Article pubs.acs.org/Macromolecules
Synthesis and Characterization of Well-Controlled Isotactic Polypropylene Ionomers Containing Ammonium Ion Groups Min Zhang, Xuepei Yuan, Lu Wang, and T. C. Mike Chung* Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
Tianzi Huang and Willem deGroot The Dow Chemical Company, 2301 Brazosport Blvd., Freeport, Texas 77541, United States S Supporting Information *
ABSTRACT: This paper discusses the synthesis of a new family of well-controlled isotactic polypropylene ionomers (iPP-NH3+Cl−) containing up to 5 mol % of NH3+Cl− ionic groups, with high molecular weight and narrow molecular weight and composition distributions, as well as good processability in melt and solution. A systematic study was conducted using various isospecific Ziegler−Natta and metallocene catalysts in the copolymerization of propylene and a high α-olefin comonomer containing a silane-protected amino group and the subsequent work-up procedures that can prevent undesirable side reactions in forming iPP-NH3+Cl− ionomers in a one-pot process. The resulting copolymers were carefully monitored by polymer solubility and a combination of NMR, GPC-triple detectors, DSC, and mechanical property measurements. Evidently, the most suitable reaction process requires a combination of the rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2 metallocene catalyst system with a purified d-MAO (TMA-free), 6bis(trimethylsilyl)amino-1-hexene comonomer during the copolymerization reaction and a work-up procedure to directly interconvert the silane-protected amino groups (−N(SiMe3)2) into −NH3+Cl− ionic groups before exposing to air. The attempt of isolating both iPP-N(SiMe3)2 and iPP-NH2 intermediates resulted in the insoluble (cross-linked) products. On the other hand, the resulting iPP-NH3+Cl− ionomers were melt processed in air and showed a systematic increase of mechanical properties and high-temperature stability with the increase of NH3+Cl− content.
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INTRODUCTION Isotactic polypropylene (iPP) represents about one-quarter of commercial polymers. It is known to be cost-effective, recyclable, and a good performing thermoplastic for a broad range of applications. Since the discovery of iPP about five decades ago, there has been constant research and industrial interests1−3 to expand largely available iPP into high-value specialty applications, such as the microporous PP separator4,5 in lithium ion batteries and biaxially oriented iPP (BOPP) thin film dielectrics6−10 in capacitors for energy storage. Specialty applications require materials with mutiple performance functions. The lack of polar group and structural diversity have limited the introduction of iPP in specialty applications. Despite significant research efforts toward the modification of the iPP structure in the past, both direct and postpolymerization approaches have yielded limited success.11,12 In the direct polymerization process, the catalyst poisoned by polar groups13−15 and side reactions are difficult to overcome and have prevented serious consideration. Current commercial functionalization methods are based on the postpolymerization process.16−20 However, due to the inert nature of the saturated © XXXX American Chemical Society
polymer chain, the functionalization reaction requires highenergy activation by breaking C−H bonds, which also cause many side reactions, such as chain degradation and crosslinking. Polyethylene ionomers with metal carboxylate salts (PECOO−M+)21−25 are well-known commercial polymers. Numerous papers discuss their superior mechanical properties (stiff, tough, high melt strength, etc.) with thermoplastic-like processability due to the formation of thermally reversible ionic aggregates (cross-linkings). However, there has been little research devoted to study iPP ionomers due to chemistry difficulties. PE ionomers are commonly prepared by free radical polymerization mechanism. On the other hand, iPP can only be prepared by Ziegler−Natta or metallocene coordination catalysts, which are highly sensitive to the heteroatoms (O, N, etc.) in polar groups. Several papers reported the chemical routes to circumvent chemistry difficulties. Weiss and Agarwal26 Received: November 11, 2013 Revised: January 4, 2014
A
