Termination reaction anionic

Ionic grafting usually leads to well defined copolymers due to the limited (if any) termination reactions. Anionic sites can be created by metallation of the backbone. This can be accomplished by complexation of several types of C-H bonds (allylic, benzylic, aromatic) with organometallic compounds such as sec-BuLi. Usually chelating compounds, for example N,N,N/>N/-tetra methyl ethylene diamine (TMEDA), act as solvating bases facilitating the reaction. By this method PB-g-PS [36-38] and Pl-g-PS [39] graft copolymers have been prepared. 

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

The chief feature of anionic polymerizations in aprotic solvents is that they involve only two reactions initiation and propagation. Spontaneous transfer or termination reactions will not take place, if proper systems and adequate reaction conditions are chosen. 

Polymer molecules with just one or a few ionic groups, in most cases terminal and anionic, are called macroions. They are encountered primarily in living polymers, polymer molecules present in a polymerizing reaction system that will grow as long as monomers (e.g., esters or nitriles of methacrylic acid) continue to be supplied. The ionic charge of the macroion always transfers to the last monomer added, keeping the macroion ready for the next such addition. 

Polymerisation are very similar in nature except in their termination reactions. Terminations are very easy in case of cationic polymerisation whereas it is absent in case of anionic polymerisation. That is... 

There are also some hydroxyl groups formed by combination of the carbonium ion with the OH from the anion BF3OH in a termination reaction ... 

Reactions in which a reagent is cloven (i) The termination reactions of the type (IX) in which an anionic fragment is abstracted from the anion, so that the chain-carrier is neutralised. This type of termination has been claimed to occur in many systems for example, in the polymerization of tetrahydrofuran by PF5 a terminal F from PF 6 was indeed found [119]. 

The termination - A termination reaction which has been favoured by many authors involves the transfer of a proton to the anion, with regeneration of the catalytic complex. This has been criticised qualitatively before [2, 20] but we can now give some precision to these arguments. 

As Skinner has pointed out [7], there is no evidence for the existence of BFyH20 in the gas phase at ordinary temperatures, and the solid monohydrate of BF3 owes its stability to the lattice energy thus D(BF3 - OH2) must be very small. The calculation of AH2 shows that even if BFyH20 could exist in solution as isolated molecules at low temperatures, reaction (3) would not take place. We conclude therefore that proton transfer to the complex anion cannot occur in this system and that there is probably no true termination except by impurities. The only termination reactions which have been definitely established in cationic polymerisations have been described before [2, 8], and cannot at present be discussed profitably in terms of their energetics. It should be noted, however, that in systems such as styrene-S C/4 the smaller proton affinity of the dead (unsaturated or cyclised) polymer, coupled, with the greater size of the anion and smaller size of the cation may make AHX much less positive so that reaction (2) may then be possible because AG° 0. This would mean that the equilibrium between initiation and termination is in an intermediate position. 

On the other hand, the very nature of the co-catalytic function implies that at least a part of the co-catalyst molecule is consumed in the course of the reaction. In other words, of the ions formed by interaction of catalyst and co-catalyst, the cation must, and the anion may be incorporated in the polymer, e.g., whenever an acid is the co-catalyst, the proton is transferred during the initiation reaction to a monomer molecule which then forms the first link in the chain. The anion may or may not become attached to the end of a polymer molecule in a termination reaction. Similarly, when an alkyl halide acts as co-catalyst [6, 11], the alkyl cation necessarily forms the start of a chain, and a halide ion may be incorporated in a termination reaction. 

The vinyloxy anion formed in this way is much less reactive than hydrocarbon anions and it might be expected that therefore the termination reaction with the propagating cation would be slower. 

The need for well defined polymer species of low polydls-perelty and of known structure arises from the Increasing Interest In structure-properties relationship In dilute solution as well as In the bulk. A great variety of methods have been attempted, to synthesize so-called model macromolecules or tailor made polymers-over the past 20 years. The techniques based on anionic polymerization, when carried out In aprotic solvents, have proved best suited for such synthesis, because of the absence of spontaneous transfer and termination reactions that characterize such systems. The "living 1 polymers obtained are fitted at chain end with carbanionic sites, which can either Initiate further polymerization, or react with various electrophilic compounds, intentionally added to achieve functionalizations. Another advantage of anionic polymerizations is that di-functlonal Initiators are available, yielding linear polymers fitted at both chain ends with carbanionic sites. In this paper we shall review the various utility of anionic polymerization to the synthesis of tailor made well defined macromolecules of various types. 

A reaction of chain transfer to alcohol or other proton-donor compounds is the most common process among the chain termination reactions in the anionic polymerization of the epoxy compounds in proton media, including the action of TA ... 

The second important chain termination reaction characteristic of the catalysis of anionic polymerization of epoxy compounds by the TA consists in the abstraction of the hydrogen atom from the p-carbon atom in the tetraalkylammonium cation by the growing alkoxy anion (P-elimination reaction)I58 164). 

Studies reported on the hydrocarbon monomers show that there are three main areas of ionicities which produce different initiation, termination and termination reactions. The strong cationic systems involve the transfer or elimination of protons or carbonium ions. This has been well reviewed by Kennedy and Langer (1). At the other extreme, strong anionic systems react by hydride transfer. For the olefinic monomers, this region extends to include alkyl aluminum which undergo easy exchange to produce dimers (72). 

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