Propylene Copolymerization to Long

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Nonconjugated α,ω-Diolefin/Propylene Copolymerization to Long Chain-Branched Polypropylene by Ziegler−Natta Catalyst: Overcoming Steric Hindrance by Introducing an Extra Electronic Pulling Effect Tingting Yang,†,‡ Yawei Qin,† and Jin-Yong Dong*,†,‡ †

CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China

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‡

S Supporting Information *

ABSTRACT: Synthesis of long chain-branched polypropylene (LCBPP) by propylene copolymerization with nonconjugated α,ω-diolefin is a steric hindrance-prevailing reaction process which involves in copolymerization not only α,ω-diolefin itself (α-olefin copolymerization) but also polymeric olefin intermediate derived from the first α-olefin copolymerization (ω-olefin copolymerization). This reaction mishap reaches its extreme when Ziegler−Natta catalysts based on MgCl2supported TiCl4 (MgCl2/TiCl4 catalysts) are considered as catalyst, which produce active sites that are highly sensitive to olefin monomer’s steric bulkiness. A proposition is put forward that such a steric difficulty may be overcome by functionalization of α,ω-diolefin with Lewis base functionality, which would usher in an extra electronic pulling effect to help with olefin coordination to the active center in both the α,ω-diolefin’s monomeric α-olefin and the polymeric ω-olefin polymerization steps. Three model compounds, including di-n-hexyldiethoxysilane, di-5hexenyldiethoxysilane, and di-5-hexenyldimethylsilane, were synthesized and used to attest to the validity of the hypothesis. The experimental results evidently proved that the Lewis base functionality enabled the functionalized α,ω-diolefin to establish dynamic electron-donating interactions with MgCl2/TiCl4 catalysts, making it far more effective in prompting LCB in copolymerization with propylene due to greatly enhanced polymerization reactivity of both its monomeric α-olefin and polymeric ω-olefin in their respective polymerization steps. The electronic promotion effect was found to be so robust that it could not be offset by reducing the initial α,ω-diolefin molecular steric hindrance. This approach is promising to solve the real issue of synthesizing LCB-PP by Ziegler−Natta catalyst.

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INTRODUCTION Branching is a major structural variation for tuning polymer properties, which is especially useful for polyolefins such as polyethylene (PE) and polypropylene (PP, isotactic).1,2 Short chain-branching in PE realized by copolymerization with high αolefins including 1-butene, 1-hexene, and 1-octene has led to renowned high-performance PE commodities such as linear-low density PE (LLDPE) and polyolefin elastomer (POE),3−5 while that in PP by copolymerization mostly with 1-butene renders random PP co- and terpolymers with excellent transparency and heat sealing properties particularly demanded by membrane applications.6−8 While short chain-branching is mainly purposed to control the polyolefins’ crystalline structure and crystallization process, long chain-branching (LCB) is however used to add to the polyolefins’ melt and processing properties. It has been common knowledge that polyolefins with LCB structure prompt strong chain entanglements in melt and thus show strain hardening in elongational flow, a unique property lacked by most polyolefins with linear chain structure, which is however required by a myriad of important applications of polyolefins including blow molding, thermoforming, and foaming where elongational flow dominates. © XXXX American Chemical Society

Free radical polymerization-based low-density PE (LDPE) naturally contain the LCB structure.9 The advent of metallocene catalyst technology has also helped researchers to realize LCB in PE based on coordination polymerization.10−14 As for PP, unfortunately, LCB has long remained a big difficulty, despite that its catalyst technology development is no less than that for PE.15−17 Propylene polymerization with isospecific stereochemistry usually occurs through 1,2-insetion, which by βhydride (β-H) transfer gives the sterically hindered vinylidene end group that makes the polymer chain polymerizationinhibited.18,19 Although β-CH3 elimination yields an allylic end group,20−22 this chain transfer mode is either weak (for heterogeneous Ziegler−Natta catalysts) or of low selectivity (for most metallocene catalysts).21 Metallocenes of types Cp* 2 MCl 2 (M = Zr, Hf) and bridged bis(fluorenyl)zirconocenes are reported to exhibit up to 90% β-CH3 elimination,21 however they are aspecific and the resultant allyl-terminated PP are atactic. Only a few chiral metallocenes Received: September 15, 2018 Revised: October 23, 2018

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DOI: 10.1021/acs.macromol.8b01958 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Non-Conjugated α,ω-Diolefin/Propylene Copolymerization to LCB-PP

are found with relatively high (∼80%) β-CH3 elimination selectivity.21,23,24 Nonmetallocene organometallic catalysts including pyridinediimine−Fe(II) complexes and group IV complexes bearing chelating phenoxyimine ligands with which the regiochemistry of propylene polymerization is dominated by 2,1-insertion give an allyl end group, with the PP main chain structure being atactic, partial-isotactic, or syndiotactic.17,25−28 In both of these cases, the allyl-terminated PP are supposed to be eligible for the subsequent synthesis of LCB-PP with an additional second polymerization involving macromonomer copolymerization. However, due to the inherent difficulty in macromonomer copolymerization that high molar mass (molecular weight) macromonomers are usually of low polymerization reactivity, this approach suffers from low efficiency, not to mention its process complicacy (two steps of polymerization).29,30 Though low molar mass macromonomers (Mn ≤ 6500 g/mol) have been successfully copolymerized with propylene using isospecific metallocene catalyst,25,31 the low molar masses of the branches are believed to be inadequate for inducing chain entanglements.32,33 Synthesis of LCB-PP is still predominantly a matter of postpolymerization modification.34−37 Another synthetic approach to LCB polyolefins is the engagement of nonconjugated α,ω-diolefins as comonomer in olefin polymerization.38 Released from the requirement of macromonomer intermediate formation, this approach is particularly welcomed in LCB-PP synthesis.39−41 As illustrated in Scheme 1, with nonconjugation, the two olefin moieties of α,ω-diolefin sequentially follow, in propylene polymerization, a copolymerization path to insert into different PP chains, first with the monomeric α-olefin, then with the intermediate polymeric ω-olefin, forming H-shaped LCB structure. With control of the α,ω-diolefin structure to avoid cyclization and selection of proper catalysts to overcome the high steric hindrance especially in the second step of α,ω-diolefin polymerization (the polymeric ω-olefin polymerization) to form branched structure, a good measure of success has been achieved with the α,ω-diolefin copolymerization chemistry in LCB-PP synthesis. For instance, chiral metallocene complexes rac-Me2Si(2-Me-4-Phenyl-Ind)2ZrCl2/modified methylaluminoxane (MMAO) and rac-Me2Si(2-MeBenz[e]Ind)2ZrCl2/ MMAO were combined with high α,ω-diolefins including 1,7octadiene and 1,9-decadiene in propylene polymerization to synthesize LCB-PP.39 Nonmetallocene complex dimethylpyridylamido-Hf/[Ph3C][B(C6H5)4]/AliBu3 was joined by diallylsilanes including 4-methyl-4-vinyl-4-sila-1,6-heptadiene to produce LCB-PP.40 Chiral metallocene complex rac-Me2Si(2-

