Electrophilic monomer pair

A. Spontaneous Initiated Alternating Copolymerization of Nucleophilic-Electrophilic Monomer Pairs... 

A variety of monomer pairs have been used in spontaneous zwitterion polymerizations. Examples of nucleophilic and electrophilic monomer pairs, genetic zwitterions, and polymer structures are shown in Table 2. 

Lewis acids have long been known to influence free radical polymerizations [117]. They have been particularly important in copolymerizations of hydrocarbon olefins with electron-poor monomers such as acrylates or acrylonitriles. In this way strictly alternating copolymers can be synthesized from monomer pairs which in the absence of Lewis acids would give more random copolymers. The Lewis acid complexes with the electron pair of the acceptor group of the acrylate or acrylonitrile to form the more electrophilic complexed monomer, which then copolymerizes in alternating fashion with the electron-rich hydrocarbon olefin. 

The last reaction Is another example of a spontaneous "alternating copolymerization between an electrophilic and a nucleophilic monomer as described by Saegusa ( ). The structure of the resulting polymer Is determined by the rate constants of the different reactions and by the concentration of the two monomers This reaction has been Investigated with the monomer pair PL-TBA, different solvents at 50 C. 

In fact, recent theoreticaP and experimental studies of small radical addition reactions indicate that charge separation does occur in the transition state when highly electrophilic and nucleophilic species are involved. It is also known that copolymerization of electron donor-acceptor monomer pairs are solvent sensitive, although this solvent effect has in the past been attributed to other causes, such as a Bootstrap effect (see Section 13.2.3.4). Examples of this type include the copolymerization of styrene with maleic anhydride and with acrylonitrile. Hence, in these systems, the variation in reactivity ratios with the solvent may (at least in part) be caused by the variation of the polarity of the solvent. In any case, this type of solvent effect cannot be discounted, and should thus be considered when analyzing the copolymerization data of systems involving strongly electrophilic and nucleophilic monomer pairs. 

Trivalent carbenium ions play a key role, not only in the acid-catalyzed polymerization of alkenes [Eq. (5.348)] but also in the polycondensation of arenes (7r-bonded monomers) as well as in the cationic polymerization of ethers, sulfides, and nitrogen compounds (nonbonded electron-pair donor monomers). On the other hand, penta-coordinated carbonium ions play the key role in the electrophilic reactions of cr-bonds (single bonds), including the oligocondensation of alkanes and alkenes (Section 5.1.5). 

Living polymers can only exist in aprotic solvents. They are killed by water, oxygen and a high number of electrophilic substances. Operating in absence of killing impurities one obtains stable living species they are ionic species whose exact form (free ion, contact ion-pair, solvent separated ion-pair1417 depends upon the concentration and the nature of the monomer, the counterion and the solvent polar... 

This part of the review has stressed the importance of the electrophilic character of the cation and the role of surfaces in stereospedfic coordinated polymerizations of non-polar monomers. The same factors hold generally for polar monomers with two important differences. The catalyst coordinates with electron pairs on the electro-negative atoms rather than with the jr-electrons of the unsaturation. Furthermore, surfaces are not essential because the electronegative groups can provide the additional points of attachment needed for consistent spatial orientation relative to the chain end. 

This review deals with polymerizations initiated by a molecule, in which at least one author believes macrozwitterions are formed. The majority are of the simple addition type and are subdivided into vinyl, carbonyl, and strained ring. The other types are all charge cancellations. Monomers are either preformed zwitterions, highly polarised molecules (i.e. CH3CNO) or zwitterions are formed in situ by nucleophile/electrophile pairs. 

The chemical details of the initiation reaction can vary from one stem to another. Thus the carbenium ion can add on to the monomer double bond in a classical electrophilic reaction or it can abstract a hydride ion or an electron from the monomer. The first process seems to be the most common according to the experimental evidence gathered to date. Whatever the nature of this interaction, it remains to be established if the active ecies formed from them are also ionic and if so, if they are predcminantly free ions or ion pairs. This is undoubtedly the most delicate problem implicit in studies of this type. It has been often assumed that since the initiator was essentially composed of free ions in the low concentrations used it would follow that the same would apply to the drain carriers derived from it. However, it has become apparent that this asmmption is generally unjustified since the stability of the carbenium ions formed from the mono-... 

Coordination of the anions to the cationic palladium center may strongly depend on the polarity of the reaction medium. Solvation of the ion-pair by protic solvent molecules, such as methanol, is expected to facilitate cation-anion dissociation and therefore render the metal center more electrophilic and more easily accessible for substrate molecules. In relatively apolar solvents, close-contact ion-pairs are generally expected to exist. Anion displacement by substrate molecules may then require the use of noncoordinating anions, such as certain tetraaryl borates [19], with a relatively strong affinity for interaction with the solvent molecules. This will lead to a reduced barrier for displacement of these anions by monomer molecules. 

Initiators include strong protonic acids and the electrophilic species generated by reaction of a Lewis acid with water, an alcohol, ester or alkyl halide (Scheme 6). In this case, initiation occurs by electrophilic addition of the vinyl monomer to the proton or carbenium ion, which may be accompanied by reversible or irreversible collapse of the ion pair (Eq. 21). 

Reaction of a nucleophile with an electrophile Indicates the ease with which the electron cloud of an atom can be distorted A molecule in which one or more subunits (called monomers) is repeated many times A solvent that is a hydrogen bond donor A mixture of equal amounts of a pair of enantiomers... 

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