Synthesis of Diverse Well-Defined Functional Polymers Based on

Jan 24, 2012 - Graduate School of the Chinese Academy of Sciences, Beijing 100039, ... property of comb-like styrene-based polymers with a high degree...
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Synthesis of Diverse Well-Defined Functional Polymers Based on Hydrozirconation and Subsequent Anti-Markovnikov Halogenation of 1,2-Polybutadiene Jun Zheng,†,‡ Feng Liu,†,‡ Yichao Lin,†,‡ Zhijie Zhang,†,‡ Guangchun Zhang,†,‡ Lu Wang,†,‡ Yan Liu,† and Tao Tang*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100039, China S Supporting Information *

ABSTRACT: For the first time, hydrozirconation and halogenolysis of 1,2-polybutadiene (1,2-PBD, by living anionic polymerization) were studied to synthesize reactive polyhalohydrocarbons, which provide a platform for preparing well-defined functional polymers via macromolecular substitution. Hydrozirconation and halogenolysis afforded quite convenience for anti-Markovnikov hydrohalogenation of 1,2PBD with controllable degree of functionalization. Diverse functional polymers and branched polymers were synthesized after macromolecular substitution reaction with a broad range of commercially available nucleophiles (amines, phenols, alcohols, carbanions, carboxylates, and azide) or macromolecular nucleophiles. NMR and GPC characterizations confirmed high conversion in substitution reaction and narrow molecular weight distribution of the resultant functional polymers, respectively. The methodology utilizing Schwartz’s reagent for hydrozirconation of macromolecules could greatly facilitate the modification of vinyl groups containing polymers with high efficiency.

1. INTRODUCTION Living polymerization methods (living radical and ionic polymerization) have widely been used to achieve a high degree of control over the resultant polymers and provide polymers with the narrowest molecular weight distribution.1,2 But relatively low functional groups tolerance restricts the synthesis of functional polymers directly by living polymerization methods. To overcome such obstacles, a combination of living radical polymerization (of monomers bearing reactive moieties) and subsequent postmodification approaches3,4 provides possibilities for preparation of various functional polymers with control over molecular weight, functionality, and tailored properties. However, living anionic polymerization seems to be extremely incompatible with most functional groups. Therefore, it is still a key challenge to prepare functional polymers directly or indirectly by living anionic polymerization. Instead of synthesis of monomers bearing reactive groups for living anionic polymerization, we try to introduce versatile reactive moieties with high efficiency into polymers for subsequent modification. By this way, a family of diverse well-defined functional polymers would be prepared indirectly by anionic polymerization, when incorporation of reactive groups was performed on an appropriate polymer precursor from living anionic polymerization. It is known that butadiene, one of common monomers, can be polymerized by living anionic polymerization with tunable 1,4- or 1,2-microstructure. The nature of unsaturated bonds in © 2012 American Chemical Society

polybutadiene (PBD) backbone or side chain offers opportunities for various modification reactions. Plenty of functional groups (e.g., hydroxyl, carbonyl, silyl, epoxy, etc.) were successfully introduced onto PBD chains via reactions with double bonds. A pioneering research has been reported on hydroboration of 1,2-PBD followed by oxidation and hydrolysis for synthesis of well-defined polyalcohol without gelation.5 By means of hydrocarboxylation and hydroacylation, PBD could be transformed to poly(carbonyl acid) or polyaldehyde, using palladium catalyst in CO pressure6 or by radical addition of aliphatic aldehydes.7 Thiol−ene click chemistry was employed to introduce functional groups onto 1,2-PBD homopolymer or block polymer.8,10 The recently reported cyclopropanation of PBD for the preparation of poly(cyclopropanyl acid) or ester would be feasible with controllable degree of functionalization.11 Epoxidation and hydrosilylation seem to be the most popular methods used in practice due to reactive epoxy ring12,13 and functional hydrosilanes14,15 for possible further transformation. Different postmodification methods selectively or nonselectively toward 1,4- or 1,2-PBD successfully afford various reactive groups grafted PBDs accompanied by side reactions in some cases. Although most of grafted groups vastly expand the properties of PBD, they are not suitable for later Received: December 4, 2011 Revised: January 10, 2012 Published: January 24, 2012 1190

