Dimerization of styrenes

This absorption is in fact due to the ions derived from l-methyl-3-phenylindane (the cyclic dimer of styrene) and its higher homologues (oligostyrenes with indanyl end groups). There can be no doubt that the ions formed at the end of the polymerisation of styrene belong to the same families of compounds (indanyl and various phenyl alkyl carbonium ions [7]). Our evidence showed that the 1-phenylethyl cation is absent from the ions formed from styrene by excess of acid its dimeric homologue, the l,3-diphenyl- -butyl cation, is a minor component of the ion mixture. We refer to this mixture of ions formed rapidly from styrene by excess acid, or at the end of a styrene polymerisation, as SD (styrene-derived) ions. 

The equilibrium VI-VII requires some comment. We reported originally [16] that the indane dimer of styrene (VI, R = CH3) did not react with strong acids. Subsequent experiments [7, 24] have shown that it does react to give the ions III, VII, and others. (We have been unable to discover the reasons for our erroneous result.)... 

Scheme 30 Dimerization of styrene mediated by BiCl3 yields a highly substituted cyclopentane...

Peng et al. report the dimerization of styrene in ILs using palladium-Lewis acid catalysts and compare these results for the same dimerization in organic solvents. The stoichiometry of the reaction was given by the following. Scheme 6 ... 

The clearest and most direct experimental evidence from zeolite studies for the existence of a free carbenium ion intermediate obtained by any means is summarized in Fig. 14 (113). We followed the dimerization of styrene to form cis and trans isomers in a series of low-temperature MAS NMR experiments. Identification of the dimeric products was further... 

Similarly, the anodic dimerization of styrene in CH3CN-H20-Et4N-pTS proceeds via an intermediate cation and its deprotonation or solvolysis with the formation of substituted 1,4-butanediols, tetrahydrofurans, tetrahydropyrroles, and some other products depending on the experimental conditions.102... 

It seems to this writer that the first alternative is the correct one. A proton transfer from NHS to styrene- ion is unlikely to be faster than a proton transfer from NH3 to poly-styryl- ion, and it was shown that the latter reaction is not too rapid. Hence, if an electron transfer does take place one might expect dimerization of styrene ions and eventually initiation of polymerization. This might be an alternative explanation for the formation of a small amount of polymer during the reduction, but nevertheless this still remains to be only a minor reaction. On the other hand, in the reduction of 1,1-diphenyl ethylene, the electron affinity of which is higher than that of styrene, the dimeric di-ion, Ph2 C. CH2. CH 2. C. Ph2 is formed in comparable amounts with the monomeric Ph2 C. CH3ion (17). 

The dimerization of styrene. IUPAC Symposium on Macromolecular Chemistry, Moscow 1960, Section II 11. 

Scheme 10 Unusual radical cyclizations and dimerizations of styrenes catalyzed by vitamin B12...

The cationic polymerization of styrene sulfide has been reinvestigated by Van Craeynest (15). With triethyloxonium tetrafluoroborate as initiator, a rapid and quantitative polymerization was observed, followed by a slow degradation of die polymer to a mixture of cis and tram 2,5-diphenyl-l, 4-dithiane and as and tram 2,6-diphenyl-1,4-dithiane. Since the BF4 counter ion is not capable of forming a covalent bond, a back-biting reaction via sulfonium ions seems the plausible mechanism for the dimer formation. The polymerization initiated with dimethyl sulfate showed the same characteristics a fast polymerization is followed by degradation to the same mixture of isomeric diphenyl- 1,4-dithianes. However, the mwts-2,5-diphenyl derivative was the only isomer that crystallized from the solution. It is therefore reasonable to accept that with dimethyl sulfate also, the cyclic dimers of styrene sulfide are formed by a back-biting type of degradation of the polymer and not by the mechanism shown above. 

Most of the dimer cations and the bonded dimer cations were produced during the pulse irradiation. This can be explained by assuming the existence of the equilibrium between styrene monomers (St) and neutral dimers of styrene (St2) under the experimental conditions [24]. 

The linear dimerization of styrene and linear polymerization DVB has been accomplished at lower temperatures (<70° C) and higher monomer concentrations (>0.10 M) without side reactions using Pd2+ derivatives as cationic initiators [Eq. (4)] [7]. The specific initiators used for this reaction were Pd(PPh3)2(BF4)2, Pd(Ph2PCH2CH2PPh2)(BF4)2, and Pd(2,6-rBu2C5H3N)2(MeN02)2(BF4)2. All of these derivatives were air-stable solids. The dimerization of styrene could be performed with or without solvent (acetonitrile or acetonitrile-chloroform) (Eq. 4). 

These Pd complexes are unique in several respects. No cross-linked polymer was detected in the reaction with p-divinylbenzene. A completely soluble linear polymer was obtained. No indane units were formed from the reaction of styrene with Pd(PPh3)2(BF4)2. a-Methylstyrene is usually much more reactive than styrene in reactions involving carbenium ion intermediates. However, the treatment of a mixture of styrene and a-methylstyrene with Pd(PPh3)2(BF4)2 resulted in the selective dimerization of styrene. 

The polymerization of styrene to form macromolecules using alkali metal catalysts has been known and extensively used. The dimerization of styrene is, however, a novel reaction (12). Dimers of styrene, a -methylstyrene and a codimer of tEe above two styrenes are obtained by heating the olefins at about 160 in the presence of catalytic amounts of anhydrous -BuOK. 