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prepared iPP-COO−M+ ionomers by a free radical grafting reaction of acrylic acid onto iPP to form low molecular weight ionomers. Breslow et al.27 have shown the synthesis of iPPCOO−M+ ionomers by the copolymerization of propylene and low levels of ethylchloroaluminium 10-undecanoate using a heterogeneous Ziegler−Natta catalyst, followed acidification with HCl and the neutralization with KOH to form potassium salts. The resulting iPP ionomers contain just a few ions per polymer chain and exhibit a very broad molecular weight distribution. Spitz et al.28 used another strategy to prepare the sulfonated iPP ionomers, which involved the copolymerization of propylene/7-methyl-1,6-octadiene using a MgCl2-supported Ziegler−Natta catalyst, followed by sulfonation of the internal double bonds in the poly(propylene-co-7-methyl-1,6-octadiene) copolymers. As expected in the heterogeneous Ziegler−Natta catalyst-mediated copolymerization and low sulfonation yield, the copolymers are complicated with broad composition and molecular weight distributions. In this paper, we will discuss the preparation of new iPP ionomers (iPP-NH3+Cl−) containing ammonium salts and derivatives. In general, the synthesis of functional polyolefin containing amine groups or its derivatives has been elusive with few successful results. To prevent the deactivation of the highly oxophilic Ziegler−Natta catalyst by the amine group (strong Lewis base) in the monomer, the general approach has been to shield the polar group from coming to contact with the catalyst site by introducing bulky alkyl groups, such as isopropyl groups in tertiary amine,29 or by precomplexing amine group with an aluminum alkyl compound30,31 employed in the catalyst preparation. As expected, the heterogeneous Ziegler−Natta catalysts show very poor comonomer incorporation and catalyst activity, namely due to low reactivity of the bulky comonomer and multiple active sites. On the other hand, the homogeneous metallocene catalysts,32−34 with favorable (well-controlled) single active sites for copolymerization reactions, are also met with some limitations. Waymouth35−37 showed in the metallocene/borate-catalyzed copolymerization of sterically hindered N,N-diisopropylamino-1-pentene, with or without α-olefins, that catalyst activities were suppressed by inter- and intramolecular coordination with the amino group resulting in polymers with relatively low molecular weights. Mülhaupt et al.38 applied two silane protecting groups to the amine group in the comonomer. In the ethylene/N,N-bis(trimethylsilyl)-1amino-10-undecene copolymerization mediated by the racMe2Si(Benz[e]Ind)2ZrCl2/MAO catalyst, the silane protecting groups seem to successfully prevent the catalyst poisoning. However, the resulting poly(ethylene-co-aminoundecene) copolymers were insoluble in common organic solvents used to dissolve LLDPE copolymers. In the end, there was no detailed molecular structure information for the resulting copolymers. Several years ago, we examined a similar copolymerization reaction between ethylene and several silane-protected α,ω-amino-olefin [CxN(SiMe3)2] using racCH2(3-tert-Butyl-Ind)2ZrCl2 and purified MAO (free of trimethylaluminum) to form ethylene/CxN(SiMe3)2 copolymers39 that were processed into thin films then interconverted to PE-NR3+Cl− ion exchange membranes showing high (Cl−) anionic conductivity. Coates et al.40−42 reported an alternative route to prepare PE-NH3+Cl− ionomers via ring-opening metathesis copolymerization of cyclo-olefins and their quaternary-ammonium derivatives, followed by the hydrogenation of unsaturated double bonds in the resulting copolymers. Recently, Wagener et al.43 also used an acyclic diene metathesis
polymerization (ADMET) mechanism with brominated α,ωdiene monomers and subsequent hydrogenation and quaternization reactions to prepare PE ionomers containing 1methylimidazolium bromide groups. However, both metathesis polymerization routes cannot be applied to the isotactic PP case.
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EXPERIMENTAL SECTION
Materials and Instrumentation. All O2 and moisture sensitive manipulations were carried out inside an argon filled Vacuum Atmospheres drybox. Methylaluminoxane (MAO, 10 wt % in toluene) was purified by vacuum-pumping to remove trimethylaluminum (TMA) at 70−80 °C for 6 h to form d-MAO solid. rac-CH2(3-tertButyl-Ind)2ZrCl2 and rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2 catalyst were prepared using the published procedures.44,45 Polymerization-grade propylene (from Matheson Gas) was used as received. Toluene (Wiley Organics) was distilled over sodium benzophenone under argon. Allylmagnesium bromide (1.0 M solution in diethyl ether), lithium bis(trimethylsilyl)amide (97%), Et(Ind)2ZrCl2, TiCl3·AA, AlEt2Cl (10 wt % in toluene), chlorotrimethylsilane, triethylamine, calcium hydride, N-methyl-N-(trimethylsilyl)acetamide (Sigma-Aldrich), and diethyl ether (anhydrous, from EMD) were