Dendritic Block and Dendritic Brush Copolymers through Anionic

Article pubs.acs.org/Macromolecules

Dendritic Block and Dendritic Brush Copolymers through Anionic Macroinimer Approach Chao Xie, Zhenhua Ju, Chao Zhang, Yuliang Yang, and Junpo He* The State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai, 200433, China S Supporting Information *

ABSTRACT: Anionic macroinimer is prepared through selective monoaddition of polyisoprenyllithium (PILi) toward 1,3-bis(1-phenylvinyl)benzene (MDDPE) in THF/cyclohexane mixture. The resulting macroinimer, possessing a living anion and a diphenylethylene (DPE) group at the same end of the PI chain, undergoes self-condensing vinyl copolymerization (SCVCP) with styrene after the solvent has been switched from mixture of THF/cyclohexane into pure cyclohexane. Dendritic block copolymers of styrene and isoprene are obtained, in which PI chains are linked to the branch points. The molecular weights of the dendritic products mainly depend on the ratio of monomer to macroinimer, whereas PI chain length has only slight effects. The PI segments are subsequently epoxidized and grafted with PILi or polystyrenyllithium (PSLi) to give dendritic polymer brushes. Both dendritic block copolymers and dendritic polymer brushes show decreased intrinsic viscosities. Dendritic block copolymer of styrene and isoprene undergoes a specially slow process of self-assembly, from unimolecular micelles and intermediate compound micelles to large compound micelles, in mixed solvent (n-heptane/THF = 19/1, v/v). In addition, conversion of large compound micelles into lamellar structure is observed for the first time in self-assembly of dendritically branched polymers.

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INTRODUCTION Self-condensing vinyl (co)polymerization (SCV(C)P) is a wellestablished method in the synthesis of dendritic addition polymers.1−3 The mechanism of SCVP is a proliferative chain branching process through continuous initiation of an inimer, the initiator monomer possessing a polymerizable moiety (such as double bond) and an initiating site.1,4 Prior to the polymerization, the initiating site in inimer is dormant, yet it is capable of initiating polymerization on demand. Owing to the rapid development of controlled radical polymerization, there have been many papers reporting the syntheses of dendritic or highly branched polymers through SCVP of inimers possessing initiating sites for persistent nitroxyl radical mediated polymerization,5 atom transfer radical polymerization (ATRP),6 reversible addition−fragmentation chain transfer process (RAFT) mediated radical polymerization,7 and metal-catalyzed “living” radical polymerization.8 Monomers containing merely nitroxyl stable radical, such as TEMPO, was used to prepare cleavable highly branched polystyrenes in which the branch point was formed through the trapping of propagating radical by the pendent TEMPO moieties.9 In addition, inimers applicable for group transfer polymerization (GTP),10 ringopening polymerization (ROP),11 and photo iniferter mediated polymerization12 have also been reported previously. In contrast, the synthesis of inimer with a carbanionic initiating site is more challenging because there is no reversible reaction that is able to mask the anionic species to form a dormant state. This is truly the case although a variety of nonlinear complex macromolecular architectures have been made using anionic polymerization.13 Baskaran explored © XXXX American Chemical Society

anionic SCVP using inimers formed in situ in the addition reaction of butyllithium and 1,3-diisopropenyl benzene.14 The activation of the initiating site in the adduct is controlled by the reaction temperature. Knauss prepared hyperbranched polymers using a styrenic monomer bearing a functional group that can couple with living chain in anionic polymerization. The functional monomer was added in a semibatch process, in which dendritically branched products were formed in a convergent way.15 Recently, we have succeeded in the synthesis of dendritic polystyrene using an anionic inimer, i.e., the monoadduct of 1,3-bis(1-phenylvinyl)benzene (MDDPE) and sec-butyllithium (s-BuLi).16 The preparation of the inimer is based on quantitative monoaddition reaction between s-BuLi and MDDPE in polar solvent such as THF.17 Dendritic polystyrene was obtained through copolymerization of styrene and the anionic inimer in cyclohexane.16 Furthermore, welldefined dendrimer-like polystyrenes were synthesized in a continuous way using the anionic inimer as a living branching agent.18 This approach greatly promoted the synthetic efficiency of dendrimer-like polymers with regular structure. For example, the dendrimer-like polystyrene of fifth generation was obtained within 12 h.18 Similar to s-BuLi, anionic living polymers such as PSLi or polyisoprenyllithium (PILi) undergo stoichiometric monoaddition reaction toward MDDPE in polar solvents as well, forming a P−AB* type macroinimer, in which the vinyl group (A) and Received: December 10, 2012 Revised: February 2, 2013