However, the probability for the reaction progression greatly depends on the monomer conversion. Because the viscosity of the dispersed phase, in the first stage, is fairly low and the quantity of styrene is sufficiently high, the decomposition process (Figure 9.4) occurs only up to the benzoyloxy radical, which can directly start the kinetic chain. The purely thermal start of chains with reactive dimers of styrene, as a result of Diels-Alder reaction, can be ignored at fairly low temperatures of suspension polymerization, in contrast to the conditions for the bulk styrene process [4-7]. 

Another reaction of this type is the nonoxidative dimerization of olefins. They very likely proceed via palladium-hydride species which may be formed by a small amount of oxidation of the olefin. Often Pd(II)-olefin 7T complexes are used. Kawamoto et al. (155) have recently reported the dimerization of styrene and vinyl compounds using the styrene Pd(II) tt complex. Also, it has been reported (254) that phosphine complexes, Pd(PPh3)4 or (PPh3)aPdX2 (X = Ng or NCO), have been employed to give a novel dimerization of malonotrile ... 

Large pore zeolites such as faujasites and zeolite beta are reported to show a significant degree of diastereoselectivity in the dimerization of styrene followed by subsequent cyclization of the open dimers to the indane dimers [96]. 

Styrene (and derivatives) also possesses the rare monomer quality that the neat material, without initiator, may be spontaneously polymerized by simply heating to 80-100°C for 24-48 hr. It is thought that this occurs via the initial Diels-Alder dimerization of styrene to the two diasteomers A and B [14]. The two diastereomers appear to have an extremely labile hydrogen, which is both doubly allylic and tertiary. However, only dimer A has the correct stereochemistry (an axial phenyl), which enables the excess styrene to abstract a hydrogen atom from it, producing two radical species (Eq. 23.5). 

Reductive dimerization of styrene in the presence of (diethylamino)dichloroborane gave a mixture of cis and tram borolane (91). Crystallization afforded a 45% yield of (92) in >99% purity (one enantiomer shown) (Scheme 14) <86TL472i>. 

Exit must be of a small radical, and this must be a species formed by chain transfer, either to chain-transfer agent or to monomer. Now, for a system such as that shown in Figure 5.2, there is no added chain-transfer agent, so the exiting species must have arisen from chain transfer to monomer. There are two points about exit which often seem strange. First, styrene contains no labile hydrogens, so transfer to monomer would appear impossible. Second, even if this were to occur, the resulting species would be so insoluble that it would never exit. In fact, the transfer process may well be [30] to a Diels-Alder dimer of styrene ... 

The dimer peaks are composed of a triplet with relative intensities of 14% 45% 41%. Almost the same triplet of the dimer peaks are also observed in the pyrogram of the blend of styrene and styrene-ds homopolymers. The first and the last peaks of the triplet are homo-dimers of styrene-ds and ordinary styrene, respectively the central peak proved to be the hybrid dimer of bofh styrene units. The fairly strong peak intensity of the hybrid dimer (45%) cannot be fully explained fhrough simple chain scissions around the minute amount of the junctions of fhe two types of styrene units in the block copolymer chains, but must also result from the contribution of some additional intermolecular reactions to form the dimers. 

A method has been developed by BASF io produce anthraquinone by dimerization of styrene followed by oxidation of the l-methyl-3-phenylindane. 

A comprehensive kinetic mechanism is proposed to describe the combined chemical and thermal free-radical polymerization of styrene. Thus, besides the commonly employed reactions (e.g., chemical initiation, propagation and termination), thermal initiation and chain transfer to monomer and to Diels-Alder adduct reactions are included. In particular, the so-called AH thermal initiation mechanism of Mayo comprises a reversible Diels-Alder dimerization of styrene to form l-phenyl-1,2,3,9-tetrahydronaphtalene (AH), the formation of a styryl (m) and a 1-phenyltetralyl radical... 

Consider, for example, the radical anion from styrene, PhCH=CH2. Dimerization of styrene to a biradical would remove the two terminal carbon atoms from conjugation. The resulting loss in n energy per styrene unit would then be Nt being the corresponding reactivity number, namely... 

As well as stereoisomerism and geometrical isomerism, polymers and copolymers can exhibit a third form of isomerism regioisomerism. Head-to-head, head-to-tail and tail-to-head isomerism is well known for simple organic compounds. Thus, a dimer of styrene monomer can exist in the following three different regioisometric forms ... 

The relatively low standard potential of the Ns /Ni redox couple in aqueous solution (+1.33 0.01 V vs. NHE) is even more reduced in organic solvents, and therefore the generation of azidyl radicals by electrochemical methods is perfectly feasible. The multigram scale dimerization of styrene represents an early synthetic application of the electrochemical process (Scheme 8.15). " However, the scope of this reaction is so far limited since other substrates give poor yields and/or significant amounts of by-products. 

The spontaneous thermal polymerization of styrene is postulated to involve an initiation step in which the reactive Diels-Alder dimer of styrene (AH in Figure lA) undergoes a molecule-assisted homolysis reaction with another styrene molecule. The evidence for this process is reviewed, and it is concluded that it is not at all conclusive. Furthermore, methylenecyclohexadiene... 

Reaction of traws-l,3-diphenyl-l-butene (D), the trans ethylenic dimer of styrene, with trifluoro-methanesulfonic acid in dichloromethane has been performed at temperatures lower than room temperature using a stopped-flow technique with real time UV-visible spectroscopic detection. The main product of the reaction was the dimer of D. A transient absorption at 340 nm has been assigned to 1,3-diphenylbutylium, a model for the polystyryl cation. Other absorptions at 349 nm and 505 nm have also been observed and were assigned to an allylic cation, l,3-diphenyl-l-buten-3-ylium, resulting from hydride abstraction from D. This species was very stable at temperatures lower than -30°C. A general mechanism was proposed based on a kinetic study of the reactions involved. 

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