Addition anionic initiation

The addition of living poly(styrene) to AIBN leads finally, especially for high coupling efficiencies, to the elimination of one nitrile group [72]. More recently, Ren et al. [73] have used bis(2-chloroethyl)2,2 -azodiisobu-tyrate (see scheme 19) to terminate anionically initiated poly(butadiene) chains. Since the azo transfer agent possesses two functional groups (Cl) that are able to termi-... 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

As is the case for cationic polymerisation, anionic polymerisation can terminate by only one mechanism, that is by proton transfer to give a terminally unsaturated polymer. However, proton transfer to initiator is rare - in the example just quoted, it would involve the formation of the unstable species NaH containing hydride ions. Instead proton transfer has to occur to some kind of impurity which is capable for forming a more stable product. This leads to the interesting situation that where that monomer has been rigorously purified, termination cannot occur. Instead reaction continues until all of the monomer has been consumed but leaves the anionic centre intact. Addition of extra monomer causes further polymerisation to take place. The potentially reactive materials that result from anionic initiation are known as living polymers. 

The monomer 19 can also be polymerized using analogous methods of initiation to those employed in organic polymer science. Radical initiators afford regioirregular polymers, whereas anionic initiators add selectively to the phosphorus atom of the P=C bond and thus yield a regioregular polymer [85]. The product of the initial addition of MeLi across the P=C bond, Mes(Me)P-CPh2Li, was identified spectroscopically. The polymers obtained from anionic initiation are spectroscopically identical to those obtained from the thermolysis. Reasonable molecular weights (ca. 5000-10,000 g mol 0 are obtained when methyllithium is used as an initiator. 

The Claisen reaction may be visualized as initial formation of an enolate anion from one molecule of ester, followed by nucleophilic attack of this species on to the carbonyl group of a second molecule. The addition anion then loses ethoxide as leaving group, with reformation of the carbonyl group. 

Alkene polymers such as poly(methyl methacrylate) and polyacrylonitrile are easily formed via anionic polymerization because the intermediate anions are resonance stabilized by the additional functional group, the ester or the nitrile. The process is initiated by a suitable anionic species, a nucleophile that can add to the monomer through conjugate addition in Michael fashion. The intermediate resonance-stabilized addition anion can then act as a nucleophile in further conjugate addition processes, eventually giving a polymer. The process will terminate by proton abstraction, probably from solvent. 

Electron-transfer initiation from other radical-anions, such as those formed by reaction of sodium with nonenolizable ketones, azomthines, nitriles, azo and azoxy compounds, has also been studied. In addition to radical-anions, initiation by electron transfer has been observed when one uses certain alkali metals in liquid ammonia. Polymerizations initiated by alkali metals in liquid ammonia proceed by two different mechanisms. In some systems, such as the polymerizations of styrene and methacrylonitrile by potassium, the initiation is due to amide ion formed in the system [Overberger et al., I960]. Such polymerizations are analogous to those initiated by alkali amides. Polymerization in other systems cannot be due to amide ion. Thus, polymerization of methacrylonitrile by lithium in liquid ammonia proceeds at a much faster rate than that initiated by lithium amide in liquid ammonia [Overberger et al., 1959]. The mechanism of polymerization is considered to involve the formation of a solvated electron ... 

The chain transfer constant for an additive or solvent in the polymerization can be determined. This value can then be compared with the transfer constants for the same substance in the polymerization of the same monomer by known radical, cationic, and anionic initiators. 

The polymerization mechanism involves addition of the anionic initiator to the carbon-carbon double bond of the monomer... 

The amide anion initiates polymerization via a similar reaction sequence by addition of L to monomer. Propagation follows in a like manner ... 

Lactones are easily polymerized by both cationic and anionic initiators, and in fact they (particularly /3-propiolactone) are known to polymerize explosively and unpredictably without addition of any initiators. Inhibitors, such as trialkylborates, have been used to prevent undesired polymerization (73GEP2255194). 

The deactivation reaction transfers an active catalyst into the inert (non-reactive) polymer. This phenomenon, when cyclic sulfonium zwitterions act as anionic initiators, can be utilized for the control of the cyclotrimerization of difunctional isocyanates. Therefore the degree of oligomerization of difunctional isocyanates can be controlled by the concentration of the initiator, rate of addition of the initiator, as well as by the temperature of the reaction system. 

In this paper graft copolymerization onto both polystyrene and styrene-butadiene block copolymer will be discussed. It will be shown that radical processes do not permit the addition of monomers onto polystyrene and that one must use anionic initiation in order... 

There are some important differences between anionic and free-radical addition. First, unlike free-radical initiators, which decompose and start chains randomly throughout the course of the reaction, anionic initiators ionize readily in fairly polar organic solvents or at low concentrations in hydrocarbons, and chains are started immediately, one for each molecule of initiator. Second, in the absence of impurities, there is no termination,... 

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