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with yield 85% and stored in nitrogen-filled glovebox (used within 1 month). 2.2. Synthesis of N,N-Dipiperidine Ethane (DPE).24 Piperidine (17.2 g, 0.2 mol) and 1,2-dichloroethane (9.4 g, 0.1 mol) were mixed in 60 mL of H2O with 8.8 g of sodium hydroxide. The mixture was vigorously stirred in 80 °C oil bath for 12 h. Upper organic layer of the final immiscible mixture was separated and washed with water, vacuum distilled in the presence of sodium hydride to afford transparent oillike liquid with yield of 70% (purity >98% GC). δH (300 M, CDCl3): 2.48 (s, 4H, N−CH2−CH2−N), 2.40 (br, 8H, 2N (CH2)2), δ 1.6−1.48 (m, 8H, 2N(CH2CH2)2), δ 1.46−1.32 (m, 4H, 2CH2).25 2.3. Polymerization of Butadiene. 1,2-Polybutadiene (1,2-PBD, with vinyl content (Cv) of 98%) was prepared in hexane with DPE at −10 °C under an argon atmosphere using sec-butyllithium (BuLi) as initiator ([DPE]/[BuLi] > 5 (mol/mol)), while 1,2-PBD with Cv of 88% was synthesized in THF at −10 °C without DPE. 2.4. Hydrozirconation of PBD and Halogenolysis. PBD (∼0.15 g) was dissolved in 20 mL of THF and stirred overnight in a N2-filled Mbraun glovebox. Cp2ZrHCl powder was added and vigorously stirred. The solution became dark red after all the powder dissolved, to which solid halogenation reagent (NBS, NCS, NIS, or I2) was added for halogenolysis. The mixture was stirred for 1 h, and drops of degassed methanol were added to finish the reaction. After concentrating by rotary evaporator, the viscous product was washed with methanol and purified by dissolution−precipitation using methane dichloride/methanol for 3−5 cycles. Isolated halogenated polymers were in ca. 80% yield after dried under vacuum at 40 °C overnight. 2.5. Synthesis of Functional Polymers by Macromolecular Substitution Reaction. A typical procedure for all the selected nucleophiles was as follows: 0.18 g of iodinated PBD (92% or 80%) was dissolved in 15 mL of THF with 400−500 mol % nucleophiles and sufficient base (NaH or K2CO3). Tetrabutylammonium bromide was used as phase-transfer catalyst when alcohols or phenols acted as nucleophiles. The mixture was stirred at 60 °C for 24 h under an argon atmosphere, and then the mixture was filtrated and concentrated. The synthesized functional polymers were isolated by precipitation with methanol and dried at 40 °C under vacuum. 2.6. Characterization. NMR spectra were performed on a Bruker AV300 (400) spectrometer by using CDCl3 as a solvent. Relative molecular weights were determined by gel permeation chromatography (GPC) on TOSOH HLC 8220 GPC at 40 °C using THF as an eluent against linear polystyrene standards. Differential scanning calorimetry (DSC, Perkin-Elmer 7 DSC instrument) was carried out in the range from −30 to 100 °C at a heating rate of 30 °C/min and then cooled down to −30 °C at a cooling rate of 30 °C/min. The secondary heating rate was 10 °C/min for glass transition temperature (Tg) determination. Thermogravimetric analysis (TGA) was performed on SDTQ600 (TA Instruments) under nitrogen flow at a heating rate of 10 °C/min.

polymer-analogous modification due to limited reactivity toward few substrates. It will be needed to incorporate a versatile group14 that can be highly reactive toward a broad range of substrates for the conversion into functional groups. Halogen groups, considered as versatile groups, are quite reactive toward abundant nucleophiles (e.g., alkoxy, azide, nitrile, organometallics for C−C coupling, and so forth). Introduction of halogen onto polymers, especially primary halogen groups (in favor of substitution than elimination), would be an excellent choice for subsequent transformation of halogens into diverse functional moieties via substitution reaction. Classical hydrohalogenation by hydrogen halides usually shows limited control over antiMarkovnikov halogenation although it has been applied industrially for rubber materials.16 Hydrozirconation and later halogenolysis are highly efficient for halogenation of alkene/ alkyne in anti-Markovnikov form (high reaction rate and mild condition compared to electrophilic addition reaction with hazardous hydrohalide17,18). However, hydrozirconation of macromolecules with unsaturated carbon−carbon bond in the backbone or pendent chains is rarely reported to our best knowledge.19,20 The named hydrozirconation reagent, generally referred to as “Schwartz’s reagent”, i.e., bis(cyclopentadienyl)zirconium hydrochloride (Cp2ZrHCl), is a well-known and convenient reagent in organic chemistry for functionalization of alkenes.21,22 In this study, we demonstrate successful hydrohalogenation of 1,2-PBD (prepared by anionic polymerization) with anti-Markovnikov form by hydrozirconation and subsequent halogenolysis (Scheme 1). The synthesized wellScheme 1. Hydrozirconation of 1,2-Polybutadiene and Halogenolysis Methods to Preparation of Polyhalohydrocarbon as a Platform for Diverse Functional Polymers

