Microwave-Assisted Syntheses in Recyclable Ionic Liquids: Photoresists Based on Renewable Resources

The copoly(2-oxazoline) pNonOx80-stat-pDc=Ox20 can be synthesized from the cationic ring-opening copolymerization of 2-nonyl-2-oxazoline NonOx and 2-dec-9′-enyl-2-oxazoline Dc=Ox in the ionic liquid n-hexyl methylimidazolium tetrafluoroborate under microwave irradiation in 250 g/batch quantities. The polymer precipitates upon cooling, enabling easy recovery of the polymer and the ionic liquid. Both monomers can be obtained from fatty acids from renewable resources. pNonOx80-stat-pDc=Ox20 can be used as polymer in a photoresist (resolution of 1 μm) based on UV-induced thiol–ene reactions.

Photolithography is as tandard procedure for the production of 2.5-dimensional polymer structures. [1][2][3] Apolymer filmisi lluminatedt hrough am ask, subjecting the illuminated areas of the photoresist film to photochemical reactions that change the solubility of the polymer.I nt he case of so-called negative photoresists, the illuminated areas become insoluble while the non-illuminated areas stay soluble and, consequently,c an be dissolvedi nasubsequent development step, yieldingastructured polymer film that reproduces the geometricp attern preset by the mask in negative fashion.
In order to further advance this toolbox of copoly(2-oxazoline)-based photoresists,w eh ave redesigned our synthetic strategym eeting the current demand for more environmentally benign syntheses,a iming at the elimination of volatile organic compounds as reaction medium[ i.e.,u sing ionic liquids (ILs) as substitute] [8][9][10][11] and the employment of reactants from renewable resources (i.e.,fatty acids as resource).
Polymerizations in ILs have previouslyf ocusedo nr adical polymerizations as prominent examples. [12] In the area of poly(2-oxazoline)s, the polymerization of 2-ethyl-2-oxazoline in ILs has been investigated in ad etailed kinetic study. [13] Notably, for the polymerizationo f2 -oxazolines, which are commonly performed under microwavei rradiation nowadays, [14][15][16] ILs offer the additional advantage that, due to their ionic character,t hey are prime absorbers of microwave irradiation, which paves the way for ah ighly energy-efficient process. [17] In this study,t he polymerization of 2-oxazolines in ILs is expandedt ot he preparation of functional materials. Based on earliers tudies on the application of copoly(2-oxazoline)s as photoresist, [4,5] two different 2-oxazoline monomers for the synthesis of the copoly(2-oxazoline) should be chosen:o ne with an unsaturated double bond in its side chain and another with an onfunctionalized side chain. In order to meet the criteria for green synthesis as comprehensively as possible, several considerations were taken into account.A mong them, organic solvents should not be involved in the monomer syntheses. This prerequisite eliminated double-bond-bearing 2-oxazoline monomers with short side chains such as 2-but-3'-enyl-2-oxazoline, the synthesiso fw hich requires vast amounts of halogenateds olvents. [18] 2-Oxazolinem onomers with longer side chains, on the other hand, can be synthesized from the reaction of ethanol aminew ith either the corresponding nitriles [19] or the corresponding carboxylates or carboxylic acids. [20] As the reactioni nvolving nitrilesr equires catalysis by toxic cadmium compounds, the reaction involving carboxylateso rc arboxylic acids (whichi sc atalyzedb yt itaniumc ompounds) was favored.
[a] C. Finally,t he two monomers for the synthesis of the copoly(2oxazoline), namely 2-nonyl-2-oxazoline (NonOx) and 2-dec-9'enyl-2-oxazoline (Dc = Ox), were synthesized from the reaction of decanoic acid (available from renewable resources such as coconut oil) [21] and undec-10-enoic acid (availablef rom renewable resources such as castor oil), [22] respectively,w ith ethanol amine (Scheme 1). Ethanol amine, which is commonly synthesized by the reaction of ethylene oxide and ammonia,i sp roduced in animal tissue as the biogenic amine of serine as well. [23] Thec ondensation reactions were catalyzed by titanium alcoholates and couldb ep erformed without the addition of any solvents, with moderate yields of approx.60% for the purified monomers after distillation from the reactionm ixture. The copolymerization of the monomers was performed as cationic ring-opening polymerization, [14,15] initiated by methyl tosylate (Scheme 2) in an IL under microwavei rradiation. Ar atio NonOx/Dc = Ox = 80:20 was chosen based on previousf indings for poly(2-oxazoline)-based photoresists. [4] n-Hexyl methylimidazolium tetrafluoroborate (HMIM BF 4 ) was chosen as reactionm edium because of its solubility properties:W hile it dissolves the monomersa nd the initiator and is miscible with the molten copoly(2-oxazoline), it does not dissolve the product pNonOx 80 -stat-pDc = Ox 20 ,w hich precipitates upon cooling of the reaction mixture.
