Metal‐Free Alkyne Polyhydrothiolation: Synthesis of Functional Poly(vinylenesulfide)s with High Stereoregularity by Regioselective Thioclick Polymerization

A new synthetic route to sulfur‐rich polymers has been developed. The alkyne polyhydrothiolations of 4,4′‐thiodibenzenethiol (1) and arylene dipropiolates (2–5) mediated by amines proceed at room temperature in a regioselective fashion, furnishing sole anti‐Markovnikov products of poly(vinylenesulfide)s (P1/2–P1/5) with high molecular weights (Mw up to 32 300) and high stereoregularities (Z content up to 81.4%) in high yields (up to 98.2%). Polymers P1/2–P1/4 are soluble in common organic solvents. They are optically transparent, allowing almost all visible and IR light to transmit through. Thanks to the high sulfur contents of the polymers, their films show high refractive indices (n = 1.73–1.70) in the wavelength region of 500–1700 nm as well as high Abbé numbers (νD' up to 539) and low optical dispersions (D' down to 0.002) at wavelengths important for telecommunications. Their refractivities can be further enhanced (n up to 2.06) by metal complexation and their films can be crosslinked by UV irradiation, which enables ready fabrication of fluorescent photopatterns.


Introduction
The exploration of effective polymerization reactions for the synthesis of new polymers with novel structures and unique properties is an important area of research in macromolecular science. Addition polymerizations of olefinic monomers have been one of the main routes to synthetic polymers. In the olefinic polymerizations, the monomers are knitted together by carbon-carbon single bonds. Because of electronic saturation of the single bonds, the olefinic or vinyl polymers are electrically inactive and have thus been commonly used as commodity materials in contemporary society.
Polymerizations of acetylenic monomers can generate polymers with p-conjugated carbon-carbon double bonds that are expected to be electronically active. Indeed, polyacetylene has been found to show metallic conductivity upon dopingthis seminal discovery has triggered much research activity in the utilization of alkynes as building blocks to construct functional polymers. [1] As a result of the enthusiastic efforts of polymer chemists, a large number of acetylenic polymers have been synthesized. [2] In almost all the acetylenic polymers, their monomer repeating units are strung together by carbon-carbon linkages. The polymerizations of acetylene monomers through carbon-heteroatom hookups are of great interest because the resultant heteroatom-containing acetylenic polymers may exhibit properties that are difficult, if not impossible, to access by the polymers with pure carbon-based skeletons.
Alkyne hydrothiolation is a reaction that is potentially useful for the synthesis of heteroatom-containing acetylenic polymers. The reaction was first reported by Truce and Simms in the 1950s: admixing alkynes with sodium thiolates resulted in the formation of vinyl sulfides. [3] The extensive studies on alkyne hydrothiolations in the past half century have revealed that the reactions proceed through Markovnikov and anti-Markovnikov addition routes to produce regio-and stereoisomers with branched and linear structures (Scheme 1). [4,5] One recent development in the area is the report by Yorimitsu and coworkers that certain cesium salts initiate hydrothiolations of aryl alkynes with alkyl thiols. [6] The inorganic bases, however, was not applicable to aryl thiols, while organic bases such as triethylamine completely failed to work as catalysts when the reactions were attempted at room temperature.
It is envisioned that alkyne hydrothiolation of alkyne and thiol with two triple bonds (diyne) and mercapto groups (dithiol) may produce a poly(vinylenesulfide) (PVS) with linear and branched conformations. The sulfur-containing polymer may exhibit properties uniquely associated with its high sulfur content, such as high light refractivity and excellent optical transparency, and thus find high-tech applications as a promising photonic material. [7] The PVS may readily react or coordinate with transition metals and the resultant macromolecular complexes may work as recyclable and reusable polymer catalysts [8] and serve as processable precursors to non-oxide ceramics upon pyrolysis. [9] Such possibilities, although attractive, have been virtually unexplored.
Our groups have been working on the development of alkyne polymerization reactions. Employing monoynes, diynes, and triynes as monomers, we have successfully synthesized a large variety of polyacetylenes, polyarylenes, polydiynes, and polytriazoles with linear and hyperbranched structures and regio-and stereoregularities by metathesis, cyclotrimerization, coupling, and click polymerizations. [10] In our previously investigated click polymerization systems, azide-alkyne cycloaddition (or 'azidoclick') reactions were utilized. Thiol-alkene addition is another well-known click reaction and has been used by several research groups to synthesize saturated macromolecules such as dendrimers and star polymers. [11,12] In contrast, thiol-alkyne addition or alkyne hydrothiolation remains unexploited, although it has the potential to be developed into a new ''thioclick'' polymerization technique for the synthesis of sulfur-containing unsaturated polymers.
In this work, we have extended our acetylene-based research to a heteroatom-mediated system and have developed alkyne hydrothiolation into a versatile polymerization technique for the preparation of sulfur-rich acetylenic polymers. In this paper, we show that alkyne polyhydrothiolations of 4,4 0 -thiodibenzenethiol (1) and arylene dipropiolates (2)(3)(4)(5) are mediated by organobases such as secondary amines at room temperature to afford regioregular linear PVSs (P1/2-P1/5) with high molecular weights and high stereoregularities in high yields (Scheme 2). This represents the first example of a metal-free, organobase-mediated, thioclick polymerization reaction. The PVSs are optically transparent and are readily metallized by complexation with cobalt carbonyls to furnish organometallic polymers with high refractive indexes (n ¼ 2.106-1.708) and low chromatic aberrations (D' down to 0.002).