defined halogenated 1,2-PBD was used as a platform for the preparation of diverse functional or grafted polymers via macromolecular substitution. Importantly, this method would not only produce novel well-defined polymers which could not be prepared directly by classical polymerization but also develop alternative applications of Schwartz’s reagent in polymer chemistry.

3. RESULTS AND DISCUSSION 3.1. Hydrozirconation of 1,2-Polybutadiene. Figure 1a shows 1H NMR spectra of 1,2-PBD (Cv = 88%) and hydrozirconated 1,2-PBD. It could be seen that the hydrozirconation of 1,2-PBD was realized with high conversion by using Schwartz’s reagent (ZrH) in equivalent molar amount of double bonds of 1,2-PBD. The typical resonance peak at 4.9 ppm ascribed to terminal protons of CHCH2 almost disappeared, and a new sharp single peak at 6.0 ppm assigned to protons of Cp rings was observed. In practice, when ZrH was added to THF solution of PBD, the transparent solution turned to yellow in a minute, and ZrH was completely dissolved after 10 min when 30% of ZrH (molar ratio of [ZrH] to [CC]) was added. If the loading of ZrH was high (70% or 100%), the solution color turned to dark red, and the reaction finished within 4 h at 30 °C (Figure 1b). Because of high reactivity of Schwartz’s reagent toward terminal alkenes, quantitative

2. EXPERIMENTAL SECTION 2.1. Materials. All the solvents (tetrahydrofuran (THF), hexane) were distilled from sodium/potassium alloy with benzophenone. Bis(cyclopentadienyl)zirconium dichloride (99%) was purchased from Alfa Aesar. N-Bromosuccinimide (NBS) was recrystallized from hot water, washed with alcohol, and vacuum-dried as white flakelike crystals, while N-chlorosuccinimide (NCS) (98+%) and N-iodosuccinimide (NIS) (97%) were used as received from Alfa Aesar. The commercially available nucleophiles and other reagents used in this work were in chemically pure grade. Schwartz’s reagent (Cp2ZrHCl) was prepared23 using diisobutylaluminum hydride (1 M in hexane) 1191

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bromo-, and N-iodosuccinimide (NCS, NBS, and NIS, respectively) or molecular iodine. All of the halogens (Cl, Br, I) could be successfully incorporated onto pendent groups of PBD in anti-Markovnikov form. Figure 2 shows the 1H NMR

Figure 2. 1H NMR spectra of halogenated (PBD-I, PBD-Cl, PBD-Br) and hydrogenated (PBD-H) of PBD with 30% degree of functionalization and pristine PBD (Cv = 88%).

