Biaxial Chain Growth of Polyolefin and Polystyrene ... - ACS Publications

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Biaxial Chain Growth of Polyolefin and Polystyrene from 1,6Hexanediylzinc Species for Triblock Copolymers Seung Soo Park, Chung Sol Kim, Sung Dong Kim, Su Jin Kwon, Hyun Mo Lee, Tae Hee Kim, Jong Yeob Jeon, and Bun Yeoul Lee* Department of Molecular Science and Technology, Ajou University, Suwon 443-749, South Korea S Supporting Information *

ABSTRACT: Synthesis of polyolefin (PO)-based block copolymers is of immense research interest. In this work, we report a strategy for the construction of polystyrene (PS)block-PO-block-PS, a useful thermoplastic elastomer, directly from olefin and styrene monomers. Multinuclear zinc species Et[Zn(CH2) 6]aZnEt were prepared through successive additions of BH3 and Et2Zn to 1,5-hexadiene. Poly(ethyleneco-propylene) chains were biaxially grown from the −(CH2)6− units in Et[Zn(CH2)6]aZnEt via “coordinative chain transfer polymerization (CCTP)” using the pyridylaminohafnium catalyst. PS chains were subsequently grown in one pot from the generated polymeryl−Zn sites by subsequent introduction of the anionic initiator Me3SiCH2Li·(pmdeta) (pmdeta, pentamethyldiethylenetriamine) and styrene monomers. The fraction of the extracted PS homopolymer grown from the Me3SiCH2 sites was low (homo-PS (g)/total PS (g), 15−22%). The gel permeation chromatography (GPC) curves shifted evidently after styrene polymerization, and change in the molecular weight (ΔMn, 39−56 kDa) was approximately twice the homo-PS Mn (20−23 kDa), in accordance with attachment of the PS chains at both ends of the PO chains. Transmission electron microscopy analysis of the thin films showed segregation of the PS domains in the PO matrix to form spherical or wormlike rippled structures depending on the PS content. The prepared triblock copolymers exhibited elastomeric properties in the cyclic tensile test, similar to the commercial PS-block-poly(ethylene-co-1-butene)-block-PS.

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INTRODUCTION Block copolymers are a remarkable class of materials with widespread applications in mundane plastics as well as in high technological devices and are well-researched in polymer chemistry.1−4 Polyolefins (POs), including polyethylene (PE) and polypropylene (PP), are the most abundant polymers. The annual production of these polymers is currently more than 120 million metric tons, and preparation of PO-based block copolymers is of immense interest in both industry and academia.5−12 In fact, Dow Chemical Company commercialized olefin block copolymers (OBCs) since a decade ago.13−15 OBCs are produced by employing the so-called “coordinative chain transfer polymerization (CCTP)” technique, which involves the use of a single transition-metal-based catalyst (e.g., Hf catalyst) and a chain transfer agent (CTA, e.g., Et2Zn) in excess ([Zn]/ [Hf] > 100). In CCTP, the growing polymeryl group in the catalyst is reversibly and rapidly exchanged with the alkyl group in the CTA, resulting in uniform PO-chain growth from the CTA.16,17 The CCTP technique is currently used for precise architectural design as well as end-group functionalization of PO.18−26 The PO-based block copolymer polystyrene (PS)-blockpoly(ethylene-co-1-butene)-block-PS (SEBS) is currently produced on an industrial scale (several hundred thousand metric tons per year). The hard PS domains segregate in the soft poly(ethylene-co-1-butene) matrix, acting as physical cross© XXXX American Chemical Society

linking sites. Consequently, SEBS exhibits thermoplastic elastomer properties. SEBS is extensively used in commodities such as rubber and plastics27−31 and is actively studied for use in new specialized areas.32−38 Despite the massive increase in the demand for SEBS, the high manufacturing cost is an obstacle to the expansion of the market. In industry, SEBS is produced by a two-step process: living anionic polymerization of styrene and butadiene and subsequent hydrogenation of the resulting PSblock-polybutadiene-block-PS (SBS).39,40 The hydrogenation of polybutadiene chains is costly and contributes significantly to the manufacturing cost of SEBS. Hence, the preparation of SEBS directly from olefin and styrene monomers without the costly hydrogenation process is a worthwhile and formidable pursuit. Strategies for generating PE−PS block copolymers directly from styrene and ethylene monomers were attempted. One method is anionic or radical styrene polymerization initiated from the end-functionalized PE chains.41−43 The other method is a sequential addition of olefin and styrene monomers during homogeneous polymerization with a living catalyst system.44 Recently, we also prepared PO-block-PS by sequentially performing CCTP and anionic styrene polymerization in one pot. 45 By feeding nBuLi·(tmeda) (tmeda: N,N,N′,N′Received: June 26, 2017 Revised: August 1, 2017

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DOI: 10.1021/acs.macromol.7b01365 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Strategy for Preparation of PS-block-PO-block-PS

Scheme 2. Synthesis of 1,6-Hexanediylzinc Species

tetramethylethylenediamine) as an initiator and styrene monomer after CCTP, PS chains were grown not only from the nBuLi·(tmeda) but also from the generated polymeryl Zn sites, generating PS-homopolymers and PO-block-PS, respectively. We then attempted to prepare PS-block-PO-block-PS triblock copolymers using dialkylzinc species bearing the αmethylstyrene moiety, i.e., [4-(isopropenyl)benzyl]2Zn; however, the resulting chain structure was a complicated, ill-defined multiblock.46 In this study, we report the successful direct synthesis of PS-block-PO-block-PS using olefin and styrene monomers (Scheme 1).

