Polymerization, anionic mechanism

The dimer behaves simultaneously as a radical and as a carban-ion, and thus the radical end might grow by a radical mechanism, anionic polymerization proceeding from the carbanion end. This behavior is particularly interesting when two monomers are present in the system, one polymerizable by a radical but not by an anionic mechanism, the other behaving in the opposite sense. In such a hypothetical case the resulting product would be a block polymer, -A—A. . . A—B—B. . . B-. 

Cyclic phosphates polymerize by both cationic and anionic mechanisms. Anionic polymerization is the method of choice for preparation of high molecular weight polymers because in cationic polymerization the reaction of exocyclic ester group leads to chain transfer ... 

Cyanoacrylate esters readily polymerize by several different mechanisms. Anionic polymerization is the most common and the most important method with respect to their use as adhesives. Typically this process is initiated by the nucleophilic contaminants found on the surfaces being bonded. The other routes by which cyanoacrylates polymerize or copolymerize involve radically initiated and photochemically initiated reactions. Radical polymerization or copolymerization of cyanoacrylates usually does... 

Glycidol polymerizes also by anionic mechanism. Anionic polymerization leading to hyperbranched multihydroxyl polyethers has been studied extensively and possible applications of polymers obtained by anionic polymerization have been indicated. "" ... 

Polymerization can take place by an ionic or by a free radical mechanism. Anionic polymerization is rare because nucleophiles do not usually react with the olefinic bond, but cationic polymerization is more frequent ... 

Just as anionic polymerizations show certain parallels with the free-radical mechanism, so too can cationic polymerization be discussed in terms of the same broad outline. There are some differences from the anionic systems, however, so the fact that both proceed through ionic intermediates should not be overextended. 

Although the anionic polymerization mechanism is the predominant one for the cyanoacryhc esters, the monomer will polymerize free-radically under prolonged exposure to heat or light. To extend the usable shelf life, free-radical stabilizers such as quinones or hindered phenols are a necessary part of the adhesive formulation. 

The mechanism of the anionic polymerization of styrenes and 1,3-dienes initiated by alkaU metals has been described in detail (3,20) as shown in equations 3—5 where Mt represents an alkaU metal and M is a monomer molecule. Initiation is a heterogeneous process occurring on the metal surface. The... 

The mechanism of anionic polymerization of cyclosiloxanes has been the subject of several studies (96,97). The first kinetic analysis in this area was carried out in the early 1950s (98). In the general scheme of this process, the propagation/depropagation step involves the nucleophilic attack of the silanolate anion on the sUicon, which results in the cleavage of the siloxane bond and formation of the new silanolate active center (eq. 17). 

Proliferous Polymerization. Eady attempts to polymerize VP anionicaHy resulted in proliferous or "popcorn" polymerization (48). This was found to be a special form of free-radical addition polymerization, and not an example of anionic polymerization, as originally thought. VP contains a relatively acidic proton alpha to the pyrroHdinone carbonyl. In the presence of strong base such as sodium hydroxide, VP forms cross-linkers in situ probably by the following mechanism ... 

Polyethylene glycol), or Carbowax, is made by anionic polymerization of ethylene oxide using NaOH as catalyst. Propose a mechanism. 

The most common mechanism of termination in anionic polymerization involves reactions with solvents or with impurities. For... 

There is a substantial literature on the thermal and photochemical degradation of PS and it is well established that polymer properties are sensitive to the manner in which a particular sample of PS is prepared. For example, it has been reported that PS prepared by anionic polymerization shows enhanced stability with respect to that prepared by a radical mechanism.2 10 This has often been attributed to the presence of "weak links" in the latter polymers. However, the precise nature of the "weak links" remains the subject of some controversy. T he situation is further confused by all PS prepared by radical mechanisms often being considered as a class without reference to the particular polymerization conditions employed in their preparation. In many cases the polymers are "commercial samples" with details of the method of preparation incomplete or unstated. 

Szwarc, M. Living Polymers and Mechanisms of Anionic Polymerization. Vol. 49, pp. 1-175. 

Anionic polymerization of lactams was shown to proceed according to what is called the activated monomer mechanism. With bischloroformates of hydroxy-terminated poly(tetramethyleneglycol) and poly(styrene glycol) as precursors for a polymeric initiator containing N-acyl lactam ends, block copolymers with n-pyrrol-idone and e-caprolactam were obtained by bulk polymerizations in vacuum at 30 and 80 °C, respectively361. ... 

An interesting feature of the ring opening polymerization of siloxanes is their ability to proceed via either anionic or cationic mechanisms depending on the type of the catalyst employed. In the anionic polymerization alkali metal hydroxides, quaternary ammonium (I NOH) and phosphonium (R POH) bases and siloxanolates (Si—Oe M ) are the most widely used catalysts 1,2-4). They are usually employed at a level of 10 2 to KT4 weight percent depending on their activities and the reaction conditions. The activity of alkali metal hydroxides and siloxanolates decrease in the following order 76 79,126). 

The mechanism of anionic polymerization of styrene and its derivatives is well known and documented, and does not require reviewing. Polymerization initiated in hydrocarbon solvents by lithium alkyls yields dimeric dormant polymers, (P, Li)2, in equilibrium with the active monomeric chains, P, Li, i.e. 

However, the mechanisms by which the initiation and propagation reactions occur are far more complex. Dimeric association of polystyryllithium is reported by Morton, al. ( ) and it is generally accepted that the reactions are first order with respect to monomer concentration. Unfortunately, the existence of associated complexes of initiator and polystyryllithium as well as possible cross association between the two species have negated the determination of the exact polymerization mechanisms (, 10, 11, 12, 13). It is this high degree of complexity which necessitates the use of empirical rate equations. One such empirical rate expression for the auto-catalytic initiation reaction for the anionic polymerization of styrene in benzene solvent as reported by Tanlak (14) is given by ... 

Yijin X. and Caiyaun P., Block and star-hlock copolymers by mechanism transformation. 3. S-(PTHF-PSt)4 and S-(PTHF-PSt-PMMA)4 from living CROP to ATRP, Macromolecules, 33, 4750, 2000. Feldthusen J., Ivan B., and Mueller A.H.E., Synthesis of linear and star-shaped block copolymers of isobutylene and methacrylates hy combination of living cationic and anionic polymerizations. Macromolecules, 31, 578, 1998. 

FIGURE 2 Anionic, cationic, and coordination mechanisms of polymerization of e-caprolactone and related lactones. 

Analogous principles should apply to ionically propagated polymerizations. The terminus of the growing chain, whether cation or anion, can be expected to exhibit preferential addition to one or the other carbon of the vinyl group. Poly isobutylene, normally prepared by cationic polymerization, possesses the head-to-tail structure, as already mentioned. Polystyrenes prepared by cationic or anionic polymerization are not noticeably different from free-radical-poly-merized products of the same molecular weights, which fact indicates a similar chain structure irrespective of the method of synthesis. In the polymerization of 1,3-dienes, however, the structure and arrangement of the units depends markedly on the chain-propagating mechanism (see Sec. 2b). 

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