Styrene, hydrogen transfer reactions

Polystyrene also forms miscible blends with PPE in which the hydrogen-transfer reaction of PS is rendered less effective due to the PPE component [Jachowicz et al., 1984]. In contrast, poly(a-methyl styrene) blends with PPE, although also reported miscible, show no evidence of any interactions during degradation. Head-to-head and head-to-tail PS structures were found to behave (hfferentiy [JCryszewski, et al., 1982]. 

Competitive reduction tests for cyclohexanone styrene, under transfer conditions, show preferential reduction of cyclohexanone however, under hydrogenation conditions the styrene is reduced exclusively.99 It is worth mentioning that the OsH2(r)2-H2)(CO)(P Pr3)2 precatalyst, formed by addition of NaBH4 to OsHCl (CO)(P Pr3)2, rapidly reduces phenylacetylene to styrene, under transfer conditions, but the reaction rate falls progressively due to the formation of Os(C=CPh)2 (CO)(P Pr3)2.72 As previously mentioned, an alkynyl-dihydrogen intermediate... 

In a recent development, a new process of preparing borane-terminated isotactic polypropylene (z -PPs) via an in situ chain-transfer reaction was achieved by a styrene/hydrogen consecutive chain-transfer reagent, which avoids the use of a B—H containing chain-transfer agent.74 This has resulted in the utilization of milder polymerization conditions due to the use of the alkylaluminoxane cocatalyst (MAO) (50) (Fig. 33), which cannot normally be used in the presence of a B—H chain-transferring... 

Not only the case of vinyl chloride but also styrene shows that the observed chain transfer to monomer is not the simple reaction described by Eq. 3-112. Considerable evidence [Olaj et al., 1977a,b] indicates that the experimentally observed Cm may be due in large part to the Diels-Alder dimer XII transferring a hydrogen (probably the same hydrogen transferred in the thermal initiation process) to monomer. 

Grafting of styrene (ST) onto polybutadiene (PB) can occur in two ways Via a chain-transfer reaction with an allylic hydrogen of the 1,4- and the 1,2-units (Case 1) via copolymerization with C=C-double bounds of polybutadiene, in particular with the vinyl groups of the 1,2-units (Case 2) ... 

The chain transfer reaction played an important role, particularly because of abstraction of the active hydrogen at a-carbon from the allyl group. Moreover, unreacted double bonds were present in the copolymer obtained. The influence of chain transfer reaction could be diminished by applying multimonomers which do not contain allyl groups. This was shown in the example of copolymerization of multimethacrylate prepared by esterification of poly(2-hydroxyethyl methacrylate) with methacryloyl chloride. Copolymerization of the multimethacrylate with vinyl monomers such as styrene or acrylonitrile can be represented by the reaction ... 

Sangalov et al., prepared arenonium ions in the form of Gustavson complexes starting from various substituted benzenes, hydrogen chloride and aluminium chloride. These complexes were drown to be active initiators for the polymerisation of styrene and isobutene at —30 and —78°C in methylene chloride. No kinetic investigation was carried out on these systems. The authors claimed that initiation took place by protonation of the olefin and that incorporation of aromatic groups from the cataylst in the products was due to transfer reactions. 

Two different types of (PS-b-PBDh) diblock can be presently synthesized. The first one by classical anionic initiation (s-buty1-lithium) and "living" propagation of the (PS-b-PBD) copolymer (8), followed by the hydrogenation procedure described here as discussed above, the resulting product will be close to a (PS-b-LLDPE) copolymer. The second one came from the discovery (9) of a "living" polymerization of butadiene into a pure (99 %) 1,4 polymer by a bis n allylnickel-tri-fluoroacetate) coordination catalyst, followed by styrene polymerization unfortunately, the length of the polystyrene block is limited (to a M.W. of ca. 20,000) by transfer reactions. 

Feringa from The Netherlands was the first to report an abnormal Wacker oxidation in 1986.21 Dec-1 -ene was oxidized in the presence of a tertiary alcohol to a mixture of aldehyde 8 and methyl ketone 9. Surprisingly, aldehyde 8 was isolated as the major product (8/9 = 70/30). Even more strikingly, styrene was transformed exclusively to its corresponding phenylacetaldehyde under the same conditions. Feringa proposed that the aldehyde formation involved an oxygen transfer reaction via initial cycloaddition of the nitro-palladium complex, followed by p-hydrogen elimination. 

By contrast, chloroperoxidase-catalyzed epoxidation of alkenes proceeds with excellent enantioselectivites [1333, 1334]. For styrene oxide it was demonstrated that all the oxygen in the product is derived from hydrogen peroxide, which implies a true oxygen-transfer reaction (path 3, Scheme 2.174) [1335]. As depicted in Scheme 2.178, unfunctionalized c/s-alkenes [1336] and 1,1-disubstituted olehns [1337, 1338] were epoxidized with excellent selectivities. On the other hand, aliphatic terminal and irans-l,2-disubstituted alkenes were epoxidized in low yields and moderate enantioselectivities [1339]. 

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