Second-Order Nonlinear Optical Polymer by Directed Lithiation of Polyethersulfone
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HAROLD E. McCARRON, CHRISTOPHER J. WALSH, RAJASHREE SEN and BRAJA K. MANDAL* Department of Biological, Chemical and Physical Sciences, Illinois Institute of Technology, Chicago, IL 60616
ABSTRACT Directed lithiation of a polyethersulfone and the reactions of the lithiated species have been investigated. This method provides a new route to second-order nonlinear optical polymers. INTRODUCTION Polymeric materials with nonlinear optical (NLO) activity have been prepared by several techniques: doping the polymer with an NLO dye, copolymerization of monomers that contain NLO pendant groups and covalent attachment of NLO dyes to polymers.l4 Herein, we report, for the first time, a method to prepare NLO polymers by directed lithiation of polyethersulfone (PES). PES is a tough, strong engineering thermoplastic with excellent strength, flexibility, and creep resistance (Figure 1).5 The aryl protons ortho to the sulfone group of PES are somewhat acidic and can be metalated by using a strong base such as butyllithium.6'7 These positions are
highly favored because the lithiated species forms a five membered ring intermediate with the ortho site and the oxygen of the sulfone group.
CHa
O
Udel®Polyethersulfone
Figure 1. The structure of polyethersulfone. In this study, the resulting lithiated polymer reacted with a variety of electrophiles to produce novel PES derivatives. The rapidity and completeness of the substitution at the lithiated sites encouraged an attempt to add NLO functionality by this technique. Particularly attractive was the possibility of adding the N-(4-nitrophenyl)-(S)-prolinoxy moiety, a smaller sized donor-rtacceptor NLO chromophore. 807 Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
EXPERIMENTAL 1-Bromooctane, ethylene oxide and n-butyl lithium (BuLi) were purchased from Aldrich and used without further purification. Tetrahydrofuran (THF) from Aldrich was distilled from sodium and benzophenone. PES (Aldrich) was purified by dissolving in THF, precipitating from methanol and drying overnight at 70°C. Methanol (histological grade) was purchased from Fisher. Argon (prepurified grade) was supplied by Linox. Carbon dioxide was obtained from dry ice by passing the evolved gas through a 14x3/4 inch tube filled with calcium chloride. N-(4-nitrophenyl)-(S)-prolinoxy tosylate (NLO-Ts)
8
This compound was prepared by a minor modification of a previously reported method.8 Into a single-necked 200 mL round bottomed flask fitted with a stirring bar was weighed 2 g (50 mmoles) of N-(4-nitrophenyl)-(S)-prolinol and 50 mL of pyridine was added. The solution was cooled in an ice-water bath and 3.43 g of p-toluenesulfonyl chloride was added in four equal portions over the course of 45 minutes. The reaction mixture was poured into water, stoppered and refrigerated overnight. The crystals were removed by filtration and redissolved in ethyl acetate for chromatography through an alumina column, eluted with ethyl acetate:hexane::4:1. After reduction of solvent in vacuo the product was crystallized f
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