A

dx.doi.org/10.1021/ma3025317 | Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of Dendritic Block Copolymer of Styrene and Isoprene from a Polyisoprene Macroinimer

presence of benzophenone until a deep purple color was attained. These solvents were redistilled from 1,1-diphenylhexyllithium (DPELi, adduct of n-BuLi or s-BuLi and DPE) or PSLi in a vacuum line before use. s-BuLi (J&K, 1.3 M solution in cyclohexane/heptane) and methyllithium (J&K, 1.6 M solution in diethyl ether) were used as received. The concentration of s-BuLi was analyzed by double titration. Methyltriphenylphosphonium bromide (Lancaster, 98%) and 1,3dibenzoylbenzene (Aldrich, 98%) were vacuum-dried at 50 °C overnight. MDDPE was synthesized from 1,3-dibenzoylbenzene via the Wittig reaction of methyltriphenylphosphonium bromide and methyllithium in ether according to the method of Schulz and Höcker25 to yield a clear oil in 50% yield (purity 99% by HPLC). 1H NMR (CDCl3): δ (ppm) = 7.36 (m, 14 H, aromatic), 5.47 (m, 4 H, =CH2). GC−MS: 99%, m/z = 282. Polymerizations. All of the polymerizations were performed in a flask connected to the vacuum line. The glass apparatus was dried by three cycles of flaming/N2-purging/evacuating and rinsed successively with a dilute solution of n-BuLi in cyclohexane and dry cyclohexane before polymerization. All reactions were conducted in nitrogen atmosphere. Synthesis of Dendritic Block Copolymer, Dendri(PS-b-PI). A solution of PILi (Mn,GPC = 4.0 × 103 g/mol, Mw/Mn = 1.06), prepared by anionic polymerization of isoprene (1.17 g, 0.0172 mol) in cyclohexane (15 mL) initiated by s-BuLi (0.39 mmol, 0.3 mL 1.3 mol/ L cyclohexane/heptane solution), was slowly dropped into a solution of MDDPE (0.110 g, 0.39 mmol) in THF (45 mL) with vigorous stirring at ca. −80 °C (liquid nitrogen/acetone bath). The solution immediately became dark red in color, indicating the addition of PILi to MDDPE. The reaction stood for 5h to yield PI end-capped by MDDPE, the macroinimer. Mn,GPC = 4.2 × 103 g/mol, Mw/Mn = 1.06. Mw,MALLS = 3.1 × 103 g/mol. The resulting PI macroinimer solution was vacuum distilled to dryness at ca. 0 °C by cooling the reaction flask with ice/water mixture. Then cyclohexane (50 mL) and styrene (4.06 g, 0.039 mol) were distilled (over MgBu2) into the reaction flask, respectively. Several minutes later, the color of the solution turned from dark brown to orange, indicating the initiation of styrene by PI macroinimer. The reaction stood at 40 °C for 6 h and was terminated using degassed methanol. The product was precipitated in methanol and dried in vacuum at 50 °C for 48 h. Mn,GPC = 6.4 × 104 g/mol, Mw/Mn = 2.39. Mw,MALLS = 27.6 × 104 g/mol. Synthesis of Dendritic Homopolystyrene. The process for the synthesis of dendritic homopolystyrene was similar to the above. PS macroinimer (Mn,GPC = 2.6 × 103 g/mol, Mw/Mn = 1.05, Mw,MALLS =

the initiating site (B*) are at the same end of a polymer (P). This reaction provides new possibility of macromolecular architecture design. In the present work, we report the synthesis of dendritic block copolymers of styrene and isoprene, in which a dangling polyisoprene (PI) chain is attached to branch point. The synthesis is through selfcondensing vinyl copolymerization of styrene and a PI macroinimer (Scheme 1). Macroinimer was first reported by Hazer in the synthesis of polymer networks using a compound that behaves as macroinitiator, macromonomer, and macrocross-linker.19 In the arena of hyperbranched polymers, macroinimer was used to describe a macromonomer possessing an initiating site.20 For example, poly(tert-butyl acrylate)- and polycaprolactone-based macromonomers containing ATRP initiating sites, poly(ethylene oxide)-b-polystyrene (PEO-bPS)-based macromonomer containing alkoxyamine and dithioester moieties, and PMMA-based macromonomer containing dithioester moiety were reported to be efficient in the synthesis of corresponding hyperbranched polymers.21−23 In all of these studies, the vinyl group and the initiator moiety are spaced by a polymer chain, denoted as A−P−B*. The spacer retains between branch points in the resulting product, like that in the system of HyperMacs.24 In the present research, however, the macroinimer is distinct with the position of a vinyl group and an initiating site at the same end of a polymer chain. This facilitates the exact positioning of desired functionalities or a polymer chain at the branch point, leading to new possibility of molecular engineering of dendritic polymers. In the following, we will show synthesis of dendritic polymer brushes from a dendritic polystyrene precursor with PI segments attached to the branch points.

