Of anionic polymerization

In the case of anionic polymerization (with 2-isoprOpenylthiazole) there is a chain-monomer equilibrium. Furthermore, lowering the temperature of polymerization increases the conversion of monomer to polymer (314). 

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). 

VEs such as MVE polymerize slowly in the presence of free-radical initiators to form low mol wt products of no commercial importance (9). Examples of anionic polymerization are unknown, whereas cationic initiation promotes rapid polymerization to high mol wt polymers in excellent yield and has been extensively studied (10). 

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 ... 

Block copolymer chemistry and architecture is well described in polymer textbooks and monographs [40]. The block copolymers of PSA interest consist of anionically polymerized styrene-isoprene or styrene-butadiene diblocks usually terminating with a second styrene block to form an SIS or SBS triblock, or terminating at a central nucleus to form a radial or star polymer (SI) . Representative structures are shown in Fig. 5. For most PSA formulations the softer SIS is preferred over SBS. In many respects, SIS may be treated as a thermoplastic, thermoprocessible natural rubber with a somewhat higher modulus due to filler effect of the polystyrene fraction. Two longer reviews [41,42] of styrenic block copolymer PSAs have been published. 

The preparation of ABA triblock polymers requires use of a telechelie bisthiol prepared by termination of anionic polymerization initiated by a difunctional initiator. The relative yields of homopolymer, di- and triblock obtained in these experiments depend critically on conversion.273... 

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

Some of the results of bulk polymerization of 61 by using different anionic catalysts are summarized in Table 858 It was easily polymerized in the presence of alkali metal compounds above 60 °C. The polymerization at 150 °C was too fast to be controlled. The yield and the viscosity number, i gp/c, of the resulting polyamide increased with the reaction time. The initial rate of the polymerization became higher with the size of the countercation, in analogy to the case of anionic polymerization of e-caprolactam59. The rate increased also with raising temperature as shown in Fig. 658. ... 

Our understanding of the intricacies of anionic polymerization of methyl methacrylate was greatly improved during the last 15years by the meticulous and persistent work of the Mainz group. To appreciate fully the progress made in this field it is advisable to summarize the older studies and the ideas developed in those days. 

The difficulties encountered in the early studies of anionic polymerization of methyl methacrylate arose from the unfortunate choice of experimental conditions the use of hydrocarbon solvents and of lithium alkyl initiators. The latter are strong bases. Even at —60 °C they not only initiate the conventional vinyl poly-addition, but attack also the ester group of the monomer yielding a vinyl ketone1, a very reactive monomer, and alkoxide 23). Such a process is described by the scheme. 

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. 

Some new initiators soluble in hydrocarbons were described during the last few years. Organo-lithium compounds form 1 1 complexes with alkyls of Mg 134,135), Zn 136) or Cd l36), and their usefulness as initiators of anionic polymerization of styrene and the dienes was established 137). 

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. 

It is necessary, however, to critically examine the possibilities of anionic polymerizations to give well defined polymers. We shall consider the monomers suited for such processes, the initiator efficiency and the solvents used. 

The synthesis of well defined block copolymers exhibiting controlled molecular weight, low compositional heterogeneity and narrow molecular weight distribution is a major success of anionic polymerization techniques 6,7,14-111,112,113). Blocks of unlike chemical nature have a general tendency to undergo microphase separation, thereby producing mesomorphic phases. Block copolymers therefore exhibit unique properties, that prompted numerous studies and applications (e.g. thermoplastic elastomers). 

The above discussion has been based on conventional free-radical catalysis. There has been substantial research on long-lived free radicals that can give a living polymer without the severe cleanliness requirements of anionic polymerizations. Unfortunately, it has not yet had commercial success. 

Slomkowski, S., and Penczek, S., Influence of dibenzo-18-crown-6 ether on the kinetics of anionic polymerization of p-propiolactone, Macromolecules, 9, 367-369, 1976. 

In an attempt to combine the syndioselectivity of half-sandwich titanium catalysts with the living characteristics of anionic polymerization initiators, the use of half-sandwich calcium-based catalysts has been described.363 364 In neat styrene complex (152) affords 76% rr triad PS. However, polydispersities are still quite high (Mw/Mn > 2.2)... 

Anionic polymerization is a powerful method for the synthesis of polymers with a well defined structure [222]. By careful exclusion of oxygen, water and other impurities, Szwarc and coworkers were able to demonstrate the living nature of anionic polymerization [223,224]. This discovery has found a wide range of applications in the synthesis of model macromolecules over the last 40 years [225-227]. Anionic polymerization is known to be limited to monomers with electron-withdrawing substituents, such as nitrile, carboxyl, phenyl, vinyl etc. These substituents facilitate the attack of anionic species by decreasing the electron density at the double bond and stabilizing the propagating anionic chains by resonance. 

The opposite effects of the initiator on the structure of 1,4-DVB polymers in a range of low (1-16 mol %) and a high (17-200 mol %) concentration of 1,4-DVB were explained by a kinetic model of anionic polymerization of 1,4-DVB [239]. Calculations indicated that, at low concentrations of the initiator, the poly-... 

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