научная статья по теме POLYMER MICELLES FROM TADPOLE-SHAPED AMPHIPHILIC BLOCK-GRAFT COPOLYMERS PREPARED BY “GRAFTING-THROUGH” ATRP Физика

Текст научной статьи на тему «POLYMER MICELLES FROM TADPOLE-SHAPED AMPHIPHILIC BLOCK-GRAFT COPOLYMERS PREPARED BY “GRAFTING-THROUGH” ATRP»

ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, 2009, том 51, № 11, с. 1947-1954

УДК 541.64:542.952:539.2

POLYMER MICELLES FROM TADPOLE-SHAPED AMPHIPHILIC BLOCK-GRAFT COPOLYMERS PREPARED BY "GRAFTING-THROUGH" ATRP1

© 2009 г. Shigeki Ohnoa, Alper Nese4, Brian Cusick4, Tomasz Kowalewski4, and Krzysztof Matyjaszewski4

a Present address: Kaneka Corporation, 5-1-1 Torikai-nishi, Settsu, Osaka, Japan b Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, USA

e-mail: km3b@andrew.cmu.edu

Abstract—A series of tadpole-shaped block-graft amphiphilic copolymers, i.e., block copolymers consisting of a cylindrical hydrophilic brush block and a coiled hydrophobic block were synthesized using "grafting-through" atom transfer radical polymerization. A tadpole-shaped block-graft copolymer from polystyrene bromide and a methacryloyl-terminated poly(tert-butyl acrylate) was prepared first. Then, hydrolysis of the poly(tert-butyl acrylate) side chains to polyacrylic acid side chains provided tadpole-shaped block-graft amphiphilic copolymers, which formed pH responsive micelles in water, the latter being confirmed by dynamic light scattering and atomic force microscopy.

INTRODUCTION

There is a growing interest in (co)polymers with well-defined structures [1, 2]. Copolymers with unique composition and topology, such as block, graft, brush, star, and hyperbranched structure are already used for a variety of applications such as impact-resistant plastics, supersoft elastomers, compatibilizers, surface modifying agents, high aspect ratio nanowires, and ionic conductors [3—19]. Amphiphilic block copolymers have exceptional solution and associative properties [20—24]. Micelle formed from amphiphilic block copolymers in block-selective solvents can be applied as nano-reservoirs for drug delivery and phase transfer catalysis [25]. Tadpole-shaped block-graft co-polymers or gradient copolymers consisting of a cylindrical brush block and a coiled block, on a surface, adopt an extended conformation with a high directional persistence [26, 27]. Such block-graft copolymers can be synthesized by three methods: "grafting-onto" (attachment of side chains to the backbone) [28], "grafting-from" (grafting side chains from the backbone) [29], and "grafting-through" (polymerization of macromonomers) [30, 31].

Controlled/"living"" polymerization [32], and especially atom transfer radical polymerization (ATRP) [4, 33—35], has provided a powerful tool for macro-molecular design. ATRP was previously used to prepare tadpole-shaped copolymers by using grafting-from procedure [26, 27, 36]. Similar structures were

1 Dedicated to the memory of Prof. N.A. Plate, our great friend

and an extraordinary polymer scientist.

also prepared by anionic ring opening polymerization of hexamethylcyclotrisiloxane followed by "grafting-through" ATRP of oligo(ethylene glycol) methyl ether methacrylate macromonomer [37].

In this study, the tadpole-shaped block-graft copol-ymers were prepared by extending polystyrene macro-initiator via "grafting-through" ATRP of methacrylate macromonomers with poly(feri-butyl acrylate) side chains, which were then hydrolyzed to poly(acryl-ic acid) moieties [38, 39]. The behavior of amphiphilic tadpole copolymers with a hydrophobic tail and a hy-drophilic head in aqueous solutions at various pH was studied by dynamic light scattering.

EXPERIMENTAL PART

Materials. tert-Butyl acrylate (BA, 99%, Aldrich) and styrene (99%, Aldrich) were purified by passing the monomers through a column filled with basic alumina to remove the inhibitor. Copper(I) bromide (CuBr, 98%, Acros) was purified via washing with acetic acid followed by filtration, and stored under nitrogen before use. All other reagents: methyl 2-bro-mopropionate (MBP, Aldrich, 98%), ethyl 2-bro-moisobutyrate (EBiB, Aldrich, 98%), N,N,N,N',N'-pentamethyldiethylenetriamine (PMDETA, 99%, Aldrich), copper(II) bromide (CuBr2, 99%, Aldrich), methacrylic acid (Aldrich, 99%), 1,8-diazabicyc-lo[5.4.0]undec-7-ene (DBU, Aldrich, 98%), trifluo-roacetic acid (99%, Aldrich), and solvents were used as received without further purification. Tris[(2-(dime-

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thylamino)ethyl]amine (Me6TREN) was prepared as previously described [40, 41].