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EXPERIMENTAL SECTION

Materials. Styrene (China National Pharmaceutical, ≥99%) and isoprene (J&K, ≥98%) were stirred with CaH2 overnight, distilled, and stored in nitrogen atmosphere at 4 °C. These monomers were redistilled over di-n-butylmagnesium (MgBu2) (for styrene) (Aldrich, 1.0 M in heptane) and PSLi (for isoprene), respectively, before polymerization. Cyclohexane (Shanghai Feida, 99.5%) and THF (Shanghai Feida, 99.5%) were refluxed over sodium (Na) in the B


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Figure 1. GPC (a), 1H NMR (b, in CDCl3), and MALDI−TOF MS (c, d) results of PI macroinimer prepared from s-BuLi (0.3 mL 1.3 mol/L cyclohexane/heptane solution, 0.39 mmol) initiated anionic polymerization of isoprene (1.72 mL, 1.17 g, 0.0172 mol) in cyclohexane, followed by mixing PILi solution with 1,3-bis(1-phenylethenyl)benzene (0.11 g, 0.39 mmol) in THF. The overall volume ratio of THF to cyclohexane is 3/1. 3.2 × 103 g/mol) was first prepared by monoaddition of PSLi (Mn,GPC = 2.4 × 103 g/mol, Mw/Mn = 1.05) toward MDDPE at ca. −80 °C, and then copolymerized with styrene at 40 °C to form dendritic polymers after solvent switching. Mn,GPC = 7.4 × 104 g/mol, Mw/Mn = 2.53, Mw,MALLS = 35.6 × 104 g/mol. Synthesis of Dendritic Polymer Brushes, Dendri(PS-b-[PI-g-PS]) and Dendri(PS-b-[PI-g-PI]). Toluene (40 mL), Dendri(PS-b-PI) (2.0 g) and formic acid (0.35 g) were mixed in a 100-mL flask. The mixture was warmed to 40 °C, and H2O2 (0.7 mL, 30 wt %) was added dropwise under stir over 20 min. The reaction stood at 40 °C for 50 min. The organic phase was washed with water until the aqueous layer reached pH 7. The polymer solution was precipitated in methanol, and then dried under vacuum for 48 h. Dendri(PS-b-PI-epoxy) was obtained. Mn,GPC = 7.2 × 104 g/mol, Mw/Mn = 2.39, Mw,MALLS = 28.3 × 104 g/mol. Solutions of PSLi (Mn,GPC = 2.3 × 103 g/mol, Mw/Mn = 1.05; Mw,MALLS = 2.9 × 103 g/mol) and PILi (Mn,GPC = 4.0 × 103 g/mol, Mw/Mn = 1.06; Mw,MALLS = 3.0 × 103 g/mol) were used respectively to graft onto dendri(PS-b-PI-epoxy) to prepare dendritic polymer brush. The reaction process and purification were conducted according to a previous literature.26 The purified products were characterized by triple-detection GPC. Dendri(PS-b-[PI-g-PS]): Mn,GPC = 11.6 × 104 g/ mol, Mw/Mn = 1.81, Mw,MALLS = 69.6 × 104 g/mol. Dendri(PS-b-[PI-gPI]): Mn,GPC = 12.3 × 104 g/mol, Mw/Mn = 1.63, Mw,MALLS = 72.8 × 104 g/mol. Morphology of Dendri(PS-b-PI). Dendri(PS-b-PI) (Mn,GPC = 6.4 × 104 g/mol, Mw/Mn = 2.39. Mw,MALLS = 27.6 × 104 g/mol) was first dissolved in THF. n-Heptane was then added dropwise under stirring until the concentration of THF was reduced to 5 vol %. The final concentration of the dendritic copolymer is 1.0 × 10−3 g/mL. During the gradual addition of n-heptane, the solution turned from clear to