Me-4-Phenyl-Ind)2ZrCl2/MAO was also reported to synthesize modified, T-shaped LCB-PP with an exotically structured α,ωdiolefin, p-(3-butenyl)styrene.41 In modern PP industry, heterogeneous Ziegler−Natta catalysts predominantly in the form of MgCl2-supported TiCl4 adduct (MgCl2/TiCl4 catalysts) are greatly advantageous over metallocene/nonmetallocene catalysts in polymer particle morphology control.7,42 Thus, this family of catalysts takes most of the credit for the production of more than 50 million tons of PP worldwide annually.7,15 With the use of organic electron donors (Lewis bases) like esters, ethers, and alkoxysilanes in the catalyst preparation (internal electron donor, Di) and polymerization processes (external electron donor, De), these adduct catalysts have been able to produce PP with high isotacticity and controllable molecular weight and molecular weight distribution at very high efficiency.43 However, Ziegler−Natta catalysts are known of their high sensitivity to steric bulkiness of olefin monomers, with which polymerization reactivity reduces quickly with substituent enlarging.1,2,44 Heavy steric bulkiness prevents olefins from accessing to the coordination site of the Ti active center, which lacks the versatile ligand setups in organometallic metallocene/nonmetallocene catalysts to improve its openness. This largely explains why the α,ω-diolefin copolymerization chemistry for LCB-PP synthesis as facile as it seems has been rarely associated with the industrially important Ziegler−Natta catalysts. As a matter of fact, there are not without examples that MgCl2/TiCl4 catalysts copolymerizing propylene with bulky diene and triene molecules stop short of the later, macromonomeric copolymerization, resulting in only linear polyolefins with pendent vinyl functionality.45,46 For the two steps of α,ω-diolefin polymerization where the first step involves a rather high αolefin (such as 1,9-decadiene) while the second is even burdened with a polymer chain-substituted α-olefin (polymeric ω-olefin) (Scheme 1), Ziegler−Natta catalysts may simply represent one extreme of the catalyst spectrum where the steric hindrance of the reaction is too overwhelming to be overcome for LCB formation. The sheer contrast between the enormity of the task and the inability of the catalyst poses an intriguing scientific challenge deserving exploration. Effects of internal and external electron donors (Di and De) in Ziegler−Natta catalysts has been the central subject of research ever since the discovery of MgCl2/TiCl4 catalysts in the early 1980s.47 The role of Di is understood as controlling the adsorption of TiCl4 on MgCl2 crystallite surfaces for catalyst preparation.48−50 When a diether is used as Di in catalyst preparation, MgCl2 crystallites prefer to form only the (110) B


Article

Macromolecules Scheme 2. Dynamic Complexation of External Electron Donor in Ziegler−Natta Catalyst

Scheme 3. LB-Functionalization Assisting α,ω-Diolefin’s Two-Step Polymerization

molecule olefin moieties of α,ω-diolefin otherwise sterically shunned from the active center would gain an extra electronic boost other than their own coordination to maneuver to the active center. Subsequently, in the intervals of the LB−Ti complexation, the olefin moieties of α,ω-diolefin would be much enhanced on their coordination to the decomplexed active center. With alleviation of the coordination steric barriers, their polymerization reactivity would in consequence be increased amid copolymerization with propylene. What is most valuable with this approach is that, as an electronic interaction, it is reasonable to expect that the LB functionality effect would not vanish because of it being attached to a polymer chain, which means that the like electronic pulling effect would also exist on the second, macromonomeric polymerization of the polymeric ω-olefin that is the most challenging part of the α,ω-diolefin/ propylene copolymerization. Overall, we assume that a LBfunctionalization would electronically assist coordination and polymerization of α,ω-diolefin in propylene polymerization with Ziegler−Natta catalyst, enabling LCB-PP synthesis, which would otherwise remain elusive due to an insurmountable steric hindrance effect. This paper serves to testify the hypothesis.

face, and the resulting MgCl2/TiCl4 catalysts have high catalyst activity and relatively uniform active species.51−53 In contrast, monoester- and diester-type Di produce catalysts with TiCl4 absorbed on both (110) and (104) edge surfaces.51 De, added during polymerization, is however used to compensate the loss of internal donor as a result of complexation and/or alkylation reactions with AlR3.50,54 Assisted by the Lewis-acidic AlR3, complexations of De (usually alkoxysilanes in the form R1R2Si[OR3]2) with catalyst are dynamic and reversible.55 Both the active center Ti and the surface Mg can be the complexation sites, with the former having a higher tendency because of its larger Lewis acidity. Complexation on the active center Ti leads to their temporary deactivation, while that on the surface Mg that connect with the central Ti through Cl bridges enhances steric hindrance of the active centers and leads to increased stereospecificity and polymerization efficiency (increased propagation rate constant, kp) (Scheme 2).56−58 Inspired by the unique electron-donating interactions of De in Ziegler−Natta catalysts, we had the proposition that the α,ωdiolefin copolymerization synthesis of LCB-PP with Ziegler− Natta catalysts, troubled by the low overall polymerization reactivity of α,ω-diolefin due to sterically hampered coordination, might be facilitated by ushering in an electronic pulling effect through functionalization of the α,ω-diolefin with a Delike Lewis base functionality (LB functionality). We assumed, as illustrated in Scheme 3, that such a LB functionality could help the α,ω-diolefin establish like electron-donating interactions with Ziegler−Natta catalyst by complexing with the active center Ti (and adjacent Mg) in a dynamic mode. If so, the same-

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EXPERIMENTAL SECTION

Materials. 5-Hexenal bromide, 1-bromohexane, magnesium powder, I2, tetrachlorosilane (SiCl4), triethylamine, anhydrous ethanol, and 1,9-decadiene were all from Alfa Aesar. 1,9-Decadiene was distilled over CaH2 before use. Hexane, heptane, and THF (AR grade) were from Beijing Chemical works and refluxed over Na before use. Two C


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Macromolecules

Table 1. Results of Propylene Polymerization with ZN1−AlEt3 in the Presence of Di-n-hexyldiethoxysilane (DA), Di-5hexenyldiethoxysilane (DO1), and Di-5-hexenyldimethylsilane (DO2)a run no.

coagent

[co-agent]e (mol/L)

1 2 3 4 5 6 7 8 9 10 11

− DAb DAb DO1c DO1c DO1c DO1c DO2d DO2d DO2d DO2d

− 0.019 0.038 0.019 0.038 0.060 0.076 0.019 0.038 0.076 0.160

[co-agent]/[Ti] [co-agent]/[Al] − 100 200 100 200 300 400 100 200 400 800

− 1 2 1 2 3 4 1 2 4 8

yield (g)

catalyst activity (×105g/mol Ti h)

co-agent in PP (mol %)

Mwf (×105g/mol)

PDIf

IIh (%)

Tm (°C)

