Propagating site

The use of an unsaturated anionic initiator—such as potassium p-vinyl benzoxide—is possible for the ring opening polymerization of oxirane [43]. Although initiation is generally heterogenous, the polymers exhibit the molecular weight expected and a low polydispersity. In this case, the styrene type unsaturation at chain end cannot get involved in the process, as the propagating sites are oxanions. 

In view of the great structural similarity between the propagating sites in the cationic polymerization of P-PIN and isobutylene and their respective polymers (4), and our considerable experience accumulated with the LC Pzn of isobutylene [1-3], efforts have been made to adapt LC Pzn conditions found to yield living polyisobutylenes for the polymerization of p-PIN. 

For NMR studies of polymer mixtures, the earliest approach proposed was the Coleman-Fox model.(5) This model assumes the coexistence of two interconverting Bemoullian propagating sites and was used extensively for poly(methyl methacrylate).(6-8)... 

There are two strategies for constructing a polymer with benzyl ester in the middle of the skeleton. One is to make polymer skeletons with the same molecular weight, and then combine two skeletons with benzyl ester. The other is to synthesize a chemical with propagation sites of polymerization at both sides of benzyl ester, and use the chemical as an initiator of living polymerization. Since the former strategy did not work well, probably due to low reactivity of polymer molecules with benzyl esters, the latter approach will be mentioned. 

The first viewpoint contradicts the autocatalytic character of the reaction, conductometric measurements in the polymerization system and some other facts (see below). Scheme (33) can be considered as completely experimentally substantiated. The following important proofs were obtained A direct experimental discovery of a quaternary ammonium alcoholate in the reaction system, 42) a full agreement of the nature of the active propagating site with all the existing kinetic and structural data l4,149 153 157 I58) establishment of the ionic behaviour of the propagating sites by comparison of the kinetic curves of the process with the character of the electric... 

Thus, in the presence of alcohols or other proton donors the polymerization of epoxy compounds under the action of TA proceeds according to the anionic mechanism to give quaternary ammonium alcoholate as the active propagating site [Scheme (33)]. 

This means that the active propagating sites such as free alkoxy ions and ion pairs are solvated with the hydroxyl groups. This must lead to an increase in the reactivity of the solvated ion pair as compared with that of the contact ion pair and decrease in the free ion reactivity, i.e. ultimately to the levelling off of the reactivity differences of these particles. 

Both reactions lead to the appearance of end hydroxyl and unsaturated groups in the polymer. The accumulation rate of these groups is expected to be controlled by the concentration of the active propagating sites (see Fig. 13) in accordance with Scheme (43). 

Determination of propagation rate constants in cationic (and in anionic) systems is complicated by the simultaneous occurrence of different types of propagating sites. In olefin polymerizations, some portion of the active centers may exist as free ions and others as ion pairs of varying degrees of solvation. In the solvents in which cationic polymerizations are normally carried out, the polymerization is mainly due to free ions. In low dielectric constant media like benzene or hydrocarbon monomers, however, ion pairs will dominate the reaction. 

In solvents of high dielectric constant, (e.g. dimethylformamide), formaldehyde polymerized sluggishly and polymers were formed in low yield. In similar solvents, aliphatic aldehydes could not be polymerized. Precipitated aldehyde polymers have the tendency to absorb monomers. The monomer concentration near the propagating site may be much hi er than that in the surrounding solution. This occurs with n-butyraldehyde polymerizations in pentane [5] the polymer precipitated during the polymerization is highly swollen by the monomer. 

This method for the preparation of poly(styrene-fc-tBuA) is based upon the procedure described by Jerome et al. Teyssie and co-workers demonstrated that the addition of LiCl can be effective in the living anionic polymerization of the acrylic monomers, because a p,-type complex" is formed between LiCl and the growing site. This complex prevents the occurrence of side-reactions at the propagating site, thus markedly narrowing the molecular weight distribution. 

The bulky anion then stabilizes the intermediate adduct from protonation of the epoxy group and then facilitates insertion of epoxide at the cationic propagation site. Rapid polymerization can then occur. Cationic photopolymerization of epoxides often involves the photo-generation of acid from an initiator such as diaryliodonium or triaryl sulfonium salts (Crivello, 1999). The anions are important in controlling the addition at the cationic site and are typically BF4 and PFg. The reactivity of the system depends also on the structure of the epoxide. 

Eor chain-growth polymerization to occur by a radical mechanism, a radical initiator must be added to the monomer to convert some of the monomer molecules into radicals. The initiator breaks homolytically into radicals, and each radical adds to an alkene monomer, converting it into a radical. This radical reacts with another monomer, adding a new subunit that propagates the chain. The radical site is now at the end of the most recent unit added to the end of the chain. This is called the propagating site. 

This process is repeated over and over. Hundreds or even thousands of alkene monomers can add one at a time to the growing chain. Eventually, the chain reaction stops because the propagating sites are destroyed. Propagating sites can be destroyed when two chains combine at their propagating sites when two chains undergo disproportionation, with one chain being oxidized to an alkene and the other being reduced to an alkane or when a chain reacts with an impurity that consumes the radical. 

Head-to-tail addition is favored for steric reasons because the propagating site preferentially attacks the less sterically hindered unsubstituted sp carbon of the alkene. Groups that stabilize radicals also favor head-to-tail addition. For example, when Z is a phenyl substituent, the benzene ring stabilizes the radical by electron delocalization, so the propagating site is the carbon that bears the phenyl substituent. 

If the propagating site abstracts a hydrogen atom from a chain, a branch can grow off the chain at that point. 

Cationic polymerization can be terminated by loss of a proton or by addition of a nucleophile that reacts with the propagating site. The chain can also be terminated by a chain-transfer reaction with the solvent (XY). 

Monomers that are best able to undergo polymerization by a cationic mechanism are those with substituents that can stabilize the positive charge at the propagating site by donating electrons inductively or by resonance. Examples of monomers that undergo cationic polymerization are given in Table 28.4. 

In anionic polymerization, the initiator is a nucleophile that reacts with the alkene to form a propagating site that is an anion. Nucleophilic attack on an alkene does not occur readily because alkenes are themselves electron rich. Therefore, the initiator must be a very good nucleophile, such as sodium amide or butyllithium, and the alkene must contain an electron-withdrawing substituent to decrease its electron density. Some alkenes that imdergo polymerization by an anionic mechanism are shown in Table 28.5. 

The chain can be terminated by a chain transfer reaction with the solvent or by reaction with an impurity in the reaction mixture. If the solvent cannot donate a proton to terminate the chain and if all impurities that can react with a carbanion are rigorously excluded, chain propagation will continue until all the monomer has been consumed. At this point, the propagating site will still be active, so the polymerization reaction will continue if more monomer is added to the system. Such nonterminated chains are called living polymers because the chains remain active until they are killed. Living polymers usually result from anionic polymerization because the chains caimot be terminated by proton loss from the polymer, as they can in cationic polymerization, or by disproportionation or radical recombination, as they can in radical polymerization. 

Chain-growth polymers are made by chain reactions— by the addition of monomers to the end of a growing chain. These reactions take place by one of three mechanisms radical polymerization, cationic polymerization, or anionic polymerization. Each mechanism has an initiation step that starts the polymerization, propagation steps that allow the chain to grow at the propagating site, and termination steps that stop the growth of the chain. The choice of mechanism depends on the stmcture of the monomer and the initiator used to activate the monomer. In radical polymerization, the initiator is a radical in cationic polymerization, it is an electrophile and in cationic polymerization, it is a nucleophile. Nonterminated polymer chains are called living polymers. 

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