cloudy. The solvents (n-heptane and THF) were passed through Millipore 0.2 μm PTFE filters prior to solution preparation. Characterization. 1H NMR and measurements were carried out on a Bruker (500 MHz) NMR instrument, using CDCl3 as the solvent and tetramethylsilane as the interior reference. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI− TOF MS) was performed on Voyager DE-STR (MALDI−TOF MS) instrument equipped with a 337-nm nitrogen laser. Polymer samples, dithranol (matrix) and silver trifluoroacetate were dissolved in THF with the concentrations of 10, 20, and 10 mg/mL, respectively. These solutions were mixed in the volume ratio of matrix/polymer/salt =10/ 2/1. A volume of 0.5 μL of the mixed solution was placed on a copper plate and air-dried at ambient temperature. Mass spectra were acquired in positive reflector mode using an acceleration voltage of 25 kV. External mass calibration was performed using a standard peptide mixture (insulin and thioredoxin). Spectra were obtained by setting the laser power close to the threshold of ionization, and generally 700 pulses were acquired and averaged. GPC analysis was performed through three TSK columns (TSK Gel H-type, pore size 15, 30, and 200 Å), calibrated by narrow polystyrene standards (Mw range: 2.2 × 103 g/mol∼5.15 × 105 g/mol), equipped with three detectors: a DAWN HELEOS (14−154°) (Wyatt multiangle laser light scattering detector, He−Ne 658.0 nm), ViscoStar (Wyatt), and Optilab rEX (Wyatt). THF was used as the eluent at a flow rate of 1.0 mL/min at 35 °C. The refractive index increment, dn/dc, for samples composed of two segments was measured based on the calibration constant of the RI detector and the weight of samples. The detector response was calibrated with polystyrene standards using the known dn/dc = 0.185 mL/g (in THF). Data acquisition was performed using Wyatt Technology WinAstra software. For dendritic homopolystyrene, the dn/dc = 0.185 mL/g (in THF) is used. Dynamic light scattering (DLS) measurements were performed at 25 °C using a Malvern C


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Figure 2. (a) Normalized gel permeation chromatography (GPC) profiles by different detectors: MALLS (solid line), viscometry (dashed line), and RI (dash-dot line). The measured Mw,MALLS, mean-square radius of gyration ⟨S2⟩, and intrinsic viscosity, [η]w, are also plotted against retention time for dendri(PS-b-PI) (sample no. 2 in Table 1). (b) Mark−Houwink plots of products with (red points) and without (blue points) solvent switching process.

g/mol, PDI = 1.05; MALDI−TOF MS gives MMS= 3.2 × 103 g/ mol, PDI = 1.04 (Supporting Information). The efficient monoaddition of PILi or PSLi toward MDDPE is a consequence of polarity of the mixture solvent (THF/ cyclohexane: 3/1, v/v), in which carbanions are generally more separated from the counterions (Li+). The resulting anion of the monoadduct impedes the addition of the second molecule of PILi or PSLi through electrostatic repulsion. In nonpolar solvents the ion pairs are more tightly contact due to the low dielectric constant of the solvent. As a result, the addition products contain both mono and diadducts.27−30 Solvent polarity is a key factor that determines the style of the addition reaction between polymeric anion and MDDPE. Synthesis of Dendritic Block Copolymer, Dendri(PS-bPI), from Macroinimer. Since the addition reaction between PILi and MDDPE is nearly quantitative under stoichiometric condition, the prepared macroinimer is used in situ to copolymerize with styrene to synthesize dendritic block copolymer of styrene and isoprene. However, the solvent has to be switched from a mixture of THF/cyclohexane into cyclohexane through flash distillation. This process guarantees the formation of dendritic polymers through SCVCP.16 Thus, the reaction mixture is distilled to dryness within ca. 15−20 min at ca. 0 °C. Decomposition of the macroinimers is avoided at low temperature. Stanetty et al. reported that n-BuLi shows a half-life time of 1039 min at 0 °C in THF.31 The DPE anion in this study is more stable than n-BuLi due to the steric hindrance and therefore can survive the flash distillation. The dry macroinimer solidifies at the bottom of the flask, and is easily redissolved into pure cyclohexane (distilled-in from another connecting flask) to form homogeneous dark brown solution. Figure 2 shows the GPC profiles of the dendritic product detected by refractive index (RI), viscometry and MALLS detectors. The product shows multimodal distribution, which is typical for the SCVCP due to intermolecular coupling reaction occurring during the polymerization. The heterogeneity in molecular weight within each sample is also implied by the substantial difference between RI and MALLS profiles, the latter being more sensitive to larger molecular weight. The dependence of intrinsic viscosity, [η]w, on molecular weight is obtained using individual dendritic product, regarding GPC as an online fractionation procedure. Mark−Houwink curves of

Autosizer 4700 instrument at a scattering angle of 90°. Transmission electron microscopy (TEM) was performed with a FEI Tecnai G2−20 instrument operated at 200 kV. The solutions were dropped onto carbon-coated copper grids, dried at room temperature before measurement.