spectra of chlorinated (PBD-Cl, by NCS), brominated (PBDBr, by NBS), and iodinated (PBD-I, by NIS) PBD with about 30% degree of functionalization. The new resonance peaks centered at 3.5 (PBD-Cl), 3.4 (PBD-Br), and 3.2 ppm (PBD-I) were assigned to methylene protons of CH2−Cl, CH2−Br, and CH2−I, respectively. The chemical shifts are in the order of halogen electronegativity. When hydrozirconated 1,2-PBD was treated with acidified methanol instead of succinimide, pendent vinyl groups was hydrogenated (PBD-H), resulting in the formation of methyl groups (δ 0.8 ppm in 1H NMR). The functionalization degree (halogenation and hydrogenation) of 30% is consistent with the feed amount ([ZrH]/[CC] = 0.3) of ZrH and succinimide, indicating that these reactions are almost quantitative. 3.2.2. Degree of Halogenation with Controllable Fashion. It is an interesting question whether hydrozirconation and halogenolysis were quantitatively controllable for halogenated PBDs with various functionalization degrees. Figure 3a shows 1 H NMR spectra of isolated brominated PBDs from varied feeding of ZrH. As the feed ratio increased, the relative intensity of peak at 3.4 ppm (CH2−Br) increased while the intensities of peaks at 5.5 and 5.0 ppm decreased, indicating that the degree of bromination was gradually enhanced. In addition, an unexpected peak (at 0.8 ppm) was also observed, the intensity of which was increased with the intensity of CH2−Br. This peak was assigned to protons from hydrogenated unit, meaning that the resultant polymers are a kind of copolymers of butadiene, 4bromo-1-butene, and 1-butene. Figure 3b shows the plot of the content of each structural unit versus the feeding ratio. The formation of 1-butene structural unit was attributed to hydrolysis (with methanol in work-up) of residual alkylzirconium from incomplete halogenolysis (Figure 4 and Figure S1 in the Supporting Information). With the increase of bromination degree, glass transition temperatures (Tg) of the resultant polymers were increased according to DSC results (Table 1 and Figure S2). Additionally, different halogenolysis reagents showed different ceiling degree of halogenations under the same reaction conditions (Figure S3). Only 50% chloronation or 77% bromination was achieved when NCS or NBS was used,

Figure 1. 1H NMR (300 MHz) spectra of (a) 1,2-PBD (Cv = 88%) (in CDCl3) and hydrozirconated 1,2-PBD (in C6D6) and (b) brominated PBDs from hydrozirconation (various reaction time) and then halogenolysis (quenched with NBS) of 1,2-PBD (Cv = 88%) at 30 °C.

conversion could take place successfully. All of the pendent vinyl groups could be hydrozircnoated, meaning no steric hindrance effect was observed. It should be mentioned that the use of THF as solvent is an important factor for such high reaction efficiency. The insolubility of Schwartz’s reagent would bring kinetic hindrance for the reactions with macromolecules. Comparatively, hydrozirconation was much faster in THF than in hydrocarbon solvents due to donor capability of THF.22 Therefore, the dissociation of ZrH cluster by THF greatly promoted the rate of hydrozirconation reaction with 1,2-PBD. Hence, hydrozirconation of PBD macromolecules is kinetically favorable in THF. Such hydrozirconated polymers are thermally stable in solution at room temperature, but sensitive to oxygen or moisture, and they can react with a variety of electrophiles including protons and halides or undergo transmetalation reactions for functional polymers.22 (For example, our preliminary study has demonstrated that this polyorganozirconium could be reactive for initiating ethylene polymerization.) Halogenolysis is one of the examples discussed in this work for preparing reactive polymers. 3.2. Halogenolysis of Hydrozirconated 1,2-PBD for Preparing Reactive Polyhalohydrocarbons. 3.2.1. Incorporation of Halogen Atoms (Chlorine, Bromine, Iodine) via Halogenolysis. Halogenolysis reaction was carried out with different sources of cationic halogen, such as N-chloro-, N1192

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Table 1. Molecular Weights of Halogenated 1,2-PBDs (Cv of 98%) polymers 1,2-PBD PBD-Br(8%c) PBD-Br(30%c) PBD-Br(51%c) PBD-Br(66%c) PBD-Br(77%c) PBD-H(99%c) PBD-I(92%c) PBD-I(92%c)d PBD-I(80%c)e

Mn (g/mol)a Mw (g/mol)a 15800 15400 13500 11900 11600 10300 10300 10200 7700 2000

16500 16300 14900 13700 13300 11700 11500 11700 8300 2200

PDI (Mw/Mn)a 1.04 1.06 1.10 1.15 1.15 1.13 1.12 1.15 1.09 1.07

Tgb (°C) −7.4 −3.1 1.7 4.2 4.4 8.0 −24 19

a Determined by GPC in THF vs linear PS standards. bDetermined by DSC (Figure S2). cDegree of functionalization determined by 1H NMR spectroscopy. dThe number molecular weights of parental PBD was about 9600 g/mol. eThe number molecular weights of parental PBD was about 2200 g/mol.

