend sample from Run 5 produced cloudy films, due to their long methylene sequences and relatively large crystal sizes, which lead to a multiphase system that could scatter visible light. On the other hand, multiblock polymer made in Run 6, just like pure poly(1-hexene), has a very transparent appearance at room temperature that could be attributed to the smaller crystal size of this microstructure.



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Figure 6. Comparing the transparency of the chosen runs: branched polyethylene made by the postmetallocene catalyst (Run 2), blend sample made by dual catalytic system (Run 5) and OBC (Run 6).

4. Conclusions A multiblock isotactic poly(1-hexene) / branched polyethylene polymer is synthesized using well-known ansa-ethylenebis(1-η5-indenyl)zirconium dichloride and

α-diimine nickel(II)

bromide catalysts in presence of diethyl zinc as the chain shuttling agent. The corresponding product is easily soluble in regular polyolefin solvents at room temperature and could be simply characterized using traditional methods. The metallocene catalyst forms linear isotactic poly(1hexene) blocks while the post-metallocene fraction creates long methylene sequences trough chain straightening, as confirmed by CNMR measurements. However, NMR cannot distinguish between the blocky and blend architectures. The bimodal molecular weight distributions formed by the dual catalytic systems turns unimodal in presence of CSA. At the same time, the melting temperatures become lower and less distinct. Formation of blocky structure is also manifested by the significantly higher transparency of the corresponding samples. 18 ACS Paragon Plus Environment

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Supporting Information Descriptive explanation of nomenclature used for distinct carbon atoms, schematic representation of sample structures, tabulated

13

C NMR chemical shifts and corresponding

equations derived for quantitative analysis of different branches.

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