Mechanistic Pathway for the Formation of Radial Polystyrenes Using

Mar 19, 2012 - The M.S. University of Baroda, Vadodara 390 002, India. #. Department of Chemistry and Division of Advanced Materials Science, Pohang ...
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Mechanistic Pathway for the Formation of Radial Polystyrenes Using Diacyl Chloride Santosh Kumar,†,⊥,§ Priyank N. Shah,†,§ Seonyoung Ahn,# Woon Sung Hwang,‡ Ik Kyung Sung,‡ Shailesh R. Shah,∥ C. N. Murthy,⊥ Taihyun Chang,# and Jae-Suk Lee†,* †

Department of Nanobio Materials and Electronics, School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdan-gwagiro (Oryong-dong), Buk-gu, Gwangju, 500-712, Korea ‡ R&D Center, Kolon Industries, Inc., Chemical Organization 294 Gajwa-dong, Seo-gu, Incheon-si 404-815, Korea ⊥ Applied Chemistry Department, Faculty of Technology & Engineering, and ∥Department of Chemistry, Faculty of Science, The M.S. University of Baroda, Vadodara 390 002, India # Department of Chemistry and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang, 790784, Korea ABSTRACT: An efficient and useful synthetic route for the synthesis of linear and star polystyrenes (PS) is described, employing living anionic polymerization to link living polystyryl anions to inexpensive and readily available malonyl chloride and other coupling agents with higher functionality that were generated in situ at room temperature. The polymers prepared in this way were analyzed and characterized by sizeexclusion chromatography (SEC), temperature gradient interaction chromatography (TGIC) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI−TOF MS). It was observed that with the increase in molecular weight of the living PS, the number of arms and weight fraction of higher arm polymers decrease significantly.

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INTRODUCTION With increasing importance of hyperbranched polymers such as dendrimers synthesis and high resolution analysis of these polymers has become pertinent and have attractive much attention in recent years.1−3 Star polymers exhibit unique and interesting properties and also have important applications4−8 that differ from those of the corresponding linear polymers. Recently, we reported the synthesis of three different block copolymers: the di-, tri-, and star-block polymers of polystyrene-b-polyisoprene (SI).9 A simple bifunctional coupling agent, malonyl chloride (MC), for the synthesis of block copolymers while controlling the molecular weights and weight fractions, employing living anionic polymerization, was demonstrated. It was envisaged that this strategy can be extended to the synthesis of polymers with a higher number of arms. With a lead from our previous findings, here in we report the experimental evidence and indepth investigation of the mechanism using state-of-the-art analytical techniques resulting in the realization of our hypothesis through the synthesis of star polystyrenes (PSs). Synthesis of PSs has been achieved by linking living polystyryl anions with various in situ generated coupling agents from (MC) at room temperature. Size-exclusion chromatography (SEC) is the most widely used method for the separation of polymers by their molecular weights.10,11 However, SEC separates the polymers relative to their hydrodynamic chain size, and it has been most successful in the analysis of linear homopolymers, in which a relatively © 2012 American Chemical Society

simple correlation exists between the molecular weight and the hydrodynamic size. The resolution in SEC is not as good for branched polymers as linear polymers because the hydrodynamic chain size does not increase with the molecular weight as significantly as with linear polymers. For a more rigorous characterization of branched polymers, interaction chromatography (IC) is preferred and shows much higher resolution.12−16 Mass spectrometry (MS) is a valuable technique for the analysis and identification of low molecular weight polymers with different end groups. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI−TOF MS) has made the analysis of these low molecular weight polymers possible for a wide variety of polymers that have rarely been directly analyzed by mass spectrometric methods.17−20 This soft ionization method has become the most widely used MS technique in polymer analyses because it is applicable to a greater variety of synthetic polymers than any other MS method leading to simple spectra, displaying only singly charged quasi-molecular ions with little or no fragmentation.21−24 The present study on the new star polystyrenes utilizes MALDI−TOF MS for the structure elucidation and end-group analysis of star polystyrenes synthesized by living anionic polymerization. The intricate details revealed by this analysis elucidate the mechanism of Received: January 30, 2012 Revised: February 29, 2012 Published: March 19, 2012 2675

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Table 1. Selected Data for the Weight Fraction, (Wt. Fr.) Molecular Weight (Mn) and Polydispersity Index (Mw/Mn) of Various Armed Polystyrene Using sec-Butyllithium (s-BuLi) as an Initiator and Malonyl Chloride (MC) as a Coupling Agent at Room Temperature wt. fr., %a (Mn/PDI)b sample

