Chain initiation anionic polymerization

What species is produced by the reaction of an anionic chain polymerization initiator and the monomer ... 

Which of the following could serve as an initiator for an anionic chain polymerization (a) AICI3 H2O, (b) BF3 H2O, (c) butyllithium, or (d) sodium metal ... 

Some monomers are also polymerized by a cationic mechanism in a series of steps not too unlike those of anionic chain-growth. Initiators are often Lewis acids such as AICI3. The polymerization is not quite as straightforward as anionic, because for one thing cationic intermediates are subject to more side reactions. Common monomers that undergo cationic polymerization include styrene, isobutylene, and vinyl acetate. Some commercial products... 

The determination of the various rate constants (ki, kp, kt, kts, ktr) for cationic chain polymerization is much more difficult than in radical chain polymerization (or in anionic chain polymerization). It is convenient to use Rp data from experiments under steady-state conditions, since the concentration of propagating species is not required. The Rp data from non-steady-state conditions can be used, but only when the concentration of the propagating species is known. For example, the value of kp is obtained directly from Eq. (8.143) from a determination of the polymerization rate when [M J is known. The literature contains too many instances where [M" "] is taken equal to the concentration of the initiator, [IB], in order to determine kp from measured Rp. (For two-component initiator-coinitiator systems, [M" ] is taken to be the initiator concentration [IB] when the coinitiator is in excess or the coinitiator concentration [L] when the initiator is in excess.) Such an assumption holds only if Ri > Rp and the initiator is active, i.e., efficiency is 100%. Using this assumption without experimental verification may thus lead to erroneous results. 

In chain polymerization initiated by free radicals, as in the previous example, the reactive center, located at the growing end of the molecule, is a free radical. As mentioned previously, chain polymerizations may also be initiated by ionic systems. In such cases, the reactive center is ionic, i.e., a carbonium ion (in cationic initiation) or a carbanion (in anionic initiation). Regardless of the chain initiation mechanism—free radical, cationic, or anionic—once a reactive center is produced it adds many more molecules in a chain reaction and grows quite large extremely rapidly, usually within a few seconds or less. (However, the relative slowness of the initiation stage causes the overall rate of reaction to be slow and the conver-... 

In the context of the anionic polymerization of CA derivatives, as considered in Section 10.3.2.2.1, it is notable that the polymerization of cyanoacrylates is also photoinitiated by substituted pyridine pentacarbonyl complexes of tungsten or chromium, i.e. M(CO)5L with M = Cr or W, and L = 2- or 4-vinylpyridine [60]. Photo-released pyridine adds to CA, and the resulting zwitterion initiates the anionic chain polymerization (see Scheme 10.16). 

The kinetic picture of anionic chain polymerization also depends mostly upon the specific reaction. For those that are initiated by metal amides in liquid anunonia, the rate of initiation can be shown to be as follows ... 

Cyanoacrylate adhesives consist mainly of alkylcyanoacrylate monomer, which undergo rapid anionic chain polymerization when the adhesive bond is closed. Hydroxide ions in absorbed water are generally thought to be the initiators, and the rapidity of anionic polymerization is due to two electron-withdrawing groups (-CN and -COOR), which stabilize the propagating anion. The initiation step is... 

Anionic chain polymerization can be initiated by metal alkoxides, aryls and alkyls and electron-transfer from sodium naphthalene. Alkyllithiums are among the most useful, being employed commercially in the polymerization of 1,3-butadiene, isoprene, and styrene. Initiation involves addition of alkyl anion to monomer... 

There are, however, several highly reactive vinyl monomers such as 2-(trifluoromethyl) acrylates and 2-cyanoacrylates that undergo anionic polymerizations in the presence of even weak bases. The photoinitiated anionic polymerizations of these monomers have been achieved using a number of photosensitive metal complexes. For example, the irradiation of alkali salts containing the trans-[Cr(NH3)2(NCS)4] anion at wavelengths in the range of 350-532 nm releases the thiocyanate anion (SCN"). As depicted in Scheme 34, the thiocyanate anion is capable of initiating the anionic chain polymerization of ethyl 2-cyanoacrylate. ... 

The second method uses the anionic chain polymerization of the heterocycle, whose mechanism (complex) was presented in Section 8.6.4. The activated monomer sodium lactamide is used as initiator. This method allows the preparation of statistical copolymers—for example, with lauryllactam, which is the monomer molecule of PA-12. 

SBR is prepared by anionic polymerization initiated by lithium alkyls in cycloaliphatic media as a solvent. The main feature of this class of anionic chain polymerization is that it is a living polymerization, i.e., the polymeric chain ends are able to survive even when monomer is completely depleted and to reinitiate the polymerization reaction when monomer is newly added. Due to the absence of termination reactions, pol5mier active chain ends do not inherently terminate, continuously growing up to tiie complete depletion of monomers this in turn means that the average pol5uner molecular weight can be predicted from the amount of starting material and tiie quantity of the initiator. 

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 ... 

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. 

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

Monomers, such as ethylene, propylene, isobutylene, and isoprene, containing the carbon-carbon double bond undergo chain polymerization. Polymerization is initiated by radical, anionic or cationic catalysts (initiators) depending on the monomer. Polymerization involves addition of the initiating species R, whether a radical, cation, or anion, to the double bond followed by its propagation by subsequent additions of monomer... 

Fig. 51. Schematic illustration of the mechanism of microgel formation in the anionic dispersion polymerization of 1,4-DVB initiated by living PBS chains in heptane. [Reprinted with permission from Ref. 247, Copyright 1995, American Chemical Society].

Pille et al. used living PBS chains to initiate the anionic polymerization of EGDM and 1,4-butanediol dimethacrylate. They obtained highly crosslinked microgels together with slightly branched oligomers of PBS of a low molar mass [260]. 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

Traditional polymerizations usually involve AB-type monomers based on substituted ethylenes, strained small ring compounds using chain reactions that may be initiated by free radical, anionic or cationic initiators [20]. Alternatively, AB-type monomers may be used in polycondensation reactions. 

FIGURE 6.2 Type of chain initiation for some common monomers in order of general decrease in electron density associated with the double bond and their tendency to undergo chain polymerization where A = anionic, C = cationic, and F = free radical. 

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