Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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One-Pot Synthesis of Block Copolymers Containing a Polyolefin Block Tianwei Yan, Dylan J. Walsh, Chengling Qiu, and Damien Guironnet* Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
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S Supporting Information *
ABSTRACT: Polyolefin-containing block copolymers were catalytically synthesized using a postpolymerization modification strategy. A traditional olefin polymerization followed by a tandem hydroformylation/hydrogenation of the olefinic terminated polymer was implemented to yield a hydroxyl terminated polyolefin. The hydroxyl terminated polyolefin was then used as a macroinitiator for the ring-opening polymerization of cyclic esters to yield the corresponding diblock copolymer. This methodology was applied to an array of different polyolefins and two cyclic monomers (ε-caprolactone and lactide) showcasing the versatility of the protocol. Most noticeably, isotactic polypropylene was quantitatively incorporated into block copolymers. Additionally, the hydrophobicity of polyethylene films was successfully decreased by the addition of small amounts of a block copolymer.
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INTRODUCTION Polyolefins account for almost half of the 300 million tons of polymers produced annually.1−3 This strong market penetration comes from a combination of inexpensive monomers, extremely efficient polymerization processes, and the tunable mechanical properties of the polymers.2−4 Despite these great characteristics, semicrystalline polyolefins suffer from a relatively narrow chemical tunability window.5,6 Polyolefins are limited to being low-polarity materials due to the high oxophilicity of the employed catalysts, which prevent the direct incorporation of polar functional groups.2,7,8 This limitation severely hinders the blending of polyolefins with other materials and limits possible surface treatments due to poor adhesion.9−11 This problem can be overcome by the addition of a small fraction of polyolefin block copolymer which modifies the surface energy.12−14 This blending approach enhances the chemical tunability of polyolefins; however, the synthesis of block copolymers containing polyolefin and polar blocks from simple olefins is nontrivial.15 The most common approaches for synthesizing polyolefincontaining block copolymers are based on the anionic polymerization of butadiene or the ring-opening metathesis polymerization of cyclic olefins followed by a postpolymerization hydrogenation. 9,16 The main limitation of these approaches is the impossibility to yield block copolymers containing isotactic polypropylene, which corresponds to ca. 25% of all polymers produced.1 Alternatively, living insertion polymerization of olefins followed by end-group modification has been used to synthesize polyolefin-containing block copolymers.15,17,18 In this polymerization, the growing © XXXX American Chemical Society
polyolefin remains attached to the metal center, and the reactivity of the metal complex toward polar groups is used to convert the polyolefin into a macroinitiator capable of reacting with polar monomers.19,20 The pitfall of living polymerizations is their scalability due to the high cost of the catalyst and the low productivity of the system; i.e., a living polymerization yields only one chain per initiator.15,21 The low productivity problem has been resolved via the living catalytic chain transfer polymerization.22−25 Chain transfer agents, such as aluminum, zinc, and magnesium alkyls, have been successfully employed for the formation of polyolefin-containing block copolymers.24,26−30 In a living catalytic chain transfer polymerization each polyolefin is attached to a metal alkyl complex. The high oxophilicity of the metal is again used to yield the corresponding macroinitiators.31,32 While successful, stereoregular living chain transfer polymerization remains very difficult to achieve; thus, examples of isotactic polypropylenecontaining block copolymers based on this method remain scarce in the literature.26,33 Another versatile approach for synthesizing polyolefincontaining block copolymers is the postpolymerization modification of polyolefins. This approach relies on the propensity of traditional insertion olefin polymerization catalysts to chain transfer (β-X elimination and chain transfer to monomer) yielding olefinic terminated polymer chains. Hydroboration, alkylation, hydroalumination, thiol−ene addition, Alder−ene, hydroformylation, and hydrosilylation have Received: November 19, 2018
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DOI: 10.1021/acs.macromol.8b02475 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules been used to functionalize the olefinic end-groups.34−42 The yield of these strategies is hindered by the solubility of the polyolefin only at elevated temperature and the low concentration and reactivity of the olefinic end-groups. As a result, these approaches have been mostly implemented on low molecular weight vinyl terminated polymers due to their higher reactivity. The incompatibility of some of these methods to modify 1,1- and 1,2-disubstituted olefinic end-groups is another major limitation, as these end-groups are the most common in polyolefins. We recently reported an approach employing cross-metathesis for the postpolymerization functionalization of semicrystalline polyolefins as an efficient strategy to convert monosubstituted and 1,2-disubstituted olefinic end-groups.43 Despite the improvement offered by this approach, the 1,1-disubstituted and tertiary olefins end-groups, the most common end-groups in polypropylene, remained unfunctionalized. Herein, we present a tandem hydroformylation/hydrogenation strategy for the quantitative postpolymerization modification of any olefin end-group into a hydroxyl group, including all polypropylene end-groups. This methodology is implemented on various grades of polyethylene and polypropylene to yield a macroinitiator capable of initiating the ROP of cyclic esters. Finally, films of polyethylene blended with block copolymers were made and evaluated to quantify the change in surface energy from the addition of block copolymers.
Table 1. Hydroformylation/Hydrogenation of Model Organic Substratesa results entry 1-1 1-2 1-3 1-4 1-5
olefin 1-hexene 1-tetradecene tetradecenesb 2-methyl-1pentene 2-methyl-1pentene
temp (°C)
time (h)
conv (%)
% ald
% alc
% alk
80 80 80 80
48 22 21 24
>99 >99 >99 83
0.2 1.6 1.3 0.1
>99 >97 >98 82
99
0.1
>98
1.4
a
Reaction conditions: N(Bu)3/Rh = 200 (molar ratio), [Rh(COD)Cl]2: 0.25 mol %. Syngas: 80 bar, CO/H2 = 1. bIsomerized from 1tetradecene; see the Supporting Information for more details.