11.9 1.7 1.1 5.1 2.2 1.6 0.4 14.6 12.6 11.1 8.0

23.2 3.3 2.2 10.0 4.3 3.2 0.9 28.3 24.5 21.5 16.2

− − − 0.09 0.11 u.d.g u.d.g u.d.g u.d.g 0.02 0.11

3.3 10.9 12.4 7.0 8.1 7.7 u.d.g 3.3 3.6 3.6 7.6

4.9 3.8 4.2 4.2 3.4 4.1 u.d.g 5.1 4.9 5.0 5.3

97.0 98.8 98.6 98.6 99.0 98.0 97.9 97.2 98.4 99.1

157.7 160.6 160.3 160.7 159.8 161.3 159.7 160.1 157.5 158.7 159.9

a

General polymerization conditions: ZN1 (MgCl2/BMMF/TiCl4)catalyst, 13.0 mg, [Ti] = 3.8 wt %; AlEt3, 0.9 mmol, [Al]/[Ti] = 100/1; temperature, 60 °C; duration, 30 min; hexane as solvent, 100 mL; propylene, 4 atm. bDi-n-hexyldiethoxysilane. cDi-5-hexenyldiethoxysilane. dDi-5hexenyldimethylsilane. eCo-agents concentration. fDetermined by GPC using PS as standards and with refractive index detector. gUnable to determine. hIsotactic index, measured by boiling heptane extraction, error range. Ziegler−Natta catalysts, MgCl 2 /TiCl 4 containing 9, 9-bis(methoxymethyl)fluorine (BMMF) as Di (ZN1) and diisobutyl phthalate (DIBP) as Di (ZN2), were kindly supplied by Xiangyang Chemical Group, Liaoning, China. Triethylaluminum (AlEt3) was purchased from Albermarle and used as a 1.8 M heptane solution. Polymerization-grade propylene was supplied by Yanshan Petrochemical Co. of China. Synthesis of Di-5-hexenyldiethoxysilane. All this and the below two compounds were synthesized following the patent report of US 2,598,728 filed by Westinghouse Electric Corp (1952). There were three steps to the final product. First, a 500 mL three-necked flask equipped with condenser and dropping funnel was filled with nitrogen. To this flask was added 200 mL of dry THF, 0.5 g of 5-hexenal bromide, and 3.6 g (0.15 mol) of magnesium powder. With stirring, a grain of I2 was added into the reaction mixture to induce the Grignard reaction. In a few minutes the reaction solution turned colorless, upon which another 19.0 g (0.12 mol) of 5-hexenal bromide was continued to be added dropwise. This being completed, the reaction mixture was subjected to refluxing for 4 h. 5-Hexenal magnesium bromide was finally obtained in THF solution after excess Mg powder had been filtered out. Then in a new, dried 500 mL three-necked flask containing 100 mL of dry THF and 8.5 g (0.05 mol) of tetrachlorosilane (SiCl4) under N2 was added slowly the above-obtained 5-hexenal magnesium bromide/THF solution. The reaction was allowed to proceed overnight at room temperature. The magnesium salt being filtered out, the filtrate was washed with hexane for three times. Di-5-hexenyldichlorosilane was obtained by rotating vacuum evaporation. In the final step, under the protection of N2, to a dried 250 mL three-necked flask was added 100 mL of dry THF, 8.08 g (0.08 mol) of triethylamine, and 3.3 mL (0.08 mol) anhydrous ethanol. To this mixture was added dropwise the above-synthesized di-5-hexenyldichlorosilane (5.30 g, 0.02 mol). The reaction was allowed to proceed overnight at room temperature. The salt being filtered out, the filtrate was distilled under reduced pressure to obtain the di-5-hexenyldiethoxysilane. 1H NMR (300 MHz, CDCl3, ppm), δ: 0.60−0.64 (t, 4H, SiCH2), 1.19−1.23 (t, 6H, CH3), 1.36−1.46 (m, 8H, CH2), 2.03−2.08 (m, 4H, CH2), 3.73−3.79 (q, 4H, OCH2), 4.92−4.50 (q, 4H, CH2), 5.75−5.86 (m, 2H, CH). 13C NMR (300 MHz, CDCl3, ppm), δ: 10.14, 32.21, 33.28 (CH2), 18.16 (CH3), 22.17 (SiCH2), 58.18 (OCH2), 114.06(CH2), 138.87 (CH). 29Si NMR (300 MHz, ppm), δ: −8.25 (Hexe2Si(OEt)2). Synthesis of Di-n-hexyldiethoxysilane. The procedure of this synthesis is similar to the above one. First, a 500 mL three-necked flask equipped with condenser and dropping funnel was filled with nitrogen. To this flask was added 200 mL of dry THF, 0.5 g of 1-bromohexane, and 3.6 g (0.15 mol) of magnesium powder. With stirring, a grain of I2 was added into the reaction mixture to induce the Grignard reaction. In a few minutes, the reaction solution turned colorless, upon which

another 19.5 g (0.12 mol) of 1-bromohexane was continued to be added dropwise. This being completed, the reaction mixture was subjected to refluxing for 4 h. n-Hexyl magnesium bromide was finally obtained in THF solution after excess Mg powder had been filtered out. Then in a new, dried 500 mL three-necked flask containing 100 mL of dry THF and 8.5 g (0.05 mol) of SiCl4 under N2 was added slowly the above-synthesized n-hexyl magnesium bromide/THF solution. The reaction was allowed to proceed overnight at room temperature. The magnesium salt being filtered out, the filtrate was washed with hexane for three times. Di-n-hexyldichlorosilane was obtained by rotating vacuum evaporation. In the final step, under the protection of N2, to a dried 250 mL three-necked flask was added 100 mL of dry THF, 8.08 g (0.08 mol) of triethylamine and 3.3 mL (0.08 mol) anhydrous ethanol. To this mixture was added dropwise the above-synthesized di-nhexyldichlorosilane (5.38 g, 0.02 mol). The reaction was allowed to proceed overnight at room temperature. The salt having been filtered out, the filtrate was distilled under reduced pressure to obtain the di-5hexenyldiethoxysilane. 1H NMR (300 MHz, CDCl3, ppm), δ: 0.59− 0.63 (t, 4H, SiCH2), 0.86−0.90 (t, 6H, CH3), 1.17−1.24 (t, 6H, CH3), 1.25−1.40 (m, 16H, CH2), 3.73−3.78 (q, 4H, OCH2). 13C NMR (300 MHz, CDCl3, ppm), δ: 12.60 (SiCH2), 14.07 (CH3), 18.43 (OCH2CH3), 22.56 (SiCH2CH2), 22.77 (CH2CH3), 31.50, 33.08 (CH2), 58.05 (OCH2). 29Si NMR (300 MHz, ppm), δ: −8.57 (Hexe2Si(EtO)2). Synthesis of Di-5-hexenyldimethylsilane. The first step of synthesis of 5-hexenal magnesium bromide is similar to that in di-5hexenyldiethoxysilane synthesis. In a dried 500 mL three-necked flask containing 100 mL of dry THF and 6.45 g (0.05 mol) of dimethydichlorosilane (SiMe2Cl2) under N2 was added slowly 5hexenal magnesium bromide/THF solution. The reaction was allowed to proceed overnight at room temperature. The magnesium salt being filtered out, the filtrate was washed with hexane for three times. Di-5hexenyldimethylsilane was obtained by rotating vacuum evaporation. 1 H NMR (300 MHz, CDCl3, ppm), δ: 0.03 (s, 6H, SiCH3),0.49−0.53 (t, 4H, SiCH2), 1.30−1.43 (m, 8H, CH2), 2.02−2.08 (q, 4H, CH2), 4.91−5.02 (q, 4H, CH2), 5.76−5.86 (m, 2H, CH). 13C NMR (300 MHz, CDCl 3 , ppm), δ: 0.37 (CH 3 ), 18.24 (SiCH 2 ), 22.80 (SiCH2CH2), 32.60 (CH2), 33.54 (CH2CHCH2), 114.11 ( CH 2 ), 139.14 (CH). 29 Si NMR (300 MHz, ppm), δ: 6.71 (Hexe2SiMe2). Polymerization. A typical procedure of polymerization is as follows. For the polymerization of run 4 in Table 1, a 450 mL autoclave reactor equipped with a mechanical stirrer was prepared by repeated vacuuming and filling with nitrogen for three cycles at 60 °C. Then propylene gas was flowed into the reactor at a slightly higher-thanatmospheric pressure. The reactor was maintained under these conditions, and 0.5 mL (1.8 M in heptane) AlEt3 was mixed with 1.9 D


Article

Macromolecules mmol of di-5-hexenyldiethoxysilane in 100 mL of hexane and syringed into the reactor. After 2 min of premixing, 13 mg of ZN1 catalyst solid was dropped into the reaction mixture to start the polymerization, and the propylene pressure was quickly brought up to 4 atm. Under these conditions the polymerization was allowed to continue for 30 min, after which the reactants were poured into excess anhydrous hexane. The precipitated product was worked up by repeatedly washing with anhydrous hexane and finally dried in vacuo at 60 °C for 12 h. Characterization. Molecular weight (Mw) and molecular weight distribution (PDI) of polymer samples were determined by a PL-GPC 220 high-temperature size exclusion chromatography equipped jointly with a two-angle laser light scattering detector (TALLS), a viscosity detector and a differential refractive index detector with 1, 2, 4trichlorobenzene (TCB) as solvent at a flow rate of 1.0 mL/min and 150 °C. Room temperature 1H, 13C, and 29Si NMR spectra were recorded on a Bruker AVANCE 400 spectrometer. All hightemperature 1H NMR spectra were recorded on a Bruker DMX 300 spectrometer at 110 °C using o-dichlorobenzene-d4 as solvent. Melting temperatures (Tm) of polymer samples were measured by differential scanning calorimetry (DSC) on a Pyris 1 PerkinElmer DSC7instrument operating at a scan rate of 10 °C/min under a flowing nitrogen atmosphere. The samples were first heated to 200 °C, held at that temperature for 3 min, and cooled to room temperature, and heated again to 200 °C at 10 °C/min during which the profile was recorded and melting temperature determined. Rheological measurements were performed on a rheometer (TA, AR2000) at 200 °C. The parallel plates with a diameter of 25 mm and a gap height of 1 mm were used. The range of the frequency sweep was from 0.01 to 500 rad/s, and a strain of 1.25% was used, which was in the linear viscoelastic regime for all samples. The rheometer oven was purged with dry nitrogen to avoid degradation of samples during measurements. The isotacticity index (II) of polymer samples was determined by boiling heptane extraction as percentage of insoluble portion.

