Rapid Modular Synthesis and Processing of Thiol−Ene Functionalized Styrene−Butadiene Block Copolymers

Diblock and triblock copolymers of poly(styrene)-block-poly(1,2-butadiene) (PS/PB) and PS/PB/PS were modified by photochemical thiol−ene chemistry to process selected functional nanopatterned polymers, with reaction completion in 1 h. PB molecular weight (MW) and thiol−ene ratios were systematically varied based on a model monomer, boc-cysteamine, to determine the efficiency of the reaction. The results demonstrate the polydispersity index (PDI) of modified block copolymers significantly increased when low thiol−ene ratios were employed and sometimes induced gelation of the reacted polymers. Using a 10-fold excess of thiol, functionalizations between 60% and 90% were obtained for amines, carboxylic acids, amides, and a pharmaceutical with a pendant thiol. Differential scanning calorimetry showed a 30−60 °C increase in the glass transition temperature of the daughter polymers. Subsequently, these polymers were spin-coated from solvents found suitable to form self-assembled block copolymer films. The microstructure domain spacing for each polymer was consistent with those originating from the parent polymer. This technique described allows for the formation of nanopatterned block copolymer films with controlled chemistries from a single source material. ■ INTRODUCTION The need for polymers with tailored properties has increased recently with applications ranging from optoelectronics to biomedical. Anionic polymerization is the predominant synthetic scheme for producing monodisperse and well-defined polymer architectures. However, the sensitivity of anionic polymerization to polar and acidic groups makes the synthesis of functional polymers difficult. Therefore, the development of facile and effective postpolymerization modification techniques which maintain monodispersity and architectural integrity of the parent polymer would be beneficial. Commodity polymers such as those containing poly(butadiene) or poly(isoprene) are ideal candidates for backbone modifications due to their widespread commercial availability. Sulfur’s reactivity with poly(isoprene) was discovered in the 19th century by Goodyear to enhance the mechanical integrity of natural rubber. More recently, thiol−ene chemistry was explored by several groups to synthesize polymer networks, block copolymers, dendrimers, liquid crystals, end-functional, and backbone-modified polymers. Thiol−ene reactions are directed by the attack of radically initiated ω-functional thiols on unsaturated bonds. Thiol−ene chemistry can be employed for interfacial modifications, including solvent dispersed nanoparticles or thin film surfaces. Low-intensity ultraviolet sources are capable of generating radicals including renewable resources like sunlight. Photochemical and thermal initiation may be used to generate sulfenyl radicals, although photochemical strategies were more effective. Some limitations of thiol−ene chemistry were observed by several research groups, including reduced efficiency and higher side product formation when using low thiol−ene ratios. Compared to low molecular weight compounds, polymeric starting materials exhibited large decreases in reactivity. Previous research confirmed the pendant vinyl groups of poly(1,2-butadiene) (PB) were over 10-fold more reactive to thiols when compared to the backbone double bonds in poly(1,4-butadiene). The modular capability of thiol−ene chemistry was demonstrated for the synthesis of low molecular weight (MW) ionomers without functional group limitations, including alcohols, amines, amino acids, carbohydrates, carboxylic acids, and fluorinated compounds. The thiol− ene reaction on PB is limited by the formation of cyclic groups when intermediate radicals react with adjacent unreacted double bonds, schematically depicted in Figure 1. Possible reaction products are no reaction, thiol addition, and thiol addition followed by cyclic group formation. These studies confirmed that sixmembered rings reduce the thiol addition yield despite full conversion of double bonds. In spite of cyclic formation in the thiol−ene reaction of PB, high yields were reported in the range of 75% modification, with near-quantitative conversion of double bonds. Received: February 13, 2012 Revised: March 18, 2012 Published: March 28, 2012 Article pubs.acs.org/Macromolecules © 2012 American Chemical Society 3161 dx.doi.org/10.1021/ma300304h | Macromolecules 2012, 45, 3161−3167 The low molecular weight polymer systems previously investigated for the thiol−ene reaction cannot be used for coating applications that require thin and bulk film processing. Styrene−butadiene block copolymers synthesized by numerous manufacturers have molecular weights in the range of 100− 150 kDa, whose mechanical properties and solution viscosity are optimal for film applications. Few studies have examined thiol−ene addition on high molecular weight PB copolymers. Passaglia and Donati found significant issues with cross-linking and low functionalization using thermal initiation on styrene− butadiene random copolymers. In order to decrease crosslinking, lower concentrations of