Novel amphiphilic conetworks by synthesis and

NOTE Novel Amphiphilic Conetworks by Synthesis and Crosslinking of Allyl-Telechelic Block Copolymers RADHAKRISHNAN KARUNAKARAN,* JOSEPH P. KENNEDY Institute of Polymer Science, The University of Akron, Akron, Ohio, 44325-3909

Received 18 March 2008; accepted 19 March 2008 DOI: 10.1002/pola.22746 Published online in Wiley InterScience (www.interscience.wiley.com).

Keywords: amphiphiles; networks; reversible addition fragmentation chain transfer (RAFT)

INTRODUCTION

EXPERIMENTAL

In the course of our investigations toward the synthesis of prepolymers suitable for crosslinking to APCNs, we postulated that amphiphilic triblocks carrying terminal allyl groups could be easily prepared by the RAFT technique,1 a technique we used previously for the preparation of well-defined multiblocks.2 Amphiphilic multiblocks fitted with allyl termini are most desirable intermediates for the preparation of APCNs because the allyl group can be readily crosslinked by various methods (hyrosilation, free radical, coupling). Even more desirable are amphiphilic prepolymers with short blocks of terminal allyl groups because the efficiency and rate of crosslinking would be enhanced in the presence of multiple allyl end groups. This article concerns the synthesis and characterization of the pentablock poly(allyl methacrylate)-b-poly (N,N-dimethyl acrylamide)-b-polydimethylsiloxane-bpoly(N,N-dimethyl acrylamide)-b-poly(allyl methacrylate) (PAMA-b-PDMAAm-b-PDMS-b-PDMAAm-bPAMA) and its crosslinking by hydrosilation to APCNs.

Materials

Paper XXXIII in the series \Amphiphilic Conetworks"; for XXXII see Y. Chen and J. P. Kennedy, Polym. Sci. Part A: Polym. Chem. 2008, 46, 174. *Present address: The Dow Chemical Company, 2301 N. Brazosport Blvd, B-1608, Freeport, TX 77541. Correspondence to: J. P. Kennedy (E-mail: kennedy@ polymer.uakron.edu) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 4254–4257 (2008) C 2008 Wiley Periodicals, Inc. V

4254

Anhydrous dichloromethane and hexyl amine (Aldrich), were used as received. HO CH2 CH2 O CH2 CH2 CH2 O [Si(CH3)2 O]n CH2 CH2 CH2 O CH2 CH2 OH, D5H, and Karstedt’s catalyst were purchased from Gelest and used without purification. The RAFT chain transfer agent (CTA) S-1-dodecyl-S0 (a,a0 -dimethyl-a00 -acetic acid) trithiocarbonate was synthesized by a literature procedure.3 DMAAm and allyl methacrylate (AMA) (Aldrich) were purified by passing the liquids through a column of basic alumina. Azobisisobutyronitrile (AIBN) (Aldrich) was purified by recrystallization from methanol. Synthesis of PAMA-b-PDMAAm-b-PDMS-bPDMAAm-b-PAMA (3) Block copolymer 2 (5 g, 1.05 3 103 mole) was placed in a 100 mL Schlenk tube, and t-butyl alcohol (20 mL), toluene (20 mL), AIBN (0.0118 g, 7.2 3 105 mole) and AMA (0.73 g, 5.8 3 103 mole) were added. The tube was closed, degassed by three freeze-thaw cycles, and placed in an oil bath at 70 8C for 12 h. After cooling to room temperature, the contents of the tube were transferred to a 100-mL beaker, and the volatiles were removed by vacuum at room temperature. Yield ¼ 5.5 g (98%) of a yellow powder. 1H NMR (CDCl3, 300 MHz): d 0.1 (s, 6H, (CH3)2Si), 0.6 (m, SiCH2), 0.8 (t, 3H, (CH2)8 CH3), 1–2 (CH2 CH C

NOTE

Scheme 1.

4255

Strategy for the synthesis of APCN.

(¼ ¼O) N ), 1.4 ( CH2 (CH2)8 CH3), 1.6 (m, Si CH2 CH2 ), 2.8 (b, N (CH3)2), 3.2 (t, 2H, S CH2 (CH2)8 ), 3.5 (t, CH2 O CH2 ), 3.6 (CH2 O CH2 ),4.2 (t, CH2 CH2 O C(¼ ¼O)), 4.4 (d, CH2 CH¼ ¼CH2), 5.1 and 5.3(m, CH2 CH¼ ¼ CH2), 6 (m, CH2 CH¼ ¼CH2).

