Polymerization mechanisms ionic

Transformation of epoxy resins, which are viscous liquids or thermoplastic solids, into network polymers is a result of interaction with alkali or acid substances by means of to polyaddition and ionic polymerization mechanisms.10 A resin solidified by to the polyaddition mechanism, is a block copolymer consisting of alternating blocks of resin and a hardener or curing agent. A resin solidified by the ionic mechanism is a homopolymer. Molecules of both resin and hardener contain more than one active group. That is why block copolymer formation is a result of multiple reactions between an epoxy resin and a curing agent.11... 

Heterocycles form a specific class of monomers. They do not usually undergo radical polymerization, and the kind of ionic polymerization mechanism is determined by the kind of heteroatom, substituent and ring size. Oxiranes and, aziridines are polymerized by both ionic mechanisms. With the exception of lactone, four-membered and larger heterocycles with oxygen and with substituted nitrogen can only be polymerized cationically heterocycles with unsubstituted nitrogen can also be polymerized anionically. 

Polymerization Catalysed by Acids and Bases. Carbonium ions and carbanions respectively are carriers of the chain transfer in cationic and anionic polymerizations respectively. Ionic polymerization mechanism was exploited for the synthesis of polymeric stabilizers in comparison with the free-radical polymerization only exceptionally. The cationic process was used for the synthesis of copolymers of 2,6-di-tert-butyl-4-vinylphenol with cyclopentadiene and/or for terpolymers with cyclopentadiene and isobutylene [109]. System SnCWEtsAlCla was used as an initiator. Poly(lO-vinylphenothiazin) was prepared by means of catalysis with titanium chlorides [110]. Polymers of 4-[a-(2-hydroxy-3,5-dimethylphenyl)ethyl]-vinylbenzene [111] and 3-allyl-2-hydroxyacetophenone [112] were also prepared under conditions of cationic polymerization. 

The lifetime of propagating polymer ion increases with decreasing temperature therefore, the higher molecular weight at the lower polymerization temperature may be reasonably explained. This is additional support for the ionic polymerization mechanism in the copolymerization at low temperatures. 

Before formulating the main features of the ionic mechanism in radiation initiation, we will consider the general concepts ensuring an ionic polymerization mechanism. 

Forty years ago, it was shown that bringing into contact Na-montmorillonite and 4-vinylpyridine at ambient temperature leads to the formation of an intercalated monolayer of poly(4-vinylpyridine) whose aromatic rings are oriented perpendicularly to the slabs (29). Melted acrylamide was also successfully intercalated and polymerized, but the attempted intercalation/polymerization of styrene was less conclusive. At the time, the assumption of an ionic polymerization mechanism initiated by chemisorbed hydronium or hydroxyl ions derived from water was made. 

This ionic polymerization mechanism offers the most convincing explantation for the data on the ECN-PN cure. Such a polymerization mechanism would show diffusion limitations only very late in the cm e because the charges are highly mobile. 

For carbon-based vinyl monomers, controlled polymerization has been traditionally achieved by ionic mechanisms [174]. The living anionic polymerizations of styrene and methyl methacrylate are quite common, resulting in preservation of the polymer functionality. However, alike the inorganic analogues the ionic polymerization mechanism is limited to a rather narrow class of monomers, under conditions of the most stringent purity. Therefore, the aim to develop a controlled free radical... 

Unlike epoxies, which cme by an ionic polymerization mechanism, modified acrylics cure by free-radical addition. Therefore, careful proportioning of components is not required. In two-component systems, no mixing is required because the adhesive is applied to one substrate, the activator to the other, and the substrates are joined. Handling strength is rapidly achieved with this fast-curing system. 

The asymmetric induction polymerizations mentioned up to now have been performed in the presence of optically active catalysts which, even without the presence of chiral components, are capable of controlling the stereochemical path of the reaction and give stereoregular polymers following an ionic polymerization mechanism. Asymmetric in-... 

The initiators which are used in addition polymerizations are sometimes called catalysts, although strictly speaking this is a misnomer. A true catalyst is recoverable at the end of the reaction, chemically unchanged. Tliis is not true of the initiator molecules in addition polymerizations. Monomer and polymer are the initial and final states of the polymerization process, and these govern the thermodynamics of the reaction the nature and concentration of the intermediates in the process, on the other hand, determine the rate. This makes initiator and catalyst synonyms for the same material The former term stresses the effect of the reagent on the intermediate, and the latter its effect on the rate. The term catalyst is particularly common in the language of ionic polymerizations, but this terminology should not obscure the importance of the initiation step in the overall polymerization mechanism. 

Ionic polymerizations, whether anionic or cationic, should not be judged to be unimportant merely because our treatment of them is limited to two sections in this text. Although there are certain parallels between polymerizations which occur via free-radical and ionic intermediates, there are also numerous differences. An important difference lies in the more specific chemistry of the ionic mechanism. While the free-radical mechanism is readily discussed in general terms, this is much more difficult in the ionic case. This is one of the reasons why only relatively short sections have been allotted to anionic and cationic polymerizations. The body of available information regarding these topics is extensive enough to warrant a far more elaborate treatment, but space limitations and the more specific character of the material are the reasons for the curtailed treatment. 

Both modes of ionic polymerization are described by the same vocabulary as the corresponding steps in the free-radical mechanism for chain-growth polymerization. However, initiation, propagation, transfer, and termination are quite different than in the free-radical case and, in fact, different in many ways between anionic and cationic mechanisms. Our comments on the ionic mechanisms will touch many of the same points as the free-radical discussion, although in a far more abbreviated form. 

