Cationic chain polymerization 1,3-dienes

The ionic chain polymerization of unsaturated linkages is considered in this chapter, primarily the polymerization of the carbon-carbon double bond by cationic and anionic initiators (Secs. 5-2 and 5-3). The last part of the chapter considers the polymerization of other unsaturated linkages. Polymerizations initiated by coordination and metal oxide initiators are usually also ionic in nature. These are called coordination polymerizations and are considered separately in Chap. 8. Ionic polymerizations of cyclic monomers is discussed in Chap. 7. The polymerization of conjugated dienes is considered in Chap. 8. Cyclopolymerization of nonconjugated dienes is discussed in Chap. 6. 

The titanium trichloride-diethylaluminum chloride catalyst converted butadiene to the cis-, trans,-trans-cyclododecatriene. Professor Wilke and co-workers found that the particular structure is influenced by coordination during cyclization between the transition metal and the growing diene molecules. Analysis of the influence of the ionicity of the catalyst shows effects on the oxidation and reduction of the alkyls and on the steric control in the polymerization. The lower valence of titanium is oxidized by one butadiene molecule to produce only a cis-butadienyl-titanium. Then the cationic chain propagation adds two trans-butadienyl units until the stereochemistry of the cis, trans, trans structure facilitates coupling on the dialkyl of the titanium and regeneration of the reduced state of titanium (Equation 14). 

Classification of Polymers Free-Radical Chain-Growth Polymerization Cationic Chain-Growth Polymerization Anionic Chain-Growth Polymerization Stereoregular Polymers Ziegler-Natta Polymerization A WORD ABOUT... Polyacetylene and Conducting Polymers Diene Polymers Natural and Synthetic Rubber Copolymers... 

These cyclic structures were foimd to be inherent to most cationically polymerized dienes in nonpolar solvents (368). It has been reported that these cyclic structures could also be produced by typical cationic polsrmerization in the presence of ubiquitous proton-donating impurities, chain-transfer reactions (to monomer and aromatic solvents), and more conventional elimination reactions (369). 

Analogous principles should apply to ionically propagated polymerizations. The terminus of the growing chain, whether cation or anion, can be expected to exhibit preferential addition to one or the other carbon of the vinyl group. Poly isobutylene, normally prepared by cationic polymerization, possesses the head-to-tail structure, as already mentioned. Polystyrenes prepared by cationic or anionic polymerization are not noticeably different from free-radical-poly-merized products of the same molecular weights, which fact indicates a similar chain structure irrespective of the method of synthesis. In the polymerization of 1,3-dienes, however, the structure and arrangement of the units depends markedly on the chain-propagating mechanism (see Sec. 2b). 

Anionic polymerization of conjugated dienes and olefins retains its lithium on the chain ends as being active moities and capable of propagating additional monomer. This distinguishing feature has an advantage over other methods of polymerization such as radical, cationic and Ziegler polymerization. Many attempts have been made to prepare block copolymers by the above methods, but they were not successful in preparing the clear characterized block copolymer produced by anionic technique. 

A vinyl branch in a diene polymer is the result of an occasional 1,2 double bond addition to the polymer chain. Branching can also occur in cationic polymerization for the same reason. 

Cationic -allylnickel complexes polymerize 1,3-butadiene to produce the cis- 1,4-polymer. Taube investigated the polymer growth via smooth and selective insertion of the diene into the -allyl-Ni bond of the growing polymer, both from experimental and theoretical aspects. The reaction catalyzed by the cationic Ci2-allylnickel(II) complex shows kinetics that agree with a chain propagation transfer model [67]. The reaction mechanism of the cis-1,4-polymerization using technical Ni catalysts is also discussed [68]. He compared the mechanism of the reaction catalyzed by allylnickel complexes [69]. 

Similarly, 2-methylpropene (isobutene) is an important monomer. It only polymerizes by a cationic mechanism, and its copolymers with dienes are known as butyl rubber. Higher 1-alkenes (1-butene, 1-hexene, 1-octene) are important copolymerization components [4, 5] they produce tailored branching of some polyethylene types prepared by a coordination mechanism. Longer-chain alkenes (Cjq, C,2, Cj ) are also sometimes used as comonomers... 

O Driscoll, Yonezawa, and Higashimura proposed a mechanism for steric control. In isoprene polymerization the terminal charges are complexed with the metal cations. These cations are close to the active centers through the occupied zr-orbitals of the chain ends and the unoccupied p-orbitals of the lithium ions, hi the transition state the monomers are conqilexed with the cations in die same way. The lithium cations are assumed to be in hybridized tetrahedral sp configurations with four vacant orbitals. The chain ends are presumed to be allylic and the diene monomers are bidentate. During the propagation steps both the monomers and chain ends complex with the same counterions ... 

The type of monomers suitable for cationic polymerization are those containing an electron-donating substituent such as 1,1-dialkyl, alkene, alkoxy, and phenyl that stabilize the propagating cationic centers. Successftil industrial examples include polyisobutylene and its copolymers with dienes such as butyl mbber. In ionic polymerization, initiator is conventionally called a catalyst. However, by definition, catalyst and initiator are two different types of reagents. Catalyst takes part in reactions but can be removed from the final product if necessary. On the other side, initiator molecules or their fragments become a part of the produced chains after polymerization. In cationic polymerization, a single catalyst is not sufficient and a cocatalyst is required. Typical catalysts are Lewis acids such as BF3, AICI3, and TiCU that must be used with a protonic cocatalyst such as H2O and methanol ... 

Controlled cationic polymerization of linear dienes, such as 1,3-pentadiene, has never been achieved, because frequent side reactions occur such as cross-linking, isomerization, cyclization, and chain transfer reaction (Figure 8). Bennevault-Celton et conducted a series of kinetic studies on the polymerization of 1,3-pentadiene initiated by AICI3 in a... 

Cationic polymerization is of little use for the preparation of homopolymers from conjugated dienes because side reactions lead to cyclic structures in the polymer chain and loss of a significant proportion of the expected residual unsaturation. 

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