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RESULTS AND DISCUSSION Synthesis of 1,5-Hexanediylzinc Species. The strategy used for preparation of the PS-block-PO-block-PS involves the biaxial growth of PO chains from multinuclear zinc species (e.g., R−[Zn−(CH2)6−]aZn−R) by CCTP and subsequent anionic styrene polymerization in one pot (Scheme 1). The targeted multinuclear zinc compound was prepared via alkyl exchange between Et2Zn and trialkylborane; the latter was generated by hydroboration of 1,5-hexadiene with BH3 (Scheme 2).47−49 During the dynamic alkyl exchange process, the transient Et3B was removed by evacuation at 0 °C, eventually leaving the desired Et[Zn(CH2)6]aZnEt (1) in the reaction pot. The 1H NMR spectrum of the hydroboration product of 1,5-hexadiene was complicated, and the signals could not be assigned;50 however, after treatment with Et2Zn, the signals were clearly assignable to Et[Zn(CH2)6]aZnEt (Figure 1a). Because of the rapid alkyl exchange among the zinc species, the average “a” value in the formula depends on the amount of Et2Zn remaining in the reaction pot and, hence, is easily controlled by deliberate addition of the calculated amount of Et2Zn after preparing 1 with a high “a” value (e.g., 6.5). Me3SiCH2Zn(CH2)6ZnCH2SiMe3 (2 in Scheme 2) was synthesized by the removal of Et2Zn after addition of (Me3SiCH2)2Zn to Et[Zn(CH2)6]aZnEt with high “a” value (a = 6.5). The 1H NMR signals were clearly assignable to Me3SiCH2Zn(CH2)6ZnCH2SiMe3 (Figure 1b). The PO

Figure 1. 1H NMR spectra (C6D6) of Et[Zn(CH2)6]6.5ZnEt (a) and Me3SiCH2Zn(CH2)6ZnCH2SiMe3 (b).

chains are thought to be selectively and biaxially grown from the Zn-(CH2)6-Zn sites, whereas the Me3SiCH2−Zn sites in 2 remain intact during the CCTP because (Me3SiCH2)2Zn functions as a poor CTA. CCTP Studies. The typical ansa-metallocene catalyst, rac[Me2Si(2-methylindenyl)2]ZrCl2 (3), which is reputed to B


Article

Macromolecules Scheme 3. PO Chain Growth from 1,6-Hexanediylzinc Species (CCTP)

Table 1. Results of CCTP Studiesa entry

monomer

catalyst

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

C2H4 C2H4 C2H4 C2H4 C2H4 C2H4 C2H4 C2H4 C2H4/C3H6 C2H4/C3H6 C2H4/C3H6 C2H4/C3H6 C2H4/C3H6 C2H4/C3H6 C2H4/C3H6 C2H4/C3H6

3 3 3 3 3 3 3 3 3 3 4 5 5 5 5 5

CTA (mmol of Zn)

time (min)

temp (°C)

yield (g)

Et2Zn (0.50) 1(a=1) (1.0) (Me3SiCH2)2Zn (0.50) 2 (2.0) 2 (1.0) 2 (1.0) 2 (1.0) 2 (1.0) 2 (1.0) 2 (1.0) 2 (1.0) 1(a=3) (0.90) 1(a=3) (0.60) 1(a=3) (0.30) 1(a=3) (0.15)

20 60 60 45 40 40 60 100 30 30 30 40 60 60 60 40

120−130 120−130 120−130 120−130 120−130 120−130 120−130 120−130 120−130 80−85 80−85 80−85 100−110 100−110 100−110 100−110

7.55 13.3 13.9 10.4 9.90 9.70 15.6 24.2 19.1 23.4 21.2 11.2 10.2 10.5 9.1 9.8

FC3b

expected Mn (kDa)c 13 9.3

0.14 0.18 0.14 0.070 0.25 0.28 0.30 0.29

9.9 19 31 48 38 47 42 7.5 9.1 14 24 52

GPC Mn (kDa)d

Mw/Mn

59 10 8.8 42 7.7 12 19 21 11 22 31 8.5 9.6 16 22 49

3.20 2.88 3.38 3.53 2.43 2.06 2.22 2.26 2.17 2.19 2.38 1.69 1.92 1.73 1.89 1.61

Polymerization conditions: methylcyclohexane (40 g), 3 or 4 (1.0 μmol) activated with MMAO (Al = 0.200 mmol), 5 (3.0 μmol) activated with [(C18H37)2MeNH]+[B(C6F5)4]− (1.0 equiv). bPropylene mole fraction in the resulting polymers measured by 1H NMR. cYield (g)/(Et− and/or −(CH2)6− units in CTA (mol)). dMeasured by GPC at 160 °C using trichlorobenzene relative to PS standards and converted to PO equivalents.

a

The use of (Me3SiCH2)2Zn led to an unstirrable viscous state in 45 min due to the generation of high-molecular-weight PE (Mn, 42 kDa), indicating that (Me3SiCH2)2Zn is a poor chain transfer agent. With the use of Me3SiCH2Zn(CH2)6ZnCH2SiMe3 (2, 2.0 mmol of Zn), low-molecular-weight PE was generated. The measured Mn (7.70 kDa) did not match with the Mn calculated upon the assumption that PE chains are grown from both the Me3SiCH2−Zn and Zn−(CH2)6−Zn sites (9.90 g/3.0 mmol = 3.3 kDa), but rather agreed with the value calculated upon the assumption that the PE chains are grown solely from the Zn−(CH2)6−Zn units without the involvement of the Me3SiCH2−Zn units (9.90 g/1.0 mmol = 9.9 kDa) (entry 5; Scheme 3a). The 1H NMR spectrum of the sample isolated after treatment with O2 showed a weak −CH2OH signal at 3.4 ppm (Figure 2a).51 In order to clearly see the end group signals, low-molecular-weight PE was prepared by stopping the polymerization at an early stage. No vinyl (CH2CH−) end group signals and a low-intensity signal of the Me3Si− end group were observed in the 1H NMR spectrum, but the −CH2OH and −CH3 end signals were clearly observed at 3.45 and 0.77 ppm, respectively (Figure S1). The action of O2 did not completely