1 arm

2 arm

3 arm

4 arm

5 arm

higher, %

C1

37.3 (2.0K/1.01) 42.1 (4.3K/1.02) 44.4 (7.0K/1.03) 47.4 (18.3K/1.02) 52.9 (53.0K/1.01) 60.8 (92.0K/1.05)

21.1 (4.3K/1.02) 21.2 (8.6K/1.05) 20.1 (14.9K/1.03) 20.1 (35.1K/1.01) 19.3 (105K/1.03) 20.7 (180K/1.04)

24.2 (6.4K/1.04) 23.3 (12.6K/1.05) 22.9 (22.9K/1.02) 23.5 (54.1K/1.03) 22.3 (150K/1.02) 16.3 (280K/1.07)

7.1 (8.9K/1.07) 5.9 (17.1K/1.05) 5.1 (30.9K/1.04) 4.4 (74.6K/1.02) 4.0 (217K/1.01) 2.2 −

6.3 (10.5K/1.02) 4.6 (23.4K/1.06) 5.6 (34.5K/1.03) 3.4 − 1.5 − −

4.0 − 2.8 − 1.8 − 1.3 −

C2 C3 C4 C5 C6

− −

a

Weight fractions were calculated using reversed phase temperature gradient interaction chromatography (RP-TGIC). bNumber-average molecular weight, Mn, and the polydispersity index, Mw/Mn was measured by size-exclusion chromatography multiangle laser light scattering (SEC-MALLS) after the fractionation of each samples. Polymerization. Samples C1−C6 were synthesized in a specially fabricated glass apparatus with break-seals under high-vacuum conditions (10−6 Torr). The synthesis of sample C1 (1:0.5 molar ratio of initiator to coupling agent) is described as an example of a typical anionic polymerization. The homopolymerization of styrene (0.790 g, 7.65 mmol) was initiated by the addition of sec-butyl lithium (s-BuLi, 0.024 g, 0.399 mmol) at room temperature using toluene as the polymerization solvent. The color of the reaction mixture turned a deep red-orange that indicated the formation of the living polystyryl anion.13 After allowing this reaction to proceed for 2 h, malonyl chloride (0.028 g, 0.197 mmol). was introduced to the living polystyryl anion and was allowed to react for 1 h. The reaction was terminated by the addition of methanol, and the resulting mixture was poured into a large amount of methanol to ensure complete precipitation. The precipitated polymer was filtered and dried. It was then dissolved in benzene and freeze-dried. A similar procedure was followed for samples C2−C6 using a similar ratio of initiator to coupling agent and varying the concentration of the initiator relative to the monomer. Characterization. RP-TGIC Analysis. For the separation by reverse-phase temperature gradient interaction chromatography (RP-TGIC), a C18 bonded silica column (Nucleosil, 500 Å pore, 250 mm × 2.1 mm i.d., 7-μm particle size) was used. The mobile phase was a mixture of CH2Cl2 and CH3CN (58/42, v/v, Samchun, HPLC grade) with a flow rate of 0.25 mL/min. The column temperature was controlled by circulating water from a bath/circulator (Haake, C25P) through a homemade column jacket. The polymer sample was dissolved in the elution solvent (2 mg/mL), and the injection volume was 100 μL. TGIC chromatograms were recorded with a UV absorption detector (Younglin, UV 730D) at a wavelength of 260 nm. SEC Analysis. For the SEC analysis of the fractionated samples, two mixed bed columns (Polymer Lab. Mixed C, 300 × 8.0 mm i.d.) were used. SEC chromatograms were recorded with multiangle laser light scattering (Wyatt, mini-DAWN TREOS) and a refractive index detector (Shodex, RI-101) using tetrahydrofuran (THF) (Samchun, HPLC grade) as the mobile phase. The polymer samples were dissolved in THF, the injection volume was 100 μL, and the flow rate of the mobile phase was 0.8 mL/min. The column temperature was maintained at 40 °C using a column oven (Eppendorf, TC-50). MALDI−TOF Analysis. For the MALDI−TOF MS measurements, a Bruker Reflex III mass spectrometer was used. The spectrometer was equipped with a nitrogen laser (λ = 337 nm), a pulsed ion extraction, and a reflector. Polymer solutions were prepared in THF at a concentration of 5 mg/mL. The matrix, 1,8-dihydroxy-9(10H)-anthracenone (dithranol, Aldrich, 97%), was dissolved in THF at a concentration of