polymerization and on the other hand makes its structure closely analogous to R1R2Si[OR3]2, the commonly used alkoxysilanetype De compounds in Ziegler−Natta catalysts, which cannot be fulfilled with other functional group settings based on amine and ether. In contrast to di-5-hexenyldiethoxysilane, di-n-hexyldiethoxysilane (DA) is saturated on the two alkenyl substituents and therefore changed to a plain De structure. Di-5hexenyldimethylsilane (DO2), on the other hand, retains the overall α,ω-diolefin structure of di-5-hexenyldiethoxysilane, only the two electron-donating ethoxyl groups being replaced by two inert methyl groups. By engaging the three compounds in propylene polymerization catalyzed by a Ziegler−Natta catalyst and parallelly comparing the polymerization and polymer properties results, we have been able to demonstrate that a Lewis base functionality in α,ω-diolefin will conjure an electronic pulling effect helping with the α,ω-diolefin’s polymerization in copolymerization with propylene by Ziegler−Natta catalyst to LCB-PP. Identification of Electronic Interaction. First, the De-like electron-donating interactions of di-5-hexenyldiethoxysilane to Ziegler−Natta catalyst induced by the diethoxysilane functionality was identified. Polymerization was carried out under slurry conditions with ZN1 as catalyst and AlEt3 as cocatalyst. No regular De compounds were used for the sake of avoiding interference. Di-5-hexenyldiethoxysilane (or its analogue) was introduced into the reaction suspension along with other reagents before polymerization started. Their concentrations were prescribed in between 0.019 and 0.16 mol/L, equivalent to molar ratios of 100−800 to [Ti]. To avoid any occurrence of sol−gel reaction of diethoxysilane,59,60 neither alcohol nor water was involved in the polymer work-up step at the end of polymerization. Table 1 summarizes the complete polymerization results. We first look at the two polymerization runs involving di-nhexyldiethoxysilane (runs 2 and 3 in Table 1). As a quintessential R1R2Si(OR3)2-formula De compound, the polymerization results clearly indicate that it established strong electron-donating interactions with catalyst. Its introduction caused quick decreases of polymerization yield/catalyst activity however sharp increases of PP molecular weight (Mw), as well as increases of II and melting temperature (Tm). Since its concentrations were well over those used in plain propylene polymerization ([DA]/[Ti] = 100 and 200 for Runs 2 and 3, respectively, whereas [regular De]/[Ti] is normally less than 50 for plain propylene polymerization), these responses are typical of complexation of the LB functionality to Ziegler−Natta catalyst (Scheme 2), where the complexation to Ti led to reduction of the number of active center ([C*]/[Ti]) while that to Mg increased both the chain propagation rate constant (kp) and isospecificity. The high concentrations of di-n-hexyldiethoxysilane in the polymerization must have spurred a feverish LB functionality/catalyst complexation that resulted in predominantly the DA-Mg-complexed active center playing out the polymerization reaction in the intervals of the DA-Ti complexation. In the case of di-5-hexenyldiethoxysilane, which is only different from di-n-hexyldiethoxysilane by the two olefin moieties on R1 and R2, the polymerization results are alike yet distinct. Its introduction also caused quick decreases of polymerization yield/catalyst activity and sharp increases of PP molecular weight (Mw) as well as increases of II and melting temperature (Tm). These polymerization results ensure that di5-hexenyldiethoxysilane also established strong electron-donat-

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RESULTS AND DISCUSSION To test the proposed hypothesis, three sterically similar yet electronically distinct model compounds, namely, di-5-hexenyldiethoxysilane, di-n-hexyldiethoxysilane, and di-5-hexenyldimethylsilane, were designed and synthesized. Chart 1 shows the molecular structures of the three compounds. Figure 1 gives their 1H and 29Si NMR spectra. Chart 1. Three Model Compounds, Di-5hexenyldiethoxysilane (DO1), Di-n-hexyldiethoxysilane (DA), and Di-5-hexenyldimethylsilane (DO2)

The compound synthesis was quite successful. Both 1H and Si NMR spectra exhibit correspondent major resonant peaks according to each compound’s structure. Though minor peaks are scrutinized, they are highly limited in intensity, which can be ascribed to impurities from both reactants and intermediates (e.g., dichlorosilane). Among the three compounds, di-5hexenyldiethoxysilane (DO1) is an external electron donorlike Lewis base-functionalized α,ω-diolefin in which the LB functionality, the diethoxysilane, is centrally placed inside an otherwise 1,11-dodecadiene structure. This design of the α,ωdiolefin on one hand minimizes the cyclization tendency of its 29

E


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Macromolecules

Figure 1. 1H and 29Si (inset) NMR spectra of (a) di-5-hexenyldiethoxysilane, (b) di-n-hexyldiethoxysilane, and (c) di-5-hexenyldimethylsilane. F


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Macromolecules ing interactions with catalyst, just like di-n-hexyldiethoxysilane did, indicating that the LB functionality of diethoxysilanyl had well kept its function albeit in an α,ω-diolefin setting. Discretely, however, it is also noticed that the di-5-hexenyldiethoxysilane/ catalyst complexation was not as strong as the di-nhexyldiethoxysilane/catalyst complexation, which can be visualized by making side-by-side comparisons between the two compounds on their respective effects on catalyst activity and polymer Mw, which are given in Figure 2. At the same

were observed for the resulting polymers before its concentration reached the high 0.16 mol/L, though at such a high concentration Mw of the resulting sample increased greatly to 7.6 × 105 g/mol, in the same order of magnitude as those of the di-5hexenyldiethoxysilane-resultant samples. The somewhat peculiar behavior of di-5-hexenyldimethylsilane in polymerization is actually quite understandable, which is similar to what was observed with trans,trans,cis-1,5,9-cyclododecatriene ([E,E,Z]CDT) in metallocene rac-Et(Ind)2ZrCl2/MAO-catalyzed propylene polymerization.61 Though the neutral dimethylsilane moiety makes di-5-hexenyldimethylsilane seemingly innocuous to the propylene polymerization, its multiple olefin moieties plus a bulky molecular structure similar to [E,E,Z]-CDT might have caused a coordination-induced congestion at the active center that posed long-term effect on propylene polymerization, increasing the stereospecificity and making the β-H chain transfer reaction difficult to occur. The emergence of this effect however hinted that this α,ω-diolefin had a very weak polymerization in its copolymerization with propylene. Summarizing the above results, it is clear that, as di-nhexyldiethoxysilane would have a full degree of De-like electrondonating interactions with Ziegler−Natta catalyst, di-5hexenyldiethoxysilane, the LB-functionalized α,ω-diolefin, would establish similar interactions only to a lesser degree, whereas di-5-hexenyldimethylsilane would rather pose inert other than its olefin coordination. The lower measure of interactions implies however that the olefin moieties in di-5hexenyldiethoxysilane were involved in its complexation with Ziegler−Natta catalyst via LB functionality. Effectiveness of Prompting LCB. Second,we evaluate the effectiveness of the two α,ω-diolefins of di-5-hexenyldiethoxysilane and di-5-hexenyldimethylsilane in prompting LCB in their copolymerization with propylene. Rheological measurement has been frequently practiced with melt-modified LCB-PP and LCB-PE to assess their LCB structure.62−67 We used this method to assess the branching structures of polymer samples resulting from di-5-hexenyldiethoxysilane and di-5-hexenyldimethylsilane. Rheological properties were measured by smallamplitude oscillatory shear (SAOS) in the linear viscoelastic regime (200 °C, 0.01−500 rad/s, 1.25% strain). The storage modulus (G′) vs frequency (ω) relationship was first studied (Figure 3). The control PP sample exhibits a typical terminal behavior of linear polymer, with G′ showing a frequency dependency of G′ ∝ ω2 (the terminal slope is derived to be 1.15). For the di-5-hexenyldiethoxysilane series samples (Figure 3A), such a terminal behavior dissipates in a fast pace at increasing of its concentration. The terminal slope reduces first to 0.69 amid a quick increase of low-frequency G′ (curve b in Figure 3A) and then to 0.30 and near 0 with less significant increases in low-frequency G′ (curves c and d in Figure 3A, respectively). For the di-5-hexenyldimethylsilane series samples, however, the terminal behavior is off only slightly in all four samples (the derived terminal slopes for the four samples are 1.03, 1.05, 1.02, 1.03, respectively), even though the lowfrequency G′ increases considerably when the α,ω-diolefin’s concentration is raised up to the high 0.16 mol/L. The nonterminal behavior of the di-5-hexenyldiethoxysilane series samples with continuously decreased terminal slopes at fast pace suggests that these samples are profoundly long chain-branched and their LCB degrees increase rapidly with increasing the α,ωdiolefin concentrations. On the other hand, the only slight offtrack of terminal behavior of all four di-5-hexenyldimethylsilane series samples indicates that they all contain very low degrees of