Monomer Preparation
To explore the possibility of utilizing alkyne hydrothiolation to construct sulfur-containing macromolecules, we adopted an A 2 þ B 2 approach and used dithiol 1 and dipropiolates 2-5 as building blocks for the synthesis of linear PVSs. The advantage of this strategy is that the monomers can be synthesized with ease and can be kept for a long time on the shelf under ambient conditions, which circumvents the self-oligomerization problem met by the AB 2 approach.
Arylene dipropiolates 2-5 were prepared by esterifications of their corresponding diols with propiolic acid in the presence of 1,3dicyclohexylcarbodiimide (DCC), 4-(dimethylamino)pyridine (DMAP), and p-toluenesulfonic acid (TsOH). All the monomers were characterized by spectroscopic techniques, from which satisfactory analysis data corresponding to their expected molecular structures were obtained (see Experimental section for details).

Polymer Synthesis
The development of non-metallic and metal-free polymerization reactions has attracted much interest among polymer chemists in recent years because of their remarkable advantages, such as environmental benignity, economic benefit, and ease in polymer purification. [13] The reactions are free of the detrimental effects of metallic ions in the catalyst residues on polymer properties, such as light emission and optical non-linearity. In addition, functional groups that are toxic to transition metals can be used, which helps widen the variety of polymers that can be synthesized by the reactions.
In our previous investigations, we found that bis(aroylacetylene)s and dipropiolates could be polycyclotrimerized in the presence of secondary amines or in refluxing N,N-dimethylformamide (DMF). [10] We thus tried to synthesize PVS from 1 and 3 under metal-free conditions. After stirring in DMF at room temperature for 24 h, a PVS with an M w of 5200 was isolated in a yield of $92% (Supporting Information, Table  S1, run 1). Changing the solvent to tetrahydrofuran (THF), toluene, and dioxane had little effect on the yield of the PVS but decreased its molecular weight. Both yield and M w dropped when the polymerization was preformed in 1,2dichlorobenzene.
In the presence of organobase, the nucleophilicity of the thiol group will be increased, which may enhance the efficiency of its We then tried to determine the optimal monomer concentration [M] in the polymerization of 1 and 3 in the diphenylamine/DMF mixture. In all the runs, an equimolar ratio of 1 and 3 was used.
With an increase in [M], the M w of the polymer is enhanced and reaches its maximum value of $32 000 at [M] ¼ 0.06 M (Fig. 1). Further increasing [M] has an adverse effect on the molecular weight. The polymer yield remains high at all [M]s studied, which demonstrates of the high efficiency of the polymerization reaction.
Temperature strongly influences the polymerization reaction. Both the yield and M w of the PVSs are decreased when the temperature is raised from 23 to 150 8C (Fig. 2). As mentioned above, dipropiolates can undergo polycyclotrimerization in refluxing DMF. [10] Some portion of monomer 3 may selfpolymerize at high temperatures to form low-molecular-weight hyperbranched oligomers that may have been removed during the purification process of the reaction product. The concentration of 3 in the solution is thus lowered, which leads to the formation of polymers with lower molecular weights in lower yields. Figure 3 shows the time course of the alkyne polyhydrothiolation. After 12 h, a polymer with a M w of 25 000 is obtained, which is high enough for practical applications. Prolonging the reaction time to     24 h slightly lowers the yield of the PVS but increases its M w to 33 000. The above systematic studies enable us to polymerize 1 with 2-5 under optimal conditions. Table 2 summarizes the polymerization results. All the polymerizations proceed smoothly, giving P1/2-P1/5 in good yields. Comparing the polymerization results of 1 and 3 obtained under nitrogen and in air, it is clear that oxygen and moisture exert little effect on the alkyne polyhydrothiolation reaction ( Table 2, runs 2 and 3). The oxygen and moisture tolerances greatly help simplify the reaction procedures.