Even though the catalyst may be only partially converted to H B", the concentration of these ions may be on the order of 10 times greater than the concentration of free radicals in the corresponding stationary state of the radical mechanism. Likewise, kp for ionic polymerization is on the order of 100 times larger than the sum of the constants for all termination and transfer steps. By contrast, kp/kj which is pertinent for the radical mechanism, is typically on the order of 10. These comparisons illustrate that ionic polymerizations occur very fast even at low temperatures. 

The existence of a single step with a collision efficiency of 10 2 in the sequence of reactions would be insufficient to account for the absence of ionic polymerization since an over-all efficiency of about 10 5 is required by this mechanism within the first five reactions. Thus, at least two low efficiency steps are required in each series. Although... 

The use of HPLC in all its forms is growing steadily and may eventually exceed that of GC. This is because all four sorption mechanisms can be exploited and the technique is well suited to a very wide range of compound types including ionic, polymeric and labile materials. The most appropriate choice of mode of HPLC for a given separation problem is based on the relative molecular mass, solubility characteristics and polarity of the compounds to be separated and a guide to this is given in Figure 4.43. 

Polymeric electrolytes can possibly be used to build safe, non-toxic modern battery systems, e.g. Li-batteries. In this context the understanding of the ionic conduction mechanism of dissolved alkali salts is of major importance. Besides macroscopic measurements of transport coefficients, the investigation of mobilities on a molecular level is essential to identify the relevant conduction mechanisms. 

Both the initiation step and the propagation step are dependent on the stability of the carbocations. Isobutylene (the first monomer to be commercially polymerized by ionic initiators), vinyl ethers, and styrene have been polymerized by this technique. The order of activity for olefins is Me2C=CH2 > MeCH=CH2 > CH2=CH2, and for para-substituted styrenes the order for the substituents is Me—O > Me > H > Cl. The mechanism is also dependent on the solvent as well as the electrophilicity of the monomer and the nucleophi-licity of the gegenion. Rearrangements may occur in ionic polymerizations. 

A change of architecture is another route that enables diversification of the properties of aliphatic polyesters. This review will focus on star-shaped, graft, macrocyclic, and crosslinked aliphatic polyesters. It must be noted that the ROP of lactones has been combined with several other polymerization mechanisms such as ROP of other heterocyclic monomers, ionic polymerization, ROMP, and radical polymerization. Nevertheless, this review will not cover these examples and will focus on polymers exclusively made up of poly(lactone)s. 

Having established that a particular polymerization follows Bemoullian or first-order Markov or catalyst site control behavior tells us about the mechanism by which polymer stereochemistry is determined. The Bemoullian model describes those polymerizations in which the chain end determines stereochemistry, due to interactions between either the last two units in the chain or the last unit in the chain and the entering monomer. This corresponds to the generally accepted mechanism for polymerizations proceeding in a nonco-ordinated manner to give mostly atactic polymer—ionic polymerizations in polar solvents and free-radical polymerizations. Highly isoselective and syndioselective polymerizations follow the catalyst site control model as expected. Some syndioselective polymerizations follow Markov behavior, which is indicative of a more complex form of chain end control. 

Like radical polymerizations, ionic polymerizations also occur by a chain mechanism. In contrast to radical polymerizations the chain carriers are macroions carbonium ions in the case of cationic polymerizations and carbanions in the case of anionic polymerization of C=C compounds ... 

Early studies (1 ) of the kinetics of polymerization of styrene, isoprene and butadiene in hydrocarbon solvents indicated a half-order rate dependency on growing chain concentration, although there were conflicting data at that time (10, 11) which suggested even lower fractional orders for the dienes. Since the apparent half-order dependency could not be rationalized, as in the case of the polar media, by an ionic dissociation mechanism, some other form of association-dissociation phenomenon offered a possible answer. In view of the known tendency of organolithium compounds to undergo molecular association in non-polar media, the following scheme was proposed by us (l) ... 

The solid n-hexadecene-1 system has also been studied.76 It was concluded that the experimental results were incompatible with a free-radical reaction and an ionic mechanism was proposed. An enhancement in the polymerization yield by a factor of 2 on going from the liquid at 20 °C. to the solid at 0°C. was observed. Solid-state ionic polymerizations induced by high-energy radiation have been reviewed by Magat.76... 

The polymerization systems discussed in this article are those in which polymerizing monomer is directly involved in the electron transferring pair, which enables the production of ion-radical on monomer. At the moment we are able to induce photosensitized ionic polymerization only in limited instances. When the charge transfer polymerization is discussed, strict distinction between radical and ionic mechanisms is impossible. As shown in Fig. 2, the difference between ion and radical and that between molecule and ion-radical is only a matter of one electron. Thermal electron transfer polymerization is demonstrated for many polymerization systems. The combination of photochemistry and electron transfer polymerization is very promising and may open up a new field in photopolymers. 

Ise, N., H. Hirohara, T. Makino, K. Takaya, and M. Nakayama Ionic polymerization under an electric field. XIII. living anionic polymerization of styrene in the Unary mixtures of benzene and dimethoxyethane by the three-state mechanism. Presented at the 17th Discussion Meeting of High Polymers, October, 1968, Matsuyama, Preprint p. 261. 

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