produce high-molecular-weight POs by suppression of the undesired β-elimination process, was evaluated as a catalyst in the CCTP (Scheme 3). After 20 min of ethylene polymerization at high temperature (120−130 °C) using rac-[Me2Si(2-methylindenyl)2]ZrCl2 activated with modified methylaluminoxane (MMAO), the reaction mixture became viscous and unstirrable due to the formation of high-molecular-weight PE (Mn, 59 kDa, entry 1 in Table 1), whereas ethylene was continuously consumed for 60 min without the stirring problem when Et2Zn (0.50 mmol of Zn) was added as a CTA. The Mn determined via GPC using PS standards and converted to PE equivalents by universal calibration was as low as 10 kDa, which roughly agrees with the expected Mn calculated by using the formula “PE weight (g)/(Et units in CTA (mol))” (i.e., 13.3 g/1.0 mmol = 13.3 kDa) (entry 2). Feeding EtZn(CH2)6ZnEt (1(a=1), 1.0 mmol of Zn) into the system resulted in the generation of low-molecularweight PE with a Mn of 8.8 kDa, which is also in good agreement with the expected Mn (13.9 g/(1.0 mmol of Et + 0.5 mmol of −(CH2)6− units) = 9.3 kDa) (entry 3). These results indicate that catalyst 3 is functional for CCTP employing ethylene feed in the presence of dialkylzinc species. C


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Macromolecules

with rac-[Me2Si(2-methyl-4-phenylindenyl)2]ZrCl2 (4), which is reputed to produce high-molecular-weight PP,56,57 the highermolecular-weight poly(ethylene-co-propylene) (Mn, 31 kDa) was generated, but the measured Mn did not reach the expected value (42 kDa) (entry 11). The undesired β-elimination process was suppressed to some extent with the use of 4 as a catalyst, but it could not be completely prevented (Figure 2d). By replacing the catalyst with the pyridylaminohafnium complex (5) activated with [(C18H37)2MeNH]+[B(C6F5)4]−, which has been efficiently used in CCTP,6,14,15,58,59 the undesirable β-elimination process was completely prevented with no observation of vinyl and vinylidene end group signals (Figure 2e). The 1H NMR spectrum showed clear Me3SiCH2− signals at 0.55 and 0.03 ppm, with −CH2OH end group signals at 3.2−3.4 ppm, indicating that the PO chains are grown from both the Zn−(CH2)6−Zn and Me3SiCH2−Zn sites in Me3SiCH2Zn(CH2)6ZnCH2SiMe3. The measured Mn (8.5 kDa) was in good agreement with the expected Mn calculated from the formula “yield (g)/(sum of Me3SiCH2− and −(CH2)6− units (mol))” (i.e., 11.2 g/1.5 mmol = 7.5 kDa). Because no advantage was gained from using Me3SiCH2Zn(CH2)6ZnCH2SiMe3 as a CTA, the more simply prepared Et[Zn(CH2)6]3ZnEt (1(a=3)) was extensively investigated as a CTA especially for controlling the molecular weight (entries 13−16). When the amount of 1(a=3) was decreased, the molecular weight increased systematically, as expected and the measured Mn values (9.6, 16, 22, and 49 kDa) were consistent with the expected Mn values calculated by using the formula “yield (g)/(sum of Et− and −(CH2)6− units (mol))” (9.1, 14, 24, and 52 kDa, respectively). The molecular weight distributions were rather broad when ansa-zirconocene catalysts 3 and 4 were used (Mw/Mn, 2.1−2.4), but the Mw/Mn values were reduced to 1.7−1.9, presumably due to the absence of the β-elimination process or fast alkyl exchange between the Hf and Zn centers.58 Insertion of the propylene monomers was regiospecific and no signals assigned to −(Me)CCH2CH2C(Me)− and −(Me)CCH2CH2CH2CH2C(Me)− were observed in the 13C NMR spectrum of poly(ethylene-co-propylene) (entry 16) (Figure S2).60 PS Chain Growth from Dialkylzinc. In a previous study, nBuLi·(tmeda) was used as an initiator to grow PS chains from the polymeryl−Zn sites.45 In that case, the PS chains were grown not only from the Zn sites (which resulted in the desired POblock-PS) but also from the initiator nBuLi, which resulted in the undesired PS homopolymer. When the [styrene]/[Zn] ratio was below 500, some portion of the polymeryl-Zn sites were not engaged in the process of PS chain growth. The molecular weight distribution was rather broad (Mw/Mn ∼ 1.5). During the

Figure 2. 1H NMR spectra of POs generated via CCTP after treatment with O2 (a, entry 5; b, entry 9; c, entry 10; d, entry 11; e, entry 12 in Table 1) obtained in C6D4Cl2 at 100 °C for (a) and in C6D6 at 70 °C for (b−e) at 20 mg/mL concentration.

convert the −CH2Zn functionality to −CH2OH; the conversion yield (62%) calculated from the integration values of the two signals agrees well with the reported value 65%.52 The Mn value calculated from the 1H NMR signals was 3100, which agrees well not only with the expected Mn (3400) calculated by using the formula “PE weight (3.4 g)/(−(CH2)6− units in CTA (1.00 mmol))” but also with the GPC-measured value (Mn, 3600, Mw/ Mn, 3.62). These results indicate that the PE chains are selectively and biaxially, though not perfectly, grown from the Zn−(CH2)6− Zn sites in Me3SiCH2Zn(CH2)6ZnCH2SiMe3 without the undesired β-elimination process, generating telechelic PE chains ended with −ZnCH2SiMe3 groups. The synthesis of telechelic PEs is currently a challenging task.53−55 However, the measured Mn deviated from the expected Mn when the amount of CTA (2) was reduced or the polymerization time was lengthened with the aim to prepare high molecular weight telechelic PEs (entries 6− 8). At the ethylene/propylene copolymerizations, low-molecularweight polymers were also generated by the addition of CTA 2, but the measured Mn deviated significantly from the expected Mn (entries 9 and 10). In the 1H NMR spectra, signals of the vinyl (CH2CH−) and/or vinylidene (CH2C(C)−) end groups were observed along with those of the desired −CH2OH end group (Figure 2b,c). No signal of the Me3Si− end group was observed, indicating that the Me3SiCH2−Zn sites are not involved in the CCTP process. When catalyst 3 was replaced