used as received. All high-temperature 1H NMR spectra were recorded on a Bruker AM-300 instrument in 1,1,2,2-tetrachloroethane-d2 at 110 °C. The polymer molecular weights were determined by intrinsic viscosity of polymer measured in decahydronaphthalene (Decalin) dilute solution at 135 °C with a Cannon-Ubbelohde viscometer. The viscosity molecular weight was calculated by the Mark−Houwink equation: [η] = KMvα where K = 1.05 × 10−4 dL/g and α = 0.80.46 The polymer molecular weights were also analyzed on a PL-220 series hightemperature gel permeation chromatography (GPC) unit equipped with triple detectors, including a Precision Detectors 2-angle laser light scattering (LS) detector Model 2040, a Viscotek model 210R viscometer, a differential refractive index detector, and four PLgel Mixed-A (20 μm) columns (Polymer Laboratory Inc.). The oven temperature was at 150 °C, and the temperatures of autosampler’s hot and warm zones were at 135 and 130 °C, respectively. The solvent 1,2,4-trichlorobenzene (TCB) containing ∼200 ppm tris(2,4-di-tertbutylphenyl) phosphite (Irgafos 168) was nitrogen purged. The flow rate was 1.0 mL/min, and the injection volume was 200 μL. A 2 mg/ mL sample concentration was prepared by dissolving the sample in N2 purged and preheated TCB (containing 200 ppm Irgafos 168) for 2.5 h at 160 °C with gentle agitation. The melting temperatures of the polymers were measured by differential scanning calorimetry (DSC) using a PerkinElmer DSC-7 instrument controller with a heating rate of 20 °C/min. All mechanical properties (tensile strength, tensile modulus, elongation at break, etc.) were measured according to the ASTM D-1708 method. The specimens (38 mm × 15 mm overall size and 5 mm × 22 mm in the gauge area) were die cut and performed using an Instron 5866 universal tester, with a load cell of 100 N and a constant cross head speed of 1 mm/min. At least five samples were tested in order to minimize possible errors. Synthesis of 6-Bis(trimethylsilyl)amino-1-hexene and 3Bis(trimethylsilyl)amino-1-propene. 6-Bis(trimethylsilyl)amino-1hexene [C6N(SiMe3)2] was prepared in two reaction steps. In a 500 mL flask equipped with a magnetic stirring bar, 50 g (0.299 mol) of lithium bis(trimethylsilyl)amide dissolved in 200 mL of THF was slowly added into a mixture of 25 mL (0.329 mol) of chloromethyl methyl ether and 50 mL of THF at 0 °C under a nitrogen atmosphere. Once the addition was complete, the solution was allowed to warm to room temperature for 2 h before evaporating the excess chloromethyl methyl ether and THF solvent. N,N-Bis(trimethylsilyl)methoxymethylamine (80% yield) was isolated by distillation. In the second step, 6-bis(trimethylsilyl)amino-1-hexene was prepared by treating N,N-bis(trimethylsilyl)methoxymethylamine with 4-pentenemagnesium bromide. In a 500 mL flask equipped with a magnetic stirring bar and a condenser, 5.2 g of magnesium powder (0.22 mol) was suspended in 200 mL of dry ether, and 25 mL (0.21 mol) of 5bromo-1-pentene diluted with 50 mL of dry ether was then introduced B
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in 50 mL of THF at 50 °C before adding dropwise 20 mL of 2 N methanolic hydrogen chloride solution. The mixture was stirred for 4 h at 50 °C, and the resulting iPP-NH3+Cl− polymer was collected by filtration and washed with the deionized water several times before drying at 80 °C under vacuum for 2 days. For the preparation of iPPNH2 polymer, the iPP-NH3+Cl− polymer solution was directly poured into 100 mL of 1 N methanolic NaOH solution. The resulting polymer was collected by filtration and washed with 1 M aqueous ammonia and water under a nitrogen atmosphere. After drying at 50 °C under vacuum for overnight, the resulting iPP-NH2 copolymer was obtained with quantitative yield. It is interesting to note that the amino group reactions are also very effective in the film form. In other words, the propylene/C6N(SiMe3)2 copolymer can be molded into film and then converted into iPP−NH3+Cl− and iPP-NH2 films by similar procedures. For GPC measurements, all amine groups in PP-NH2 copolymers were reacted with succinic anhydride to form imide groups. Under an Ar atmosphere, 1.0 g of iPP-NH2 polymer (with 0.4−3.0 mol % NH2 content) was dissolved in 25 mL of anhydrous chlorobenzene at 130 °C. After adding 0.2 g of succinic anhydride (excess amount) into the solution, the solution mixture was stirred and refluxed at an elevated temperature for 1 h. The mixed solution was cooled to room temperature and then poured into 250 mL of acetone. The filtered polymer was further washed with acetone three times and dried under vacuum at 60 °C for 24 h.
