Polymerization methods, three anionic

Although the core-first method is the simplest, success depends on initiator preparation and quantitative initiation under living conditions. This method is of limited use in anionic polymerization because of the generally poor solubility of multifunctional initiators in hydrocarbon solvents [12]. Solubilities of multifunctional initiators are less of an issue in cationic polymerizations, and tri- and tetrafunctional initiators have been used to prepare well-defined three- and four-arm star polymers by this method [7] Except for two reports on the synthesis of hexa-arm polystyrene [27] and hexa-arm polyoxazoHne [26], there is a dearth of information in regard to well-defined multifunctional initiators for the preparation of higher functionality stars. 

The nature of the active species in the anionic polymerization of non-polar monomers, e. g. styrene, has been disclosed to a high degree. The kinetic measurements showed, that the polymerization proceeds in an ideal way, without side-reactions, and that the active species exist in the form of free ions, solvent-sparated and contact ion pairs, which are in a dynamic equilibrium (l -4). For these three species the rate constants and activation parameters (including the activation volumes), as well as the rate constants and equilibrium constants of interconversion have been determined (4-7.) Moreover, it could be shown by many different methods (e. g. conductivity and spectroscopic methods) that the concept of solvent-separated ion pairs can be applied to many ionic compounds in non-aqueous polar solvents (8). 

Synthetic methods have limited the preparation of saturated perfluoropolyethers. The most successful perfluoropolyether synthetic chemistry has been DuPont s anionic polymerization of perfluoroepoxides, particularly hexafluoro-propylene oxide and tetrafluoroethylene oxide (39). Their synthetic procedure is a three-step scheme for saturated perfluoropolyether production involving oxidation of perfluoroolefins to perfluoroepoxides, anionic polymerization to acyl fluoride terminated perfluoropolyethers, and conversion of acyl fluoride end groups to unreactive end groups by decarboxylation reactions or chaincoupling photolytic decarboxylate reactions. 

Star polymers of chemically different arms are usually called miktoarm stars. Although there are several individual methods for the synthesis of miktoarm stars four general methodologies have been developed. Three of them are based on anionic polymerization and the fourth on cationic polymerization. In all of them the use of appropriate linking agents is necessary. 

Narrow polydispersity diblock copolymers of PS—PMMA and PS—PEO were produced by anionic polymerization using conventional high-vacuum methods. The average AB copolymer composition was determined by NMR (model EM30, Varian, UK). Narrow dispersity PS and PMMA standards (Polymer Laboratories) were used for both instrument and SEC column calibrations. Samples were prepared as nominally 1-mg/mL solutions in the eluent and spiked with toluene as a flow rate marker before full loop 100-fxh injection. Each copolymer was analyzed three times. 

Transformation of Anionic Polymerization into Cationic Polymerization. Richards et al. (26. 27, 73-75) proposed several methods for the transformation of a living anionic polymeric chain end into a cationic one. Such a process requires three distinct stages polymerization of a monomer I by an anionic mechanism, and capping of the propagating end with a suitable but potentially reactive functional group isolation of polymer I, dissolution in a solvent suitable for mechanism (2), and addition of monomer II and reaction, or change of conditions, to transform the functionalized end into propagating species II that will polymerize monomer II by a cationic mechanism (73). 

Block Copolymer Synthesis by Three-Step Sequential Monomer Addition The preparation of block copolymers by sequential addition of monomers using living anionic polymerization and a monofunctional initiator is the most direct method for preparing well-defined block copolymers. Detailed laboratory procedures for anionic synthesis of block copolymers are available [37, 230], Several important aspects of these syntheses can be illustrated by considering the preparation of an important class of block copolymers (Scheme 7.22), the polystyrene-fe-polydiene-( -polystyrene triblock copolymers. 

Sequential monomer addition by using a monofunctional initiator (Figure 3(c)) could be followed only if the reactivity of both monomers is almost the same. The maaoinitiator formed from monomer A can polymerize monomer B and vice versa. Styrene (St) and dienes in anionic polymerization and various methacrylates in anionic or GTP are characteristic examples of this method. The sequential monomer addition route, due to partial termination and/or deactivation reactions, proposed from the addition of three different quantities of monomer allows the synthesis of asymmetric ABA ttiblock copolymers, where the two end blocks exhibit different molecular characteristics. 

Vinyl polymerization, as illustrated by the above reactions, involves a three-part process, namely initiation, in which is formed an active species capable of starting polymerization of the otherwise unreactive vinyl compound propagation, in which high molecular weight polymer is formed termination, in which deactivation occurs to produce the final stable polymer. The active species in vinyl polymerizations may be of three different types, namely free radicals, anions and cations and these possibilities give rise to three distinct methods of accomplishing polymerization. 

Three generations of latices as characterized by the type of surfactant used in manufacture have been defined (53). The first generation includes latices made with conventional (/) anionic surfactants like fatty acid soaps, alkyl carboxylates, alkyl sulfates, and alkyl sulfonates (54) (2) nonionic surfactants like poly(ethylene oxide) or poly(vinyl alcohol) used to improve freeze—thaw and shear stabiUty and (J) cationic surfactants like amines, nitriles, and other nitrogen bases, rarely used because of incompatibiUty problems. Portiand cement latex modifiers are one example where cationic surfactants are used. Anionic surfactants yield smaller particles than nonionic surfactants (55). Often a combination of anionic surfactants or anionic and nonionic surfactants are used to provide improved stabiUty. The stabilizing abiUty of anionic fatty acid soaps diminishes at lower pH as the soaps revert to their acids. First-generation latices also suffer from the presence of soap on the polymer particles at the end of the polymerization. Steam and vacuum stripping methods are often used to remove the soap and unreacted monomer from the final product (56). 

During this, the electrons of the partial X—Z multiple bond are used. Experiments show that the ester can be further active in the polymerization. Its reactivity, however, is reduced in comparison with ion pairs. From a mechanistical point of view, the chain propagation should proceed in the manner of a SN2 reaction, that is with the monomer as nucleophile and the ester as substrate. With the assistance of quantum chemical calculations using the CNDO/2 method, the differences between covalent species and free ions should be examined. The following contains the three types of anions used ... 

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