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RESULTS AND DISCUSSION The synthetic route is composed of two successive steps, as outlined in Scheme 1. First, PILi reacts with equimolar amount of MDDPE in a mixture of THF and cyclohexane (v/v, 3/1) to afford an anionic macroinimer. Then the solvent is switched from the mixed solvent into pure cyclohexane, followed by styrene polymerization initiated by the anionic macroinimer. Synthesis of Polyisoprene Macroinimer. The addition reaction of PILi with MDDPE is performed by mixing equimolar amount of PILi in cyclohexane and MDDPE in THF at ca. −80 °C. Thus, the reaction medium is THF/ cyclohexane (3/1 v/v). The reaction solution became dark red immediately upon mixing the reactants, indicating the addition of carbanion toward MDDPE. The reaction completes within 5 h and the analytical results are presented in Figure 1. GPC (Figure 1a) shows clear monomodal distribution of PILi before and after addition to MDDPE. No coupling product is observed, indicating that monoaddition indeed takes place exclusively to form macroinimer. 1H NMR spectrum (Figure 1b) shows clearly vinylic protons at 5.40−5.50 ppm, methine proton at 3.8−3.9 ppm and methyl protons of sec-butyl at 0.8− 0.9 ppm. The integrations of these protons give 91% endfunctionality. MALDI−TOF MS spectrum (Figure 1c) of PI macroinimer is also monomodal with narrow distribution. The molecular weight measured by MS correlates well with the expected molecular structure of the macroinimer. For example, the signal at m/z = 2970.5 corresponds to PI macroinimer of 37 repeating units and one MDDPE end unit (Figure 1d). Unfunctionalized PI chains give very small peaks as indicated by one signal at m/z = 2960.6. In addition, the molecular weight measured by GPC is notably larger than that measured by MS because polystyrene standards are used in GPC measurement. The result of MS is more close to the theoretical value (Mn,theo = 3.2 × 103 g/mol). For comparison, PS macroinimer is prepared in a similar way. The molecular weight measured by GPC is Mn,GPC= 2.6 × 103 D


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Table 1. Polymerization Conditions and Characterization Results of Dendritic Products PI macroinimer

products

runa

Mw,MALLS (103 g/mol)

amt (mmol)

γb

Mw,GPC (104 g/mol)

Mw/Mn (GPC)


dn/dcc

[η]w (mL/g)

αd

g′e

1 2 3 4 5 6 7 8 9 10

3.2 3.1 3.1 1.2 6.1 3.1 3.2 3.2 3.1 3.1

0.39 0.39 0.39 0.39 0.39 0.39 0.39 0.39 0.39 0.39

50 100 150 100 100 100 50 100 150 100

4.8 15.3 30.3 14.4 14.3 11.5 6.1 18.7 55.4 20.0

1.61 2.39 2.33 2.49 1.95 1.62 1.97 2.53 3.53 1.54

7.2 27.6 58.1 26.1 24.5 11.3 9.9 35.6 127.1 23.0

0.144 0.167 0.176 0.175 0.158 0.178 0.185 0.185 0.185 0.185

20.6 46.4 96.3 46.7 45.9 60.3 28.8 47.0 94.1 95.1

0.50 0.48 0.48 0.45 0.47 0.60 0.51 0.47 0.42 0.54

− − − − − − 0.52 0.38 0.31 0.98

a

Samples 1−5 and 7−9: products of copolymerization systems with solvent switching. Samples 6 and 10: products of copolymerization systems without solvent switching. bγ is the feed ratio of styrene to macroinimer. cMeasured in THF at 35 °C using Optilab rEX (Wyatt). dα is the Mark− Houwink exponent. eg′ = [η]branched/[η]linear

