Rubber cyclization

As natural rubber is a product of nature, its properties are determined by the biochemical pathway by which the polymer is synthesized in the plant. In the case of natural rubber the polymerization process cannot be tailored like that of synthetic rubbers. The only option to modify natural rubber is after it has been harvested from the tree. The important modified forms of natural rubber include hydrogenated natural rubber, chlorinated natural rubber, hydro-halogenated natural rubber, cyclized natural rubber, depolymerised liquid natural rubber, resin modified natural rubber, poly(methyl methacrylate) grafted natural rubber, poly(styrene) grafted natural rubber, and epoxidized natural rubber [33,34]. Thermoplastic natural rubber prepared by blending natural rubber and PP is considered as a physically modified form of natural rubber. 

Efforts to bond rubber to metal without the use of metal plating led to what is believed to be the first research efforts in surface preparation prior to adhesive bonding. Strong and durable bonds of rubber to metal were necessary for rubber shock mounts for automobiles in the late 1920s, but they were limited to proprietary formulations used on specific metals. In 1927 solvent-based thermoplastic rubber cements for metal-to-rubber bonding were prepared from rubber cyclized by treatment with sulfuric or other strong acids. With these rubber cements strong bonds could be made to either vulcanized or unvulcanized rubber. 

Polymers related to natural rubber include ebonite, chlorinated rubber, oxidized rubber, cyclized rubber, and gutta-percha and balata. 

Acyclic C5. The C5 petroleum feed stream consists mainly of isoprene which is used to produce rubber. In a separate stream the linear C5 diolefin, piperylene (trans and cis), is isolated. Piperylene is the primary monomer in what are commonly termed simply C5 resins. Small amounts of other monomers such as isoprene and methyl-2-butene are also present. The latter serves as a chain terminator added to control molecular weight. Polymerization is cationic using Friedel-Crafts chemistry. Because most of the monomers are diolefins, residual backbone unsaturation is present, which can lead to some crosslinking and cyclization. Primarily, however, these are linear acyclic materials. Acyclic C5 resins are sometimes referred to as synthetic polyterpenes , because of their similar polarity. However, the cyclic structures within polyterpenes provide them with better solvency power and thus a broader range of compatibility than acyclic C5s. 

Cyclization is generally very effective in improving the adhesion of TR and SBR to polyurethane adhesives. Rubbers treated with concentrated sulfuric acid yield a cyclized layer on the surface. This layer is quite brittle, and when flexed develops microcracks, which are believed to help in subsequent bonding by favoring the mechanical interlocking of the adhesive with the mbber. 

Chlorination of natural rubber, involving both addition and substitution (with some cyclization), yields a product with improved chemical and corrosion resistance. Chlorination of polyethylene in the presence of sulfur dioxide results in substituting both chloride and sulfonyl chloride groups into the polymer. A commercially useful material is one which contains about 12 chlorides and one sulfonyl chloride per 40-45 repeating units. This extensive substitution converts the polyethylene, a plastic, into an elastomer by destroying crystallinity. 

Schematic representation of the chemistry of the cyclized rubber - lns arylazide negative resists.

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 ). 

The first modern day negative photoresists were developed by the Eastman Kodak Company which utilized cyclized rubbers and cinnamic acid derivatives as photosensitive crosslinking agents (42). The first commercially important photoresist based on this chemistry was known as KPR, which was of a cinnamate ester of polyvinyl alcohol. It was introduced by Kodak in 1954. 

The most familiar negative photoresists are examples of two-component, resist materials. These include Kodak s KTFR, Merck s Selectilux N, Hunt s HNR, etc., all of which consist of a cyclized synthetic rubber matrix resin which is radiation insensitive but forms excellent films. This resin is combined with a bis-arylazide sensitizer. 

Figure 13. Bisarylazide-rubber resists. The matrix resin is cyclized poly(cis-isoprene). The sensitizers are bisarylazides. A typical structure of one commonly employed sensitizer is provided.

Subramaniam, 1988]. Hydrochlorination, usually carried out at about 10°C, proceeds by electrophilic addition to give the Markownikoff product with chlorine on the tertiary carbon (Eq. 9-33) [Golub and Heller, 1964 Tran and Prud homme, 1977]. Some cyclization of the intermediate carbocation (XXVI) also takes place (Sec. 9-7). The product, referred to as rubber hydrochloride, has low permeability to water vapor and is resistant to many aqueous solutions (hut not bases or oxidizing acids). Applications include packaging film laminates with metal foils, paper, and cellulose films, although it has been largely replaced by cheaper packaging materials such as polyethylene. 

Chlorination of natural rubber (NR) is carried out with chlorine in carbon tetrachloride solution at 60-90°C to yield a chlorinated rubber containing about 65% chlorine, which corresponds to 3.5 chlorine atoms per repeat unit. The process is complex and includes chlorine addition to the double bond, substitution at allylic positions, and cyclization. Chlorinated rubber has high moisture resistance and is resistant to most aqueous reagents (including mineral acids and bases). It is used in chemical- and corrosion-resistant paints, printing inks, and textile coatings. Bromination of butyl rubber is also practiced [Parent et al., 2002]. 

Natural rubber and other 1,4-poly-1,3-dienes are cyclized by treatment with strong protonic acids or Lewis acids [Golub, 1969 Subramaniam, 1988]. The reaction involves protonation of the double bond (Eq. 9-40) followed by cyclization via attack of the carhocation on the double bond of an adjacent monomer unit (Eq. 9-41). Some bicyclic and polycyclic... 

Golub, M. A., Cyclized and Isomerized Rubber, Chap. 10A in Polymer Chemistry of Synthetic Elastomers, Part II, J. P. Kennedy and E. G. Tornqvist, eds., Wiley-Interscience, New York, 1969. 

Rubber appears to be a metabolic dead-end because there have been no findings of enzymes capable of breaking down the rubber in latex. The exact termination reaction of the rubber polymerization is not known. Different end-groups have been detected by NMR in rubber purified from a range of species, indicating that molecule dephosphorylation and release may involve esterification, cyclization, or hydrolysis [262]. 

ODUR-110-WR, but no detailed lithographic evaluation has been published. Since the matrix resin is cyclized rubber, one would expect the same swelling limitation on resolution evident in conventional negative photoresists. 

Fig. 5. Chemistry of cyclized rubber—bis-azide negative acting resist, (a) Preparation of cyclized rubber resin from polyisoprene (b) photochemistry of aromatic bis-azide sensitizers. 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.

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cyclized rubber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenolic structure renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxylic acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubility is controlled by chemical and polarity differences rather than molecular size. 

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