Removal of the RAFT-CTA Residue from the Block Copolymer (3 ? 4) The 3 (5 g, 1 3 103 mole) was placed in a 100-mL Schlenk tube, dissolved in anhydrous THF (30 mL), Journal of Polymer Science: Part A: Polymer Chemistry DOI 10.1002/pola

and the solution was deaerated by bubbling N2 for 20 min. Hexyl amine (0.1 g, 1 3 103 mole) was added and the mixture was stirred at room temperature for 16 h. The THF was removed (rotovap), and the white residue was extracted with hexanes to remove the RAFT-CTA. Yield ¼ 4.3 g (86%) of a colorless powder. 1 H NMR (CDCl3, 300 MHz): d 0.1 (s, 6H, (CH3)2Si), 0.6 (m, Si CH2), 1–2 (CH2 CH C(¼ ¼O) N ), 1.6 (m, Si CH2 CH2 ), 2.8 (b, N (CH3)2), 3.5 (t, CH2 O CH2 ), 3.6 (CH2 O CH2 ), 4.2 (t, CH2 CH2 O C(¼ ¼O)), 4.4 (d, CH2 CH¼ ¼CH2), 5.1 and 5.3 (m, CH2 CH¼ ¼CH2), 6 (m, CH2 CH¼ ¼CH2).

4256

J. POLYM. SCI. PART A: POLYM. CHEM.: Vol. 46 (2008)

Network Synthesis The synthesis of a representative network was carried out as follows: 4 (0.4g, Mn ¼ 10,000 g/mol, 8.5 3 105 mole), D5H (0.255 g, 8.5 3 104 mole) and THF (1 mL) were placed in a 4-mL vial and shaken well to form a homogeneous solution. Karstedt’s catalyst solution (50 lL) was added, and the charge was poured into rectangular (8 3 8 cm2) cavities of a Teflon mold. Crosslinking was effected by placing the system in a heating oven at 70 8C for 4 h. After crosslinking was deemed complete, the film was removed from the mold cavity by submerging the device in water. The film was extracted by placing in isopropanol and gently shaking for 8 h. Characterization 1

H spectra were acquired by a Varian Unity plus 300 MHz spectrometer by the use of CDCl3 solvent. GPC traces were obtained with a Waters GPC instrument equipped with a series of six Styragel columns (HR 0.5, HR 1, HR 3, HR 4, HR 5, and HR6; Waters) calibrated with narrow molecular weight distribution polystyrene standards, and a refractive index (RI) detector (Optilab, Wyatt Technology). The flow rate was 1 mL of THF/min. Swelling in water (equilibrium water content, EWC) was determined by the method described earlier.2 RESULTS AND DISCUSSION Scheme 1 summarizes the synthetic strategy to the target APCN. The procedure consists of two steps:

1. The synthesis of the allyl telechelic pentablock (4) The key intermediate 4 was obtained in 5 steps. Recently we described the synthesis of the macrochain transfer agent 1 of 5000 g/mol molecular weight and its use for the polymerization of DMAAm to afford 2 having 1:1 wt % of PDMS and PDMAAm.2 Subsequently, we used 2 to effect the RAFT polymerization of 5 mol % AMA, and thus obtained 3 carrying 5 terminal AMA units. Finally the RAFT-CTA was removed by reductive elimination (aminolysis) with hexyl amine to obtain the thiol ended block copolymer 4.4,5 The absence of yellow color of the final product indicated the successful removal of the RAFT-CTA. This reductive elimination method is superior to free radical induced elimination6,7 as the possibility of reaction of allyl groups with free radicals is avoided. The intermediates were characterized by 1H NMR spectroscopy. Triblock copolymer 2 showed the signature resonances of Si(CH3)2 of PDMS and N(CH3)2 of PDMAAm, as well as those associated with the RAFT-CTA. The introduction of terminal allyl methacrylate groups resulted in additional resonances at 4.3 ppm due to CH2 CH¼ ¼CH2 and in the 5–6 ppm

Figure 1. 1H NMR spectrum of sample 4. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.] range due to CH2 CH¼ ¼CH2. The key product 4 did not show any resonances associated with the RAFT-CTA indicating its complete removal. Figure 1 shows the 1H NMR spectrum of 4. The molecular weight distribution of samples 2 and 3 were 1.2–1.3 typical of controlled radical polymerization. The molecular weight as well as molecular weight distribution of the block copolymer remained unchanged after the reductive elimination. 2. Crosslinking by hydrosilation with D5H Orienting experiments were carried out by reacting 4 with a large (10 fold) stoichiometric excess of D5H, and the 1H NMR spectrum of the D5H telechelic product was analyzed. The absence of resonances associated with the allyl termini indicated complete hydrosilation, while new resonances appeared at 0.5, 1.7, and 3.7 ppm associated with the presence of Si CH2, Si CH2 CH2, and Si CH2 CH2 CH2 O groups, respectively. Subsequently, the APCN was obtained by the use of 1.2 stoichiometric excess of D5H in respect to 4. To remove the sol fraction the network was extracted with isopropanol for 24 h at room temperature. The low amount of sol (