Structural Characterization
To verify that dithiol 1 and dipropiolates 2À5 have indeed been polymerized in an alkyne polyhydrothiolation mechanism, we conducted a model reaction using thiophenol (6) and phenyl propiolate (7) as reactants (Scheme 3). While a thiol can undergo Markovnikov addition to an alkyne to give a branched vinyl sulfide, the reaction can also proceed in an anti-Markovnikov fashion to yield linear adducts with E and Z conformations (cf., Scheme 1). When 6 and 7 are reacted under conditions similar to those given in Table 2, run 2, product 8 is isolated in a high yield (94%). Spectroscopic analysis reveals that 8 is a linear anti-Markovnikov product with a predominant Z conformation (Z : E ¼ 4 : 1). No branched product is obtained at all, which indicates that the reaction proceeds in a regioselective fashion.
To collect direct structural information, we characterized the polymers by spectroscopic methods. Examples of the IR spectra of P1/3 and its monomers 1 and 3 are given in Figure 4. The SÀH stretching vibration of 1 is observed at 2600 cm À1 , while the --CÀH and C --C stretchings of 3 occur at 3266 and 2124 cm À1 , respectively. No such peaks are observed in the spectrum of P1/3. Instead, new peaks associated with C¼C stretching and bending vibrations are observed at 1580 and 940 cm À1 , and reveal that all the mercapto groups and triple bonds of 1 and 3 have been transformed by the polymerization reaction into vinyl sulfide units in P1/3.
The model reaction suggests that the alkyne polyhydrothiolation is regioselective. To prove whether this is the real case, we analyzed the NMR spectra of the polymers. Figure 5 shows the 1 H NMR spectra of P1/3 and its monomers 1 and 3 in chloroformd; for better peak assignment, the spectrum of model compound 8 is also given in the figure. The thiol and acetylene protons of 1 and 3 resonate at d 3.46 and 3.06, respectively, which completely disappear after the monomers have been subjected to alkyne polyhydrothiolation reaction. By comparison with the spectra of monomers 1 and 3 and model compound 8, the resonance peaks in the spectrum of P1/3 can be readily assigned. The resonances of the phenyl protons in P1/3 are located at similar chemical shifts to those in 1 and 3.
New peaks assigned to the proton resonances of the linear vinyl groups newly formed by the alkyne polyhydrothiolation are observed at d 5.86, 6.15, and 7.92. While the first and third peaks stem from the resonances of the PVS segments with an E-conformation, the second one originates from the resonance of the chain segments that comprise Z-isomeric units. Thanks to the large difference in the chemical shifts of the stereoisomers, the www.afm-journal.de www.MaterialsViews.com   E and Z contents of the polymer can be calculated using their integrals and the Z/E ratio is found to be 81.4%. Other polymers are also Z-rich. Evidently, the alkyne polyhydrothiolation produces polymers with high stereoregularity. The 13 C spectrum of P1/3 shows no resonance peaks of the acetylenic carbons of monomer 3 at d 76.6 and 74.3 (Supporting Information, Fig. S1). New peaks that correspond to the resonances of the olefinic carbons are observed at d 164.8, 163.5, 115.0, and 112.8, because of the conversion of the acetylene triple bonds in 3 into the double bonds in P1/3.

Solubility and Stability
Except for P1/5, all other polymers are completely soluble in common organic solvents, such as toluene, dichloromethane, chloroform, THF, and dioxane, and can be readily fabricated into tough solid films by spin-coating and static-casting processes. Among the PVSs, P1/5 has the highest sulfur content (F S > 22 wt %; Table 3). Its intractability may thus be caused by the strong interaction between the sulfur atoms in different polymer chains, which leads to the formation of physical crosslink networks. This hypothesis is further supported by the good solubility of its sulfonylated counterpart of P1/4. The oxidation of the thio group between the phenyl rings reduces the likelihood of the formation of a disulfur bridge between the polymer chains, thus making P1/ 4 soluble in common organic solvents.
All the polymers are thermally stable. As can be seen from Fig. S2 in the Supporting Information, the temperatures for 5% weight loss or the degradation temperatures (T d ) of the polymers are all higher than 320 8C under nitrogen, which is indicative of their strong resistance to thermolysis (Table 3). Except for P1/4, the residues of the polymers after pyrolysis at 600 8C are higher than 30%. The exceptionally low weight residue for P1/4 ($14 wt %) is probably a result of the ready evaporation of the sulfur dioxide gas formed in the pyrolysis process of the sulfonylated polymer at the high temperature.