Figure 2. Comparison of effects of (a) di-5-hexenyldiethoxysilane and (b) di-n-hexyldiethoxysilane on (A) catalyst activity and (B) PP molecular weight (Mw) of propylene polymerization with ZN1 catalyst.

concentrations, both the activity-decreasing and Mw-increasing effects of di-5-hexenyldiethoxysilane are surely lower than those of di-n-hexyldiethoxysilane, suggesting that the electrondonating function of the former is weaker than that of the latter. Di-5-hexenyldimethylsilane, on the other hand, behaved far different from either di-5-hexenyldiethoxysilane or di-nhexyldiethoxysilane in the polymerization. Though structurally very similar to di-5-hexenyldiethoxysilane with both being α,ωdiolefin and having almost the same degree of steric hindrance, di-5-hexenyldimethylsilane could hardly be detected signs of complexation with catalyst as those shown by di-5-hexenyldiethoxysilane and di-n-hexyldiethoxysilane. It did not cause significant decreases of polymerization yield/catalyst activity. As a matter of fact, at some lower concentrations (Runs 8 and 9), the electronically inert α,ω-diolefin even exhibited an effect of increasing polymerization yield/catalyst activity. Only when its concentration was increased to a high 0.16 mol/L did it start to cost the polymerization efficiency. However, even then their absolute values were still quite high (at around 2/3 of those of the control run). At the same time, only marginal increases of Mw G


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the contribution to G′ increases in the di-5-hexenyldiethoxysilane series samples as compared to the control sample is largely from LCB while that from Mw increases is limited. In complex viscosity (η*) vs frequency (ω) relationship study (Figure 4), the results correspond well with those of the G′-ω

Figure 3. Comparisons of storage modulus (G′) vs angular frequency (ω) at 200 °C between (a) control PP (run 1 in Table 1) and (A) di-5hexenyldiethoxysilane series samples with di-5-hexenyldiethoxysilane concentrations being (b) 0.019, (c) 0.038, and (d) 0.060 mol/L (runs 4−6 in Table 1), respectively, and (B) di-5-hexenyldimethylsilane series samples with di-5-hexenyldimethylsilane concentrations being (b′) 0.019, (c′) 0.038, (d′) 0.076, and (e′) 0.160 mol/L (runs 8−11 in Table 1), respectively.

Figure 4. Comparisons of complex viscosity (η*) vs angular frequency (ω) at 200 °C between (a) control PP (run 1 in Table 1) and (A) di-5hexenyldiethoxysilane series samples with di-5-hexenyldiethoxysilane concentrations being (b) 0.019, (c) 0.038, and (d) 0.060 mol/L (runs 4−6 in Table 1), respectively, and (B) di-5-hexenyldimethylsilane series samples with di-5-hexenyldimethylsilane concentrations being (b′) 0.019, (c′) 0.038, (d′) 0.076, and (e′) 0.160 mol/L (runs 8−11 in Table 1), respectively.

LCB, which can hardly be improved by increasing the α,ωdiolefin concentrations. Analysis of the low-frequency G′ helps draw the same conclusions. Both Mw and LCB contribute to the low-frequency G′. The control sample is recorded a G′ value of 9.7 Pa at ω = 0.01 rad/s. The three di-5-hexenyldiethoxysilane series samples give significantly increased G′ values of 3812, 11 180, and 40 390 Pa at the same frequency, while the four di-5hexenyldimethylsilane series samples only moderately change (first decrease and then increase) their G′ values to 7.0, 21.4, 27.8, and 374.3 Pa, respectively. For the di-5-hexenyldiethoxysilane series samples, the initial increase of G′ should be partly ascribed to increase of Mw, since their Mw (700k, 810k, and 770k g/mol, respectively) are more than double that of the control PP (330k g/mol). The rest of the increases (from curve b through to curve d in Figure 3A), however, can be considered as the net result of LCB degree increasing in these samples, because they have similar Mw. For the di-5-hexenyldimethylsilane series samples, on the other hand, the tiny increases of G′ up to 0.076 mol/L of di-5-hexenyldimethylsilane could be jointly contributed by their small-scale increases in Mw and the presence of LCB though with very limited degrees. It is of particular interest to note that the high 0.16 mol/L sample with a Mw very close to those of the di-5-hexenyldimethylsilane series samples gives a G′ value far lower than those of the latter. This result suggests that though both Mw and LCB contribute to the low-frequency G′

study. In the entire ω range the di-5-hexenyldiethoxysilane series samples exhibit increased η* as compared to the control PP, with the increment at low frequency being far greater than that at high frequency. As the control sample shows an unmistakable Newtonian plateau, it can no longer be seen in the di-5hexenyldiethoxysilane samples. With increasing the α,ω-diolefin concentration, the samples are only increased on the low frequency η* while that at high frequency kept unified. The di-5hexenyldimethylsilane series samples, on the other hand, retain the Newtonian zone without noticeable shrinkage up to 0.076 mol/L. Even for the high 0.16 mol/L sample, Newtonian plateau still exists though it becomes thinner and the transitioning to shear-thinning regime occurs at lower frequency. Similar to lowfrequency G′, both Mw and LCB contribute to the low-frequency η*. The control sample is recorded a η* value of 5930 Pa·s at ω = 0.01 rad/s. The three di-5-hexenyldiethoxysilane series samples give significantly increased η* values of 664 000, 1 358 000, and 4 796 000 Pa·s at the same frequency, while the four di-5hexenyldimethylsilane series samples only moderately change (first decrease and then increase) their η* values to 4204, H


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Figure 5. 1H NMR spectra of (a) control PP (run 1 in Table 1) and (b) PP/di-5-hexenyldiethoxysilane prepared with 0.019 mol/L of di-5hexenyldiethoxysilane (run 4 in Table 1), and (c) PP/di-5-hexenyldimethylsilane prepared with 0.076 mol/L of di-5-hexenyldimethylsilane (run 8 in Table 1).