Table 2. Results of Anionic Styrene Polymerization in the Presence of (Hexyl)2Zna entry

initiator

[Li]/[Zn]

[styrene]/[Zn]

Mn (Da)b

Mw/Mn

[PS growing sites]/[Zn]c

1 2 3 4 5 6 7 8 9

Me3SiCH2Li Me3SiCH2Li·(tmeda) Me3SiCH2Li·(tmeda) Me3SiCH2Li·(pmdeta) Me3SiCH2Li·(pmdeta) Me3SiCH2Li·(pmdeta) Me3SiCH2Li·(pmdeta) Me3SiCH2Li·(pmdeta) Me3SiCH2Li·(pmdeta)

1.00 1.00 0.700 1.00 0.700 0.500 0.300 1.00 0.700

500 500 500 500 500 500 500 250 250

88600 28100 31200 23200 23000 20800 22000 13200 12500

1.33 1.32 1.37 1.24 1.23 1.26 1.30 1.31 1.36

0.59 1.85 1.67 2.24 2.26 2.50 2.36 1.97 2.08

a Polymerization conditions: (hexyl)2Zn (22.6 mg, 96 μmol), methylcyclohexane (27 g), 90 °C, 3 h (full conversion). bMeasured by GPC at 40 °C using toluene eluent. cCalculated as [styrene]/DP/[Zn], where DP = Mn/104.

D


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Macromolecules Scheme 4. PS Chain Growth from (Hexyl)2Zn

polymerization process, the color of the solution gradually changed from orange to opaque black, indicating that the chain growing anionic species gradually decomposed. We aimed to develop another type of initiator to overcome the drawbacks observed with the use of nBuLi·(tmeda). Me3SiCH2Li readily forms zincate species ([R2(Me3SiCH2)Zn]+[Li]−) with R2Zn,61 and selective growth of the PS chains from R-Zn sites in [R2(Me3SiCH2)Zn]+[Li]− can be expected, leaving the ZnCH2SiMe3 sites intact, based upon the fact that Me3SiCH2Li is a poor initiator for styrene polymerization. In fact, high-molecularweight PS (Mn, 280 kDa) with a broad molecular weight distribution (Mw/Mn, 1.80) was generated with a use of a [styrene]/[Me3SiCH2Li] feed ratio of 250, indicating that only ∼10% of Me3SiCH2Li participates in the initiation process. Feeding Me3SiCH2Li in the presence of (hexyl)2Zn ([Li]/[Zn] = 1.00) at a [styrene]/[Zn] ratio of 500 also resulted in the formation of high-molecular-weight PS (Mn, 88.6 kDa), indicating that either the Me3SiCH2− or hexyl−Zn sites was partially involved in the initiation process (entry 1 in Table 2). When Me3SiCH2Li·(tmeda) was used instead of Me3SiCH2Li, substantially low-molecular-weight PS (Mn, 28.1 kDa) was generated with full styrene conversion (entry 2). No signal of the Me3SiCH2− end group was observed in the 1H NMR spectrum of the isolated PS, indicating that the PS chains are not grown from the Me3SiCH2− sites but are selectively grown from the hexyl−Zn sites. The [PS growing sites]/[Zn] ratio was calculated by applying the formula “[styrene]/DP/[Zn]” (i.e., [PS growing sites] = [styrene]/DP, DP (degree of polymerization) = Mn/ 104); the obtained value of 1.85 is close to the value of 2.0, which is anticipated upon the assumption that the PS chains are grown from all the hexyl−Zn sites whereas the Me3SiCH2− sites remain intact. However, during the course of polymerization, the color of the solution gradually changed from orange to opaque dark brown. When the [Li]/[Zn] ratio was reduced from 1.0 to 0.70, the [PS growing sites]/[Zn] value was unsatisfactorily low (1.67), deviating from the desired value of 2.0. By replacing tmeda with the more powerful chelating agent pentamethyldiethylenetriamine (pmdeta),62 the PS chains were successfully grown from all the hexyl−Zn sites (Scheme 4). In the styrene polymerizations performed with a [Me3SiCH2Li· (pmdeta)]/[Zn] ratio of 1.0, 0.70, 0.50, or 0.30 and a [styrene]/ [Zn] ratio of 500, a low intensity signal of the Me3SiCH2− end group signal was observed at ∼0 ppm in 1H NMR spectra of the isolated PS (Figure 3); the [PS growing sites]/[Zn] values were desirably in the range of 2.24−2.50 (entries 4−7). The molecular weight distributions were narrow (Mw/Mn, 1.23−1.30). The color of the solution remained persistently yellow during the course of polymerization. Even at a low [styrene]/[Zn] ratio of 250, the [PS growing sites]/[Zn] values were 1.97 and 2.08 with no Me3SiCH2− end group signal in the 1H NMR spectra, though the molecular weight distributions were somewhat broad (Mw/ Mn = 1.31 and 1.36). nBuLi·(pmdeta) rapidly decomposes in

Figure 3. 1H NMR spectra showing the end groups of PS generated by Me3SiCH2Li·(pmdeta) + (hexyl)2Zn (a, entry 5 in Table 2) and homoPS extracted from the triblock copolymer (b, entry 4 in Table 3).