incomplete halogenolysis, especially when NCS is used. The low degree of chlorination was also observed in hydrozirconation of small molecular alkenes.26 Fortunately, using molecular iodine indeed promotes halogenolysis, which leads to the formation of almost complete iodinated PBD. Moreover, another great advantage of using molecular halogen is the regeneration of bis(cyclopentadienyl)zirconium dihalide, which could be separated and reused.26,27 Owing to the existence of unreacted zirconium moieties on polymer chains, additional care was required for purification. Deoxygenated methanol should be used to convert all the unreacted organic zirconiums to methyl groups before exposed in air. Otherwise, possible oxygen-induced radical coupling28 would bring a shoulder peak in GPC trace of halogenated polymer (Figure S4). Highly halogenated 1,2-PBDs were slight yellow (PBD-Br) or brown (PBD-I) solid, which were stable at room temperature in desiccator for several months (GPC traces and 1H NMR spectra did not change). 3.2.3. Molecular Weight and Polydispersity of the Resulting Polyhalohydrocarbons. It is crucial to maintain lengths of parental polymers after postpolymerization modification. Thus, side reactions, such as chain-scission and crosslinking, should be avoided during polymer modification. Unfortunately, chain-scission reaction indeed happened during the hydrozirconation of 1,2-PBD with Cv of 88%. However, the chain-scission could be greatly alleviated when 1,2-PBD with Cv of 98% was used as parental materials (Figure 5), which gave the viability of the halogenation methodology based on hydrozirconation and encouraged us to make deep investigation. As we know, carbon−carbon double bond of the backbone is a key factor causing chain-scission; i.e., internal double bonds hydrozirconated compounds undergo β-alky elimination, which resulted in chain cleavage. A similar depolymerization process has been reported in hydrogenolysis of polyethylene and polypropylene.29 Only increasing Cv content could the degree of chain scission be reduced. Increasing molecular weight of parental PBDs was useless for PBD with Cv of 88% to maintain high molecular weight after hydrozirconation and halogenolysis. The new generated endgroups from chain scission would be halogens after halogenolysis. (Systematical work related to chain scission behavior and possible mechanism will be discussed particularly in another paper.) Therefore, this method is only suitable for

Figure 3. (a) 1H NMR (300 MHz, CDCl3) spectra of brominated polybutadiene from varied feeding addition and starting polybutadiene (Cv = 98%). (b) Microstructure content of brominated PBDs plotted with varied feeding addition (calculated from 1H NMR (a)).

Figure 4. 13C{H} NMR (CDCl3, 100 MHz) spectra of brominated (77%) and iodinated (92%) PBD and DEPT135 NMR spectrum of iodinated PBD (92%).

respectively. It was helpless solely by extending halogenolysis time when succinimide was used as electrophilic halogenation reagent. However, when I2 was used, iodinated PBD with a higher degree of halogenation (92%) was obtained. Consequently, the incomplete halogenolysis limited the complete halogenation. It is possible that the reactivity of grafted zirconium is weakened as the halogenolysis goes, resulting in 1193

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Table 2. Summary of Molecular Weights of Starting 1,2PBDs (Cv of 98%) and the Corresponding Iodinated PBDs polymers b

PBD2K PBD2K-0.8Ic PBD10Kb PBD10K-0.9Ic PBD15Kb PBD15K-0.9Ic PBD30Kb PBD30K-0.3Ic

Mn (g/mol)a

Mw (g/mol)a

PDI (Mw/Mn)a

2200 5800d 9700 24400d 15900 32400d 28100 21000d

2400 6200d 10000 26200d 16600 37900d 29600 26600d

1.07 1.07 1.04 1.09 1.04 1.15 1.05 1.27

a

Determined by GPC in THF vs linear PS standards. bParental PBDs with different molecular weights (2K, 10K, 15K, 30K) for hydrozirconation. cIodinated polymers from corresponding PBD with certain degree of halogenation (0.8, 0.9, 0.9, and 0.3). dCalculated according to molecular weights from GPC after considering degree of halogenation. Figure 5. GPC traces of brominated 1,2-PBDs (Cv of 88% and 98%) from hydrozirconation with varied feeding ratios ([ZrH]).