the dendritic product and linear polystyrene standard are plotted in Figure 2b. The slope of the fitted line is 0.48 for dendritic product, while that for linear PS is 0.70. Moreover, the magnitude of intrinsic viscosity of the former is much smaller than that of the latter under equal molecular weight. It is also interesting to find that the product without solvent switching is more similar to linear PS in both viscosity values and Mark−Houwink plot, despite that the product is a copolymer instead of a homopolymer of styrene. The above results on viscosity−molecular weight relations for the dendritic products may have been complicated by the chemical heterogeneity of the copolymer. Therefore, dendritic structure containing homopolystyrene is also synthesized from a polystyrene macroinimer, using similar solvent-switching procedure. The characterization results by triple-detection GPC are presented in Supporting Information. The product shows multimodal distribution on GPC profiles and it has indeed lower intrinsic viscosity than polystyrene linear counterparts. Mark−Houwink plot shows a slope of 0.47, indicating branched structure of the product. The contraction factor, defined as the ratio of intrinsic viscosity of branched polymers to that of its linear analogues, g′ = [η]branched/[η]linear,32,33 is remarkably less than 1.0, as shown in Table 1. In addition, products from reactions without solvent switching process give very similar viscosity and Mark−Houwink plot to linear polystyrene. These results demonstrate that copolymerization of the macroinimer with vinyl monomer in nonpolar solvent indeed produces dendritic product. Theoretical studies on kinetics of SCVP by Yan et al. indicate that there should be an abrupt increase in molecular weight along with monomer conversion at late stage of the polymerization.4 It is, however, difficult to observe this phenomenon in the present work due to the very slow reaction rate at the later stage. Instead, the molecular weight of the products depends largely on the ratio of monomer to macroinimer, as shown in Figure 3 and Table 1 (nos. 1, 2, and 3). Larger molecular weight is obtained from higher ratio of monomer to macroinimer. This is not only due to longer primary segment itself, which is determined by the ratio of monomer to anionic species, but also due to more efficient branching reaction as a consequence of less steric hindrance. Contrarily, lower ratio of monomer to macroinimer results in more compact molecular conformation because the spacer between branch points is short. This will inevitably lead to higher steric hindrance.

Figure 3. Normalized GPC results of copolymerization reaction mixture of styrene and PI macroinimers at various feed ratios. The polymerizations are conducted in 50 mL of cyclohexane at 40 °C for 5 h.

On the other hand, the molecular weight of the hyperbranched copolymers only slightly depends on the molecular weight of the macroinimer. As shown in Figure 4, PI macroinimers of molecular weight Mw,MALLS = 1200, 3100, and 6100 g/mol (nos. 4, 2, and 5 in Table 1) results in molecular weights of Mw,MALLS = 261000, 276000, and 245000 g/mol, respectively, of the dendritic product. However, the amount of unreacted macroinimer is more significant for larger

Figure 4. Normalized GPC results of the polymerization reaction mixture of styrene and PI macroinimer of different molecular weights at the same feed ratio of styrene to PI macroinimer (100:1). The polymerizations are conducted in 50 mL of cyclohexane at 40 °C for 5 h. E


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Scheme 2. Synthesis of Dendritic Polymer Brush Based on Dendri(PS-b-PI)

Figure 5. 1H NMR spectra of (a) dendri(PS-b-PI) and (b) dendri(PS-b-PI-epoxy).

molecular weight of the macroinimer, which is attributed to the steric hindrance caused by the macroinimer. Synthesis of Dendritic Polymer Brush from Dendri(PS-b-PI). Using the anionic macroinimer, a PI chain has been successfully located at each branch point of the dendritic structure. These PI segments are used as backbones to prepare polymer brushes by grafting onto technique. The grafting reaction is accomplished through epoxidation and subsequent ring-opening addition of polymeric anions (PSLi and PILi), as shown in Scheme 2. For this purpose, the polymerization of isoprene for macroinimer is conducted in cyclohexane to ensure a high content of 1,4-microstructure. As shown by 1H NMR spectrum in Figure 5, the molar percent of 1,4-structure is as high as 91%, calculated from the integration of signals at 5.0−5.2 (main chain vinyl) and 4.6−4.8 ppm (pendent vinyl). The double bond of 1,4-structure is easily epoxidized using performic acid as the oxidation agent, as reported by Gauthier and coworkers.34 After epoxidation, a new peak appears at 2.7 ppm while that of 1,4-structure shows remarkably reduced intensity. The epoxidation efficiency is 46% as estimated from the signal integrations of these protons. The epoxidized dendritic products are subject to further nucleophilic attack by polymeric anions to form dendritic polymer brushes, dendri(PS-b-[PI-g-PS]) or dendri(Ps-b-[PI-gPI]). For example, the former is prepared using an excess amount of PSLi in the presence of LiCl. As shown in Figure 6, the GPC profile of the grafting product shifts clearly to larger molecular weight than that of the precursor. The product is also characterized by triple-detection GPC, and the results are concluded in Table 2. Using the molecular weights measured by light scattering, the average number of side chains per PI block is estimated to be 8−9. The grafting efficiency is ca. ∼45%, indicating steric hindrance in the grafting process.