dropwise through the condenser. The solution was refluxed overnight before adding 41 g (0.20 mol) of N,N-bis(trimethylsilys)methoxymethylamine over a period of 2 h under room temperature. The reaction was allowed to proceed at room temperature for another 2 h before adding 100 mL of aqueous NaOH solution (30%). The organic layer was separated and dried with anhydrous Na2SO4, and the crude product was then distilled over CaH2 to obtain 6-bis(trimethylsilyl)amino-1-hexene in 73% yield. 1H NMR (300 MHz, 25 °C, CDCl3): 5.81 (m, 1 H, −CHCH2), 5.07 (m, 2 H, −CH CH2), 2.77 (t, J = 7.8 Hz, 2 H, −CH2−N−), 2.14 (m, 2 H, −CH2− CHCH2−), 1.38 (m, 4 H, −CH2−CH2−), 0.21 (s, 18 H, −NSi2(CH3)6). 13C NMR (75 MHz, 25 °C, CDCl3): 139.34 (−CHCH2), 114.75 (−CHCH2), 45.88 (−CH2−NSi2(CH3)6), 35.36 (−CH2−CH2−NSi2(CH3)6), 34.04 (−CH2−CHCH2), 26.90 (−CH 2 −CH 2 −CHCH 2 ), 2.52 (−NSi 2 (CH 3 ) 6 ). 3-Bis(trimethylsilyl)amino-1-propene [C3N(SiMe3)2] was prepared by similar procedures, except using allymagnesium bromide instead of 4-pentenemagnesium bromide. 1H NMR (300 MHz, 25 °C, CDCl3): 5.81 (m, 1 H, −CHCH2), 5.07 (m, 2 H, −CHCH2), 2.77 (t, J = 7.8 Hz, 2 H, −CH2−N−), 2.14 (m, 2 H, −CH2−CHCH2−), 0.21 (s, 18 H, −NSi2(CH3)6). Both 1H and 13C NMR and spectra of 6bis(trimethylsilyl)amino-1-hexene and 3-bis(trimethylsilyl)amino-1propene are shown in Figures S1 and S2. Copolymerization of Propylene/C6N(SiMe3)2 by Metallocene Catalyst. In a typical copolymerization reaction, 45 mL of toluene and 0.35 g of solid d-MAO (Al/Zr = 1000) were charged into a Parr 450 mL stainless autoclave equipped with a mechanical stirrer in a drybox. After removal from the box, the reactor was injected with 4 mL of C6N(SiMe3)2 and then charged with 40 psi of propylene to saturate the toluene solution at ambient temperature. About 5 × 10−6 mol of rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2 in 2 mL of toluene solution was then injected into the reactor with 120 psi of propylene under rapid stirring to initiate the copolymerization. Additional propylene was fed continuously into the reactor to maintain a constant pressure (120 psi) during the entire course of the polymerization. After a 15 min reaction time elapse at 30 °C, the reaction solution was quenched by THF, filtered, and washed extensively with THF to remove residual monomer. The isolated propylene/6-bis(trimethylsilyl)amine-1-hexene copolymer was dried under vacuum at room temperature for 12 h. Alternatively, the reaction solution was directly quenched with methanolic hydrogen chloride solution to convert the incorporated −N(SiMe3)2 groups to the stable −NH3+Cl− groups. Slurry Copolymerization Using Liquid Propylene. In a typical liquid propylene copolymerization, 50 mL of toluene and 1.4 g of dMAO were charged into a Parr 450 mL autoclave equipped with a mechanical stirrer in a drybox. After removing from the box, the reactor was injected with a certain amount of C6N(SiMe3)2 and then charged with 60 mL of liquid propylene at ambient temperature. The reactor was heated to 45 °C before 2 × 10−6 mol of rac-Me2Si[2-Me-4Ph(Ind)]2ZrCl2 in 2 mL of toluene solution was injected into the reactor by argon gas to initiate the reaction. After rapid stirring the solution for 10 min, the copolymerization was terminated by adding 10 mL of methanol. The reaction mixture was then cooled to ambient temperature before releasing the pressure. The resulting polymer was discharged into an acidified methanol solution and washed with a large amount of the deionized water before drying in vacuum at 60 °C for 24 h. Copolymerization of Propylene/6-bis(trimethylsilyl)amine1-hexene