Polymerization Reactivity Assessment. We used a combination of NMR technique and rheological measurement to assess the polymerization reactivity of di-5-hexenyldiethoxysilane and di-5-hexenyldimethylsilane in their first (monomeric α-olefin) and second (macromonomeric ω-olefin) polymerizations. With 1H NMR detecting the characteristic resonant signals of the diethoxysilanyl and dimethysilanyl functionalities, we were able to get a glimpse on the incorporations of di-5hexenyldiethoxysilane and di-5-hexenyldimethylsilane which are directly related to their monomeric α-olefin reactivity. Figure 5 gives typical 1H NMR spectra of a PP-di-5-hexenyldiethoxysilane polymer (run 4 in Table 1) and a PP-di-5hexenyldimethylsilane polymer (run 8 in Table 1) as compared to that of control PP sample (run 1 in Table 1). Di-5hexenyldiethoxysilane incorporations were estimated based on the 3.75 ppm signal (−Si[OCH2CH3]2) (Figure 5b) which was integrated as four protons and compared to the six-proton methyl, methylene, and methine integration sums. Likewise, the 0.05 ppm signal (Figure 5c) representing the −Si(CH3)2 protons was used to estimate the incorporations of di-5hexenyldimethylsilane. As summarized in Table 1, despite the relatively low numbers, the results indicate evidently that di-5hexenyldiethoxysilane was incorporated much faster than di-5hexenyldimethylsilane. At a concentration of 0.019 mol/L, the former was detected a 0.09 mol % of polymer incorporation, which increased to 0.11 mol % when its concentration was further increased to 0.038 mol/L. At the same concentrations, however, the latter could not be meaningfully detected in polymers. Only by increasing its concentration to 0.076 mol/L was a doubtful 0.02 mol % incorporation obtained. A 0.11 mol % incorporation, which was realized by di-5-hexenyldiethoxysilane at a concentration of 0.038 mol/L, could only be achieved by di5-hexenyldimethylsilane at a highly increased concentration of 0.16 mol/L. Note that when its concentration increased to a concentration of 0.076 mol/L, di-5-hexenyldiethoxysilane already rendered insoluble, gelated polymers. Originally, efforts were made to distinguish and integrate the allylic proton resonances of the 1H NMR spectra, which, by being compared to those of the characteristic diethoxysilanyl (−Si[OCH2CH3]2) and dimethysilanyl (−Si[CH3]2) protons,

11 220, 12 580, and 120 700 Pa·s, respectively. Again, for the di5-hexenyldiethoxysilane series samples, the initial increase of η* should be partly ascribed to an increase of Mw, since their Mw (700k, 810k, and 770k g/mol, respectively) values are more than double that of the control sample (330 kg/mol). The rest of the increases (from curve b through to curve d in Figure 4A), however, can be considered as the net result of LCB content increasing in these samples, because they have similar Mw. For the di-5-hexenyldimethylsilane series samples, on the other hand, the much smaller increases of η* up to 0.076 mol/L of di5-hexenyldimethylsilane could be jointly contributed by their small-scale increases in Mw and the presence of LCB though with very limited degrees. It is also of particular interest to note that the high 0.16 mol/L sample with Mw very close to those of the di-5-hexenyldimethylsilane series samples gives a η* far inferior to those of the latter. This result suggests that, similar to lowfrequency G′, the contribution to low-frequency η* increases in the di-5-hexenyldiethoxysilane series samples as compared to the control sample are largely from LCB while that from Mw increases is limited. In case there is any doubt on LCB of the di-5hexenyldiethoxysilane series samples, the rheological properties of the two di-n-hexyldiethoxysilane-resultant PP samples with even higher Mw (1.09 and 1.24 M g/mol, respectively) were examined (Figure S1). Known to be LCB-absent, these two samples exhibit rheological features typical of high molecular weight linear PP: though increased greatly on low-frequency G′ as compared to the control PP sample, they both keep as strong terminal behavior. Furthermore, in the η* vs ω relationship, the two overly high Mw samples are revealed low frequency (ω = 0.01 rad/s) η* values barely equivalent to that of the lowest concentration (0.019 mol/L) di-5-hexenyldiethoxysilane sample. On the part of the di-5-hexenyldiethoxysilane samples, having much lower Mw, LCB is the only logical explanation for their extraordinary rheological performance. Overall, the above analyses have unambiguously indicated that the electronically active di-5-hexenyldiethoxysilane is far more effective than the inert di-5-hexenyldimethylsilane in prompting LCB in copolymerization with propylene by Ziegler−Natta catalyst. I


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Macromolecules respectively, were expected to be able to render the coveted knowledge of the polymerization reactivity of the macromonomeric ω-olefin of the two α,ω-diolefins. Unfortunately, due to the low absolute number of the diolefin incorporations, the exhaustion in NMR measurements unintentionally surfaced the intrinsic unsaturation groups at PP chain end (vinylidene and allyl), which gave signals (from 4.5 to 6.0 ppm) overlapping those of the ω-olefin residues (Figure 5a), leaving such efforts fruitless. We again turned to rheological measurement for solution. The PP-di-5-hexenyldiethoxysilane sample prepared at a di-5hexenyldiethoxysilane concentration of 0.038 mol/L was determined by 1H NMR an overall diolefin incorporation of 0.11 mol %. The PP-di-5-hexenyldimethylsilane sample prepared at a di-5-hexenyldimethylsilane concentration of 0.16 mol/L was determined a same-amount of diolefin incorporation (0.11 mol %). The two polymer samples were not significantly different in Mw and PDI either, with the former having a Mw of 810 kg/mol and PDI of 3.4 and the latter placing the two parameters at 760 kg/mol and 5.3, respectively. With such comparable main chain structures, the significantly different rheological behaviors in G′ ∼ ω and η* ∼ ω relationships (curve c vs curve e′ in Figure 3 and curve c vs curve e′ in Figure 4) unequivocally suggest that di-5-hexenyldiethoxysilane had more of its polymer chain-bound ω-olefins enchained forming LCB than did di-5-hexenyldimethylsilane. Such an assertion was further corroborated by directly comparing the two samples in the so-called Cole−Cole (η″ ∼ η′), Han (G′ ∼G″), and vGP (δ ∼ G*) plots of the rheological measurement. Figure 6A shows Cole−Cole plots of the two samples, which are accompanied by the same plot on the control PP sample as reference. The Cole− Cole plot of linear PP is close to a semicircle, and the higher the molecular weight is, the larger the radius will be. When LCB is present, depending on its degrees, the plot would deform from a semicircle and gradually upturn at high viscosity, and the higher the LCB degree is, the greater the deformation would be. As shown in Figure 6A, the control sample poses a small-radius semicircular curve, indicating its linear chain structure. Enlarged in radius because of increased Mw, the 0.16 mol/L di-5hexenyldimethylsilane sample however still largely exhibits a semicircular curve without sign of high viscosity up-tilting. In contrast, plot of the 0.038 mol/L di-5-hexenyldiethoxysilane sample is seen greatly upwardly detached from that of the 0.16 mol/L di-5-hexenyldimethylsilane sample, indicating a much higher degree of LCB. Figure 6B shows the Han plots. For Han plot, linear PP poses a straight line, irrespective of molecular weight and measurement temperature. If LCB is present, its long relaxation mechanism will render a deviation, and the higher the LCB degree is, the greater the deviation will be. The 0.16 mol/L di-5-hexenyldimethylsilane sample virtually renders no deviation in its Han plot off from that of the control sample, whereas the 0.038 mol/L di-5-hexenyldiethoxysilane sample does so evidently. The same contrast is seen in vGP plot comparison (Figure 6C). The high degree of LCB leads to a δ-reduced terminal plateau in the vGP plot of the 0.038 mol/L di-5hexenyldiethoxysilane sample, while that of the 0.16 mol/L di-5hexenyldimethylsilane sample only exhibits an increased Mw effect. With similar main chain structure settings and nominally identical diolefin incorporations, the above analyses suggesting that the PP-di-5-hexenyldiethoxysilane sample is significantly branched while its di-5-hexenyldimethylsilane counterpart is rather weak in that regard indicate that the di-5-hexenyldiethoxysilane-resultant macromonomeric ω-olefin has a polymer-

Figure 6. Comparison of (A) Cole−Cole plots (η″ ∼ η′), (B) Han plots (G′ ∼G″), and (C) vGP plots (δ ∼ G*) of (a) di-5-hexenyldiethoxysilane series sample prepared with 0.038 mol/L of di-5-hexenyldiethoxysilane concentration (run 5 in Table 1), (b) di-5-hexenyldimethylsilane series sample prepared with 0.160 mol/L of di-5hexenyldimethylsilane (run 11 in Table 1), and (c) control sample (run 1 in Table 1), measured at 200 °C.

