Polymerization cationic method

Finally, combination of anionic with other polymerization methods (cationic, living radical, ROMP, metallocenes, etc.) will open new horizons for the synthesis of more complex and more fascinating macromolecular structures. 

The chemistry of polymerization of the oxetanes is much the same as for THE polymerization. The ring-opening polymerization of oxetanes is primarily accompHshed by cationic polymerization methods (283,313—318), but because of the added ring strain, other polymerization techniques, eg, iasertion polymerization (319), anionic polymerization (320), and free-radical ring-opening polymerization (321), have been successful with certain special oxetanes. 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

In Section 6.21 we listed three main methods for polymerizing alkenes cationic, free-radical, and coordination polymerization. In Section 7.15 we extended our knowledge of polymers to their stereochemical aspects by noting that although free-radical polymerization of propene gives atactic polypropylene, coordination polymerization produces a stereoregulai polymer with superior physical properties. Because the catalysts responsible for coordination polymerization ar e organometallic compounds, we aie now in a position to examine coordination polymerization in more detail, especially with respect to how the catalyst works. 

Polystyrene is unusual among commodity polymers in that we can prepare it in a variety of forms by a diversity of polymerization methods in several types of reaction vessel. j Polystyrene may be atactic, isotactic, or syndiotactic. Polymerization methods include free radical, cationic, anionic, and coordination catalysis. Manufacturing processes include bulk, solution, suspension, and emulsion polymerization. We manufacture random copolymers ... 

Why is it possible to manufacture polystyrene by radical, anionic, cationic and coordination polymerization methods ... 

Cationic polymerization was considered for many years to be the less appropriate polymerization method for the synthesis of polymers with controlled molecular weights and narrow molecular weight distributions. This behavior was attributed to the inherent instability of the carbocations, which are susceptible to chain transfer, isomerization, and termination reactions [48— 52], The most frequent procedure is the elimination of the cation s /1-proton, which is acidic due to the vicinal positive charge. However, during the last twenty years novel initiation systems have been developed to promote the living cationic polymerization of a wide variety of monomers. 

Generally, at least in theory, an important aspect of cation-radical polymerization, from a commercial viewpoint, is that either catalysts or monomer cation-radicals can be generated electrochem-ically. Such an approach deserves a special treatment. The scope of cation-radical polymerization appears to be very substantial. A variety of cation-radical pericyclic reaction types can potentially be applied, including cyclobutanation, Diels-Alder addition, and cyclopropanation. The monomers that are most effectively employed in the cation-radical context are diverse and distinct from those that are used in standard polymerization methods (i.e., vinyl monomers). Consequently, the obtained polymers are structurally distinct from those available by conventional methods although the molecular masses observed so far are still modest. Further development in this area would be promising. 

Most of the methods for synthesizing block copolymers were described previously. Block copolymers are obtained by step copolymerization of polymers with functional end groups capable of reacting with each other (Sec. 2-13c-2). Sequential polymerization methods by living radical, anionic, cationic, and group transfer propagation were described in Secs. 3-15b-4, 5-4a, and 7-12e. The use of telechelic polymers, coupling and transformations reactions were described in Secs. 5-4b, 5-4c, and 5-4d. A few methods not previously described are considered here. 

In conclusion, crystallization polymerization is cationic in nature, and is not an easy method for obtaining the polyacetalic polymer in fact, the yield was rather low (about 15%), and high monomer purity and a careful control of the experimental techniques are required. 

Presently the main technique for the synthesis of copolymers are free-radical polymerization methods [11-17]. For a limited range of comonomers, anionic and cationic polymerizations are also used [236,237]. 

Polyphosphazene block copolymers were synthesized by these chain-growth polymerization methods. The successive anionic polymerization of N-silylphosphoranimines 19d and 19a at 133 °C yielded the block copolymer with Mw/Mn of 1.4-2.3 (Scheme 80) [278,279]. However, due to the presence of two possible leaving groups in 19d, this approach yielded block copolymers where one of the block segments contained a mixture of side groups. On the other hand, the cationic polymerization of 19b with PCI5 was carried out at ambient temperature, followed by addition of a second phosphoranimine to yield a block copolymer with Mw/Mn of 1.1-1.4, where each block segment had one kind of side chain (Scheme 81) [280]. 

Techniques derived from anionic or cationic living polymerization methods have widely been used. They are efficient because of the long lifetime of the active sites. Once polymerization is completed these sites are used for functionalization purposes. Alternately, unsaturated ionic initiators have been used but to a lesser extent because of the requirement involved that the polymerizable groups remain unscathed during the macromonomer formation. The versatile inifer method has also been applied to the synthesis of macromonomers. 

The all-cis structure of natural rubber is vita) to its elasticity. The all-trans compound is known and it is hard and brittle. Though dienes such as isoprene can easily be polymerized by cationic methods, the resulting rubber is not all-cis and has poor elasticity and durability. However, polymerization of isoprene in the Ziegler-Natta way gives an all-cis (90-95% at least) polyisoprene very similar to natural rubber. 

Typical monomers that may be polymerized by cationic methods include styrene, isobutylene, and vinyl ethers. Unlike radical polymerizations, solvent polarity can influence the rate of polymerization. This is due to the presence of the counterion (see Fig. 15.13). For example, more polar solvents can increase the degree of separation between the growing end and the counterion during the propagation step, increasing the rate of propagation.17... 

A polymerization method for cationically polymerizing liquefied isobutylene using the initiator pair l,2-bis(9-bora-l,2,3,4,5,6,7,8-octafluoro-fluorenyl)-3,4,5,6-tetrafluorobenzene and tri-n-octylaluminum is described. The method is particularly unique in that the polymerization occurs in a nonchlorinated solvent. Isobutylene catalyzed using these co-reagents had molecular weights upto 258,000 daltons. 

Both unsubstituted bicyclic monomers and related anhydrosugars polymerize by cationic mechanism [148]. In sugar derivatives, the remaining free hydroxyl groups have to be blocked before polymerization by typical methods known in carbohydrate chemistry. 

Although several cyclic amides (lactams) can be polymerized by cationic mechanism, this method of polymerization is of little practical importance because the anionic or hydrolytic polymerization provides much more convenient route to corresponding polyamides. Polyamides obtained by cationic polymerization of lactams are less stable and oxidize faster than those obtained by anionic polymerization [213). 

Aromatic polysulfones are a commercially important class of thermoplastic polymers [127]. They have highly desirable qualities such as chemical inertness, thermal stability, and flame retardency [128,129]. Although a number of methods are available for the synthesis of polysulfones [130,131,132], step polymerization methods are the most widely used industrially [127]. Polysulfones have been synthesized with the involvement of sulfonylium cations as propagating species. 

The cation radical intermediate and the process of electron (hole) transfer have recently been shown to constitute the basis for a fundamentally new addition to the repertoire of polymerization methods [85], Both cation radical chain cyclobutana-tion polymerization (Scheme 44) and Diels-Alder polymerization have been demonstrated under the typical aminium salt conditions. 

Another method to include polymeric moieties within the films was reported by Shen et the polymeric PVOP cation is the counter-ion... 

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