by Ziegler−Natta Catalyst. The similar synthetic procedure was applied, except the catalyst system was changed to TiCl3·AA (0.10 g) and AlEt2Cl (5 mL, 10 wt % toluene). The reaction temperature kept at 60 °C for 15 min. The pressure of propylene gas was maintained at 60 psi. The products were washed with THF and methanol three times before drying. Functional Group Interconversion. The incorporated bis(trimethylsilyl)amine (−N(SiMe3)2) groups in the iPP copolymers were further interconverted to the desired −NH3+Cl− and −NH2 groups. As discussed, these reactions can also be carried out consecutively in the polymerization reactor. For this systematic study, about 3 g of propylene/C6N(SiMe3)2 copolymer was suspended
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RESULTS AND DISCUSSION There is a clear scientific interest and industrial importance to expand iPP compositions and properties for high-value specialty applications. The well-defined metallocene catalysts with tunable single active sites offer the opportunity for developing new iPP structures. However, there are many hurdles in the preparation of amino-group-containing iPP copolymers. The reactive amino moieties can engage in many undesirable side reactions (discussed later). In this paper, we will focus on a systematic study to identify the most suitable chemical route to prepare processable iPP ionomers (iPPNH3+Cl−) and the associated derivatives with well-controlled molecular structures and examine their thermal and mechanical properties. Synthesis of iPP-NH2 and iPP-NH3+Cl− Copolymers. Scheme 1 illustrates the reaction scheme to prepare isotactic Scheme 1. Synthesis of iPP-NH2 and iPP-NH3+Cl− Copolymers
poly(propylene-co-6-ammonium chloride-1-hexene) (iPPNH3+Cl−) ionomer (III) in a one-pot reaction process and the subsequent treatment of iPP-NH3+Cl− with NaOH to obtain the corresponding neutral poly(propylene-co-6-amino-1hexene) (iPP-NH2) (IV) copolymer. The copolymerization reaction between propylene and 6-bis(trimethylsilyl)amino-1C
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Table 1. Summary of Propylene/α,ω-Bis(trimethylsilyl)aminoolefin Copolymerization Using Various Coordination Catalyst Systems reaction conditions runa
propylene (psi)
[CnN(SiMe3)2] (mL)
A-1 A-2 A-3 B-1 B-2 B-3 C-1 C-2 C-3 D-1 D-2 D-3 D-4 D-5 D-6
60 60 60 120 120 120 120 120 120 120 120 120 120 120 30
4 (n = 3) 4 (n = 6) 6 (n = 6) 4 (n = 3) 4 (n = 6) 6 (n = 6) 4 (n = 3) 4 (n = 6) 6 (n = 6) 4 (n = 3) 4 (n = 6) 6 (n = 6) 8 (n = 6) 12 (n = 6) 4 (n = 6)
copolymerization results
b
temp (°C)
time (min)
polymer yield (g)
[CnN(SiMe3)2]c (mol %)
Mvd (kg/mol)
Tm (°C)
ΔH (J/g)
60 60 60 40 40 40 40 40 40 40 40 40 40 40 40
15 30 30 30 30 30 30 30 30 30 30 30 30 30 30
1.4 1.4 0.8 3.2 5.1 6.3 5.1 3.7 2.6 4.2 4.8 6.4 3.3 2.3 1.9
0 0.1 0.2 0 0.4 0.5 0 0.2 0.4 0 1.0 1.2 2.2 3.0 4.8
311 245 206 58 44 38 158 121 96 388 332 235 286 269 146
160.9 156.9 156.3 138.3 130.7 125.2 156.7 152.9 146.8 156.9 140.7 137.7 135.6 131.7 125.1
76.3 75.3 70.2 50.1 41.2 37.8 78.2 75.5 61.9 105.5 84.7 76.8 70.4 66.3 46.9
a
Catalysts used in set A: TiCl3.AA/AlEt2Cl; set B: rac-Et(Ind)2ZrCl2/d-MAO; set C: rac-CH2-bis(3-tert-butylindene)ZrCl2/d-MAO; set D: racMe2Si[2-Me-4-Ph(Ind)]2ZrCl2/d-MAO. b3-Bis(trimethylsilyl)amino-1-propene (n = 3); 6-bis(trimethylsilyl)amino-1-hexene (n = 6). cComonomer content (mol %) in the copolymers determined by 1H NMR under 110 °C in 1,1,2,2-tetrachloroethane-d2. dEstimated by intrinsic viscosity of polymer/decalin dilute solution at 135 °C with the standard of polypropylene (K = 10−2 and α = 0.8).