polymer formation and contribute to a greater understanding of both the polymerization process and the end-functionalization mechanisms. There are a few reports on the synthesis of star-shaped polystyrenes with the help of bifunctional coupling agents such as 4-(chlorodimethylsilyl)styrene (CDMSS)25,26 and divinylbenzene (DVB)27−31 employing living anionic polymerization. Synthesis of CDMSS is not easy, requires several steps, stringent conditions and also results in low yields. The use of DVB as a coupling agent has its own disadvantages.32,33 Its reaction with low-molecular-mass organolithium compounds (e.g., n- and s-BuLi) is extremely difficult to control. Even at nearly equimolar DVB: initiator ratios, large aggregates are formed in the system. The polydispersity (structural heterogeneity) of such systems is quite high. In comparison, our methodology employed here is a highly valuable tool even for the synthesis of a variety of starshaped block copolymers at once as was observed with the living polystyrene-b-polyisoprenyl anion.9 The effect of chain length on the number of arms formed in the star-shaped PS has also been studied. We found that longer chains (with molecular weights ranging from 2.0K to 92.0K) formed fewer arms in the star-shaped polymers, as explained below. In the present communication, we investigated this mechanism for the first time and confirmed our hypothesis regarding the synthesis of star polystyrene by linking living polystyryl anions to the coupling agents produced from a simple bifunctional coupling agent, MC at room temperature.

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EXPERIMENTAL SECTION

Materials. The styrene monomer (Aldrich, 99%) was passed through activated alumina, washed with an aqueous solution of sodium hydroxide and then with distilled water to remove inhibitors, dried over calcium hydride and vacuum distilled under reduced pressure. Malonyl chloride (Aldrich, 97%) was dried over calcium hydride, vacuum distilled, diluted with toluene, and sealed in glass ampules equipped with break-seals. Toluene (Aldrich, 99%) was purified using a standard high-vacuum technique described elsewhere.34,35 secButyllithium (s-BuLi) (Aldrich, 1.6 M in hexane), calcium hydride (Junsei, 95%), n-butyllithium (Aldrich 1.6 M in hexane) and 1,1diphenylethylene (Aldrich, 97%) were used as received without further purification. 2676

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Scheme 1. Mechanism of Formation of Various Armed Star Polystyrene from Malonyl Chloride and in Situ Generated Multifunctional Coupling Agents

Figure 1. Size-exclusion chromatography (SEC) chromatograms of samples C1−C6, showing the effect of increasing molecular weight (MW) of the PS precursor. Note: The molecular weight of the precursor (PS having one arm) is also shown.

(Aldrich, 98%) solution (1 mg/mL) in THF. A 0.5-μL portion of the final solution was deposited onto a sample target plate and allowed to dry in air at room temperature.

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RESULTS AND DISCUSSION The synthesis of the star-shaped polystyrene was achieved through the anion-initiated polymerization of styrene by the addition of MC to the living polystyryl anions. This reaction sequence resulted in a mixture of five or more armed polystyrenes, depending upon the molecular weight of the living polystyryl anion. The living anionic polymerizations were performed using a 1:0.5 molar ratio of initiator to coupling agent and by varying the concentration of the initiator relative to the monomer in samples C1−C6. The experiments resulted in mixtures of star-shaped PS that had a varying number of arms with controlled molecular weights and relatively low PDIs (1.01−1.09) (Table 1) with nearly a 100% yield in each case. The extent of the linking reaction decreases as the molecular weight of the parent polymeric chain increases from C1−C6.

Figure 2. (a) TGIC curves obtained from sample C1, F1−F5 showing its fractionation (dotted lines): column, C18 bonded silica, 100 Å, 250 × 4.6 mm; eluent, CH2Cl2/CH3CN (55/45, v/v); flow rate, 0.5 mL/min. Some minor products (up to 4%) of PS having six or more arms are marked with a circle. (b) Size-exclusion chromatography (SEC) curves obtained from sample C1 and the SEC curves of five temperature gradient interaction chromatography (TGIC) fractions (F1−F5) after the fractionation of sample C1. 20 mg/mL. A 5 μL aliquot of the polymer solution was mixed with 50 μL of the matrix solution and 1.5 μL of a silver trifluoroacetate 2677

