Styrene transfer reactions

Styrene Transfer reactions were also evidenced with borohydride precatalysts associated to BEM in styrene polymerization. In a study centered around the structure/reactivity relationships of the precatalyst, it was shown that Ln(BH4)3(THF)j x = 3, Ln = Nd, La) as well as the mixed La(BH4)2Cl(THF)2g led to an efficient transmetalation of the growing polystyrene chain with the Mg-CTA (Scheme 27.5). However, NMR and MALDI-TOF studies established the simultaneous occurrence of some fi-W abstraction. Such uncontrolled termination reactions were absent with LaCl3(THF)3,... 

By polymerising styrene in solution many problems associated with heat transfer and the physical movement of viscous masses are reduced, these advantages being offset by problems of solvent recovery and the possibility of chain transfer reactions. In 1955 Distrene Ltd started a plant at Barry in South Wales for the production of styrene by such a solution polymerisation process and some details have been made available. The essential details of this process are indicated by Figure 16.7. 

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

Ruthenium porphyrin complexes are also active in cyclopropanation reactions, with both stoichiometric and catalytic carbene transfer reactions observed for Ru(TPP)(=C(C02Et)2> with styrene. Ru(Por)(CO)orRu(TMP)(=0)2 catalyzed the cyclopropanation of styrene with ethyidiazoacetate, with aiiti.syn ratios of 13 1... 

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... 

The cis alkenes are more reactive and more selective than their trans counterparts. As with the Evans system, this reaction is not stereospecific. Acyclic cis alkenes provide mixtures of cis and trans aziridines. cis-p-Methylstyrene affords a 3 1 ratio of aziridines favoring the cis isomer, Eq. 67, although selectivity is higher in the trans isomer. A fascinating discussion of this phenomenon, observed in this system as well as the Mn-catalyzed asymmetric oxo-transfer reaction, has been advanced by Jacobsen and co-workers (83). Styrene provides the aziridine in moderate selectivity, Eq. 68, not altogether surprising since bond rotation in this case would lead to enantiomeric products. 

The cationic polymerization of arylenes differs in some respects from that of alkenes. The most notable features are that the degree of polymerization is generally lower and that the proportion of unsaturated end groups is always small [21, 22] and often very variable [10]. In the system styrene-TiCl4-CCl3COOH-toluene low polymers are formed which have tolyl end groups [11]. It is not possible to decide at present whether the transfer reaction involved in this is (VIII) or (IX) ... 

The most important of these in chemically initiated polymerizations are the transfer reactions with solvent, rate Rs, and rate-constant ks, and with monomer, rate Rm, and rate-constant km. Solvent transfer was shown to be important by Ueno etal. (1966c) for the polymerization of styrene in toluene, and it will be discussed below. The chemistry of the transfer with an aromatic compound ArH, discovered by Plesch et al. (Plesch 1953 Brackman Plesch 1958 Penfold Plesch 1961), can be represented as... 

The species IA, IB, IC represent the chain-propagating ester molecules, stabilised by styrene, and they are equivalent from the point of view of the polymerisation. At the end of the polymerisation, the now unstabilised ester II reacts with its own kind to give the indanyl ion III and a saturated linear polymer IV. It is also in equilibrium with unsaturated polymer V, and it reacts with acid and/or V to give the polymer with indanyl end groups VI. This is equivalent to the transfer reaction with monomer which gives the indanyl end groups [23]. The oligostyryl ion VII can only be present in very small concentration, as it is much less stable than the other SD ions which co-exist with ion III [7] these other SD ions have been omitted from the scheme so as not to complicate it unnecessarily. 

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) ... 

Mononuclear oxazolines have been less studied. Moderate enantioselectivities (up to 60%) were obtained using PyrOx as ligands for the copper(l)-catalyzed carbene-transfer reaction of ethyl diazoacetate to styrene." " " However, the diastereo-selectivities (cis/trans) in these reactions were generally poor. 

In the product, there should be a ladder-type blocks linked by segments composed of p-cresyl methacrylate units. This type of structure was confirmed by IR and NMR spectrometry. However, by preparation of such copolymers with labeled end-groups (using radioactive AIBN), and by fractionating and radiometric analysis, it was shown that copolymers obtained are slightly branched. There is slightly more branch points than in the case of copolymers with styrene. It could be an effect of chain transfer reaction. 

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 ... 

Catalytic methods are suitable for nitrene transfer," and many of those found to be effective for carbene transfer are also effective for these reactions. However, 5- to 10-times more catalyst is commonly required to take these reactions to completion, and catalysts that are sluggish in metal carbene reactions are unreactive in nitrene transfer reactions. An exception is the copper(ll) complex of a 1,4,7-triaza-cyclononane for which aziridination of styrene occurred in high yield, even with 0.5 mol% of catalyst. Both addition and insertion reactions have been developed. 

Evidence for the absence of termination or transfer reactions in the organolithium-initiated polymerization of styrene and iso-prene is shown in Table I for representative examples of these polymers. It can be seen that these polymers exhibit the expected low kfo/Mn values, except in the case of the isoprene polymerized in the H4furan, where a slow side reaction seems to occur between the solvent, on the one hand, and both the initiator (Ifo vs Mg) and the growing chains (1% vs Mn)>on the other hand. 

Monomers devoid of polar groups generally undergo anionic polymerization in a predictable manner. With polar monomers sometimes side reactions occur during the process transfer reactions in the case of acrylonitrile, or propylene oxide, and even more so with alkylacrylates deactivations (or "killing") reactions in the case of halogen substituted styrene or dienes. 

The first results of anionic polymerization (the polymerization of 1,3-butadiene and isoprene induced by sodium and potassium) appeared in the literature in the early twentieth century.168,169 It was not until the pioneering work of Ziegler170 and Szwarc,171 however, that the real nature of the reaction was understood. Styrene derivatives and conjugated dienes are the most suitable unsaturated hydrocarbons for anionic polymerization. They are sufficiently electrophilic toward carbanionic centers and able to form stable carbanions on initiation. Simple alkenes (ethylene, propylene) do not undergo anionic polymerization and form only oligomers. Initiation is achieved by nucleophilic addition of organometallic compounds or via electron transfer reactions. Hydrocarbons (cylohexane, benzene) and ethers (diethyl ether, THF) are usually applied as the solvent in anionic polymerizations. 

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