Figure 6. Normalized gel permeation chromatography (GPC) profiles of dendri(PS-b-PI) (black) and dendri(PS-b-[PI-g-PS]) (red). The latter is purified in order to remove the unreacted PSLi.

Incomplete epoxidation also causes low grafting efficiency. The pendent vinyl groups, e.g., those in 1,2- and 3,4-microstructure, are difficult to be epoxidized in the conditions used in this study.34 Since the product by PSLi grafting, dendri(PS-b-[PI-g-PS]), contains as low as ∼8% PI by weight, the molecular conformation and intrinsic viscosity are directly compared with linear homopolystyrene, neglecting the effect of PI segments. Figure 7 shows the dependence of radius of gyration (Rg) and intrinsic viscosity on polymer molecular weights of Dendri(PS-b-PI) and its grafting product, Dendri(PS-b-[PI-gPS]), in double logarithmic style. The slopes of the plots, or the α exponential of scaling laws, Rg = kMws and [η] = k′Mαw, of the dendritic brushes products are 0.26 and 0.37, respectively, clearly lower than those of the linear product. These values are also lower than those of the dendritic precursor, dendri(Ps-bF


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Table 2. Polymerization Conditions and Characterization Results of Polymer Brushes Prepared from Dendritic Precursor grafts description

molecular properties

samplesa

graft


Mw/Mn (GPC)

Mw,GPC (104 g/mol)

Mw/Mn (GPC)


dn/dcb

[η]w (mL/g)

αc

dendri(PS-b-[PI-gPS]) dendri(Ps-b-[PI-g-PI])

PSLi

2.9

1.05

21.0

1.81

69.6

0.181

47.0

0.37

PILi

3.0

1.06

20.1

1.63

72.8

0.149

63.1

0.33

The grafting products are prepared from Dendri(PS-b-PI) (No. 2 in Table 1). bMeasured in THF at 35 °C using Optilab rEX (Wyatt). cα is the Mark−Houwink exponent. a

Figure 7. Molecular weight dependence of radius of gyration, Rg (a), and intrinsic viscosity (b) for dendri(PS-b-PI) (red), purified dendri(PS-b-[PIg-PS]) (blue), and linear polystyrene standard (black).

PI), which indicate more compact molecular conformation after grafting reaction. Self-Assembly of Dendri(PS-b-PI) in n-Heptane/THF. In recent years, self-assembly of hyperbranched or dendritically branched polymers has received much attention due to some unique characters of the morphology.35,36 Yan and co-workers reported a variety of assembly morphologies ranging from large spherical micelles, mesoscopic vesicles to macroscopic multiwall tubes by varying the structure of hyperbranched poly(3ethyl-3-oxetanemethanol) (HBPO) based amphiphilic copolymers.37−40 They also performed real time observation of vesicle fission and fusion based on the suitable fluidity and stability of the membranes formed by hyperbranched polymers.41 These studies open a new way of cytomimetic chemistry in the research of cellular budding, endocytosis and exocytosis.42 It is known that PS-b-PI block copolymers self-assemble into micelles and vesicles in selective solvents.43−47 The hyperbranched products in the present study are subjected to similar conditions with nonsolvent titration. Thus, copolymer No. 2 in Table 1 is dissolved in THF. n-Heptane is added dropwise to the solution to a volume fraction of 95 vol % and the final concentration of the copolymer is 1.0 × 10−3 g/mL. During the gradual addition of n-heptane, the solution becomes cloudy, indicating the formation of large-sized aggregates. DLS results in Figure 8 show average hydrodynamic radius around RH = 400 nm for the aggregates in n-heptane/THF, while RH of the hyperbranched molecule dissolved in THF is 16 nm. The size distribution of the aggregates is broad after 1 day of the dispersion preparation, but becomes narrower along with the aging period up to 3 months. The peak value of the diameter also slightly reduces during the storage. The evolution of the aggregation morphology in the dispersion was followed within a period of 3 months using TEM observation on copper-grid collected samples. As shown