initiator were used at the expense of reduced functionalization. More recently, David and Kornfield were able to graft thiol groups to high molecular weight PB using thermal initiation with a controlled range of yields within 2−6 h. These polymers could be further reacted to create functionalities with potential optoelectronic or liquid crystalline properties. In this study, modification of styrene−butadiene block copolymers of varying molecular weight by radical thiol addition is reported using a photochemical strategy that allows reaction completion within 1 h. While alkylation of PB was shown to control the orientation of self-assembled butadiene− ethylene oxide block copolymers, this study seeks to synthesize and process a modular collection of highly functionalized BCP nanostructured films with similar morphologies. Low thiol−ene ratios increased the polydispersity index and in some cases induced gelation of the modified or “daughter” polymers. Increased thiol concentrations allowed for efficient grafting of various functional groups, including amines, acids, amides, and a pharmaceutical with a pendant thiol. Various characterization methods are shown and support the efficiency of the thiol−ene reaction. Additionally, microphase separation of the modified polymers into nanostructured domains from solution film casting is described. This work demonstrates the versatility of the thiol−ene reaction in creating a novel class of patterned block copolymers with tunable chemistry. ■ EXPERIMENTAL SECTION Materials and Methods. Captopril, thioglycolic acid, boc-cysteamine, thiosalicylic acid, 2-(diethylamino)ethanethiol hydrochloride (DAET), anhydrous tetrahydrofuran (THF), phenylbis(2,4,6trimethylbenzoyl)phosphine oxide (BAPO), propylene glycol monomethyl ether acetate, dimethylformamide, and chloroform were all purchased from Sigma-Aldrich (St. Louis, MO) and used as received. Deuterated chloroform and tetrahydrofuran were purchased from Cambridge Isotopes (Andover, MA). EasiVial poly(styrene) standards for gel permeation chromatography (GPC) calibration were purchased from Agilent Technologies (Santa Clara, CA). Various poly(styrene)block-poly(1,2-butadiene) diblock and poly(styrene)-block-poly(1,2-butadiene)-block-poly(styrene) triblock copolymers (PS/PB) were purchased from Polymer Source (Montreal, Canada). The molecular weights and relative molar percentages, as determined by GPC and H nuclear magnetic resonance (NMR) spectroscopy, are summarized in Table 1. Synthesis. PS/PB and PS/PB/PS were modified with various thiol compounds radically initiated using BAPO and UV irradiation. The thiol compounds investigated in this study are summarized in Figure 2. 50 mg of each respective block copolymer (BCP), 25 mg of BAPO photoinitiator, and thiol were preweighed into vials with Teflon septa and purged with nitrogen for 10 min. During the purging process, a minimal amount of anhydrous THF was added via syringe, yielding an ∼5 wt % solution with respect to polymer. Because of the low solubility of captopril and 2-(diethylamino)ethanethiol hydrochloride (DAET) in THF, a minimal amount of chloroform was used. After purging, the vials were placed into an UltraLum photo-cross-linking oven (λ = 365 nm) and irradiated for 60 min to generate sulfenyl radicals. The distance between the UV lamps and the base of the vials was ∼15 cm. After UV irradiation, the polymers were concentrated and precipitated three times in hexane or acetone, followed by redissolution in THF or chloroform each time. Finally, they were dried under vacuum at room temperature until reaching constant mass. It is vital to note that applying heat during vacuum drying often yielded an insoluble product. For determination of the effects of PB molecular weight on the functionalization and/or gelation of styrene−butadiene BCPs, molar ratios of thiol to double bonds were systematically varied from 1:1 to 10:1. The results reported were calculated from H NMR spectra. After recognizing that excess amounts of thiols were required, 10-fold excess of thiol monomers were used to synthesize modified PS/PB block copolymers for calorimetry and thin film analysis. Characterization. Gel permeation chromatography (GPC) confirmed molecular weight distribution and polydispersity of the stock and modified polymers using a Waters 515 HPLC pump, in-line degasser, a Waters 2410 refractive index detector, and PolyPore columns in series. PS/PB block copolymers were dissolved at 2 mg/mL in THF. THF was used as the eluent at a flow rate of 1 mL/min, and the poly(styrene) equivalent molecular weights reported were determined by constructing a multipoint calibration curve using EasiVial standards (Agilent). H NMR spectra were recorded with a Bruker AV-400 highresolution NMR operating at 400 MHz to assess reaction completion. The stock polymers as received and modified polymers were scanned Figure 1. Reaction scheme of the thiol−ene reaction on PS/PB block copolymers. Possible reaction routes include thiol addition, no reaction, and cyclic group formation. Table 1. Summary of the Poly(styrene)-block-poly(1,2butadiene) Diblock and Triblock Copolymers Used in This Study mol wt (kDa) PDI mol % PB