4-Acetoxystyrene

PHOST is often prepared by polymerization of 4-acetoxystyrene followed by base-catalyzed hydrolysis (Fig. 29). The acetoxystyrene monomer s stabihty and polymerization kinetics allow production of PHOST of higher quaUty than is easily obtained by direct radical polymerization of HOST. The PHOST homopolymer product is then partially or fully derivatized with an acid-cleavable functionaUty to produce the final resist component. 

Several 4-(3-alkyl-2-isoxazolin-5-yl)phenol derivatives that possess liquid crystal properties have also been obtained (533-535). In particular, target compounds such as 463 (R = pentyl, nonyl) have been prepared by the reaction of 4-acetoxystyrene with the nitrile oxide derived from hexanal oxime, followed by alkaline hydrolysis of the acetate and esterification (535). A homologous series of 3-[4-alkyloxyphenyl]-5-[3,4-methylenedioxybenzyl]-2-isoxazolines, having chiral properties has been synthesized by the reaction of nitrile oxides, from the dehydrogenation of 4-alkyloxybenzaldoximes. These compounds exhibit cholesteric phase or chiral nematic phase (N ), smectic A (S4), and chiral smectic phases (Sc ), some at or just above room temperature (536). 

Either addition sequence works if the two monomers are in the same family (e.g., methyl acrylate and butyl acrylate or methyl methacrylate and butyl methacrylate or styrene and 4-acetoxystyrene), because the equilibrium constants (for activation) for both types of chain ends have about the same value. The situation is usually quite different for pairs of monomers from different families. Chain ends from monomers with large equilibium constants can initiate the polymerization of monomers with lower equilibrium constants thus, cross-propagation is efficient. Methacrylate works well as the first monomer to form methacrylate-acrylate and methacrylate-styrene blocks. 

Authur et al. (1) prepared 4-(2-hydroxyethoxy)-styrene monomers and oligomers, (I), by the base-catalyzed reaction of 4-acetoxystyrene with ethylene oxide as illustrated in Eq. (1). 

A reactor flask was charged with methacrylate ester of 9-anthracene methanol (4.2 g), 4-acetoxystyrene (13.8 g), 2,2 -azobisisobutylonitrile (0.8 g), and propyleneglycol monomethylether and then vented for 15 minutes. The reaction mixture was then heated to 70°C for 5 hours and then cooled to ambient temperature and treated with 26 wt% aqueous tetramethylammonium hydroxide (7 g). The reaction temperature was then raised to 40°C for 3 hours and then further raised to 60°C for 8 hours. The mixture was then recooled to ambient temperature and acidified to a pH of 6 using acetic acid. The polymer was precipitated in 600 ml of methanol and the solid filtered, washed with methanol and deionized water, and dried. The precipitated polymer was redissolved in propyleneglycol monomethylether (60 g) and reprecipitated in 600 ml methanol. The solid was refiltered, rewashed, dried at 40°C, and the product isolated having an Mw of 12,800 Da with an Mn of 5400 Da. 

The required monomer, 4-t-butoxycarbonyloxystyrene, is widely described in the literature. Because 4-hydroxy styrene is difficult to isolate due to its rapid polymerization, BOC-oxystyrene is generally prepared by treatment of 4-acetoxystyrene with strong base thus giving the corresponding phenoxide, immediately followed by addition of (B0C)20 in THF solution (Ref. 98). 

Add THF (1 ml) and 4-acetoxystyrene (10 g, 62 mmol). Vigorously stir the reaction mixture at room temperature until the oily layer has completely dissolved in the aqueous part (approximately 1 h). 

A copolymer approach can provide more flexibility to the resist design because all the necessary functions do not have to reside on one component. Today s advanced positive deep UV resists are exclusively based on this concept with 4-hydroxystyrene as one component. However, early copolymer systems and some of the 193-nm resists consisted of lipophilic components only. Incorporation of 4-acetoxystyrene to poly(4- er -butoxycarbonyloxystyrene sulfone) has already been mentioned. This section deals with copolymer resists composed of lipophilic comonomers first and then the currently dominant hydroxystyrene copolymers. Co- and terpolymers for ArF excimer laser lithography will be described in a separate section. 

A new type of copolymer resist named ESCAP (environmentally stable chemical amplification photoresist) has recently been reported from IBM [163], which is based on a random copolymer of 4-hydroxystyrene with tert-butyl acrylate (TBA) (Fig. 37), which is converted to a copolymer of the hydroxystyrene with acrylic acid through photochemically-induced acid-catalyzed deprotection. The copolymer can be readily synthesized by direct radical copolymerization of 4-hydroxystyrene with tert-butyl acrylate or alternatively by radical copolymerization of 4-acetoxystyrene with the acrylate followed by selective hydrolysis of the acetate group with ammonium hydroxide. The copolymerization behavior as a function of conversion has been simulated for the both systems based on experimentally determined monomer reactivity ratios (Table 1) [164]. In comparison with the above-mentioned partially protected PHOST systems, this copolymer does not undergo thermal deprotection up to 180 °C. Furthermore, as mentioned earlier, the conversion of the terf-butyl ester to carboxylic acid provides an extremely fast dissolution rate in the exposed regions and a large... 

To address these shortcomings the carboxylic acid was replaced with weaker phenol for a better shelf life and dry etch stability and the vinyl ether functionality was separated from the acidic functionality employing a three component design. A water soluble phenolic copolymer was prepared by radical copolymerization of 4-acetoxystyrene and sodium styrenesulfonate, followed by deacetylation with ammonium hydroxide. A water soluble bis(vinyl ether)... 

