Lithium naphthalene styrene reaction

Difunctional initiators such as sodium naphthalene are useful for producing ABA, BABAB, CAB AC, and other symmetric block copolymers more efficiently by using fewer cycles of monomer additions. Difunctional initiators can also be prepared by reacting a diene such as /n-diisoprope ny I benzene or l,3-bis(l-phenylethenyl)benzene with 2 equiv of butyl-lithium. Monomer B is polymerized by a difunctional initiator followed by monomer A. A polymerizes at both ends of the B block to form an ABA triblock. BABAB or CABAC block copolymers are syntehsized by the addition of monomer B or C to the ABA living polymer. The use of a difunctional initiator is the only way to synthesize a MMA-styrene-MMA triblock polymer since MMA carbanion does not initiate styrene polymerization (except by using a coupling reaction—Sec. 5-4c). 

A naphthalene supported polymer was prepared by radical copolymerization of 2-vinylnaphthalene, styrene, and divinylbenzene.70 This catalyst was used to mediate metallation of alkyl chlorides by lithium. The reaction was done in the presence of electrophiles to afford, after quenching, the desired addition products. 

The initiation step is normally fast in polar solvents and an initiator-free living polymer of low molecular weight can be produced for study of the propagation reaction. The propagation step may proceed at both ends of the polymer chain (initiation by alkali metals, sodium naphthalene, or sodium biphenyl) or at a single chain end (initiation by lithium alkyls or cumyl salts of the alkali metals). The concentration of active centres is either twice the number of polymer chains present or equal to their number respectively. In either case the rates are normalized to the concentration of bound alkali metal present, described variously as concentration of active centres, living ends or sometimes polystyryllithium, potassium, etc. Much of the elucidation of reaction mechanism has occurred with styrene as monomer which will now be used to illustrate the principles involved. The solvents commonly used are dioxane (D = 2.25), oxepane (D = 5.06), tetrahydropyran D = 5.61), 2-methyl-tetrahydrofuran (D = 6.24), tetrahydrofuran (D = 7.39) or dimethoxy-ethane D = 7.20) where D denotes the dielectric constant at 25°C. 

Summary l-[2,6-bis(dimethylaminomethyl)phenyl]silenes (2a-d) were prepared by treatment of the (dichloromethyl)oligosilanes R (Me3Si)2Si-CHCl2 la-d (a R = Me b R=tert-Bu c R=Ph d R = Me3Si) with 2,6-bis(dimethylaminomethyl)phenyl-lithium and were characterized by NMR studies and (in part) by X-ray stmctural analyses as well as by their reactions with water to give silanols. Treatment of 2a-d with benzaldehyde produced 2,2-bis(trimethylsilyl)styrene (4) and a silanone polymer as the expected products. For the reaction of 2a and 2c with benzaldehyde, an interesting side reaction was observed leading to the 2-oxa-l-sila-l,2,3,4-tetrahydro-naphthalenes 6a and 6c, respectively. 

An alternative method of initiation is through the use of the radical anion produced from the reaction of sodium (or lithium) with naphthalene. Such radical anions react with styrene by electron transfer to form styrene radical anions these dimerize to produce a dianion, which initiates polymerization as outlined in Scheme 14. One particular feature of this method is that polymerization proceeds outwards from the centre. Subsequent reaction of the living chains ends with another suitable monomer system produces a triblock copolymer. This is the principle by which styrene-butadiene-styrene triblock copolymers (formed when butadiene is polymerized in the same way. and styrene is added as second monomer) are produced commercially. This material behaves as a thermoplastic elastomer, since the rigid styrene blocks form cross-links at room temperature on heating these rigid styrene portions soften, allowins the material to be remoulded. ... 

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