Diene based polymers

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

With 1,3-diene based polymers, greater scope for structural variation is introduced because there arc two double bonds to attack and the propagating species is a delocalized radical with several modes of addi tion possible (see 4.3.2). 

There is greater scope for structural variation in the diene based polymers than for the monoene polymers already discussed. The polymers contain units from overall 1,2- and cis- and 1,4-addition. Two mechanisms for overall 1,2-addition may be proposed. These are illustrated in Scheme 4.9 and Scheme 4.10 ... 

Diene-based polymers such as polybutadiene have other structural distinctions. The linear versions of these polymers have one residual double bond for each mer. When the double bonds are in the polymer chain, the cis and trans stereoisomers are possible. The double bonds can appear as pendant vinyl groups, which can then exhibit tacticity. 

Diene polymers refer to polymers synthesized from monomers that contain two carbon-carbon double bonds (i.e., diene monomers). Butadiene and isoprene are typical diene monomers (see Scheme 19.1). Butadiene monomers can link to each other in three ways to produce ds-1,4-polybutadiene, trans-l,4-polybutadi-ene and 1,2-polybutadiene, while isoprene monomers can link to each other in four ways. These dienes are the fundamental monomers which are used to synthesize most synthetic rubbers. Typical diene polymers include polyisoprene, polybutadiene and polychloroprene. Diene-based polymers usually refer to diene polymers as well as to those copolymers of which at least one monomer is a diene. They include various copolymers of diene monomers with other monomers, such as poly(butadiene-styrene) and nitrile butadiene rubbers. Except for natural polyisoprene, which is derived from the sap of the rubber tree, Hevea brasiliensis, all other diene-based polymers are prepared synthetically by polymerization methods. 

Numerous methods have been employed for hydrogenating conjugated diene-based polymers in the presence of suitable and effective hydrogenation catalysts. Typical hydrogenation catalysts can be classified into two types heterogeneous and homogeneous catalysts. 

Homogeneous catalysts, which are soluble in the solutions of the diene-based polymers to be hydrogenated, include ... 

This chapter provides a review of the progress in reaction art, reactor techniques and process technology with respect to homogeneous catalytic hydrogenation of diene-based polymers, in accordance with the homogeneous hydrogenation theme of this handbook. 

Table 19.1 Outline of major patents with respect to catalysts for diene-based polymer hydrogenation. 

RhCl(PPh3)3 has been used for the homogeneous hydrogenation of various diene-based polymers, and its catalytic mechanism is understood to a considerable extent. Parent et al. [81] proposed a mechanism which has been found to be consistent with the kinetic data for various diene-based polymer hydrogenation systems and an understanding of the coordination chemistry of RhCl(PPh3)3 in solution. The main points comprising the mechanism are outlined as follows ... 

Table 19.2 Kinetic parameters for homogeneous hydrogenation of diene-based polymers.

Ruthenium catalysts, such as Ru(CHCH(Ph))Cl(CO)(PCy3)2, have been found to be active for catalyzing the hydrogenation of various diene-based polymers. The catalytic mechanism for the hydrogenation of NBR, SBR and PB has been investigated [68]. 

Guo X, Farwaha R, Rempel G L. (1990) Catalytic hydrosilylation of diene-based polymers. 1. Hydrosilylation of Polybutadiene. Macromolecules 23 5047-5054... 

NR and synthetic diene based elastomers are a very versatile material for grafting. Chain transfer grafting may be carried out by means of a functionalized monomer-initiator system in a rubber solution, in the latex phase, in swollen rubber or during vulcanization. The chain transfer can be accompanied in diene-based polymers by addition reactions involving both the starting radicals and growing polymer radicals [217]. 

Substituted aminophenols and phenolic or anilino moieties bearing N-hetero-cycles contain two distinguished CB functional centres. Substituted BQMI 57 are formed via 4-aminophenoxyl after ROO trapping by aminophenols. For example, the aminophenolic moiety creates the active part of 2,4-bis(octylthio)-6-(3,5-di-ter -butyl-4-hydroxyanilino)-1,3,5-triazine, a very efficient AO for diene based polymers. The respective QI 173 is formed transiently and 2,6-di-fm-butyl-l, 4-benzoquinone is released by hydrolysis of 173 in the ultimate phase of its lifetime [251]. The respective QI are also formed in sacrificial transformations of 6-hydroxy- or 6-anilino-2,2,4-trimethyl-1,2-DHQ, 6- or 8-hydroxy-2,2,4-trimethyl-1,2,3,4-tetrahydroquinolines or 6-hydroxycarbazole [37]. All these bifimctional heterocyclic amines are strong AO. 

Friedel-Crafts alkylation of lib with 1,3-di-isopropenylbenzene was used to prepare an oligomeric stabilizer for diene based polymers. SBR may serve as a co-reactant in the alkylation of the pendant phenyl group with DHQ and yields... 

Representative diene-based polymers include natural rubber (NR), polyisoprene (PIP), PBD, styrene—butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR), which together compose a key class of polymers widely used in the rubber industry. These unsaturated polyolefins are ideal polymers for chemical modifications owing to the availability of parent materials with a diverse range of molecular weights and suitable catalytic transformations of the double bonds in the polymer chain. The chemical modifications of diene-based polymers can be catalytic or noncatalytic. The C=C bonds of diene-based polymers can be transformed to saturated C—C and C—H bonds (hydrogenation), carbonyls (hydrofbrmylation and hydrocarboxylation), epoxides (epoxidation), C—Si bonds (hydrosilylation), C—Ar bonds (hydroarylation), C—B bonds (hydroboration), and C—halogen bonds (hydrohalogenation). ... 

Hydroformylation involves the reaction of C=C bonds with syngas (i.e., a mixture of carbon monoxide and hydrogen) and produces aldehyde functional groups. Hydroformylation of diene-based polymers is mostly performed by means of providing sites for further derivations. The most commonly explored secondary modification of aldehyde functional groups is hydrogenation to give primary alcohol functionality however, aldehyde may also be converted to nitrile, acetate, or amine functionalities. " ... 

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