Nucleophilic Initiators

A variety of basic (nucleophilic) initiators have been used to initiate anionic polymerization [Bywater, 1975, 1976, 1985 Fontanille, 1989 Hsieh and Quirk, 1996 Morton, 1983 Morton and Fetters, 1977 Quirk, 1995, 1998, 2002 Richards, 1979 Szwarc, 1983 Young et al., 1984]. These include covalent or ionic metal amides such as NaNFU and LiN(C2H5)2, alkoxides, hydroxides, cyanides, phosphines, amines, and organometallic compounds such as n-C4H9Li and ) MgBr. Initiation involves the addition to monomer of a nucleophile (base), either a neutral (B ) or negative (B ) species. 

Alkyllithium compounds are probably the most useful of these initiators, employed com-merically in the polymerizations of 1,3-butadiene and isoprene. Initiation proceeds by addition of the metal alkyl to monomer 

The extensive use of alkyllithium initiators is due to their solubility in hydrocarbon solvents. Alkyls or aryls of the heavier alkali metals are poorly soluble in hydrocarbons, a consequence of their more ionic nature. The heavier alkali metal compounds, as well as alkyllithiums, are soluble in more polar solvents such as ethers. The use of most of the alkali metal compounds, especially, the more ionic ones, in ether solvents is somewhat limited by their reactivity toward ethers. The problem is overcome by working below ambient temperatures and/or using less reactive (i.e., resonance-stabilized) anions as in benzylpotassium, cumylcesium and diphenylmethyllithium. 

Alkyl derivatives of the alkaline-earth metals have also been used to initiate anionic polymerization. Organomagnesium compounds are considerably less active than organolithiums, as a result of the much less polarized metal-carbon bond. They can only initiate polymerization of monomers more reactive than styrene and 1,3-dienes, such as 2- and 4-vinylpyridines, and acrylic and methacrylic esters. Organostrontium and organobarium compounds, possessing more polar metal-carbon bonds, are able to polymerize styrene and 1,3-dienes as well as the more reactive monomers. 

In the relatively few anionic polymerizations initiated by neutral nucleophiles such as tertiary amines or phosphines the proposed propagating species is a zwitterion [Cronin 

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The first step of NCA polymerization is usually accomplished by the use of nucleophilic initiators. These initiators can be alkoxides, alcohols, amines, transition metals, and even water [53,54]. In order to synthesize a copolymer diblock, the polymerization of the second block and its connection to the previously formed block are performed in a single process. This is achieved by initiating the polymerization of the second NCA monomer using the first homopolypeptide as a macroinitiator. Precipitation and purification processes follow to isolate the... 

Trifluoromethyl)trimethylsilane has been prepared by a modification5 of the procedure originally published by Ruppert.4 The optimized yield is 75%. Other less convenient methods are also available for its preparation. (Trifluoromethyl)trimethylsilane acts as an in situ trifluoromethide equivalent under nucleophilic initiation and reacts with a variety of electrophilic functional groups. Carbonyl compounds such as aldehydes, ketones and lactones react rather readily5 7 with (trifluoromethyl)trimethylsilane under fluoride initiation. The reagent also reacts with oxalic esters,8 sulfonyl fluorides,9 a-keto esters,10 fluorinated ketones,11 and... 

Fig. 6) can activate hydrogen under mild conditions [219]. In contrast to transition metals, that act as electrophiles towards hydrogen, the (alkyl)(amino)carbenes mainly behave as nucleophiles initially creating a hydride like species, which then attacks the positively polarized carbene carbon atom. 

For cationic polymerization with an acid whose anion Z is nucleophilic, initiation involves the sequence described by Eq. 7-51 plus the formation of XXIX. XXIX propagates by a sequence similar to that described by Eqs. 7-52 and 7-53 except that a growing polymer chain possesses a Z—CO— end group instead of a lactam end group. 

Solution polymerization of these compounds can be brought about by nucleophilic initiators including n-butyllithium, triethylamine, and sodium cyanide. In the absence of such initiators, solution polymerization proceeds very slowly. As an example, l-(p-chlorothiobenzoyl)aziridine at a concentration of 0.5 mole percent in tetrahydrofuran polymerizes at room temperature when initiated with n-butyllithium to give a 94% yield of polymer. Melting point of the polymer is 90-100° C and its reduced viscosity in N-methylpyrrolidone (1% concentration at 30° C)i is 0.15. 

As mentioned above, the new method Lewis acid promoted living polymerization of methacrylic esters, by using enolatealuminum porphyrin (2) as nucleophilic initiator in conjunction with organoaluminum compounds, such as methylaluminum bis(2,6-di-tert-butyl-4-methylphenolate) (3e), as Lewis acids has enabled us to synthesize poly(methacrylic ester) of narrow molecular-weight distribution [51]. On the other hand, some reactions of aluminum por-... 

The nitro group, which most successfully facilitates both the SNAr reaction and the aliphatic version of the SrnI reaction, appears to prevent the aromatic SrnI reaction under nucleophile-initiated conditions,39 7077 with the possible exception of the reaction of o-iodonitrobenzene with "d COBu. 69... 

For addition reactions, the absolute magnitude of kHi is not the real concern. Instead it is the k,t khi ratio compared to the k,2 km ratio that is important. Reducing both Hi and kH2 may not provide a significant advantage. An ideal reagent would donate hydrogen slowly to nucleophilic initial radicals but rapidly to electrophilic product radicals. 