The reaction parameters for the microwave-assisted polymerization wereo ptimized in aM onowave 300 reactorw ith magnetic stirring (600 rpm) at as mall scale (reaction mixtures of 10 ge ach). Ar eactiont emperature of 140 8Ca nd irradiation times of 90 min were found to be sufficient for complete monomer conversion.T he large-scale polymerizations (reaction mixtures of 750 g) were performed at theseoptimized parameters in aM asterwave BTR (bench top reactor), equipped with mechanical stirring. After the initial heatingo ft he reaction mixture (lasting 5.5 min with an average poweri nput of 935 W; maximum input power: 1700 W), an average power input of only 170 Ww as sufficient to maintain the targeted reaction temperature for the targeted polymerization time of 90 min.
The polymer precipitated spontaneously from the IL and could be recovered after cooling in crude quality by filtration. As some IL was occluded in the polymer,t he product needed to be furtherp urified by dispersion in distilled water and ultrasonification for 30 min. The polymerr ecovered by this second filtration did not show anyi mpurities according to NMR analysis. The aqueous phasew as combined with the IL from the first filtration.A fter thorough drying at reduced pressure and elevated temperatures, the IL did not show any impurities either and could be reused as reactionm edium. Both, the copoly(2-oxazoline) as well as the IL, were recovered in quantities of 95 %.
The targeted composition of the copoly(2-oxazoline) (ratio NonOx/Dc = Ox = 80:20) was confirmed by 1 HNMR analyses. The weight dispersity of the polymer was determined by size exclusion chromatography to be 1.34 (calculated from M w = 10.8 kDa and M n = 8.1 kDa, according to the PS calibration) (Scheme 2), which is of ar easonable range for this synthetic scale (250 go fp olymer/batch from 750 gr eaction mixtures) and is assumed to originate in part from the efficient stirring provided.
As crosslinking method, the UV-induced thiol-ene reaction was chosen because of the high yields obtainable with this reaction. Hence, for the formulation of the photoresist,i na ddition to the 'ene' component [the copoly(2-oxazoline) pNonOx 80 -stat-pDc = Ox 20 ], also an oligofunctional thiol component, ap hotoinitiator,a nd as olvent needed to be chosen. Because of its high reactivity in thiol-ene reactions, pentaerythritol-tetra-(3-mercaptoproprionate) (4SH) was chosen as thiol.
The photoresist formulations could be appliedo nto various substrates such as FR4, gold-coated FR4, glass, and calcium Scheme1.Representation of the synthesis of the two 2-oxazoline monomers (with atom numbering) from carboxylic acids derived from renewable resources. www.chemsuschem.org fluoride, by either drop-or spincasting.A fter removal of the residual solventa te levated temperatures, smooth nonsticky surfaces were obtained. Hence, masks could be put on the polymer films during illumination in direct contact.
Due to the oligofunctionality of the thiol and the copoly(2oxazoline) and am aintenance of the thiol/ene ratio of 1:1, efficient crosslinking could be achieved in UV illumination times as short as 1min. According to sol-gel analyses of photoresists that were illuminated without am ask, the thiol-ene reaction proceeded to ad egree that no soluble fractionsc ould be detected;I Ra nalyses of illuminated films failed to reveal the signal of the thiol group. The illuminated photoresists could be developed in ethyl lactate;b est results were obtained if the ethyl lactatew as applied at slightly elevated temperatures of 40 8C. The heightp rofile of the polymer film after development ( Figure 1) revealed comparably smooth surfaces (height changes of AE 5nmi naf ilm with ah eight of 100 nm) ando nly weakly pronouncedf lank angles at its edges.
The resolution attainable with the pNonOx 80 -stat-pDc = Ox 20based photoresist was determined on the example of calcium fluorides ubstrates, onto whicht he photoresist formulation was spincast. After illumination through am ask aligner system equipped with aq uartz-chromium mask, the polymer films were developedi ne thyl lactate. Optical microscopy of the developedf ilms (Figure 2) revealed resolutions as high as 1 mm, representing the highest resolution reachable and detectable with the set-up employed. [25] The 2-oxazoline monomers NonOx and Dc = Ox were synthesized from renewabler esources;o nly three types of solvents were required from the synthesis of the monomers to the readily-produced2 .5-dimensional polymer films:t he IL HMIM BF 4 as reactionmediumfor the cationic ring-opening polymerization of the 2-oxazolines( which can be recovered and reused), water for the purification of the crude pNonOx 80 -stat-pDc = Ox 20 ,a nd ethyl lactatef or the formulation and development of the photoresist.O fs pecial additional notice are the monomer syntheses from renewable resources and the low powerc onsumption for the microwave-assisted polymerization and the straightforward recovery of the IL HMIM BF 4 and the copolymer pNonOx 80 -stat-pDc = Ox 20 .H ence, in comparison with previously reportedc opoly(2-oxazoline)-based photoresists, the congenerr eported in here combinesn umerousa pproaches towards an environmentally more benign synthesis and application,w hile not showing any disadvantages in the final material:T he 2.5-dimensional structures of the developed polymer film exhibit smooth surfaces and resolutions of 1 mm, and good adhesion to various substrates.