Figure 8. Normalized hydrodynamic radius (RH) distributions determined by DLS of Dendri(PS-b-PI) in THF (black) and in mixed solvent (n-heptane/THF = 19/1 (v/v)) after 1 day (blue), 1 week (green), and 3 months (red).

in Figure 9, after one day of the preparation of the dispersion, large spheres of irregular shape, with a radius of ca. 500 nm, coexist with small micelles of radius ca. 20 nm and intermediate aggregates with variable sizes (frames in Figure 9a). The samples stored for 7 days shows similar morphology except that the large size aggregates become more compact and have smoother edges. After 3 months, small aggregates disappear almost completely while particles with more compact structures are observed, Figure 9c. Close analysis of the particles reveals that these are aggregates with homogeneous contrast inner part and irregular flower-like corona (Figure 9d). We propose that dendri(PS-b-PI) forms unimolecular micelle in the mixed solvent with a dendritic PS core and PI shell. However, the interaction between PI segment and the solvent is not enough to balance the attractive forces between PS cores. Therefore, aggregation occurs among unimolecular micelles to G


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Figure 9. TEM images of dendri(PS-b-PI) in mixed solvent (n-heptane/THF = 19/1 (v/v)) after 1 day (a), 1 week (b), and 3 months (c). Highmagnification images of two compact particles after 3 months are also presented (d).

separation within confined superparticles has been achieved recently in the fabrication of bicontinuous nanostructure.48

form large compound micelle. Yan and co-workers proposed a multimicelle aggregation (MMA) mechanism for the selfassembly of multiarm copolyether with a dendritic core.37 From the results of the present study, we point out that the aggregation of the unimolecular micelle undergoes three stages. First, collision between two or among several unimolecular micelles forms intermediate aggregates. Second, the intermediate may further assembly into large compound micelle. Meanwhile, the unimolecular micelle may adsorb to the large size micelle, as indicated by the arrows in Figure 9a. Third, the large compound micelles finally develop into lamellar structure as indicated by the homogeneous contrast within the core in Figure 9d (upper). In another observation on the same sample, layered structure is clearly observed, Figure 9d (lower). It is noted that the conversion of compound micelles into vesicles (vesicles themselves are lamellar structure) also takes place during the intermediate stage, as indicated by the arrows in Figure 9b. The conversion of compound micelle into lamellar structure is driven by the microphase separation between PS and PI segments with the confinement of a PI corona. The orientation of PI segment to the solvent forms large area round surface that encapsulates a number of molecules of hyperbranched copolymers in the inner part. Owing to the asymmetric character of the outmost layer, i.e., with a PI-rich convex and a PS-rich concave, PS and PI segments tends to segregate into template layered structure. This process may be very slow due to chain entanglement of the dendritic structure. Microphase

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CONCLUSION We have explored the synthesis of dendritically branched block copolymers (or segmented hyperbranched copolymers) using a novel anionic macroinimer approach. The macroinimer is prepared through highly selective monoaddition of polyisoprenyllithium toward bifunctional 1,1-diphenylethylene derivative, MDDPE, in polar solvents. The resulting monoadduct (e.g., PI macroinimer) undergoes SCVCP with styrene after the solvent has been switched from polar THF/cyclohexane mixture to nonpolar cyclohexane. This P−AB* approach is featured by locally attachment of a pendent PI segment to each branch point in the dendritically structured product. The product is further grafted, by anionic living chain attachment after epoxidation of PI blocks, to make novel segmented polymer brushes interconnected by a dendritic skeleton. The prepared dendritic block copolymer composed of PS and PI segments undergoes unique slow process of selfassembly that may last for months in mixed solvent of nheptane and THF (19/1 v/v). At the initial stage, unimolecular micelles and intermediate and large compound micelles coexist as observed by TEM. Within extended aging period, the unimolecular micelles and the intermediate compound micelles adsorb to the large sized compound micelles. Finally the former two disappear completely, whereas the latter form more compact spheres with flower-like corona surrounding lamellar H


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structure arising from microphase separation in the confined core.

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ASSOCIATED CONTENT

S Supporting Information *

GPC, 1H NMR, and MALDI−TOF MS results, normalized gel permeation chromatography (GPC) profiles, and Mark− Houwink plots. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-21-65643509. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the financial support by National Nature Science Foundation of China (NSFC) (Grant No. 21074024) and the Research Fund for the Doctoral Program of Higher Education of China (20100071110013).

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Dendritic Block and Dendritic Brush Copolymers through Anionic

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