cyclohexane at high temperature (90 °C), making it unsuitable for use as an initiator. Synthesis of PS-block-PO-block-PS. The β-elimination process is detrimental to the preparation of PS-block-PO-blockPS (Scheme 1) because its occurrence results in the generation of diblock or homo-PO chains at the expense of generation of the desired triblock chains. Hence, CCTP was performed by using the pyridylaminohafnium complex 5 (3.0 or 4.0 μmol) activated with [(C18H37)2MeNH]+[B(C6F5)4]− (1.0 equiv) in the presence of CTA 1(a=3) (150 μmol). Generation of some POblock-PS chains (∼25 wt %) due to the presence of the Et− group in 1(a=3) is inevitable, but the commercial-grade triblock copolymer SEBS also contains some portion of diblock chains.63 The mole fraction of propylene in the POs (FC3 = [C3]/([C2] + [C3])) was varied in the range of 0.22−0.29 by controlling the propylene/ethylene ratio in the feed gas (Table 3). CCTP was simply switched to the anionic styrene polymerization by simply introducing the initiator Me3SiCH2Li·(pmdeta) (110 or 120 μmol depending on the amount of catalyst used in the CCTP; [Li]/[Zn] = 0.73 or 0.80) and styrene monomer in sequence in one pot. The amount of styrene was fixed at [styrene]/[Zn] = 500 (7.8 g), and styrene was completely converted to PS. The stryene content was varied in the range 28−44 wt % by the E


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Macromolecules Table 3. Results of Sequential CCTP and Anionic Styrene Polymerizationa PO Mn (kDa); PDI entry

yield (g)

[C3]/([C2] + [C3])b

(PS, g)/ (yield, g)

(homo-PS, g)/ (PS, g)c

homo-PS Mn (kDa); PDId

expectedf

PO equivg

PS equive

triblock copolymer Mn (kDa); PDIe

1 2 3 4 5 6 7

17.6 23.0 28.2 20.4 24.7 18.0 20.8

0.29 0.29 0.28 0.26 0.27 0.23 0.22

0.44 0.34 0.28 0.38 0.32 0.43 0.38

0.17 0.15 0.21 0.18 0.20 0.17 0.22

20; 1.63 23; 1.76 23; 1.81 20; 1.52 23; 1.78 20; 1.63 22; 1.51

52 81 109 67 89 54 69

49; 1.61 57; 1.70 61; 1.81 45; 1.90 57; 1.78 43; 1.81 50; 1.81

78; 1.62 91; 1.71 96; 1.84 75; 1.92 93; 1.80 74; 1.82 88; 1.83

128; 1.37 127; 1.56 129; 1.62 120; 1.64 141; 1.51 130; 1.48 135; 1.58

a Polymerization conditions: CTA 1(a=3) (150 μmol), catalyst 5 (3.0 μmol for entries 1, 4, 6; 4.0 μmol for entries 2, 3, 5, 7) activated with [(C18H37)2MeNH]+[B(C6F5)4]− (1.0 equiv), 95−125 °C, 40 min, then Me3SiCH2Li·(pmdeta) (110 μmol for entries 1, 4, 6; 120 μmol for entries 2, 3, 5, 7), 100−110 °C, 5.0 h (complete conversion of styrene). bPropylene mole fraction in POs measured by 1H NMR spectra. c(PS weight extracted with acetone and chloroform (2:1 weight ratio))/(consumed styrene weight). dMeasured with GPC at 40 °C eluting with toluene using PS standards. eMeasured with GPC at 160 °C eluting with 1,2,4-trichlorobenzene using PS standards. f(PO (g)/(Et− and −(CH2)6− units in CTA (mol)). gConverted to PO equivalents by universal calibration.

variation of the amount of POs generated during the CCTP. During the course of styrene polymerization at 100−110 °C, the color of the solution remained persistently yellow, indicating that the active anionic species were not destroyed. The PS homopolymers grown from the Me3SiCH2Li initiator could be extracted from the generated block copolymers using chloroform and acetone (1:2 w/w). In the 1H NMR spectra of the extracted polymers, signals of the Me3SiCH2− end group were clearly observed around 0 ppm (Figure 3b), and in the 1H NMR spectra of the isolated solid, PS signals were observed at 6.5−7.2 and 1.5−2.2 ppm in addition to the typical poly(ethylene-co-propylene) signals at 0.8−1.6 ppm (Figure S3). The content of homo-PS (i.e., extracted PS (g)/total PS (g)) was low (15−22%). From the Mn values of the extracted homo-PS (20− 23 kDa), the number of PS-growing sites (calculated from the formula (total PS weight, i.e., 7.8 g)/(PS-Mn)) were 390−340 μmol. It is assumed based upon the above model studies that 300 μmol (i.e., 150 μmol-Zn × 2) of the PS growing sites correspond to those attached to the PO chains grown from the polymeryl Zn sites, while the rest (90−40 μmol) correspond to homo-PS grown from the initiator, Me3SiCH2Li. The content of homo-PS calculated based on the aforementioned assumption (i.e., homePS growing sites (μmol)/total PS growing sites (μmol)) was 12− 23%, which is in agreement with the measured values (15−22%). The GPC curve showed a clear overall shift after the anionic styrene polymerization (Figure 4) and the molecular weight (both Mn and Mw) increased by 50 kDa after anionic styrene polymerization, which is roughly twice the homo-PS Mn (20 kDa), in accordance with the biaxial chain growth of PS from both ends of most of the PO chains. In most cases, the ΔMn values (39−56 kDa) were roughly twice the homo-PS Mn (20− 23 kDa) with a clear shift in the GPC curves (Figure S4). When the yield of PO was high (20.4 g, entry 3), the measured POequivalent Mn (61 kDa) deviated severely from the expected Mn (20.4 g/0.188 mmol = 109 kDa), and the ΔMn value was unsatisfactorily low (33 kDa) with a marginal shift in the GPC curve. The molecular weight distributions became narrow after styrene polymerization (Mw/Mn, 1.37−1.64). Because the PS chains are immiscible with the PO chains, phase separation occurs and the PS domains that are selectively stained by RuO4 are clearly seen as dark regions in the transmission electron microscopy (TEM) images of the thin films (Figure 5 and Figure S5).64 Systematically increasing the PS content from 28 to 32, 38, and 44 wt % caused a change in the

Figure 4. GPC curves for the samples before (a) and after (b) the anionic polymerization (entry 1 in Table 3).