the halogenation of PBDs with high Cv (>98%) for preservation of molecular weights. Although DPE is a powerful agent for synthesizing 1,2-PBD with high Cv by living anionic polymerization,30 it failed in preparing 100% 1,2-PBD with high molecular weight. However, high iodinated PBD with preserved molecular weights and PDI would be prepared after hydrozirconation of 1,2-PBD (Cv of 98%, Mn < 15 kg/mol) and halogenolysis (Figure 6 and Table 2). In the case of 1,2-PBD

Figure 7. Hydrohalogenation of poly(p-(3-butenyl)styrene) by hydrozirconation and halogenolysis: 1H NMR (300 MHz, CDCl3) and GPC results of parental polymer (PVSt) and brominated polymer (PSt-Br).

reagent mediated hydrogenation of polybutadiene or polyisoprene.31 However, it is believed that this chain scission will be useful on polymer recycling. Although chain-scission took place, the halogenated polymer from 98% 1,2-PBD retained relatively high molecular weights and low polydispersity index (Tables 1 and 2). Such halogenated 1,2-PBDs could be used as reactive building blocks for functional materials and polymer architecture design (grafted or polymer brush) via macromolecular substitution reaction,32−37 which would facilitate the preparation of polymers with low PDI described in following section. 3.3. Preparation of Functional Polymers by Macromolecular Substitution. Versatile reactivity of incorporated halogens toward various nucleophiles facilitates abundant functionality of synthesized polyhalohydrocarbon after macromolecular substitution. Typical chemical reagents (Figure 8) available in our laboratory were used to demonstrate this idea. As summarized in Table 3, the experiments with all the selected nucleophiles indicated the success of substitution reaction on versatile halogen groups to form a diverse of functional

Figure 6. GPC traces of iodinated PBDs and corresponding parental PBDs.

with high molecular weight (30 kg/mol), the changes of PDI and molecular weights restricted the application of hydrozirconation of PBD (Cv = 98%) due to the existence of 1,4 structure. We speculated that hydrozirconation of 100% 1,2PBD would be free of such molecular weight limitation and chain-scission reaction. For a special example, no chain-scission happened in the hydrozirconation of poly(p-(3-butenyl)styrene) (60 kg/mol, PDI = 1.2, from living anionic polymerization) and its subsequent halogenolysis due to free double bonds in the backbone (Figure 7). As we can see, Schwartz’s reagent was quite sensitive to backbone double although it was thought that terminal vinyl groups were favored for hydrozirconation than internal double bonds.26 This phenomenon would be helpful for microstructure characterization of PBD and diagnosis of microstructure change in polymerization. Chain scission was also observed in Schwartz’s 1194

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perfluoro-1-octanol into side chains of PBD. Cyclopentadiene containing PBD could be a kind of self-vulcanized rubber materials with thermoreversible cross-linking−de-cross-linking properties (Diels−Alder and retro-Diels−Alder reaction of grafted cyclopentadienes). Azobenzene-functionalized polymer is a kind of photoresponsive polymer39 and also would be used in host−guest supermolecular chemistry,40 while carbazolemodified polymers have been demonstrated to photoconductive or electric conductive polymers.41 Alkyne, furan, and azide substituted polymers are perfect candidates for “click” reactions, especially in the combination of living anionic polymerization and click chemistry for synthesis of well-defined polymer brush or comb polymers.42 It is possible to use mono-end-functional polymers as macronucleophiles,43−45 which provides a method to synthesize grafted (or branched) polymers via substitution reaction. For example, methoxypoly(ethylene glycol)s (MPEGOH) were grafted onto halogenated PBD in the presence of sodium hydride by the Williamson ether approach. PBD-g-PEG was synthesized with low PDI (1.05), and the graft ratio determined by GPC was about 20 mol % (Figure S6). When living polybutadienyllithium (1,4-PB-Li) was used as a macromolecular nucleophile to react with iodinated PBD, monodispersed PBD-I grafted 1,4-PBD (PBDI-g-PBD) was prepared with the graft ratio of 40 mol %, although dimeric products were formed46 (Figure S7a). The length of branch chains and density of grafting would be easily adjusted by controlling the molecular weight of PB-Li and PB-Li/PBD-I ratio (Figure S7b). Hence, monohydroxyl, carboxyl, or other nucleophilic moieties terminated polymers could undoubtedly undergo similar approach to form branched or comb polymers. In these situations, living anionic macromolecular species were utilized to graft onto a well-defined polymer chain, which would be fascinating for synthesizing well-defined nonlinear polymer by living anionic polymerization. This feasibility should be attributed to the high reactivity of halogen groups toward various nucleophiles. Therefore, well-defined polyhydrohalo-