hexene [C6N(SiMe3)2] (I) was first examined using four isospecific coordination catalyst systems, including a heterogeneous TiCl3·AA/Et2AlCl (AA: aluminum activated) Ziegler− Natta catalyst and three homogeneous metallocene catalysts, including rac-Et(Ind) 2 ZrCl 2 /d-MAO, rac-CH 2 -bis(3-tertbutylindene)ZrCl 2 /d-MAO, and rac-Me 2 Si[2-Me-4-Ph(Ind)]2ZrCl2/d-MAO. In the metallocene-mediated copolymerization reactions, it is essential to use purified d-MAO cocatalyst. A trace amount of trimethylaluminum (TMA), commonly presented in MAO, causes an aversive effect in forming the completely insoluble propylene/C6N(SiMe3)2 (PPC6N(SiMe3)2) copolymer (II). TMA may in situ activate the reactive N−Si bonds in the pendent N(SiMe3)2 groups of the PP-C6N(SiMe3)2) copolymer to engage in an intermolecular dimerization to form cyclosilazane rings,47 which results in the cross-linked polymer network. The resulting iPP-C6N(SiMe3)2 copolymer (II), with the pendent silane-protected amino groups, were directly deprotected and neutralized with HCl/CH3OH solution under an inert atmosphere to form stable iPP-NH3+Cl− (III) during the sample work-up process. Some attempts to prepare iPPC6N(SiMe3)2 and iPP-NH2 solids were met with completely insoluble products. In the iPP-C6 N(SiMe 3) 2 case, the intermolecular dimerization47 between pendent N(SiMe3)2 groups may be slowly happened at ambient temperature and greatly enhanced under elevated temperatures during the sample drying process. On the other hand, any two pendent amine groups in iPP-NH2 copolymer can engage in a facile reaction with one CO2 molecule in air to form an ammonium carbonate salt48−50 that serves as a cross-linker between iPP polymer chains. In fact, several amine molecules are commonly used in CO2 capture in the oil and natural gas industry and have also been demonstrated to remove CO2 from flue gas in fossil fuel power plants.51 This iPP-NH2 copolymer could potentially be a solid CO2 absorbent material. Table 1 summarizes four comparative sets of propylene/ CnN(SiMe3)2 copolymerization reactions using four catalysts,
propylene gas (maintained at constant pressure during the reaction), and two comonomers, including 3-bis(trimethylsilyl)amino-1-propene [C3N(SiMe3)2] and 6-bis(trimethylsilyl)amino-1-hexene [C6N(SiMe3)2]. Overall, all copolymerization reactions produce some polymers, indicating no significant catalyst poisoning by the silane-protected amino group. However, runs A-1, A-2, and A-3 show the formation of high molecular weight iPP polymers with very low CnN(SiMe3)2 comonomer incorporation. Because of the large difference in two comonomer sizes and reactivity ratios, the heterogeneous Ziegler−Natta catalyst is incapable of incorporating the bulky α,ω-bis(trimethylsilyl)aminoolefin during the propylene copolymerization. Furthermore, all reactions (runs A-1, B-1, C-1, and D-1) involving the C 3 N(SiMe 3 ) 2 comonomer also show no comonomer incorporation in the resulting iPP polymers, despite good polymer yields. Evidently, two −Si(CH3)3 protecting groups to each amine group in C3N(SiMe3)2 comonomer are adequate in preventing catalyst poisoning. However, the small spacer (one methylene unit) is insufficient to disengage the bulky silane-protected amino group from the α-olefin coordination insertion process. On the other hand, the C6N(SiMe3)2 comonomer, with a fourmethylene spacer, shows the formation of propylene/C6N(SiMe3)2 copolymers (II) by all three metallocene catalysts. In two comparative sets (between runs B-2, C-2, and D-2 and between runs B-3, C-3, and C-3), the rac-Me2Si[2-Me-4Ph(Ind)]2ZrCl2 catalyst shows best performance with high C6N(SiMe3)2 incorporation and high copolymer molecular weight. Although rac-Et(Ind)2ZrCl2 offers good catalyst activity, the resulting copolymers show low molecular weight and low melting temperature due to the large special opening at the active site that increases the chain transfer reaction and reduces steric control in the monomer insertion. On the other hand, rac-CH2-bis(3-tert-butylindene)ZrCl2 catalyst, with a small opening at the active site, shows low comonomer incorporation. Comparing runs D-2, D-3, D-4, and D-5, with the systematical increase of the [C6N(SiMe3)2] comonomer feed in D
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Figure 1. 1H NMR spectra of (a) iPP-C6N(SiMe3)2 copolymer with 2.2 mol % of C6N(SiMe3)2 units (run D-4) and (b) the corresponding iPP-NH2 copolymer.
