Living anionic systems

In the first step the oxirane polymerization initiated by the isomerized EN dianion has been studied. The results reported in Table III and the low polydispersity observed (about 1.1 Figure 5) are conclusive for a living anionic system. Moreover the initiation rate of which is higher than the propagation one. 

Living anionic systems are among the most suitable to study the mechanism of polymerizations. Indeed, the high efficiency of anionic Initiators, combined with the good stability of active centers, allows one to study directly the active species by common physico-chemical methods. 

As was stated above, the interpretation that the field affects the dis-sodation state of the growing chain ends was not uniquely substantiated by the experimental data, except those on copolymerizations. Thus it is interesting to investigate the field influence on much simpler systems than cationic homopolymerizations. For this purpose we have chosen living anionic systems in which only propagation steps are involved. The system first studied was a living anionic polymerization of styrene with n-butyllithium in the binary mixtures of benzene and tetrahydrofuran (17,24) and in the binary mixtures of benzene and dimethoxyethane (15). 

Besides the field influence on the monomer reactivity ratio mentioned in the previous sections, living anionic systems present strong evidence against the electroinitiated polymerization mechanism. First of all, the experimental fact, that the apparent rate constant of propagation was increased by the presence of an electric field, rules out a possibility that the observed field-accelerating effect resulted only from the initiation reaction enhanced by the field. The finding that the field had no influence on the dependences of the polymerization rate on monomer and initiater concentrations, but did influence the rate constant, implies that the reaction mechanism was unaltered by the application of the field. These results confirm our very low opinion of the electroinitiated polymerization mechanism. 

The new controlled/living carbocationic polymerizations were, in some sense, quite similar to new controlled/living anionic systems devel-... 

Living polymers do not live forever and even in the absence of terminating agents decays with time [2]. The most stable of all living anionic systems are polystyryl carbanions, as they are stable for weeks in hydrocarbon solvents. The mechanism for the decay of polystyryl carbanions on aging, referred to as spontaneous termination, is not completely established. The generally accepted mechanism consists of hydride elimination ... 

KINETICS AND MOLAR MASS DISTRIBUTION IN LIVING ANIONIC SYSTEMS... 

Thermoplastic elastomers have the important advantage over conventional elastomers that there is no need for the additional chemical crosslinking reaction and fabrication is achieved in the same way as for thermoplastics. However, only certain polymerization methods can be used to synthesis block copolymers — primarily living anionic chain polymerization and certain step polymerizations. Triblock copolymers are produced by living anionic polymerization by sequential addition of different momomer charges to a living anionic system, for example, a styrene-isoprene-styrene is synthesized by the sequence... 

In 1956 Szwarc and his associates discovered an anionic polymerization of styrene without chain transfer and termination , and this reaction was called living polymerization. Subsequent studies revealed a number of examples of living anionic systems . In the field of cationic polymerization, two research groups reported in 1965 that the propagating species in ring-opening polymerization of tetrahydro-furan have an unusually extended lifetime . ... 

Anionic polymerization in suitable systems allows the preparation of polymers with controlled molecular weight, narrow molecular weight distributions and functional termination. The functional termination of a living anionic polymerization with a polymerizable group has been used frequently in the preparation of macromonomers (4). Our research has encompassed the anionic homo and block copolymerizations of D- or hexamethyl cyclotrisiloxane with organolithiums to prepare well defined polymers. As early as 1962 PSX macromonomers were reported in the literature by Greber (5) but the copolymerization of these macromonomers did not become accepted technique until their value was demonstrated by Milkovich and... 

The range of monomers that can be incorporated into block copolymers by the living anionic route includes not only the carbon-carbon double-bond monomers susceptible to anionic polymerization but also certain cyclic monomers, such as ethylene oxide, propylene sulfide, lactams, lactones, and cyclic siloxanes (Chap. 7). Thus one can synthesize block copolymers involving each of the two types of monomers. Some of these combinations require an appropriate adjustment of the propagating center prior to the addition of the cyclic monomer. For example, carbanions from monomers such as styrene or methyl methacrylate are not sufficiently nucleophilic to polymerize lactones. The block copolymer with a lactone can be synthesized if one adds a small amount of ethylene oxide to the living polystyryl system to convert propagating centers to alkoxide ions prior to adding the lactone monomer. 

In the present section we describe the living anionic polymerization of meth-acrylonitrile by two initiating systems such as the aluminum porphyrin-Lewis acid system and the aluminum porphyrin-Lewis base system which enables the synthesis of poly(methyl methacrylate-h-methacrylonitrile)s of controlled molecular weights. 

By using the aluminum porphyrin-Lewis acid system, we attempted the synthesis of a narrow MWD block copolymer from oxetane and methyl methacrylate (MMA). Methacrylic monomers can be polymerized radically and anioni-cally but not cationically, so a block copolymer of oxetane and methyl methacrylate has never been synthesized. As already reported, methacrylic monomers undergo accelerated living anionic polymerization with the (TPP)AlMe (1, X= Me)-3e system via a (porphinato)aluminum enolate as the growing species. 

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