Figure 8. Chemical structures of various nucleophiles used in macromolecular substitution for functional and branched polymers.

polymers. As confirmed by 1H NMR (Figure S5a and Table S1), the macromolecular substitution reaction proceeds with good conversion. Both molecular weight and molecular weight distribution were well preserved as elucidated by GPC measurement (Figure S5b). These results verified the success of the mentioned-above proposal, i.e., to prepare diverse functional polymers with controlled structure by incorporation of versatile reactive groups on well-defined polymer chains and subsequent polymer-analogy modification. The resultant functional moieties on the 1,2-PBD would provide a potential as advanced materials. For example, poly(ionic liquid) was formed by the incorporation of methylimidazole, which could be applied in full cell alkaline anion exchange membranes38 and antibacterial or antistatic materials. Fluororubber was made by incorporation of

Table 3. Summary of Nucleophiles in Experiments and Degree of Conversion, Molecular Weight, and Molecular Weight Distribution of the Resultant Functional Polymers (PBD-Nu) after Macromolecular Substitution nucleophiles (Nu) imide or amines

phenols carbanions

alcohols

carboxylates azide

d

phthalimide 1-methylimidazole carbazole phenol hydroxyazobenzene allylmagnesium chloridef cyclopentadienyl sodiumf 1,4-polybutadienyllithium (Mn = 2200 g/mol) 1,4-polybutadienyllithium (Mn = 14300 g/mol) propargyl alcohol furfuryl alcohol 1H,1H-perfluorooctanol MPEG-OH (Mn = 2000 g/mol) sodium acetate sodium azide

conva (%)

Mn (g/mol) (PBD-I)

Mnb (g/mol) (PBD-Nu)

Mw/Mnc

80 100 100 100 100 100 >90g 40j 47j 47 (48i) 41 (59i) 39 (51i) 20j 50 100k

32400 24400 5 800 24400 24400 24400 24400 5800 24400 5800 32400 5800 24400 24400 24400

35700 18400 7000 19500 33600 12900 h 25000 863000 3000 17200 7100 68000 20600 13300

1.26 e 1.07 1.12 1.15 h 1.05 1.20 1.10 1.22 e 1.05 1.07 1.08

a Determined from 1H NMR (Figure S4a); 100% for all the halogen was substituted and converted to functional groups by nucleophiles. bCalculated from the conversion and the characteristics of the parental polymers. cDetermined from GPC in THF against linear PS standards (Figure S4b). d Phthalimide potassium was used. eNot measured due to insolubility of polymer in THF. fReaction was carried out below 0 °C. gEstimated by TGA. h After purification, insoluble solid polymer was obtained because of D−A cross-linking. iRatio of elimination reaction. jGraft ratio determined from GPC (Figures S6 and S7). kTHF/DMF (v/v 4:1) was used.

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51073149) and the Fund for Creative Research Groups (no. 50921062).

carbons provide an excellent platform for synthesis of diverse functional polymers as well as nonlinear polymers. However, it should be noted that phase separation would happen in some case during postpolymerization reaction. For example, when 1-methylimidazole and perfluoro-1-octanol were used as nucleophiles, the precipitation of polymers was observed, which usually results in a low conversion. Despite precipitation, high conversion was still obtained for the reaction of 1-methylimidazole with polyhalohydrocarbon. Optimizing reaction conditions (such as solvents and reaction temperature) would be possible to avoid such problem. It is easy to remove excess small molecules by precipitation in methanol in most of the above experiments. Furthermore, dialysis47 or precipitation fractionation45 is possible for isolation of functional polymers, for example, when m-PEG or polybutadienyllithium was used for the preparation of branched polymers in this work.

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4. CONCLUSIONS First, we demonstrated a novel and quite convenient chemical modification for 1,2-PBD by hydrozirconation and halogenolysis under mild reaction conditions. Highly halogenated 1,2PBD (up to 92% for iodination, 77% for bromination, and 50% for chlorination) was prepared with low PDI (