Lithium-catalyzed polymerization

The first examples employed a lithium-terminated polymer chain, utilizing the primary product of the -butyl-lithium-catalyzed polymerization of a masked disilene (see Section 3.11.4.1.2) and a reactive siloxy alkylbromide anchor-derivatized quartz surface, affording the end-grafted polysilane 79, as shown in Scheme 27.191... 

Such "living" polymer systems, however, are not limited to polymerizations in solvating media, such as ethers. Thus, the lithium-catalyzed polymerizations, which can lead to the synthesis of cis-1,4-pol yisoprene, also demonstrate the virtual absence of... 

Coupling to produce dimeric product was a side reaction in these systems also, e.g. 75 % dimer formation was reported for poly(styryl)lithium and 23 % dimer formation with the poly(styryl)-Grignard reagent 326). However, it should be noted that the only reported characterizations of these reactions were size exclusion chromatography traces and silver catalyzed polymerization of tetrahydrofuran using the polymeric halogen compounds as co-initiator. 

The effect of small amounts of cyclic ethers on the rates of lithium catalyzed reactions can be more pronounced, especially in styrene polymerization (Fig. 12). The observed propagation rate first increases rapidly, passes through a maximum, and then decreases for both dioxane... 

Lithium differs from the other alkali metals in that it directs the polymerization of butadiene or isoprene predominantly to 1,4 addition structures. In the case of lithium-catalyzed polyiso-prene, the 1,4 addition structures are all cis. The other alkali metals direct the polymerization of butadiene largely to the 1,2 addition structure and isoprene largely to the 3,4 addition structure. The differences in physical properties, accompanying the structural variations mentioned above, are illustrated by the example of lithium-catalyzed polybutadiene. Lithium, sodium, and potassium are used most conveniently as polymerization catalysts by converting them to metal dispersions in petroleum jelly or other inert hydrocarbons. Special care must be used in handling rubidium or cesium metal. 

The microstructures of the polybutadienes, butadiene-styrene copolymers, and polyisoprenes were determined by infrared spectroscopic methods (1,3). The spectra of alkali metal-catalyzed polybutadienes and polyisoprenes show that other reactions occur during polymerization in addition to those involving cis- and trans 1,4, 1,2, and 3,4 additions. For sodium and potassium polybutadienes and polyisoprenes, the absorbances of the bands arising from these additional structures could be taken into account satisfactorily by the methods described. No foreign structures are found in lithium-catalyzed polyisoprenes and the additional band foimd near 14.2 microns in polybutadiene spectra does not appear to affect the cis-1,4 band at 14.7 microns. (Cesium and rubidium, as well as additives such as dimethoxy-tetraglycol, affect the polymerization of butadiene so markedly that it was not possible to obtain satisfactory analyses of such polymers. The effect of these catalysts in isoprene polymerizations does not appear to be so marked and satisfactory analyses were obtained by the method described. 

Addition of a catalyst modifier during the preparation of solution-polymerized, lithium-catalyzed BR results in an increase in the vinyl-1,2-butadiene level in the polymer and causes an increase in Fg. There is a corresponding drop in abrasion resistance and an increase in wet traction. 

Styrene butadiene n. A group of thermo plastic elastomers. They are linear co-polymers of styrene and butadiene, produced by lithium catalyzed solution polymerization, with a sandwich molecular structure containing a long Polybutadiene center surrounded by shorter polystyrene ends. A co-polymer of styrene and butadiene made by emulsion polymerization for use in latex paints. 

The melt temperature of nylon-1 pol5uners depends on the legth of the side chain. The longer the side chain, the lower the melting point of the polsrmer. Steric requirements for the homopolymerization of monoisocyanates are severe. No homopolymers are obtained from isopropyl, cyclohexyl, o-methox5tphenyl, and a-naphthyl isocyanate. All alkyl nylon-1 polymers, with the exception of poly(allyl isocyanate), have a low order of crystallinity as shown by X-ray diffraction. However, the polymerization of re-butyl isocyanate and phenyl isocyanate in various solvents at — 78°C, initiated by lithium or sodium alkyls, produces crystalline pol5uners (28). The mechanism of the sodium cyanide catalyzed polymerization in DMF involves intermediates having a spiro structure (29). 

In the 1952 paper mentioned above [3], Gilman reported on the formation of lithium dimethylcuprate from polymeric methylcopper and methyllithium. These so-called Gilman cuprates were later used for substitution reactions on both saturated [6] and unsaturated [7, 8, 9] substrates. The first example of a cuprate substitution on an allylic acetate (allylic ester) was reported in 1969 [8], while Schlosser reported the corresponding copper-catalyzed reaction between an allylic acetate and a Grignard reagent (Eq. 2) a few years later [10]. 

The concept of using group I metal initiators was applied in order to minimize the toxicity generated by heavy metal residues in the end product PLAs when using metals like aluminum, tin, and lanthanides as initiators. In recent years, dinuclear lithium and macro-aggregates with phenolate ligands have attracted substantial interest, mainly due to uncommon strucmral feamres and their ability to catalyze formation of polyester and various other polymeric materials via ROP [28]. A series of lithium complexes supported with 2, 2-ethylidene-bis (4, 6-di-tert-butylphenol) (EDBP-H2) 2-6, (Scheme 6) are excellent initiators for the ROP of L-lactide in CH2CI2 at 0 °C and 25 °C [33-35]. In this case, the PDIs of the obtained PLAs were quite narrow (1.04—1.14) and a Unear relationship between and the monomer-to-initiator ratio ([M]o/[I]o) existed at 0 °C. Dimeric complexes 4 and 6 were the... 

The initiation step for the lithium diethylamide catalyzed butadiene polymerization can be postulated in terms of the following equations ... 

Catalysts. Acrolein and methacrolein 1,4-addition polymerization is catalyzed by lithium complexes of quinoline. The pcracctic acid epoxidation of a wide range of alkenes is catalyzed by 8-hydroxyquinoline. 

It was observed that ammonolysis of B(C2H,Si(R)H2)3 (Scheme 2, route A) requires basic catalysts such as n-butyl lithium. The reaction is performed in analogy to the potassium hydride-catalyzed cross-linking of cyclic silazanes described by Seyferth et al. [8]. Most probably, n-BuLi initially deprotonates the weak nucleophile ammonia with the formation of lithiiun amide and evaporation of n-butane. The stronger nucleophilic amide then replaces a silicon-bonded hydride, which subsequently deprotonates ammonia, leading to the evolution of molecular hydrogen. The silylamines that arise are not stable under the reaction conditions applied (refluxing solvent), and by fast condensation of ammonia the polymeric precursors form [6]. 

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