Anionic polymerization reaction media

The electron-releasing R group helps stabilize this cation. As with anionic polymerization, the separation of the ions and hence the ease of monomer insertion depends on the reaction medium. The propagation reaction may be written as... 

Based on this approach Schouten et al. [254] attached a silane-functionalized styrene derivative (4-trichlorosilylstyrene) on colloidal silica as well as on flat glass substrates and silicon wafers and added a five-fold excess BuLi to create the active surface sites for LASIP in toluene as the solvent. With THF as the reaction medium, the BuLi was found to react not only with the vinyl groups of the styrene derivative but also with the siloxane groups of the substrate. It was found that even under optimized reaction conditions, LASIP from silica and especially from flat surfaces could not be performed in a reproducible manner. Free silanol groups at the surface as well as the ever-present impurities adsorbed on silica, impaired the anionic polymerization. However, living anionic polymerization behavior was found and the polymer load increased linearly with the polymerization time. Polystyrene homopolymer brushes as well as block copolymers of poly(styrene-f)lock-MMA) and poly(styrene-block-isoprene) could be prepared. 

It still represents a great challenge to conduct anionic polymerizations in an automated parallel synthesizer. Above all, the technique requires an intensive purification of the reagents and the polymerization medium in order to obtain well-defined polymers. Therefore, a special procedure has been described for the inertization of the reactors [55]. It is called chemical cleaning, which is essentially rinsing all the reactors with. yec-butyllithium (.y-BuLi) prior to the reaction in order to eliminate all chemical impurities. This process can be performed in an automated manner. Due to the extreme sensitivity of the polymerization technique to oxygen, moisture, and impurities, detailed investigations on the inertization procedure and the reproducibility of the experiments need to be conducted. 

Since then, the process has been extended to a wide variety of lactones of different size and to several lipases, as recently reviewed [93-96]. Interestingly, large-membered lactones, which are very difficult to polymerize by usual anionic and coordination polymerizations due to the low ring strain, are successfully polymerized by enzymes. Among the different lipases available, that fi om Candida antarctica (lipase CA, CALB or Novozym 435) is the most widely used due to its high activity. An alcohol can purposely be added to the reaction medium to initiate the polymerization instead of water. The polymerization can be carried out in bulk, in organic solvents, in water, and in ionic liquids. Interestingly, Kobayashi and coworkers reported in 2001 the ROP of lactones by lipase CA in supercritical CO2... 

This review deals with current ideas on the mechanisms operative in acrylonitrile polymerization. The topic is of importance in its own right and also because the study of acrylonitrile has cast light on heterogeneous polymerizations in general. It is an active field of research and the interpretations are still controversial. We shall look first at free-radical polymerization in homogeneous solution, where the monomer behaves in a more or less classical fashion. Next we shall consider the complications that arise where the monomer is at least partially soluble in the reaction medium but where the polymer precipitates. These conditions are encountered in bulk polymerization and in most aqueous or organic diluents. Finally we shall examine the less extensive literature on anionic polymerization and show important differences between the radical and the ionic processes. 

The fact, that almost all basic functions in caprolactam or polyamide medium occur in the form of amidic anions [-CO-N-]- arose the idea that these particular anions are directly responsible for the polymerization reaction. In analogy to the transesterification mechanism an intermediate addition complex might be formed by combining the amidic group with the respective anion... 

The low activity of bromide and inactivity of chloride do not arise from their decomposition since the introduction of the iodide anion into the reaction medium activates the system and transforms its stereospecificity with regard to polymerization in hydrocarbon. Iodide anion addition activates (ir-C iNil also (Table II). 

The characteristics of the active centers in free-radical polymerizations depend only on the nature of the monomer and are generally independent of the reaction medium. This is not the case in ionic polymerizations because these reactions involve successive insertions of monomers between a macromolecular ion and a more or less tightly attached counterion of opposite charge. The macroion and counterion form an organic salt which may exist in several forms in the reaction medium. The degree and nature of the interaction between the cation and anion of the salt and the solvent (or monomer) can vary considerably. 

The propagating anion and its counterion exist in relatively nonpolar solvents mainly in the form of associated ion pairs. Different kinds of ion pairs can be envisaged, depending on the extent of solvation of the ions. As a minimum, an equilibrium can be conceived between intimate (contact) ion pairs, solvent-separated ion pairs, and solvated unassociated ions. The nature of the reaction medium and counterion strongly influences the intimacy of ion association and the course of the polymerization. In some cases the niicrostructure of the polymer that is produced from a given monomer is also influenced by these variables. In hydrocarbon solvents, ion pairs are not solvated but they may exist as aggregates. Such inter-molecular association is not important in more polar media where the ion pairs can be solvated and perhaps even dissociated to some extent. 

The choice of reaction medium is much more significant in anionic and cationic reactions than in free-radical polymerizations because the character of the growing chain end is altered if the ion pair is more or less solvated. 

The direction of temperature effects in anionic polymerizations is conventional, with increased temperature resulting in increased reaction rates. Observed activation energies are usually low and positive. This apparent simplicity disguises complex effects, however, and the different ion pairs and free ions do not respond equally to temperature changes. Overall activation energies for polymerization will be influenced indirectly by the reaction medium because the choice of solvent shifts the equilibria of Eq. (9-1). 

The reaction medium in cationic polymerizations is usually a moderately polar chlorinated hydrocarbon like CHjCI (dielectric constant = 12.6 at —2(TC). A greater proportion of the macroions are free of their counterions in cationic than in anionic polymerizations in the usual solvents for the latter processes. Cationic polymerizations are characterized by extremely fast propagation rates. 

Although the presence of water is generally not an issue in free-radical chain polymerization (indeed water may be a suitable medium for polymerization as in Protocols 5-7) unlike, for example, chain-growth polymerization initiated by anionic species, it is always advisable to use solvents of the highest purity and this will generally include some element of predrying. In general, solvents should be distilled, particularly as a number of suitable solvents for polymerization reactions contain stabilizers which usually serve to mop up free radicals and therefore inhibit the polymerization... 

Ionic polymerizations, as we shall see later, involve successive insertion of monomer molecules between an ionic chain end (positive in cationic and negative in anionic polymerization) and a counterion of opposite charge. The macroion and the counterion form an organic salt which may, however, exist in several forms depending on the nature and degree of interaction between the cation and anion of the salt and the reaction medium (solvent/monomer). Considering, for example, an organic salt a continuous spectrum of ionicities ( Winstein... 

Unlike free-radical and homogeneous anionic polymerizations, cationic polymerizations cannot be described by conventional kinetic schemes involving reactions like initiation, propagation, and so on. This is due to the complexity of the cationic initiation and the variable extents of ion-pair formation at the propagating chain end [cf. Eq. (8.1)]. The possibihty of the reaction being heterogeneous due to the limited solubility of the initiator m the reaction medium further adds to the complexity of the kinetics. 

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