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Figure 3. Matrix-assisted laser desorption/ionization time-of flight mass spectrometry (MALDI−TOF MS) spectra of only the F1−F3 fractions separated from sample C1. (a) MALDI−TOF mass spectrum of one arm, F1. Inset: magnification of the m/z 20-mer and 22-mer peaks of the desired product, Bu(C8H8)20 H(Ag+). (b) MALDI−TOF mass spectrum of two-arm fraction F2. Inset: magnification of the m/z 34-mer and 36-mer peaks of the desired product, 2Bu(C8H8)34C3H2O2(Ag+), (c) MALDI−TOF mass spectrum of three-armed fraction F3. Inset: magnification of the m/z 55-mer and 57-mer peaks of the desired product, Bu3(C8H8)55C6H3O4(Ag+).

peak, RP-TGIC elution peaks of C1 were fractionated, as marked with dotted vertical lines in Figure 2a. The weight fractions of each fraction were calculated and listed in Table 1. Figure 2b depicts the individual SEC chromatogram of C1 and its RP-TGIC fractions, F1−F5. The absolute molecular weight can be measured more easily by SEC than RP-TGIC since a single solvent is used in SEC while a mixed solvent is used in the TGIC separation.37 Their individual SEC values (Table 1) indicate that the molecular weights of the each TGIC peak increases 2-, 3-, 4-, and 5-fold indicating that they corresponds to two arm, three arm, four arm and five arm star-shaped polystyrenes. As can be seen from the results included in Table 1, the reactivity of living polystyryl anions remains same in all cases but the number of arms and their weight fractions decrease gradually from C1−C6. It is also depicted in the table that as the molecular weight increases the number of arms decreases which is due to the steric hindrance generated by bulkier polystyryl chains inhibiting the living polystyryl anions to approach for further substitution. Scheme 1 depicts the mechanistic path leading to the concurrent synthesis of various armed star polystyrenes. The proposed mechanism shows the in situ generation of the new coupling agents (MC-1, MC-2, MC-3, etc.) and the results from the novel synthesis of the three-, four-, and five armed

The molecular weights (Mn) of the polymers were determined by SEC-MALLS (Table 1) after separating them with a RPTGIC technique.9,19,20,36 Figure 1 depicts the SEC chromatograms of the six starshaped PSs prepared with PS anions of different molecular weights. The chromatograms clearly show that the samples are complex mixtures of the star-shaped PS with a varying number of arms. To further investigate the constituents in the PS mixtures that we prepared, sample C1 was used as a representative example. The resulting polystyrenes, after being subjected to the RP-TGIC analysis, indicated that the formation of the anion − (MC ) was dominant over the coupling, leading to the threearmed polymer via the trifunctional coupling agent MC-1, which was generated in situ because of the acidity of the methylene hydrogens of MC.9 Figure 2a displays the RP-TGIC chromatograms of C1. These chromatograms show three to five distinct peaks for the homopolystyrene. In addition higher-armed star polymers are clearly observed as minor products (up to 4%) and are marked with a circle in Figure 2a. Under RP-TGIC conditions, the retention of the polymer species increases as its absolute molecular weight increases. Therefore, the elution sequence under these conditions contains 2-, 3-, 4-, and 5-armed star polystyrene. To identify the species that correspond to each elution 2678

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Scheme 2. Structure Elucidation of One (F1), Two (F2), and Three Arm (F3) Fractions from Sample C1

explains the simultaneous formation of all the observed products, and the investigations are well supported by the TGIC and MALDI−TOF analyses discussed hereafter. To further investigate the mechanism (Scheme 1), the MALDI−TOF analysis of each fraction was conducted. The analysis was performed for the first three fractions that were obtained from sample C1. Structural elucidation was also performed for the first three fractions, F1−F3. The latter fractions were not analyzed because the resolution becomes poor for high molecular weight polymers using this technique.38,39 The expanded trace of the mass spectrum of F1 is shown in Figure 3a. The spectrum contains a major distribution A (inset), whose m/z values correspond to the Ag+ adducts of the PS oligomers with a sec-butyl end group [(C4H9)(C8H8)nH(Ag+)]. For example, the 20-mer of this distribution is expected to produce a signal at m/z = 57.12 (C4H9) + 20 × 104.06 (20 × C8H8) + 1 (H) + 106.91 (Ag+) = 2246.2 (Scheme 2a). For the two arm fraction (F2), all the oligomers were also detected as Ag-cationized ions. The expanded trace of the mass

