Anionic polymerization of alkenes

Other additions, such as addition of alkyl halides and carbonyl compounds, are discussed in Chapter 5, whereas Chapter 7 covers addition reactions involving carbon monoxide (hydroformylation, carboxylations). Hydrogen addition is discussed in Chapter 11. The nucleophilic addition of organometallics to multiple bonds is of great significance in the anionic polymerization of alkenes and dienes and is treated in Chapter 13. 

Anionic polymerization of alkenes is quite difficult to achieve because few anions (or nucleophiles) are able to add readily to alkene double bonds (see Section 10-6). Anionic polymerization occurs readily only with alkenes sub-... 

Section 27.9 Anionic polymerization of alkenes that bear a carbanion-stabilizing substituent (X) can be initiated by strong bases such as alkyllithium reagents. 

Homogeneous Catalysis for Ziegler-Natta Coordination Polymerization (Section 29.6B) Initiation of Anionic Polymerization of Alkenes (Section 29.6D)... 

Anionic polymerizations of alkenes, which do not suffer from the termination problems of radical polymerizations, can be initiated by nucleophiles or one-electron reduction. 

The synthesis and properties of heat-resistant polyazomethines containing 2,5-disubstituted oxadiazole fragments, being insulators convertible into semiconductors by doping with iodine, have been described. The radical copolymerization of alkenes with the fluorescent co-monomer 2-/-butyl-5-(4 -vinyl-4-biphenylyl)-l,3,4-oxadiazole has resulted in useful macromolecular scintillators. Anionic polymerization of 2-phenyl-l,3,4-oxadiazolin-5-one has produced a nylon-type product <1996CHEC-II(4)268>. 

Problem 6.63 Describe (a) radical-induced, and (b) anion-induced, polymerization of alkenes. (c) What kind of alkenes undergo anion-induced polymerization M... 

Most technically important polymerizations of alkenes occur by chain mechanisms and may be classed as anion, cation, or radical reactions, depending upon the character of the chain-carrying species. In each case, the key steps involve successive additions to molecules of the alkene, the differences being in the number of electrons that are supplied by the attacking agent for formation of the new carbon-carbon bond. For simplicity, these steps will be illustrated by using ethene, even though it does not polymerize very easily by any of them ... 

Polyphenylsilane, (PhSiH) , can be derivatized by free-radical hydrosilylation in the presence of a radical initiator. Alkenes, ketones and aldehydes react readily, to replace up to 93% of the Si-H bonds. This route can be employed to make polysilanes with hydrophilic groups, such as hydroxy, amino and carboxylic acid functions.43 Dialkylamino substituted polysilanes, made by the anionic polymerization of masked disilenes (see equation (17)), when treated with acetyl chloride give chloro-substituted poly silanes. The chlorine can then be displaced by other nucleophiles.27... 

Additions occur more easily if a carbanion with resonance or inductive stabilization is formed in the addition. Thus, fulvenes are very reactive, vinylsilanes and highly fluorinated alkenes somewhat less so. Styrene, 1,3-dienes, and enynes are more reactive than isolated alkenes, and Grignard reagents may be used to initiate anionic polymerization of styrenes, dienes, and acryhc monomers. Strained alkenes such as norbomenes and cyclopropenes are also more reactive. Examples of additions facilitated by resonance or substitution are shown in Scheme 8. 

Anionic polymerization (Section 30.2C) Chain-growth polymerization of alkenes substituted by electron-withdrawing groups that stabilize intermediate anions. 

C and 9.5 x 10 at 5 °C in cyclohexane [29]. Although chain transfer would be expected for p-methylstyrene, controlled polymerizations can be effected when the temperature is maintained at room temperature or below. The observations of broad molecular weight distributions and a low molecular weight tail by SEC analysis have provided evidence for chain transfer during the anionic polymerization of 7-isopropyl-a-methylstyrene [108]. Significant chain transfer effects have also been reported for alkyllithium-initiated polymerizations using alkenes as solvents [3]. 

The equilibria between these different species in anionic polymerizations of nonpolar alkenes are dynamic enough relative to the rate of propagation to generate monomodal and narrow molecular weight distributions [26, 27]. 

Chain polymerization of alkenes can also be catalyzed by anionic reagents. Anionic polymerization occurs most readily when the alkene carries a carbanion stabilizing substituent. Even ethylene can be polymerized anionically, however, if a... 

The key difference between poly( styrene) and styrene is the presence of a double bond in the latter. In poly(styrene), there are two sigma bonds cormecting neighboring styrene units as opposed to the one pi bond in monomeric styrene. The polymerization of alkenes like styrene represents one of the most common polymer synthesis techniques and leads to ubiquitous materials like poly(ethylene), poly(propylene), and poly(vinyl chloride) in addition to poly(styrene). There are many ways to convert alkenes into polymers by chain polymerization and while they are all different in detail, there are some common features. As a starting point, let us first consider the anionic polymerization of styrene to prepare the poly(styr-ene) molecule depicted in Figure 1(a). 

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