Cholesteryl 4-vinylphenyl carbonate was synthesized by reacting 4-vinylphenol (obtained by hydrolyzing 4-acetoxystyrene) and cholesteryl chloroformate (commercially available) in dry THF. To the mixture of this carbonate ester (5 mol%) and ethylene glycol dimethacrylate (95 mol%) in hexane (2 mL/g, solvent/monomers), was added AIBN (1 mol% with respect to the C=C bonds). The solution was degassed and sealed at a reduced pressure. The polymerization was carried out at 65 °C for 24 h. The resulting polymer was washed with methanol, ground (average size 30 pm), extracted with methanol in a Soxhlet apparatus for 12-18 h, and dried in vacuo. In order to hydrolyze the carbonate esters, the polymer particles were suspended in 1 M NaOH/ methanol. After the suspension was refluxed for 6 h, it was added to dilute hydrochloric acid and filtered. The polymer was further washed with water, methanol, and ether, extracted with methanol and then with hexane, and finally dried in vacuo [3]. 

Another route to PHOST synthesis, pioneered at Hoechst-Celanese, is the radical polymerization of 4-acetoxystyrene, followed by base hydrolysis in ammonium hydroxide. The 4-acetoxystyrene is readily prepared via Friess rearrangement of phenyl acetate to form 4-hydroxyacetophenone, followed by protection of the OH group with the acetyl group, and followed by reduction to carbi-nol, and finally, dehydration to yield the monomer. " ... 

It has been reported that living radical polymerization of 4-acetoxystyrene with a TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) adduct as the initiator, followed by base hydrolysis produces PHOSTs with narrow polydispersity, 1.1-1.4, which tend to have a 10-20°C higher 7g than their conventional PHOST counterparts (with polydispersity of 2.0-2.4), whose 7g ranges from 140 to 180°C. Hirao et al. have demonstrated the synthetic route to monodisperse PHOST, involving the living anionic polymerization of 4-tert-butyl(dimethyl)siloxystyrene... 

The poly(hydoxystyrene-co-iert-hutyl acrylate) copolymer can he readily prepared hy direct radical copolymerization of 4-hydroxystyrene with tert-hutyl acrylate or alternatively via radical copolymerization of 4-acetoxystyrene with the t-hutyl acrylate, followed hy selective hydrolysis of the acetate group with ammonium hydroxide (see Scheme 7.35). ... 

Acetoxystyrene CO2 cylinder or some solid CO2 (dry ice) in a stoppered Buchner flask as a CO2 source... 

Recently, Lam et al. presented the synthesis of a hypergrafted ( randomly dendronized ) homopolymer. The strategy employed the simultaneous cationic polymerization of 4-acetoxystyrene in combination with the branching transesterification process of 3,5-diacetoxybenzoic add that is reversible under thermal conditions. The mechanism of the reaction and the homogeneity of the products still have to be explored. Fr chet and co-workers prepared a hypergrafted PS and its copolymers via SGVP. A linear backbone bearing latent... 

NMRP has been used for the preparation of diblock copolymers of St, other styrenic derivatives, and various different monomers such as PAcOSt-l -PSt (AcOSt 4-acetoxystyrene), PVBCl-li-PSt (VBCl vinylbenzylchloride), PBA-b-PSt, PSi-b-P2VP, PMMA-li-PSt, and rod-coil block copolymers consisting of p-acetoxystyrene as the coil segment and... 

Materials. 4-Acetoxystyrene (1) (Hoechst Celanese), styrene (2) (Aldrich) and t-butylacrylate (3) (Aldrich) were distilled under reduced pressure prior to use. 1-... 

Random Copolymerization. 4-Acetoxystyrene (1) (15.68 g, 0.097 mol) and styrene (2) (4.32g, 0.042 mol) were placed in a 100 mL round bottom flask and purged with N2.1-Phenyl-l-(2, 2, 6, 6 -tetramethyl-r-piperidinyloxy)ethane (4) (0.52g, 0.002 mol) was then added. After addition of the initiator, the polymerization mixture was heated to 125-130°C, under N2, and stirred for 48 hours. During the polymerization the polymer solidified in the reaction vessel. The reaction was then cooled to room temperature. The polymer dissolved in acetone (100 mL), and isolated by precipitation into hexanes (1000 mL). The poly(4-acetoxystyrene-co-styrene) (5) was then filtered, washed with hexanes and dried in a vacuum oven overnight at 50°C. Isolated yield 92% of theory. M = 9230, = 10330, polydispersity PD = 1.12... 

A similar synthetic procedure was used to prepare the various random copolymers of (a) 4-acetoxystyrene and styrene or (b) styrene and t-butyl acrylate. Deacetylation of Poly(4-acetoxystyrene-co-styrene) (5). To a slurry of poly(4-acetoxystyrene-co-styrene) (70 30) (2) (10.0 g, 0.069 mol) in methanol at reflux (50 mL), under N2, ammonium hydroxide (5.33 g, 0.152 mol) dissolved in water (10 mL) was added dropwise over 15 minutes. After addition, the reaction mixture was heated at reflux for 18 hours, during which time the polymer went into solution. The reaction is then cooled to room temperature, and the polymer isolated by precipitation into water (500 mL), filtered, washed well with water, and dried in a vacuiun oven overnight at 50°C. Isolated yield of poly(4-hydroxystyrene-co-styrene) (6) 85% of theory. M = 7278, = 8297, PD = 1.14. 

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