pattern of the PS domains from spherical to wormlike ripples (Figure 5a−c,e).65 The pattern resembled, but not as regular as that of commercial-grade SEBS, which was prepared through the hydrogenation of SBS with a narrow PDI (Figure 5f). In the case where PO chains were somewhat crystalline due to the low propylene fraction (FC3, 0.22), the morphology of the PS domains remained persistently spherical, even though the PS content was as high at 38 wt % (Figure 5d). The prepared triblock copolymers show weak melting signals in the broad temperature region of 0−90 °C with no discrete PS Tg signal at ∼100 °C, as shown in the DSC thermogram of SEBS (Figure S6). Duplicate measurements of the tensile properties are performed for the samples in each batch after the polymer lumps were compressed into films with ∼1 mm thickness. For the batches in entries 3 and 6 in Table 3, the two tests were inconsistent, whereas the tests for the other batches produces reproducible data, which are summarized in comparison with the data for commercial-grade SEBS and poly(ethylene-co-propylene) (PS-equivalent Mn, 79 kDa; PDI, 2.16; FC3, 0.26) in Table 4 and Figure 6. Poly(ethylene-co-propylene) with no attached PS chains underwent elongation with the application of a small stress with no break up to 1600%, whereas the prepared triblock copolymers, especially with those with high PS contents (38 and 44%), exhibited the same stress−strain trend as commercialF


Article

Macromolecules

Figure 5. TEM images of thin films prepared using PS-block-poly(ethylene-co-propylene)-block-PS (a, 28% PS, entry 3; b, 32% PS, entry 5; c, 38% PS, entry 4; d, 38% PS, entry 7; e, 44% PS, entry 1) and commercial-grade PS-block-poly(ethylene-co-1-butene)-block-PS (f).

Table 4. Results of the Tensile Test and the Cyclic Tensile Testa tensile test entry

FC3

PS (%)

Mn (kDa); PDI

1 2 3 4 5

0.29 0.29 0.26 0.22 SEBS

44 34 38 38

128; 1.37 127; 1.56 120; 1.64 135; 1.58

a

cyclic tensile test

tensile strength (N/mm2)

elongation at break (%)

elastic recovery at first cycle (%)

elastic recovery at 10th cycle (%)

9.72 (8.37) 2.49 (2.33) 6.94 (4.15) 9.56 (8.50) 19.2 (14.8)

889 (843) 609 (595) 785 (512) 900 (837) 723 (739)

63 (63) 84 (84) 75 (78) 58 (57) 89 (88)

56 (58) 71 (73) 71 (73) 43 (46) 86 (86)

Tests were performed in duplicate for the samples in each batch.

lene copolymerization in the presence of the multinuclear Zn species; the GPC-measured Mn values agree well with the expected Mn values calculated by using the formula: “PO (g)/ growing sites in CTA (mol)”. Me3SiCH2Li·(pmdeta) was found to be an appropriate initiator for the preferential growth of PS chains from the Zn−alkyl sites during styrene polymerization in the presence of (hexyl)2Zn. By combing these two findings, the desired PS-block-PO-block-PSs were successfully prepared with the variation of the propylene fraction ([C3]/([C2] + [C3], 0.22−0.29) as well as the styrene content (28−44 wt %), by sequentially performing ethylene/propylene copolymerization using 5 as a catalyst and Et[Zn(CH2)6]aZnEt (a = 3) as a CTA and styrene polymerization with the addition of the Me3SiCH2Li·(pmdeta) initiator and styrene monomer. PS homopolymers were generated in small quantities (15−22%). The GPC curves were shifted after the anionic styrene polymerization, and the ΔMn values (39−56 kDa) were approximately twice homo-PS Mn (20−23 kDa), agreeing with the assumption that the PS chains are biaxially grown from both

grade SEBS, even though the tensile strengths of the former were not as high as that of SEBS. The prepared triblock copolymers exhibited elastomeric behavior similar to that of commercialgrade SEBS (Figure 7 and Figure S7). In the cyclic tensile tests, the first cycle resulted in some amount of permanent deformation, followed by minimal deformation in subsequent cycles. The permanent deformation was somewhat greater than that observed for SEBS.

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CONCLUSION With the aim of preparing PS-block-PO-block-PS through successive CCTP and anionic styrene polymerization in one pot, multinuclear zinc species Et[Zn(CH2)6]aZnEt were prepared by successive additions of BH3 and Et2Zn to 1,5hexadiene. Whereas the β-elimination process is unavoidable with typical ansa-metallocene catalysts rac-[Me2Si(2-methylindenyl) 2 ]ZrCl 2 (3) and rac-[Me 2 Si(2-methyl-4-phenylindenyl)2]ZrCl2 (4), it is completely prevented by the use of the pyridylaminohafnium (5) catalyst during ethylene/propyG