Hydroformylation/hydrogenation reactions of linear alkenes reached high conversion with a catalyst loading of 0.25 mol % (Table 1). Less than 1% of hydrogenated products and less than 2% of aldehyde products were formed at 80 °C. At this temperature, the reaction of 2-methyl-1-pentene did not reach completion, with only 82% of the alcohol being formed (Table 1, entry 1-4). Detailed analysis of the 1H NMR spectroscopy and GC chromatograph showed that the initial geminal olefin had been isomerized in situ into a tertiary olefin: 2-methyl-2pentene, a less reactive olefin. The formed 2-methyl-2-pentene could be converted into the desired alcohol by performing the tandem catalytic reaction at a higher temperature (140 °C). Analysis of the reaction mixture showed that about 20% of the product had undergone in situ isomerization into the tertiary olefin (Table 1, entry 1-5). The increased temperature also resulted in a slight increase in the hydrogenated product (Table 1, entries 1-4 and 1-5, from 0.4% to 1.4%). Overall, the high conversions obtained with these model compounds suggest that the rhodium catalyst employed should quantitatively convert any polyolefin end-group into primary alcohol. Hydroformylation/Hydrogenation of Polyolefins. A series of industrially relevant polyolefins were synthesized using existing catalyst systems. The chemical structure, molecular weight dispersity, and the chain-end functionality of these polymers are listed in Table 2. Two grades of linear polyethylene, l-HDPE and h-HDPE (Table 2, entries 2-1 and 2-2), were used for this study. The quantitative characterization of the postpolymerization reaction is easier with the low molecular weight polymers as their end-group concentration is higher, although GPC analysis is not suitable on the low molecular weight polymers (Table 2, entries 2-1 and 2-3). Therefore, l-HDPE was used to fine-tune the reaction conditions and the characterization methods. The tandem hydroformylation/hydrogenation conditions developed for small molecules were applied directly to the lHDPE, and the product was characterized by 1H NMR spectroscopy. High conversion of the unsaturated end-group was illustrated by the complete disappearance of the olefinic peak signals between 5 and 6 ppm and the formation of new signals between 3.4 and 3.8 ppm corresponding to the hydroxymethyl −CH2OH (Figure 1).44 The fraction of aldehyde present in the sample was also quantified by 1H NMR (δ = 9.6−9.8 ppm) and was initially higher than desired. The tandem hydroformylation/hydrogenation was further finetuned in which a reaction temperature of 140 °C with 2 mol % Rh, 100 equiv of NBu3 to rhodium, for 22 h resulted in a complete consumption of the olefin terminated l-HDPE, and
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RESULTS AND DISCUSSION Hydroformylation/Hydrogenation of Model Compounds. A [(COD)RhCl]2 precatalyst with NBu3 ligand was identified as a prime candidate for the hydroformylation/ hydrogenation of polyolefins based on the reported high conversions and ability to transform geminal and tertiary olefins.44−46 A common undesired side reaction of any tandem hydroformylation/hydrogenation is the hydrogenation of the olefin to yield the corresponding saturated alkane.47 We presumed that it would be difficult to quantify the saturated polyolefin end-groups, i.e., the extent of the hydrogenation. Therefore, we performed the tandem reaction with a series of small organic molecules to identify the reaction conditions necessary to achieve high selectivity and yields. A series of primary, secondary, and tertiary olefins were explored for the tandem hydroformylation/hydrogenation reaction (Scheme 1). The hydroformylation/hydrogenation Scheme 1. Hydroformylation/Hydrogenation of Model Compounds and Their Corresponding Side Products
reactions were performed at different temperatures, pressures, and catalyst loadings to identify the most efficient reaction conditions. The product mixtures were analyzed by a gas chromatograph (GC) equipped with a flame ionization detector (FID), and the key experimental results can be found in Table 1. B
DOI: 10.1021/acs.macromol.8b02475 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Table 2. Polyolefins and Their End-Group Functionalities % end-groupc entry
polyolefin
2-1 2-2 2-3 2-4 2-5
l-HDPE h-HDPE aPP synPP iPP
−1 b
a
Mn (g mol )
Đ
primary
1,2-disubstituted
1,1-disubstituted
tertiary
800c 18000 450c 5800 18000
n.d. 1.6 n.d. 1.8 2
43 83 0 >99 0
57 17 2 0 62
0 0 >97 0 34
0 0 0 0 4
b
a
See the Supporting Information for synthesis of these polymers. bHT-GPC using triple detection. cFrom 1H NMR, Mn below GPC lower limit.
Figure 1. 1H NMR spectra for the synthesis of l-HDPE-b-PCL. Top: alkene terminated l-HDPE. Middle: l-HDPE after hydroformylation/ hydrogenation, Table 3, entry 3-5. Bottom: l-HDPE-b-PCL, Table 5, entry 5-1.
>95% of the desired hydroxyl terminated polymer was formed (Table 3, entry 3-5). The improved reaction conditions were then directly applied to the other polyolefins (Table 4). The hydrogenation of aPP catalyzed by palladium on charcoal was performed as a control experiment to confirm that the hydrogenation of polypropylene was not significant in the tandem reaction (Figure S33). The conversion of the olefinic end-groups appeared in all cases
Table 4. Hydroformylation/Hydrogenation of Other Polyolefins product characterizationb
entry
NBu3/Rh
temp (°C)
3-1 3-2 3-3 3-4 3-5 3-6
200 200 200 40 100 100
80 110 140 140 140 140
olefin conv (%)
% ald
% alc
% alk
22 22 22 22 22 16
>99 >99 >99 >99 >99 >99
7 3 98 >98 >98
95
