Halogenoaryl acrylates: preparation, polymerization and optical properties
Introduction
The recent large demand for the construction of optical telecommunication networks has been stimulating intense development in the field of integrated optics. Organic polymeric materials offer a versatile medium for the creation of low-cost optical guided structures [1]. For that purpose, polyacrylates, especially poly(methylmethacrylate) (PMMA), have been widely studied because of their optical clarity, and the availability of a wide range of monomers. However, the major disadvantage of polymers is their lack of transparency in the near-infrared region 1.2–1.6 μm, which has been used increasingly for telecommunication.
When no O–H or N–H bonds are present in the material, intrinsic lightloss is mainly due to absorption by C–H bonds. This loss may be reduced to a more acceptable level by resorting to fluorinated compounds.
Within the framework of the European Commission Program RACE (Project R2010 POPCORN), a low-cost mass fabrication technique for polymeric passive waveguide devices has been developed [2]; the formation of the waveguide structure involves in situ photopolymerization of the core monomer. Optical polymeric materials derived from fluoroalkyl acrylates, methacrylates, and α-fluoroacrylates have been studied widely during the last 20 years 3, 4. Although several alternatives to acrylic polymers were described leading to highly transparent materials: fluorinated polyimides 5, 6 and perfluorinated polyethers like CYTOPTM 7, 8 and TEFLONTM AF 7, 9, these polymers are usually prepared by solution polymerization and are thus not suitable for POPCORN technology which require in situ bulk polymerization.
Recently, we have studied a variety of fluorinated monomers which could be used as core raw materials (preliminary communication [10]). Acrylate derivatives were preferred to methacrylates and α-fluoroacrylates; on one hand methacrylate derivatives should exhibit lower transparency than acrylate ones, owing to their higher hydrogen content, and on the other hand, fluoroacryloyl chloride is not commercially available.
However, one major drawback of polyacrylates is their relatively low glass transition temperature. Therefore, acrylates bearing perhalogenoaryl or perhalogenophenyl-hexafluoroisopropyl residues were prepared. Halogen atoms were either fluorine or chlorine. Using an aryl group as the residue, instead of commonly used alkyl chains, was aimed at increasing both the Tg and the refractive index of the resulting polymers. Actually, the refractive index of the core material in a light-guiding structure must be slightly higher than that of the cladding (or substrate), and polyfluorinated compounds are known to show very low indices [11]: in the polymers we designed, the index may be tuned by modulating the relative amounts of fluorine and chlorine atoms borne by the aryl moiety.
Variously substituted poly(perhalogenoaryl acrylates) were prepared and their optical and thermal properties investigated.
Section snippets
Preparation of the acrylates
Seven substituted acrylates (Scheme. 1) were prepared by acylation of the corresponding phenols with acryloyl chloride. An hindered pyridine (2,6-lutidine) was used as proton acceptor in place of pyridine itself in order to prevent the in situ polymerization of the monomers [10]. The monomers 2a, 3a, 5a, 6a, bearing one or two chlorine atom(s) on the aryl moiety, consist of mixtures of regioisomers (see Section 3for more details).
Polymerization of the acrylates
Polymerization of the acrylates 1a–7a was performed in bulk, in
General
, and spectra were determined on a Bruker AC 300 spectrometer, at 300.1, 75.5, and 282.4 MHz, respectively, using CDCl3 as solvent. Chemical shifts are reported in δ (ppm) from internal TMS ( and ) or from internal CFCl3 ). Mass spectra were measured at the mass spectrometry service of the University of Paris VI.
IR spectra were obtained on a Perkin-Elmer 1420 spectrometer as solutions in CCl4 contained in a sodium chloride cell. UV spectra were recorded on a
Conclusion
In conclusion, the thermal and optical qualities of the poly(perhalogenoarylacrylates) we have studied make them promising candidate materials for the construction of waveguiding devices [15].
Acknowledgements
This work enters partly in the frame of the European TMR programme ERB FMRX-CT97.0120, entitled: “Fluorine As a Unique Tool for Engineering Molecular Properties”.
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