Macromolecules

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Article

EXPERIMENTAL SECTION

All manipulations were performed under an inert atmosphere using a standard glovebox and Schlenk techniques. Methylcyclohexane was purchased from Sigma-Aldrich and purified over a Na/K alloy. The ethylene/propylene mixture gas was purified over trioctylaluminum (0.6 M in methylcyclohexane) in a bomb reactor (2.0 L). Styrene purchased from Sigma-Aldrich was purified by stirring over nBuLi·AlEt3 (2.3 mmol/300 g of styrene) for 3 days. tmeda and pmdeta were purchased from Sigma-Aldrich and purified over CaH2. The 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded using a Varian Mercury Plus 400 instrument. The gel permeation chromatograms (GPC) of the PS samples were obtained in toluene at 40 °C using a Waters Millennium apparatus. The GPC data for the POs and block copolymers were obtained in 1,2,4-trichlorobenzene at 160 °C using a PL-GPC 220 system equipped with a RI detector and two columns (PLgel mixed-B 7.5 × 300 mm from Varian (Polymer Lab)). Transmission electron microscopes (TEM) images were recorded on a Carl Zeiss LIBRA 120 instrument. (Me3SiCH2)2Zn,66 3−4,56 and 567 were prepared according to the reported method. Et[Zn(CH2)6]aZnEt. A solution of iodine (18.5 g, 73.0 mmol) in diglyme (73.0 mL) was introduced dropwise via a syringe into a solution of NaBH4 (5.5 g, 146.1 mmol) in diglyme (73.0 mL) at room temperature over the course of 5 h.68 The generated diborane (B2H6) and H2 gases were carried off through a distillation apparatus, of which the distillate collecting tip was specially designed to be long; the gases were bubbled through a solution of 1,6-hexadiene (6.0 g, 73.0 mmol) in hexane (24 g) and diethyl ether (17 g). The entire closed system was connected to a manifold equipped with mercury bubbler through which H2 gas was removed during the course of reaction. The solution in the receiving flask was transferred to a bomb reactor which was then pressurized with ethylene gas at 25 bar. The solution was stirred overnight under ethylene pressure to convert any residual B−H groups to B−Et. Removal of the solvent produced an yellow oil (6.4 g), which was mixed with diethylzinc (17.4 g, 141 mmol). The reaction pot was connected to a distillation apparatus, and the whole system was thoroughly evacuated at −20 °C. Under the closed vacuum system, the generated Et3B was selectively transferred from the reaction pot, the temperature of which was maintained at 0 °C with an ice bath, to a receiving flask that was cooled in dry ice/acetone bath. The system was transferred to a glovebox, and hexane (ca. 10 mL) was added to the reaction pot. Some black insoluble species were removed by filtration. The hexane and excess Et2Zn were transferred under vacuum to a flask to obtain a viscous gray oil, which solidified when evacuated further using a high-vacuum line. The 1H NMR spectrum indicated that the obtained solid has the formula Et[Zn(CH2)6]aZnEt (a = 6.5, 6.5 g) (Figure 1a). Et[Zn(CH2)6]aZnEt with an “a” value of 1.0 or 3.0 was prepared by gentle heating after addition of the calculated amount of Et2Zn to the obtained Et[Zn(CH2)6]aZnEt (a = 6.5). Et2Zn is volatile and pyrophoric, and any collected solvent and byproduct were carefully quenched with water after dilution in mineral oil. Et2Zn collected in the vacuum trap was also quenched by deliberate evaporation of isopropanol before exposure to air. Me3SiCH2Zn(CH2)6ZnCH2SiMe3. (Me3SiCH2)2Zn (12.3 g, 51.3 mmol) was added to a flask containing Et[Zn(CH2)6]6.5ZnEt (5.0 g, 4.6 mmol) inside a glovebox. The solution was stirred for 30 min at 40 °C while removing the generated Et2Zn and excess (Me3SiCH2)2Zn under vacuum (Figure 1b). Typical Procedure for Ethylene Polymerization (Entry 5 in Table 1). A bomb reactor (125 mL) was charged with a solution of trimethylaluminum (14.4 mg, 200 μmol of Al) in methylcyclohexane (17.0 g). After stirring for 1 h at 100 °C using a mantle, the solution was removed using a cannula. This washing procedure was performed to clean up any catalyst poisons. The reactor was again charged with a solution of Me3CH2Zn(CH2)6ZnCH2SiMe3(389 mg, 2.0 mmol of Zn) in methylcyclohexane (18.0 g), and the temperature was set to 100 °C. The catalyst solution was prepared by reacting rac-[Me2Si(2methylindenyl)2]ZrCl2 (0.48 mg, 1.0 μmol) and MMAO (AkzoNobel, 7.0 wt % Al in heptane, 77 mg, 200 μmol of Al) in methylcyclohexane (2.0 g) for 40 min. After injection of the solution of the activated catalyst

Figure 6. Comparison of tensile stress−strain curves for PS-blockpoly(ethylene-co-propylene)-block-PSs in Table 4 with that of poly(ethylene-co-propylene) and SEBS.

Figure 7. Plots of the cyclic tensile tests.

ends of the PO chains. TEM images of the thin film confirmed the segregation of the PS domains in the PO matrix, where the morphology of the PS domains changed from spherical to wormlike ripples with an increase in the PS content. The elastomer properties of the prepared triblock copolymers followed a trend similar to that of commercial-grade SEBS based on the tensile tests as well as cyclic tensile tests. H