Metal Complexation
The polymers contain p-electron-rich aromatic rings and vinyl double bonds as well as metal-affinitive or metallophilic thio groups that are expected to form organometallic complexes with cobalt carbonyls. [14] Indeed, when the soluble PVSs (P1/2, P1/3, and P1/4) are admixed with Co 2 (CO) 8 in THF, their solutions are darkened, which is accompanied by gas evolution. The solutions remain homogeneous throughout the whole process of the cobaltcomplexation reaction. However, the isolated macromolecular complexes P1/2(Co)-P1/4(Co) are insoluble, possibly because of the formation of supramolecular aggregates during the product purification processes (precipitation into poor solvent and drying under vacuum).
We used elemental analysis (EA) and energy-dispersion X-ray analysis (EDX) to estimate the compositions of the PVS(Co) complexes. The cobalt contents of P1/2(Co), P1/3(Co), and P1/ 4(Co) determined by EA are 0.09, 6.25, and 1.86%, respectively (Table 4). The severe steric effect of the bulky, non-planar tetraphenylethene (TPE) units in P1/2 may have greatly shielded the vinyl sulfide groups and hampered them from reacting with Co 2 (CO) 8 . On the other hand, the higher M w value of P1/3 together www.MaterialsViews.com www.afm-journal.de  with its less sterically hindered bisphenol-A moieties may have enabled more efficient metal complexations, which leads to the higher cobalt content in its complex. Similar results are obtained from the EDX measurements. The analysis by X-ray photoelectron spectroscopy (XPS) offers information about chemical compositions on the surfaces of PVS(Co) powders. The cobalt contents estimated by the XPS analysis are generally lower than those obtained from the EA and EDX analyses, which suggests that the cobalt species are aggregated in the bulk rather than on the surface. The ceramic materials obtained from the pyrolyses of the PVS(Co) complexes are practically non-magnetic, in sharp contrast to the very high magnetizabilities of the ceramics obtained from the cobalt complexes of the hyperbranched polymers in our previous studies. [9] This is probably because of the formation of cobalt sulfides Co x S y by the pyrolysis of the PSV(Co) complexes, which are known to be magnetically insusceptible. [15]

Optical Transparency
Polymers with high optical clarity are promising candidate materials for advanced photonic applications. [16] The PVSs absorb little light in the visible spectroscopic region and allows almost all the light at wavelengths longer than 400 nm to transmit through (Fig. 6). For example, the light transmittance of P1/3 is already 99% at a wavelength as short as 370 nm. Although the optical transparencies of the polymers are decreased after complexation with cobalt metal, the light transmittances are still higher than 90% beyond 400 nm. For example, the light transmittance of P1/3(Co) at 370 nm is 98%, just 1% lower than that of its parent form (P1/3). The excellent transparencies of the PVSs and PVS(Co)s may be attributed to the existence of two ester groups in every one of their repeat units, which weaken the electronic communications between the neighboring aromatic and vinyl groups, thereby decreasing their absorptivities in the long wavelength region.