polystyrenes. As shown in step 2, with the introduction of MC into the living polystyryl anions, the living anion abstracts an acidic proton from the active methylene group of MC, forming a new resonance-stabilized anion (MC−) and terminating the living polymer to give PS. At the same time, the coupling agent (MC) undergoes a substitution reaction at both ends to form the linear two-armed PS (step 3). The anion MC− further reacts with another molecule of the coupling agent MC in a nucleophilic substitution reaction that leads to the coupled malonyl chloride (MC-1, step 4). The reaction of the living polystyryl anion with the in situ generated trifunctional coupling agent MC-1 leads to the synthesis of the three-armed polystyrene (Step 5). Furthermore, the living MC− also react with the newly formed MC-1 (or a new anion from MC-1 may react with MC) to give MC-2 as a new coupling agent, which leads to the formation of the four-armed polystyrene. Similarly MC− can also react with MC-2 to form a five-armed coupling agent MC-3 in a similar reaction sequence that gives higher branched polystyrene. The proposed mechanism satisfactorily 2679

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Korea Institute for Advancement of Technology (KIAT) through the Inter-ER Cooperation Projects. T.C. acknowledges the supports from NRF via the NRL (R0A-2007-000-20125-0), SRC (R11-2008-052-03002), and WCU (R31-2008-000-10059-0) programs.

spectrum of F2 is shown in Figure 3b. The spectrum contains a major distribution A (inset), whose m/z values correspond to the Ag+ adducts of the PS oligomers with two sec-butyl end groups [(C4H9)2(C8H8)nC3H2O2(Ag+)]. For example, the 34mer of this distribution is expected to produce a signal at m/z = 2 × 57.12 (C4H9) + 34 × 104.06 (34 × C8H8) + 70 (C3H2O2) + 106.91 (Ag+) = 3829.11 (Scheme 2b). There are two additional unexpected minor products, which result in series B and C. The oligormers of series B and series C appear at m/z values that are 68 and 86 Da greater than those of the A oligomers (Scheme 2b). These minor products must form because of the different termination modes of the four branched MC-2 and three branched MC-1 coupling agents that did not completely react, as shown in Scheme 2b. Figure 3c shows the MALDI−TOF mass spectrum of the three arm fraction (F3) of sample C1. The major product series A (inset) results from the expected termination of the completely reacted coupling agent MC-1 that is generated in situ. (Scheme 2c) viz. Bu3(C8H8)nC6H3O4(Ag+). The 55-mer of this distribution is expected to produce a signal at m/z = 3 × 57.12 (3 × C4H9) + 55 × 104.06 (55 × C8H8) + 140.06 (C6H3O4) + 106.91 (Ag+) = 6140.45. The oligomers of series B appear at m/z values that are 96 Da greater (6236.45) than those of the polymers A and result from a different termination of the halfreacted coupling agent MC-3 (Scheme 2c).