PL, unlike other lactones, undergoes polymerization with weakly nucleophilic initiators such as metal carboxylates, tertiary amines, phosphines, and a variety of other initiators [81-83]. This is primarily due to the high ring-strain in the four-membered ring. Pyridine and other tertiary amines initiate the anionic polymerization via a betaine that rapidly transforms into a pyridinium salt of acrylic acid. In order to minimize the chain transfer reactions, the polymerization is performed at a temperature between 0 and 10 °C (Scheme 9). 

The differentiation between the coordination polymerisation of heterocyclic and heterounsaturated monomers and their nucleophilically initiated anionic polymerisation lies in the covalent nature of the metal-heteroatom bond (but being polarised as in the Mt 5+— Xs bond) in the coordination catalyst, which activates the monomer by its coordination, enhancing the nucleophilicity of the metal substituent simultaneously, and the ionic character of the metal-heteroatom bond in the nucleophilic initiators. 

It is worth mentioning in this connection that the anionic polymerisation of heterocyclic and heterounsaturated monomers requires the application of nucleophilic initiators, involving mostly alkali metal compounds, which are characterised by a high nucleophilicity of the monomer attacking agent and by low Lewis acidity of the positive counterion in the initiator. Thus, nucleophilic initiation of anionic polymerisation does not require any monomer coordination on to the metal, although interaction of the monomer with an electrophilic counterion is considered commonly to occur to some extent. 

It may be mentioned that the use of ionic nucleophilic initiators, instead of zinc-based coordination catalysts, in order to promote propylene oxide/carbon dioxide copolymerisation, did not result in the formation of any copolymer but led to the cyclic carbonate, propylene carbonate [194,236,237]. Also, zinc-based coordination catalysts with non-condensed zinc atoms in their molecules (formed by the reaction of diethylzinc with a monoprotic compound such as... 

The group polymerized mechanically the dispersed methyl or ethyl cyanoacrylate in aqueous acidic medium in the presence of polysorbate-20 as a surfactant under vigorous mechanical stirring to polymerize alkyl cyanoacrylate. The polymerization follows an anionic mechanism since it is initiated in the presence of nucleophilic initiators like OH- in an acidic medium (pH 1.0-3.5). The same group coated PACA nanoparticles with various polysaccharides introducing modifications in the method (Couvreur et al. 1978 Vauthier et al. 2003 Betrholon-Rajot et al. 2005). 

The nucleophile initially adds at the multiple bond, forming carbanion A. Further transformations of A occur in line with electronic and steric effects, depending on the reaction conditions and on the use of nucleophilic catalysis. Several routes are possible, leading to different reaction products. Note that the use of nucleophilic catalysis is a general technique in the chemistry of compounds with electrophilic multiple bonds in particular, it is widely employed for dimerization and trimerization of activated olefins, keteneimines, etc. 

The authors explained the formation of these compounds as follows. The O-nucleophile initially attacks the C=C bond of the perfluoroolefin, forming a carbanion. The latter may be stabilized by elimination of the fluoride ion from the y-position of the CF2 fragment, giving a multiple bond. The olefin has a mobile fluorine atom in the allyl position, leading to intramolecular nucleophilic cyclization induced by the O- or N-nucleophiles. This process may be regarded as an example of the important role of the intermediate from the highly active perfluoroolefin, determining the direction of the subsequent attack of the O- and N-nucleophiles. 

If the N,S- or Y,Y-binucleophile has an a-b-c triad, the character of the heterocyclic product depends on the nature of the nucleophile initiating the reaction with perfluoroolefins and perfluoroazaalkens (Table Y) (03PU1). 

Certainly, the reactive species has to be an enolate. The conceivable candidates are the enolate D (malonic acid monoanion, Figure 10.49), the enolate E (malonic acid dianion ), and the enolate F (malonic add trianion ). The nucleophilicity increases greatly in this order, but the concentrations of these species also drastically decrease. The combined effect of nucleophilicity and abundance determines which nucleophile initiates the Knoevenagel condensation. It is therefore important to know the concentrations of the various species. 

Besides the Michael addition of heteroatomic nucleophiles initiating cyclocondensations, acceptor substituted unsaturated systems can also be reacted with carbon nucleophiles stemming from aldehydes in the sense of an umpolung, generally referred to as the Stetter reaction [244-246]. This process is organocatalytic and furnishes in turn 1,4-dicarbonyl compounds, intermediates that are well suited for Paal-Knorr cyclocondensations giving rise to furans or pyrroles. Among numerous heterocycles furans and pyrroles have always been the most prominent ones since they constitute important classes of natural products [247-249], of synthetic... 

New monomers like tetracyanotetrathiafulvalene (96) and tetranitriles (97) were examined - . (96) was converted with various metal acetylacetonates in bulk at 548 K to dark coloured polymers in yields of 61-87%. The amount of incorporated metal is in good agreement with theoretical values. From different nitriles (97) polymers were produced by heating solely or in the presence of SnCl2 or Cu (and partly using a nucleophilic initiator) " Only in one case the strucutre was determined. It was found... 

It is thought that intermediate 45a, upon quenching with 2-propanethiol, undergoes fragmentation and subsequent nucleophile-initiated cleavage to give 1,2-dithiolane 2a, although in an unspecified yield. Similarly, the related 1,2-diselenolane 2b and 1,2-ditellurolane 2c were synthesized. 

Oxasilacyclopentenes have been synthesized either by silacyclopropenation of alkynes utilizing Ag3P04 as catalyst <2004JA9522>, or by reaction of ketones and alkynylsilanes in the presence of a catalytic amount of a nucleophilic initiator (Equation 48) <2005OL4995>. 

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