Light Refractivity
The PVSs are comprised of polarizable aromatic rings, ester groups, and sulfur atoms and may show high refractive indices. [17] This is indeed the case: as shown in Figure 7a, P1/2 displays high refractivities (n ¼ 1.7300-1.6569) over a wide spectroscopic region (500-1700 nm). Polymers P1/3 and P1/4 display similarly high refractive indexes of 1.7148-1.6611 and 1.7345-1.7003, respectively, in the same spectroscopic region. It is noted that the n value www.afm-journal.de www.MaterialsViews.com   increases with the sulfur content (F S ) in the polymer (cf., Table 3). When the F S value is increased from $13% in P1/2 to $21% in P1/ 4, the n value at 1550 nm is increased from 1.6583 to 1.7004. The refractivity of the PVS can thus be tuned through molecular engineering endeavors. P1/5 has the highest sulfur content among the polymers but unfortunately its refractivity cannot be measured because of its insolubility in common organic solvents.
The n values of P1/2, P1/3, and P1/4 are around 1.7 at 632.8 nm and larger than 1.65 even at 1550 nm (Table 5), which are much higher than those of the commercially important optical plastics [e.g., n $ 1.49 for poly(methyl methacrylate) (PMMA) and n $ 1.59 for poly(ethylene terephthalate) (PET) and polycarbonate (PC)]. The PVSs are thus highly refractive polymers. No or little birefringence is detected, which is indicative of the amorphous nature of their thin solid films.
Interestingly, after metal complexation, the n values of the polymers are further enhanced (up to 2.0593). Inorganic and organometallic materials often show higher refractive indexes than pure organic materials. [17,18] The high refractive indexes of PSV(Co)s are thus likely because of their metallic components. The differences in the refractivity (Dn) between PVSs and PVS(Co)s at the telecommunication important wavelength of 1550 nm can be as large as 0.3322 (Fig. 7b). The Dn value of the P1/2 and P1/2(Co) pair is relatively small (0.0514), which is understandable because of the low cobalt content in P1/2(Co).

Chromatic Dispersion
For a material to be useful for practical applications, its optical aberrations should be small. The Abbé number (v D ) of a material is a measure of the variation or dispersion in its refractive index with wavelength, which is defined as where n D , n F , and n C are the refractive indexes at wavelengths of the Fraunhofer D, F, and C spectroscopic lines of 589.2, 486.1, and 656.3 nm, respectively. A modified Abbé number (n D 0 ) has been proposed to evaluate the application potential of an optical material, using its n values at the non-absorbing wavelengths of 1064, 1319, and 1550 nm. [19] The first two wavelengths are chosen in view of the practical interest of a commercial laser wavelength (Nd:YAG), while the last one is the wavelength of telecommunication importance. The modified Abbé number is defined as where n 1319 , n 1064 , and n 1550 are the refractive indexes at 1319, 1064, and 1550 nm, respectively. The chromatic dispersion (D 0 ) is the constringence of the Abbé number (n D 0 ): The n D and n D 0 values of P1/4 are 26.3 and 643, which correspond to D and D 0 values of 0.038 and 0.002, respectively (Table 5). After metal complexation, the D and D 0 values of P1/ 4(Co) drop to 0.006 and 0.002, respectively, which are lower than those of PC (D ¼ 0.0297) and PMMA (D ¼ 0.0175), [20] and are comparable to our recently synthesized poly(triazole)s by the metal-free azido-click polymerizations (D 0 ¼ 0.006-0.004). [10] Other PVS(Co)s also show small chromatic dispersions. The low spectroscopic aberrations of the polymers, coupled with their high optical transparencies and light refractivities, may enable them to find technological applications as coating materials in advanced optical display systems, such as microlens components for charge-coupled devices and high-performance complementary metal-oxide semiconductor image sensors. [7] 2.9. Photoresponsive Patterning The PVSs contain a large number of photosensitive vinyl and ester groups. The polymers may be readily photo-crosslinked because of the formation of free radicals by UV irradiation and can thus be used as negative photoresist materials for pattern generation. [21] When a thin solid film of P1/2 spin-coated on a silicon wafer is irradiated in air through a copper photomask, the exposed region of the thin film is readily photo-crosslinked and a negative photoresist pattern with clear-cut edges is generated after the development of the irradiated polymer film by 1,2-dichloroethane (Fig. 8).
The TPE moieties in the backbone of P1/2 are well-known luminogenic units that exhibit an unusual phenomenon of aggregation-induced emission (AIE). [22,23] Although light emission of a 'conventional' luminophore is often quenched when its molecules are aggregated in the solid state and fabricated into a thin film, the film of P1/2 is still luminescent, thanks to its unique AIE feature. The photopattern generated by the photolysis of P1/2 emits a yellow light upon illumination with a handheld UV lamp, as evidenced by the luminescence image taken under a fluorescence microscope shown in Figure 8. Clearly, the polymer can be used as a photoresist material for the creation of fluorescent images.
www.MaterialsViews.com www.afm-journal.de [a] Abbreviations: n ¼ refractive index, n D ¼ Abbé number (calculated from equation 1), n D ' ¼ modified Abbé number (calculated from equation 2), D ¼ chromatic dispersion in the visible region, and D ' ¼ chromatic dispersion in the IR region.

