Nucleophilic monomer pair

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. 

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. 

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 choice of counterions (anions) in the cationic polymerization of heterocyclic monomers can be almost as wide as in anionic polymerization, but only for the most nucleophilic monomers (i. e. cyclic amines). Unfortunately, in the polymerization of cyclic ethers, this choice is much more restricted. Thus, the small anions like F or OH cannot be used because, due to their high nucleophilicity and ability to form covalent bonds, they give rise to fast termination. In order to suppress or even to eliminate termination by collapse within an ton pair (cf. Sect. 5.1.), it is necessary to use complexed anions having large ionic radii. These are shown below (rctyst)-... 

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. 

During this, the electrons of the partial X—Z multiple bond are used. Experiments show that the ester can be further active in the polymerization. Its reactivity, however, is reduced in comparison with ion pairs. From a mechanistical point of view, the chain propagation should proceed in the manner of a SN2 reaction, that is with the monomer as nucleophile and the ester as substrate. With the assistance of quantum chemical calculations using the CNDO/2 method, the differences between covalent species and free ions should be examined. The following contains the three types of anions used ... 

These and related heterocyclic monomers are usually highly polar and strongly nucleophilic conpounds. During polymerization chains containing heteroatoms are formed and they can, as well as monomers themselves, interact with components of ionic growing species. The interaction of the macroion-pairs with the elements of the chains has well been documented for the polymerization of ethylene oxide [Z]. 

These centres are formed by the addition of monomer to a suitable anion. They are almost always simpler than their cationic reverse part. The counter ion is usually a metal cation able to interact with the electrons of the growing end of the macromolecule, and to bind in its ligand sphere monomer or solvent molecules or parts of the polymer chain. This changes the properties of the whole centre. Therefore, by selection of the metal, the stability of the centre, the tendency of the centres to aggregation, the position of the equilibrium between the contact and solvent-separated ion pairs and free ions, and the stereoselectivity of the centre [the ability to produce polymers with an ordered structure (tacticity, see Chap. 5, Sect. 4.1)] are predetermined. The chemical reactions of the metal cations are, however, very limited. Most solvents and potential impurities are of nucleophilic character. They readily solvate the cation, leaving the anion relatively free. The determination... 

The overall consumption rate of the covalent precursor (ks) is determined by HPLC and/or titration measurements this correlates with monomer consumption in propagation. The rate of racemization of optically active 1-phenylethyl chloride (ka) is determined by polarimetric measurements, Racemization is usually faster than solvolysis, confirming that activation is reversible and that internal return may occur before the carbenium ion reacts with an external nucleophile, Racemization requires not only that the C—Cl bond of the covalent precursor is broken, but that the lifetime of the ion pair is long enough for the flat carbenium ion to rotate, such that both sides of the carbenium ion are completely equivalent as shown in Eq. 18. 

The styrene/methyl methacrylate pair contains monomers with different relative reactivity levels in Table 9-1. Polystyryl anion will initiate the polymerization of methyl methacrylate, but the anion of the latter monomer is not sufficiently nucleophilic to cross-initiate the polymerization of styrene. Thus the anionic polymerization of a mixture of the two monomers yields polyfmethyl methacrylate) while addition of methyl methacrylate to living polystyrene produces a block copolymer of the two monomers. 

The ether oxygen is a Lewis base (electron donor), and polymerization of vinyl and cyclic ethers can be initiated by reaction with an ion pair comprising an acidic cation and a weakly nucleophilic base. These monomers do not polymerize by free-radical or anionic processes. Thio ethers behave similarly. 

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