Characterization. Monomer conversions were determined by a Shimadzu GC-14A gas chromatograph (GC), equipped with a capillary column (DB-Wax, 30 m x 0.54 MA x 0.5 MA, J&W Scientific), using acetone as an internal standard. The polymer samples were analyzed by GPC equipped with Polymer Standards Services (PSS) columns (guard, 105, 103, and 102 A) and differential refractive index (RI) detector (Waters, 2410) in THF eluent at 35°C, flow rate 1.0ml/min using calibration based on PS standards and WinGPC 6.0 software from PSS. The absolute molecular weights (Mw) were measured by using a triple detector system containing RI detector (Wyatt Technology, Optilab REX), viscometer detector (Wyatt Technology, ViscoStar) and a multi-angle laser light scattering detector (Wyatt Technology, DAWN EOS) with the light wavelength at 690 nm using ASTRA software from Wyatt Technology. 1H-NMR spectra were recorded using a 300 MHz Bruker spectrometer. The CDCl3 singlet at 7.24 ppm and TDF-d8 singlet at 3.57 ppm were selected as the reference standards. Particle size and size distribution were measured by dynamic light scattering (DLS) on a High Performance Particle Sizer, Model HP5001 from Malvern Instruments, Ltd. Atomic force microscopy (AFM) studies were conducted using tapping mode under ambient conditions with the aid of a Nanoscope III (Veeco Metrology Group, Santa Barbara, CA) equipped with phase extender module, "vertical engage" J-scanner; imaging in air was carried out with silicon cantilever with a nominal spring constant of 40 N/m (MikroMasch USA, Wilsonville, OR).

Synthesis of the Methacryloyl-terminated Poly(feri-butyl acrylate) Macromonomer (PBA—MA).

In the first step PBA—Br was synthesized by ATRP in acetone at 50°C with the ratio of reagents [BA]0 : : [MBP]0 : [CuBr]0 : [CuBr2]0 : [PMDETA]0 = 40 : 1 : : 0.4 : 0.02 : 0.42 [43]. BA (100 ml, 0.68 mol), PMDETA (1.5 ml, 7.2 mmol), CuBr2 (76 mg, 0.3 mmol), and acetone (20 ml) were added to a 300 mL Schlenk flask. The resulting mixture was deoxygenated by five freeze-pump-thaw cycles. The reaction flask was filled with nitrogen and then CuBr (0.98 g, 6.8 mmol) was added to the frozen solution. The flask was closed, and then evacuated with vacuum and backfilled with nitrogen three times. The mixture was thawed and then the N2-bubbled initiator MBP (1.9 ml, 17 mmol) was injected into the reaction system via a purged syringe. An initial sample was taken via syringe and then the flask was immersed in an oil bath preheated at 50°C to start the polymerization. Aliquots were withdrawn at different time intervals during the polymerization to monitor conversion by GC. The polymerization was stopped at 53% conversion. The resulting mixture was diluted with THF and filtered through a neutral alumina column to remove the copper catalyst. The sol-

vent and non-reacted monomer were removed under vacuum to yield PBA—Br; GPC: Mn = 3,300 g/mol, Mw/Mn = 1.13, 1H-NMR: the functionality of bromine end groups was 94%.

The PBA-Br (25 g, 7.6 mmol) was dissolved in ethyl acetate (13 mL), and methacrylic acid (6.5 g, 76 mmol) was added under stirring followed by DBU (11 g, 76 mmol). The resulting mixture was stirred for 24 h at room temperature then diluted with ethyl acetate and filtered through a neutral alumina column. The solvents were removed at 80oC under reduced pressure to yield PtBA—MA; GPC: Mn = 3,400 g/mol, Mw/Mn = 1.12, 1H-NMR: the functionality of meth-acryloyl group was 94%.

Synthesis of the Polystyrene Macroinitiator (PS—Br). PS—Br was synthesized by conducting ATRP in toluene at 80°C with the ratio of reagents [styrene]0 : [EBiB]0 : [CuBr]0 : [CuBr2]0 : [PMDETA]0 = 300 : 1 : : 1 : 0.05 : 1.05. St (100 mL, 0.87 mol), PMDETA (0.64 mL, 3.0 mmol), CuBr2 (32 mg, 0.1 mmol), and toluene (10 mL) were added to a 300 mL Schlenk flask. The resulting mixture was deoxygenated by five freeze-pump-thaw cycles. The reaction flask was filled with nitrogen and then CuBr (0.42 g, 2.9 mmol) was added to the frozen solution. The flask was closed, and then evacuated with vacuum and backfilled with nitrogen three times. The mixture was thawed and then the N2-purged initiator EBiB (0.43 mL, 2.9 mmol) was injected into the reaction system via a purged syringe. An initial sample was taken via syringe and then the flask was immersed in an oil bath preheated at 80°C to start the polymerization. Aliquots were withdrawn at different time intervals during the polymerization to monitor conversion by GC. The polymerization was stopped at 58% conversion. The resulting mixture was diluted with THF and filtered through a neutral alumina column to remove the copper catalyst. The solvent and non-reacted monomer were removed under vacuum to yield a PS-Br; GPC: Mn = 18,000 g/mol, Mw/Mn = 1.09.

Synthesis of the PS-b-P(PBA-MA) Block-Graft Copolymer. PS-6-P(PBA—MA) (Scheme 1) block-graft copolymer was synthesized by conducting "grafting-through" ATRP of the PBA—MA macromonomer

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Scheme 1. Structure of PS-£-(PBA-MA).

using the PS—Br as the macroinitiator in anisole at 70°C with the ratio of reagents [PBA-MA]0 : [PS-Br]0 : [CuBr]0 : [Me6TREN]0 = 50 : 1 : 4 : 4. The PBA-MA (2.54 g, 0.77 mmol), PS-Br (0.28 g, 15 mmol), Me6TREN (16 mL, 62 mmol) and anisole (2.5 mL) were added to a 25 mL Schlenk flask. The resulting mixture was deoxygenated by five freeze-pump-thaw cycles. The reaction flask was filled with nitrogen and then CuBr (8.8 mg, 62 mmol) was added to the frozen solution. The flask was sealed, and then evacuated with vacuum and backfilled with nitrogen three times. An initial sample was taken via syringe and the flask was then immersed in an oil bath preheated at 70°C to start the polymerization. Aliquots were withdrawn at

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