Conclusions
In this work, we have developed a thioclick polymerization technique for the synthesis of heteroatom-containing acetylenic polymers. The alkyne polyhydrothiolations of dithiol 1 and dipropiolates 2-5 proceed smoothly in the presence of diphenylamine at room temperature, producing sulfur-rich polymers P1/ 2-P1/5 with high molecular weights in high yields. The organobase-mediated alkyne hydrothiolation reaction is strictly regioselective and furnishes polymers with high stereoregularities. The polymerization reaction is tolerant to functional groups as well as air and moisture. All these advantageous attributes make the alkyne polyhydrothiolation a true click reaction. [11] Polymers P1/2-P1/4 are completely soluble, film-forming, thermally stable, and optically transparent. Their films exhibit high refractive indices and low chromatic aberrations. Their refractivities can be tuned by their sulfur contents and greatly enhanced by complexation with transition-metal compounds such as cobalt carbonyls. The PVSs are photoresponsive and their films can be readily photo-crosslinked to create negative photoresist patterns that emit strong visible light upon photoexcitation. These unique features make the polymers promising photonic materials for high-tech applications.

Experimental
Materials: THF (Labscan), toluene (BDH), and dioxane (Aldrich) were distilled under nitrogen from sodium benzophenone ketyl immediately prior to use. Dichloromethane was distilled under nitrogen over calcium hydride. DMF was stirred with calcium hydride overnight, distilled under reduced pressure, and kept under nitrogen. Other solvents, such as triethylamine and dimethyl sulfoxide (DMSO), were purified using standard procedures. All the chemicals used in this work, such as 4,4 0thiodibenzenethiol (1), were purchased from Aldrich and used as received without further purification. 1,2-Bis(4-hydroxyphenyl)-1,2-diphenylethene was prepared according to our previously published procedures [23b].
Instrumentation: IR spectra were recorded on a Perkin-Elmer 16 PC FTIR spectrophotometer. 1 H and 13 C NMR spectra were measured on Bruker ARX 300 NMR spectrometers using CDCl 3 or DMSO-d 6 as solvents. Light transmission spectra were measured on a Milton Roy Spectronic 3000 array spectrophotometer. MALDI-TOF spectra were recorded on a GCT Premier CAB048 mass spectrometer operating in a chemical ionization (CI) mode with methane as carrier gas. M w , M n , and M w /M n values of the polymers were estimated by a Waters Associates GPC system equipped with refractive index (RI) and UV detectors. THF was used as eluent at a flow rate of 1.0 mL min À1 . A set of monodisperse linear polystyrenes were used as standards for the molecular weight calibration.
EA experiments were conducted on an Elementary Vario EL analyzer. XPS measurements were conducted on a PHI 5600 spectrometer (Physical Electronics), and the core level spectra were measured using a monochromatic Al Ka X-ray source (hn ¼ 1486.6 eV). The analyzer was operated at 23.5 eV pass energy and the analyzed area was 800 mm in diameter. EDX analyses were preformed on a JEOL-6300 SEM system with quantitative elemental mapping and line scan capacities operating at an accelerating voltage of 15 kV. Thermogravimetric analyses (TGA) were carried out under nitrogen on a Perkin-Elmer TGA 7 analyzer at a heating rate of 10 8C min À1 .
Refractive indices of the polymers were determined on a J. A. Woollam variable angle ellipsometry system with a wavelength tunable from 300 to 1700 nm. To fit the acquired C and D curves with the data obtained from the three-layer optical model that consisted of a crystalline silicon substrate, a 2 nm silicon dioxide layer, and a uniform polymer film, the Levenberg-Marquardt regression algorithm was employed. The Cauchy dispersion law was applied to describe the polymer layer from the visible to the IR spectroscopic region.
Photoresist Patterning: Photo-crosslinking reactions of the polymer films were conducted in air at room temperature using a 365 nm light obtained from a Spectroline ENF-280C/F UV lamp as light source at a distance of 1 cm. The incident light intensity was $18.5 mW cm À2 . The film was prepared by spin-coating the polymer solution (10% w/w in 1,2dichloroethane) at 2000 rpm for 1 min on a silicon wafer. The polymer film was dried in a vacuum oven at room temperature overnight. The photoresist patterns were generated by irradiating the polymer films for 3 min through a copper photomask. The films were developed in 1,2dichloroethane for 40 s and dried at room temperature overnight under reduced pressure. The three-dimensional patterns were images on an optical microscope (Olympus B202) using a normal light and a fluorescent optical microscope (Olympus BX41) with a 330-385 wideband UV excitation.