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(1) Roovers, J.; Comanita, B. Adv. Polym. Sci. 1999, 142, 179−228. (2) Inoue, K. Prog. Polym. Sci. 2000, 25, 453−571. (3) Vogtle, F.; Gestermann, S.; Hesse, R.; Schwierz, H.; Windisch, B. Prog. Polym. Sci. 2000, 25, 987−1041. (4) Bywater, S. Adv. Polym. Sci. 1979, 30, 89−116. (5) Roovers, J. Encyclopedia of Polymer Science and Engineering, 2nd ed.; Kroschwitz, J. I., Ed.; Wiley-Interscience: New York, 1985; p 478. (6) Hsieh, H. L.; Quirk, R. P. Anionic Polymerization: Principles and Applications; Marcel Dekker: New York, 1996; p 333. (7) Grest, G. S.; Fetters, L. J.; Huang, J. S.; Richter, D. Adv. Chem. Phys. 1996, 94, 67−163. (8) Hirao, T.; Yoo, H. S.; Ozama, Y.; Abou El-Magd, A.; Sugiyama, K.; Hirao, A. J. Inorg. Organomet. Polym. 2010, 20, 445−456. (9) Kumar, S.; Shah, P. N.; Kang, B. G.; Min, J. K.; Hwang, W. S.; Sung, I. K.; Shah, S. R.; Murthy, C. N.; Ahn, S.; Chang, T.; Lee, J. S. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 2636−2641. (10) Yau, W. W.; Kirkland, J. J.; Bly, D. D. Modern Size-Exclusion Liquid Chromatography, Practice of Gel Permeation and Gel Filtration Chromatography; John Wiley & Sons: New York, 1979. (11) Mori, S.; Barth, H. G. Size Exclusion Chromatography; SpringerVerlag: New York, 1999. (12) Chang, T. Adv. Polym. Sci. 2003, 163, 1−60. (13) Lee, H. C.; Chang, T.; Harville, S.; Mays, J. W. Macromolecules 1998, 31, 690−694. (14) Lee, H. C.; Lee, W.; Chang, T.; Yoon, J. S.; Frater, D. J.; Mays, J. W. Macromolecules 1998, 31, 4114−4119. (15) Perny, S.; Allgaier, J.; Cho, D.; Lee, W.; Chang, T. Macromolecules 2001, 34, 5408−5415. (16) Choi, H.; Im, K.; Chang, T. Macromol. Res 2012, 20, 101−105. (17) Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshida, T. Rapid Commun. Mass Spectrom. 1988, 2, 151−153. (18) Karas, M.; Hillenkamp, F. Anal. Chem. 1988, 60, 2299−2301. (19) Chang, T. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 1591− 1607. (20) Park, S.; Park, I.; Ryu, C. Y.; Chang, T. J. Am. Chem. Soc. 2004, 126, 8906−8907. (21) Pasch, H.; Schrepp, W. MALDI−TOF Mass Spectrometry of Polymers; Springer: Berlin, 2003. (22) Arnould, M. A.; Polce, M. J.; Quirk, R. P.; Wesdemiotis, C. Int. J. Mass Spectrom. 2004, 238, 245−255. (23) Morelle, W.; Michalski, J. C. Curr. Anal. Chem. 2005, 1, 29−57. (24) Hong, J.; Cho, D.; Chang, T.; Shim, W. S.; Lee, D. S. Macromol. Res. 2003, 11, 341−346. (25) Knauss, D. M.; Al-Muallem, H. A.; Huang, T.; Wu, D. T. Macromolecules 2000, 33, 3557−3568. (26) Im, K.; Kim, Y.; Chang, T.; Lee, K.; Choi, N. J. Chromatogr. A 2006, 1103, 235−242. (27) Worsfold, D. J. Macromolecules 1970, 3, 514−517. (28) Kohler, A.; Zilliox, J. G.; Rempp, P.; Polacek, J.; Koessler, I. Eur. Polym. J. 1972, 8, 627−639. (29) Eschwey, H.; Burchard, W. Polymer 1975, 16, 180−184. (30) Iiohles, A.; Polacek, J.; Koessler, I.; Zilliox, J. G.; Rempp, P. Eur Polym. J. 1972, 8, 627−639. (31) Young, R. N.; Fetters, L. J. Macromolecules 1978, 11, 899−904. (32) Vinogradova, L. V. Russ. J. Appl. Chem. 2010, 83, 351−378. (33) Hadjichristidis, N. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 857−871. (34) Hadjichristidis, N.; Iatrou, H.; Pispas, S.; Pitsikalis, M. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3211−3234.

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CONCLUSIONS A novel, inexpensive and readily available coupling agent has been employed in an anionic polymerization at room temperature, leading to the concurrent synthesis of star polystyrenes with a varying numbers of arms. As the molecular weight of the polymeric chains increases, the number of arms decreases. This trend is due to the steric hindrance which inhibits the approach of the living polystyryl anion to the bulky polymeric chains and prevents the formation of multiple arms. This case study demonstrates that MALDI−TOF MS is a sensitive and reliable probe for the detailed analysis of the polymer coupling reactions. This characterization tool can be used for the quantitative determination of the end groups of similarly sized oligomers. The analysis of the products from novel coupling reactions by MALDI−TOF MS provides fast, accurate and detailed insight into the corresponding end group structures. This strategy may prove to be highly valuable in the synthesis of various star-block copolymers by applying the living anionic polymerization technique. Work in this area is in progress.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Telephone: +82 62 970 2306. Fax: +82 62 970 2304. E-mail: [email protected]. Author Contributions §

Both of these authors have contributed equally to the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Program for Integrated Molecular System (PIMS, GIST), World Class University (WCU) (R31-2008000-10026-0) program at GIST. This research was financially supported by the Ministry of Knowledge Economy (MKE), 2680

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(35) Shah, P. N.; Min, J. K.; Kim, H. J.; Park, S. Y.; Lee, J. S. Macromolecules 2011, 44, 7917−7925. (36) Park, S.; Chang, T. Macromolecules 2006, 39, 3466−3468. (37) Lee, W.; Park, S.; Chang, T. Anal. Chem. 2001, 73, 3884−3889. (38) Rader, H. J.; Schrepp, W. Acta Polym. 1998, 49, 272−293. (39) Thomson, B.; Suddaby, K.; Rudin, A.; Lajoie, G. Eur. Polym. J. 1996, 32, 239−256.

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