ization reactivity much higher than its di-5-hexenyldimethylsilane-resultant counterpart. Overall, the electronically active di-5hexenyldiethoxysilane overwhelms the inert di-5-hexenyldimethylsilane on both the monomeric α-olefin’s and macromonomeric ω-olefin’s polymerization reactivity, which is the reason that the former excelled in prompting LCB. Extra Electronic Pulling Effect. Combining the results of the electronic interaction identification with those of the branching effectiveness and polymerization reactivity assessment, we are now able to derive the extra electronic pulling effect that enhances the overall polymerization reactivity of di-5hexenyldiethoxysilane as compared to di-5-hexenyldimethylsilane. In Ziegler−Natta-catalyzed propylene polymerization, when Lewis base-natured De is used, it will quickly complex to the catalyst on both Ti and Mg in a dynamic mode. The saturated diJ


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Scheme 4. Polymerization Mechanism of Di-5-hexenyldiethoxysilane in Its Copolymerization with Propylene by MgCl2/TiCl4 Catalyst to LCB-PP

n-hexyldiethoxysilane is a quintessential De compound, its LB functionality of diethoxysilane would complex to Ti and Mg of the catalyst in a mode as described in Scheme 2. The experimental results indicated that the di-n-hexyldiethoxysilane/catalyst complexations were highly intense because of the

high concentrations of the compound. With decreased polymerization efficiencies and significantly increased PP molecular weights, the introductions of di-n-hexyldiethoxysilane did not cause substantial broadening of PP molecular weight distribution, suggesting that the active centers were relatively uniform. K


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Macromolecules Table 2. Results of LCB-PP Synthesis with 1,9-Decadiene (DD)a run no.

co-agent

[co-agent] (mol/L)

[co-agent]/[Ti]

yield (g)

catalyst activityb

Mwc

PDI

IId (%)

Tm (°C)

1 12 13 14

− DD DD DD

− 0.019 0.038 0.076

− 100 200 400

11.9 14.8 15.5 15.1

23.2 28.8 30.1 29.3

3.3 3.0 3.3 3.1

4.9 5.2 5.0 5.1

97.0 97.1 97.2 95.9

157.7 154.7 155.9 153.1

a

General polymerization conditions: ZN1(MgCl2/BMMF/TiCl4)catalyst, 13.0 mg, [Ti] = 3.8 wt %; AlEt3, 0.9 mmol, [Al]/[Ti] = 100/1; temperature, 60 °C; duration, 30 min; hexane as solvent, 100 mL; propylene, 4 atm. bUnit: ×105 g/mol Ti h. cUnit: ×105 g/mol. dIsotactic index, measured by boiling heptane extraction.

olefin moiety coordinating to catalyst active center. Followed by insertion and continuous propylene polymerization, the first step of polymerization−the α-olefin polymerization of di-5hexenyldiethoxysilane would be completed by forming PP containing macromonomeric ω-olefin that is still attached by the LB functionality posing potential complexation to the catalyst. Thus, despite the soaring steric hindrance caused by the polymer chain, the extra electronic pulling effect induced by the LBcatalyst complexation would still pitch in in the second, macromonomeric ω-olefin coordination and help with its polymerization in a similar manner. Overall, with a dialkoxylsilane LB functionality coexisting with α,ω-diolefin, the extra electronic pulling effect would in an all-round way assist di-5hexenyldiethoxysilane in its copolymerization with propylene to LCB-PP by Ziegler−Natta catalyst. Robustness of the Extra Electronic Pulling Effect. Di-5hexenyldiethoxysilane in itself is a sterically highly hindered α,ωdiolefin. From the previous results we have known that without the LB functionality of diethoxysilane the similar sterichindrance di-5-hexenyldimethylsilane is rather weak in prompting LCB. 1,9-Decadiene compared to di-5-hexenyldiethoxysilane as well as di-5-hexenyldimethylsilane is significantly reduced in steric bulkiness, which should sterically benefit its polymerization reactivity. With seldom report on its cyclization during Ziegler−Natta- or metallocene-catalyzed olefin polymerization, 1,9-decadiene makes an excellent referencing α,ωdiolefin to test the robustness of the extra electronic pulling effect in di-5-hexenyldiethoxysilane, to see if it could be easily offset by reducing the initial α,ω-diolefin molecular steric hindrance. As such, 1,9-decadiene was engaged in propylene polymerization under similar conditions to check on its results of LCB-PP synthesis, which are summarized in Table 2. In general, the effect of 1,9-decadiene on the polymerization was somewhat close to, but not the same as, that of di-5hexenyldimethylsilane and far different from that of di-5hexenyldiethoxysilane. Within the employed concentrations, 1,9-decadiene caused significant increases in polymerization yield/catalyst activity however did not result in any noticeable increase in either polymer Mw or II. Polymer Tm decreased notably with increase of [1,9-decadiene]. These results suggest that 1,9-decadiene poses a clear-cut relation with the catalyst solely based on α,ω-diolefin without other interactions (e.g., the LB functionality-induced electronic complexations). The samples’ rheological G′ ∼ ω and η* ∼ ω relationship curves are shown in Figure S2. Compared to the control PP sample, the three PP-1,9-decadiene samples show concentration-dependent increases of both low frequency G′ and η*. The terminal behavior in G′ ∼ ω relationship and the Newtonian plateau in η* ∼ ω only wane very slowly. The terminal slopes of the G′ ∼ ω curves and the ω = 0.01 rad/s G′ and η* values are plotted against the polymerization concentrations of the three α,ωdiolefins and compared in Figure 7. These comparisons made it

The structure of the active centers should have the adjacent Mg constantly complexed by the LB functionality while the central Ti in a state of decomplexation with LB amid a dynamic complexation. The same LB functionality-containing di-5hexenyldiethoxysilane exhibited a similar effect on polymerization as di-n-hexyldiethoxysilane, suggesting it had the same mode of complexation with the catalyst on both the central Ti and adjacent Mg. However, its same-molecule olefin moieties caused interferences in the otherwise dynamic yet equilibrated complexations. Compared to di-5-hexenyldimethylsilane, which is of similar degree of molecular bulkiness yet missing the diethoxysilane LB functionality, the verified higher polymerization reactivity of di-5-hexenyldiethoxysilane on both the monomeric α-olefin and macromonomeric ω-olefin leading to significantly increased branching effectiveness can only be ascribed to an extra electronic pulling effect induced by the α,ωdiolefin’s LB functionality. We assume that, in the polymerization path of di-5hexenyldiethoxysilane, the LB-functionalized α,ω-diolefin is distinctively different from that of the inert di-5-hexenyldimethylsilane or other conventional α,ω-diolefin without a like electronic complexation with catalyst in copolymerization with propylene by Ziegler−Natta catalyst. In the latter copolymerization case, the steric hindrance effect would overwhelm the α,ωdiolefin polymerizaiton. In both steps of its polymerization the monomeric α-olefin and macromonomeric ω-olefin would have to blantantly coordinate to the catalyst active center, which is highly disfavored due to much increased steric hindrance as compared to propylene. Particularly, the degree of steric hindrance would increase exponentially when it comes to the macromonomeric ω-olefin coordination in the second step of its polymerization. This explains why di-5-hexenyldimethylsilane could not prompt effective branching. With a LB functionality, however, the polymerization of di-5-hexenyldiethoxysilane (Scheme 4) would start by a LB-catalyst complexation (on both Ti and Mg) in a dynamic fashion rather than by its olefin moieties directly coordinating to catalyst active center. This electronic interaction would serve to draw the otherwise sterically unfavored α,ω-diolefin to the catalyst active center. The LB-catalyst complexation on Mg would be constant in a complexation-decomplexation equilibrium, with the complexed state rendering more active Ti to account for polymerization. When the LB-catalyst complexation is on Ti, however, the otherwise complexation−decomplexation equilibrium would be interrupted by coordination of the same-molecule olefin moieties, which have been accompanying the LB functionality all along to the catalyst and now gain a preemptiveness in coordination to the active center in the intervals of its complexation with LB (the decomplexation state). Thus, reduced in its De-like effect, the LB functionality in di-5hexenyldiethoxysilane would conjure an extra electronic pulling effect to overcome the inherent counteractive steric effect in its L


