Polyisoprenes cyclization

Fig. 5. Chemistry of cyclized mbbei—bis-a2ide negative acting resist, (a) Preparation of cyclized mbber resin from polyisoprene (b) photochemistry of aromatic bis-a2ide sensiti2ers. The primary photoproduct is a highly reactive nitrene which may combine with molecular oxygen to form oxygenated products, or may react with the resin matrix by addition or insertion to form polymer—polymer linkages.

Cyclized polyisoprene has been used as a photoresist by being sensitized with bisazides(1-3). Recently, H.Harada et al. have reported that a partially cyclized 1,2-polybutadiene showed good properties as a practical photoresist material in reproducing submicron patterns (U ). S.Shimazu et al, have studied the photochemical cleavage of 2,6-di(h -azidobenzal)cyclohexanone in a cyclized polyisoprene rubber matrix, and have reported that the principal photoreaction is the simultaneous cleavage of the both azido groups by absorption of a single photon with a U3% quantum yield(5 ). Their result does not support the biphotonic process in the photolysis of bisazide proposed by A.Reiser et al.(6 ). 

Figure 9. Photochemical transformations of a bisazide sensitizer in negative photoresists based on cyclized polyisoprene.

Negative Resists for Deep UV. There has been considerable effort recently devoted to the design of negative resists for deep-UV application. Iwayanagi and co-workers (37) have reported on the properties of resists composed of cyclized polyisoprene and several bisazides whose absorption maxima lie within the deep-UV region. Since the sensitizers do not absorb in the visible region they are referred to as "white resists" and are claimed to be 60-450 times more sensitive than PMMA. One of these resist is available commercially as... 

Triflic acid has been used to induce cyclization of polyisoprene (xylene, 75°C, 5-30 min) and the various ring moieties (215-217) were identified by 1H... 

Polystyrene-b/ock-polyisoprene-idock-polystyrene (SIS) with different compositions were synthesized and cyclized by the coupling reaction between two chain end groups on the same molecules as depicted in Fig. 14 [62], From... 

The type of alkylaluminum compound has only a secondary influence on structure in the polymerization of diolefins in contrast to its strong effect on the structure of polypropylene. An exception is AlEtCb, which, apparently in connection with its cationogenic character, with j3-TiCl3 induces polymerization to trans-1,4 polybutadiene and, even without transition metal compound, leads to formation of cyclized polyisoprene. Incidentally, this indicates that poly-butadiene is much more stable towards cyclization than polyisoprene. 

There are a few points to be raised regarding this interpretation. To the best of our information, the resist employed consists of cyclized polyisoprene, having some residual double bonds, containing a few percent of 2,6-bis(4-azidobenzylidene) cyclohexanone as the photosensitive crosslinking reagent (55). 

Synthetic polyisoprenes are superior to natural rubber in terms of consistency of properties. The are also freer of contaminants, and are preferred for applications that require lighter color, for personal care items and for derivatization to chlorinated and cyclized rubber products that are used in the adhesives and coatings industries. 

Polyenes (i.e., unsaturated aliphatic polymers) such as polyisoprenes, and polybutadiens may be hydrogenated, halogenated, hydrohalo-genated, cyclized, and epoxidized. 

Several bisazides with absorption maxima between 240 and 290 nm have been examined as DUV sensitizers for cyclized polyisoprene (76). The sensitivity of DUV resist formulated with cyclized polyisoprene and 1 wt % of 3,3 -bisazidophenyl sulfone (structure 3.10) is 75 times higher than that of PMMA. These azide DUV resists suffer from the resolution limit imposed by low contrast and the swelling phenomenon, which is as expected from the preceding discussion. 

Typical resists include cyclized polyisoprene with a photosensitive crosslinking agent (ex bisazide) used in many negative photoresists, novolac resins with diazoquinone sensitizers and imidazole catalysts for positive photoresists, poly(oxystyrenes) with photosensitizers for UV resists, polysilanes for UV and X-ray resists, and polymethacrylates and methacrylate-styrenes for electron-beam resists (Clegg and Collyer, 1991). Also note the more recent use of novolac/diazonaphthoquinone photoresists for mid-UV resists for DRAM memory chips and chemically amplified photoacid-catalysed hydroxystyrene and acrylic resists for deep-UV lithography (Choudhury, 1997). 

In summary, cationic polymerization gives polymers of structure P8a Ziegler-Natta complexes mainly lead to blocks of structure P8b. This study gives further evidence for cyclization reactions of high-molecular-weight polyisoprenes. This point of view has been confirmed by the study of the cyclization of model polyisoprene molecules with two, three, or four monomer units toward the catalysts able to initiate such a reaction (24). 

Acid Catalyzed Rearrangements. An example of a photoresist based on an acid catalyzed rearrangement is the diaryliodonium salt photoinduced cyclization of cis-1,4-polyisoprene shown in Equation 18. This facile cyclization which has been reported previously (11) by non-photochemical processes results in a polycyclic polymer whose physical properties and solubility characteristics are considerably different than the initial polymer. Exploitation of these differences in the exposed and unexposed regions of the polymer film permit their use as either positive or negative tone resists. 

The efficient light-initiated decomposition of azides has been the basis for commercially important photoresist formulations for the semiconductor industry. A common approach is to mix a diazide, such as diazadibenzyIidenecyclohexanone (I), with an unsaturated hydrocarbon polymer. Excitation of the difunctional sensitizer produces highly reactive nitrenes which crosslink the polymer by a variety of paths including insertion into both carbon-carbon double bonds and carbon-hydrogen bonds, and by generation of radicals. The polymer component in the most widely used resists is polyisoprene which has been partially cyclized by reaction with p-toluenesuI fonic acid (3). Other polymers used include polycyclopentadiene and the copolymer of cyclopentadiene and a-methyI styrene (40. 

The second example of a basic reaction is the bisazide bisnitrene crosslinking of cyclized polyisoprene... 

The dependence of the pyrolysis on the sample structure may be used in applying Py-GC to solve various physico-chemical problems. In particular, Py-GC has been used to study the kinetics of cyclization of polyisoprene rubber the results were later corroborated by other methods [268,269]. 

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