Article

Macromolecules with a syringe, ethylene gas was immediately charged at 5 bar. After sustaining the pressure at 5 bar for 3 min, the flow rate of ethylene gas was controlled at a constant value of 0.250 g/min for 37 min by using a mass flow controller. In the initial stage, the pressure decreased, reaching 3.5 bar in 3 min, and then gradually increased, finally reaching 11 bar. After polymerization for 40 min, the remaining ethylene gas was vented off, and O2 gas was charged at 5 bar. The thick solution was stirred for 1 h at 120 °C. After venting O2 gas, acetic acid (1.0 mL) and ethanol (30 mL) were successively injected. The precipitated polymer powders were collected by filtration and dried overnight in a vacuum oven at 120 °C (9.9 g). Typical Procedure for Anionic Styrene Polymerization in the Presence of (Hexyl)2Zn (Entry 5 in Table 2). Me3SiCH2Li (6.3 mg, 67 μmol) and pmdeta (11.7 mg, 67 μmol) were added to a flask containing (hexyl)2Zn (22.6 mg, 96 μmol) and methylcyclohexane (27 g) inside a glovebox. Styrene (5.0 g, 48.0 mmol) was added, and the anionic polymerization was performed at 90 °C for 3 h. Complete conversion of the styrene monomer was evident from analysis of the 1H NMR spectrum of an aliquot. Aqueous HCl (2N, 0.3 mL) was added, and the resulting solution was stirred for 30 min at 90 °C to destroy the zinc species. The solution was filtered through a short pad of silica gel which was subsequently washed with toluene. To isolate PS, toluene was removed by using a rotary evaporator; the isolated sample was dried in a vacuum oven at 130 °C for 5 h. The weight of the isolated PS was identical to that of the styrene monomer. Typical Procedure for the Synthesis of PS-block-PO-block-PS (Entry 5 in Table 3). A bomb reactor (125 mL) was charged with a solution of trimethylaluminum (14.4 mg, 200 μmol of Al) in methylcyclohexane (17.0 g). After stirring for 1 h at 100 °C using a mantle, the solution was removed using a cannula. This washing procedure was performed to clean up any catalyst poisons. The reactor was again charged with a solution of Et[Zn(CH2)6]3ZnEt (21.5 mg, 150 μmol of Zn) in methylcyclohexane (45.0 g) under an inert atmosphere, and the temperature was set to 70 °C. The catalyst stock solution was prepared by reacting the Hf complex 5 (18.1 mg, 25.0 μmol) with [(C18H37)2MeNH]+[B(C6F5)4]− (30.4 mg, 25.0 μmol) in benzene (4.0 g). The catalyst stock solution (583 mg, 4.0 μmol of Hf complex) was injected into the reactor using a syringe and ethylene/propylene mixture gas was immediately charged into the system at a pressure of 20 bar. The temperature increased to ∼125 °C within 5 min due to the exothermic reaction, even though the reactor was cooled with a fan. The temperature gradually decreased due to catalyst deactivation and was controlled in the range 95−100 °C. The catalyst was deactivated during the course of CCTP, and the use of a lower amount of the catalyst resulted in generation of a smaller amount of PO and consequent reduction of the molecular weight. The pressure gradually decreased and finally reached 16 bar due to consumption of the monomers and the stirring rate also gradually decreased from 300 to 40 rpm due to formation of a thick viscose solution. After performing the polymerization process for 40 min, the remaining gas was vented off. During this procedure, the polymer solution swelled up to plug the valve and was withdrawn for GPC and 1H NMR analyses. When the temperature reached 90 °C, a solution of Me3SiCH2Li·(pmdeta) prepared by mixing Me3SiCH2Li (11.3 mg, 0.120 mmol) and pmdeta (20.8 mg, 0.120 mmol) in methylcyclohexane (1.0 g) was added. The temperature was maintained at 90 °C for 30 min under stirring, after which styrene (7.8 g, 750 mmol) was injected. The temperature was controlled in the range 100−110 °C using a mantle. The viscosity gradually increased to reach an almost unstirrable state in 5 h. Full conversion of styrene was evident from 1H NMR analysis of an aliquot. After the complete conversion of styrene, acetic acid and ethanol were successively injected. The obtained polymer lump was dried overnight in a vacuum oven at 160 °C (24.7 g). After dissolving the polymer (3.0 g) in chloroform (30.0 g) at 60 °C, acetone (60.0 g) was added to precipitate the block copolymers. PShomopolymers, which are freely soluble in the chloroform/acetone cosolvent, were isolated via filtration. After filtration, the solvent in the filtrate was removed using a rotary evaporator, and the residual solvent was completely removed in a vacuum oven at 80 °C over the course of several hours to obtain the PS homopolymers (0.19 g). GPC, TEM,

DCS, and tensile test data were collected for the samples containing the PS homopolymers. High-Temperature GPC Studies. Sample solutions (200 μL) with concentrations of 3000 ppm were eluted in 1,2,4- trichlorobenzene at a flow rate of 1.0 mL/min at 160 °C. The mobile phase was stabilized with 2,6-di-tert-butyl-4-methylphenol (0.04%). The PS-based molecular weight distributions were calculated from a calibration curve on the basis of narrow PS standards (Mp: 480; 915; 4715; 10 009; 20 681; 73 143; 197 736; 419 672; 1 047 000; 2 906 000; 4 714 000 Da). For calculation of PE-based molecular weight distributions, the PS standard molecular weights (MPS) were converted to PE equivalents (MPE) using the reported Mark−Houwink−Sakurada parameters for PS (K = 0.000 121; a = 0.707) and PE (K = 0.000 406; a = 0.725) using the equation MPE = [(0.000 121/0.000 406) × MPS(1+0.707)](1/(0.725+1)) = 0.495 × MPS0.990. In the case of the poly(ethylene-co-propylene) samples, the converted MPE values were further converted to PO equivalents using the equation MPO = MPE/(1 − S), where S is the mass fraction of the CH3− side chains (i.e., S = (15 × FC3)/[(1 − FC3) × 28 + (FC3 × 42)]); here, FC3 is the mole fraction of propylene in the poly(ethylene-co-propylene) samples).14,69 Sample Preparation for Transmission Electron Microscopy (TEM). The triblock copolymer (5 mg) was completely dissolved in toluene (5 mL) at 100 °C. A drop of the hot solution was loaded on a carbon-coated copper TEM grid (200 mesh). After a slow evaporation of the solvent at room temperature overnight, the sample on the grid was annealed in an oven at 150 °C for 6 h. The sample was stained with RuO4 by suspending the TEM grid coated with the film for 30 min in a closed chamber containing an aqueous solution of RuO4, which was prepared by reacting RuO2 (30 mg) with NaIO4 (0.20 g) in water (5 mL) at 0 °C for 4 h. Tensile Test. The polymer samples were compressed between hot plates at 135 °C while sustaining the pressure at 5 MPa for 20 min and subsequently at 10 MPa for 100 min. The obtained polymer films with ∼1 mm thickness were cut into four pieces (100 × 10 mm2 size). Tensile tests were performed in duplicate for each batch according to ASTM D882 using a UTM (WL2100) at a drawing rate of 500 mm/min with a gage length of 50 mm at 25 ± 2 °C and 45 ± 5% humidity. The cyclic tensile test was also performed in duplicate. Each specimen was extended over 10 cycles to half of the distance at break measured in the tensile test.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01365. 1 H and 13C NMR spectra, GPC curves, TEM images, DSC thermograms, plots of the cyclic tensile test (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel 82-31-219-1844 (B.Y.L.). ORCID

Bun Yeoul Lee: 0000-0002-1491-6103 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Korea CCS R&D Center (KCRC) grant (No. 2012-0008935) and the Priority Research Centers Program (No. 2012-0006687) funded by the Korean Government (Ministry of Science, ICT and future planning).

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