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Macromolecules

generation. Though with reduced initial steric hindrance, the fact that 1,9-decadiene is far from rivaling di-5-hexenyldiethoxysilane in ability to prompt LCB is a rock-solid testament to the robustness of the extra electronic pulling effect, which promotes the overall polymerization reactivity of α,ω-diolefin including not only that of the first, monomeric α-olefin but also that of the second, more critical macromonomeric ω-olefin. The initial molecular steric hindrance-reducing effect in 1,9-decadiene can benefit on its first α-olefin polymerization, which could however be of little help to its second, macromonomeric ω-olefin polymerization. On the other hand, the fact that di-5hexenyldimethylsilane exhibited an evidently lower LCBgeneration capability than 1,9-decadiene suggests that, without the LB functionality-induced electronic effect, steric effect can still play a role. Relevantly, for the proposed electronic pulling effect, there may be one argument that di-5-hexenyldiethoxysilane may have enhanced adsorption to the Ziegler−Natta catalyst due to the diethoxysilane group, which would increase its local concentration around the catalyst and eventually lead to similarly enhanced copolymerization. Such an adsorption-led-concentration increasing effect might be highly plausible. However, like the case of 1,9-decadiene, it would mostly be helpful in the monomeric α-olefin polymerization while little influence should be expected when it comes to the second macromonomeric ωolefin polymerization. Generality with Ziegler−Natta Catalysts. In chemical structure Ziegler−Natta catalysts MgCl2/TiCl4 are mainly diversified by the variation of Di which renders respective catalysts with characteristically different properties in polymerization efficiency, polymer molecular weight distribution, hydrogen response, etc. Nevertheless, based on interactions directly with the active center, the LB functionality-induced electronic effect promoting LCB copolymerization of α,ωdiolefin should not be limited to Ziegler−Natta catalysts with any particular type of Di. Other than ZN1 with a diether-type Di, we also tested the chemistry with ZN2 having a phthalate-type Di. The results turned out to be just as expected. As shown in Table 3, the LB-functionalized α,ω-diolefin, di-5-hexenyldiethoxysilane, posed as strong interactions with ZN2, the polymerization yield/catalyst activity quickly reduced with the α,ω-diolefin’s concentration while an up to 3-fold increase of polymer Mw. The bare ZN2 without the use of De produced PP with poor isotacticity, which however was significantly improved by addition of di-5-hexenyldiethoxysilane. The terminal behavior in G′ ∼ ω relationship (Figure S3-A) and the Newtonian plateau in η* ∼ ω dissolved (Figure S3-B) as well quickly.

Figure 7. Plots of (A) terminal slopes of the G′ ∼ ω curves, (B) ω = 0.01 rad/s G′ values, and (C) ω = 0.01 rad/s η* values as against polymerization concentrations of (a) di-5-hexenyldiethoxysilane, (b) di-5-hexenyldimethylsilane, and (c) 1,9-decadiene.

clear that di-5-hexenyldiethoxysilane is far more effective than either di-5-hexenyldimethylsilane or 1,9-decadiene for LCB

Table 3. Results of Propylene Polymerization with ZN2−AlEt3 and Di-5-hexenyldiethoxysilane (DO1) as α,ω-Diolefina run no.

co-agent

[co-agent](mol/L)

[co-agent]/[Ti]

yield (g)

catalyst activity

Mwc

PDI

IId (%)

15 16 17 18 19 20

− DO1 DO1 DO1 DO1 DO1

0 0.005 0.010 0.019 0.038 0.060

− 25 50 100 200 300

8.4 5.3 4.5 2.4 0.67 0.60

23.1 14.7 12.7 6.6 1.6 1.5

3.1 7.1 7.1 8.3 7.0 9.5

9.7 5.8 5.5 5.1 3.7 6.6

73.8 94.1 94.3 97.0 93.9 97.6

a

General polymerization conditions: ZN2(MgCl2/DIBP/TiCl4)catalyst, 13.0 mg, [Ti] = 2.5 wt %; AlEt3, 0.9 mmol, [Al]/[Ti] = 100/1; temperature, 60 °C; duration, 30 min; hexane as solvent, 100 mL; propylene, 4 atm. bUnit: ×105g/mol Ti h. cUnit: ×105g/mol. dIsotactic index, measured by boiling heptane extraction. M


Article

Macromolecules The terminal slopes of the G′ ∼ ω curves and the ω = 0.01 rad/s G′ and η* values are plotted against the concentrations of di-5-hexenyldiethoxysilane. Figure 8 compares these plots with

Natta catalysts, di-5-hexenyldiethoxysilane exhibited similar polymerization reactivity in copolymerization with propylene toward LCB-PP due to the prevailing electronic pulling effect of LB functionality. In practical applications, Ziegler−Natta catalysts would certainly be accompanied by regular external electron donors to ensure high stereospecificity in propylene polymerization. In such circumstances, we envisage that the electronic pulling effect would still be able to function and not be significantly affected of its efficacy considering the usually much lower dosages of their uses.

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CONCLUSION

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ASSOCIATED CONTENT

In conclusion, using three model compounds, namely, di-nhexyldiethoxysilane, di-5-hexenyldiethoxysilane, and di-5-hexenyldimethylsilane, as well as a conventional nonconjugated α,ω-diolefin, 1,9-decadiene, in combination with two MgCl2/ TiCl4 Ziegler−Natta catalysts of different internal electron donors, we have been able to confirm the hypothesis that, by establishing dynamic electron-donating interactions between α,ω-diolefin and Ziegler−Natta catalyst through functionalization of the α,ω-diolefin with Lewis base functionality, the overwhelming steric hindrance effect baffling synthesis of LCBPP by α,ω-diolefin/propylene copolymerization with Ziegler− Natta catalyst would be overcome. With the Lewis base functionality, both steps of α,ω-diolefin polymerization during copolymerization with propylene to LCB-PP would be initiated, rather than the olefin moieties directly coordinating to the catalyst active center as with conventional α,ω-diolefins, its electron-donating functionality first establishing dynamic complexation with the catalyst pulling the same-molecule olefin moiety to the catalyst active center, thus eliciting an additional electronic pulling effect to help with active site-coordination (preemptive coordination) of the α,ω-diolefin in its two steps of polymerization. The electronic pulling effect is found to be so robust in promoting α,ω-diolefin’s LCB capability that it cannot be offset by reducing the initial α,ω-diolefin molecular steric hindrance. This effect is also found its generality with different types of Ziegler−Natta catalyst. By further optimizing the Lewis base-functionalized α,ω-diolefin structure in order to balance between the electronic pulling effect and its accompanying catalyst deactivation effect, this approach is promising to solve the real issue of synthesizing LCB-PP by Ziegler−Natta catalysts.

* Supporting Information S

Figure 8. Plots of (A) terminal slopes of the G′ ∼ ω curves, (B) ω = 0.01 rad/s G′ values, and (C) ω = 0.01 rad/s η* values as against concentrations of di-5-hexenyldiethoxysilane for Ziegler−Natta catalysts of ZN1 and ZN2.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01958. Rheological properties of two di-n-hexyldiethoxysilaneresultant PP samples, 1,9-decadiene series samples, and samples prepared with ZN2 (PDF)

those that are derived from the di-5-hexenyldiethoxysilane series samples from the ZN1 catalyst one by one. The control PP sample by ZN2 (run 15 in Table 3) gave a lower starting value of terminal slope than that by ZN1 (run 1 in Table 1), due likely to the former’s much broader molecular weight distribution. In spite of that, the two curves of terminal slope vs concentration are almost parallel to each other, same concentrations of diolefin resulting in same extents of terminal slope reductions. Moreover, both the G′ value and the η* value vs di-5-hexenyldiethoxysilane concentration curves are nearly overlapped between the two series of samples. Clearly, though catalyzed by different Ziegler−

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AUTHOR INFORMATION

Corresponding Author

*(J.-Y.D.) Telephone: ++86 10 82611905. E-mail: jydong@ iccas.ac.cn. ORCID

Jin-Yong Dong: 0000-0001-7873-9364 N


Article

Macromolecules Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Natural Science Foundation of China is gratefully acknowledged for the financial support (Grant Nos. 21574143 and 51373178).

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Propylene Copolymerization to Long

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