Figure 2. 13C NMR spectrum of the iPP-NH2 copolymer (run D-4). Inset: the chemical shift assignments and the expanded methyl region.
the rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/d-MAO-mediated copolymerization reactions, the comonomer content in the
copolymer proportionally increases, while the catalyst activity systematically decreases. After incorporating the bulky [C6NE
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(SiMe3)2] comonomer unit, the propagating chain may slow down the coordination/insertion of propylene. All propylene/C6N(SiMe3)2 copolymers in Table 1 were handled under inert gas atmosphere at ambient temperature. They are soluble at elevated temperatures, which are very different from the reported insoluble ethylene/C11N(SiMe3)2 copolymers38 prepared by the rac-Me2Si(Benz[e]Ind)2ZrCl2/ MAO catalyst. They were first examined by 1H NMR to determine their copolymer compositions. Figure 1 shows the typical 1H NMR spectra of a iPP-C6N(SiMe3)2 copolymer containing 2.2 mol % of C6N(SiMe3)2 units (run D-4) and its corresponding poly(propylene-co-6-amino-1-hexene) (iPPNH2) copolymer. In the Supporting Information, Figure S1 shows the 1H NMR spectrum of the corresponding [C6N(SiMe3)2] comonomer. In Figure 1a, in addition to three major chemical shifts at 0.95, 1.35, and 1.65 ppm, corresponding to the methine, methylene, and methyl groups in the iPP chain, there are two additional chemical shifts around δ = 0.21 ppm (−NSi2(CH3)6) and 2.77 ppm with a coupling constant J = 7.8 Hz (−CH2−NSi2), respectively, in the propylene/C6N(SiMe3)2 copolymer. In Figure 1b, the combination of a complete disappearance of a sharp peak at δ = 0.21 ppm and the coexistence of the chemical shift at δ = 2.77 ppm (J = 7.8 Hz) indicates a successful deprotection reaction of bis(trimethylsilyl)amino groups by HCl to form NH2 groups. Figure 2 shows the 13C NMR spectrum of the same iPP-NH2 copolymer (run D-4). In addition to three major peaks (δ = 21.9, 28.5, and 46.2 ppm), corresponding to the CH3, CH, and CH2 groups in the iPP backbone, there are several minor chemical shifts corresponding to the incorporated comonomer units, as shown in the inset with the chemical shift assignments. The peak at 46.7 ppm corresponds to CH2−NH2 in the side chains of the iPP-NH2 copolymer. In the expanded CH3 region shown in the other inset, the major CH3 peak at 21.95 ppm (mmmmmm) is accompanied by two minor peaks at 21.88 ppm (mmmmmr) and 21.82 ppm (rmmmmr), respectively.52 The estimated isotacticity (based on the mmmm pentad content) is about 95% similar to that of the i-PP homopolymer prepared by the same rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/dMAO catalyst system under similar reaction conditions. Evidently, the incorporation of C6N(SiMe3)2 comonomer units in the iPP chain does not affect the stereo (isospecific) or regio (1,2-insertion) selectivity of propylene during the coordination/insertion process. The overall results are also consistent with the DSC results discussed later. In order to understand the detailed molecular weight information, all copolymers were examined by a hightemperature GPC measurement with triple detectors, including a light scattering (LS) detector to determine the absolute polymer molecular weight. Before GPC measurements, amine groups in all PP-NH2 copolymers were converted to imide groups (V) to prevent polymer chain interactions, as illustrated in Scheme 2. Figure 3 compares the FTIR spectra of an iPP-
Figure 3. FTIR spectra of (a) iPP-NH2 and (b) iPP-NH2-SAH copolymers. Inset: the local expansion of the iPP-NH2 FTIR spectrum.
NH2 copolymer, containing 4.8 mol % of NH2 comonomer units (run D-6), and the corresponding succinic anhydride caped iPP-NH2-SAH copolymer (V). In Figure 3a, two absorption peaks at around 3400 and 1640 cm−1, which indicate the presence of amine groups in iPP-NH2 (IV), have completely disappeared and are replaced with two new absorption peaks at 1776 and 1708 cm−1 as seen in Figure 3b; these correspond to the symmetric and asymmetric stretching of the two CO groups in the resulting imide group of iPP-NH2-SAH (V). Figure 4 compares absolute GPC (LS) curves of three reaction runs A-2, C-2, and D-2 (Table 1). They are prepared
Scheme 2. Chemical Conversion of iPP-NH2 to iPP-NH2SAH Polymers
Figure 4. GPC molecular weight distribution overlay of three iPPNH2-SAH copolymers prepared by (a) TiCl3·AA/Et2AlCl, (b) racMe2Si[2-Me-4-Ph(Ind)]2ZrCl2/d-MAO, and (c) rac-CH2-bis(3-tertbutylindene)/d-MAO catalysts. F
dx.doi.org/10.1021/ma402328k | Macromolecules XXXX, XXX, XXX−XXX
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Table 2. Summary of Propylene/[C6N(SiMe3)2] Copolymerization Reaction at Low Conversion reaction conditiona
iPP-C6N(SiMe3)2 copolymers
run
propylene/[C6N(SiMe3)2] (mole ratio)
temp (°C)
polymer (g)
conv of [C6N(SiMe3)2] (%)
[C6N(SiMe3)2] (mol %)b
F-1 F-2 F-3 F-4 F-5
61.4/1 30.7/1 27.1/1 20.5/1 13.5/1
30 30 30 30 30
1.86 1.41 1.35 0.99 0.93
2.33 1.90 3.78 1.36 2.21
0.50 1.08 1.12 1.65 1.90
General reaction conditions: 5 μmol of rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2 catalyst, 0.5 g of dMAO, 50 mL of toluene. bThe comonomer content determined by 1H NMR under 110 °C in 1,1,2,2-tetrachloroethane-d2. a
k11/k12) and [C6N(SiMe3)2] (r2 = k22/k21) were estimated using the Fineman−Ross equation. Figure 5 shows the plots of F/f(f
under similar reaction conditions but with three different catalyst systems. The iPP-NH2-SAH copolymer (curve a), prepared by a heterogeneous TiCl3·AA/Et2AlCl Ziegler−Natta catalyst, shows a broad molecular weight distribution (Mn = 44 000, Mw= 203 000, and PDI = 4.61). It is important to note that this polymer (run A-2) also exhibits very low functional group content (0.1 mol %) and a broad composition distribution. Most of the C6N(SiMe3)2 comonomer units are located in low molecular weight polymers that are highly undesirable molecular structure. The iPP-NH2-SAH copolymer (curve c) formed by the homogeneous rac-CH2-bis(3-tert-butylindene)/ d-MAO catalyst shows much narrower molecular weight distribution but relative low molecular weight (Mn = 47 000, Mw= 105 000, and PDI = 2.24). As discussed (run C-2), this catalyst system is also not effective in incorporating C6N(SiMe3)2 comonomers. On the other hand, the iPP-NH2-SAH copolymer (curve b, run D-2), prepared from rac-Me2Si[2-Me4-Ph(Ind)]2ZrCl2/d-MAO mediated propylene/C6N(SiMe3)2 copolymerization, shows high molecular weight and narrow molecular weight distribution (Mn = 97 900, Mw= 210 600, and PDI = 2.15) as well as good C6N(SiMe3)2 comonomer incorporation (as was previously discussed). Overall, the GPC results are also consistent with the molecular weight results (Mv) measured by intrinsic viscosity. The narrow molecular weight distribution (PDI ∼ 2) in both iPP-NH2-SAH copolymers, prepared by rac-CH2-bis(3-tert-butylindene)ZrCl2 and rac-SiMe2[2-Me-4-Ph(Ind)]2ZrCl2/MAO catalysts, indicates a well-defined single-site polymerization mechanism. Evidently, the rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/d-MAO catalyst is most suitable in the propylene/C6N(SiMe3) 2 copolymerization. The active site, defined by dimethylsilane bridged bisindenyl ligands with 2-methyl and 4-phenyl substitutions and C2 symmetry, provides a sufficient stereo opening for 1,2-insertion of α-olefin groups in both propylene and C 6 N(SiMe 3 ) 2 monomers, but not too much for regioirregular 2,1-insertion53,54 and the formation of agostic chain conformation55 at the propagating site; both promote chain transfer reactions and reduce the polymer molecular weight. Reactivity Ratios. The quantitative method to understand the copolymerization reaction and the resulting copolymers is to measure the reactivity ratios of the comonomers. To obtain meaningful results, a series of experiments were carried out by varying monomer feed ratio (F = [M1]/[M2]; [M1] = [propylene] and [M2] = [C6N(SiMe3)2]) and compared with the resulting copolymer composition ( f = d[M1]/d[M2]) at a low monomer conversion (
