Diene cationic polymerization

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). 

Cationic polymerization of dienes using boron trifluoride or aluminum chloride as catalysts seems also to favor the rans-1,4 structure, although 1,2 and 3,4 units also are present. These catalysts also cause cyclization of the structural units, with a consequent decrease in the unsaturation in the polymer. 

Sulfur-containing spiro orthocarbonates, cationic polymerization of, 23 729 Sulfur-cured EPDM, 21 8041. See also Ethylene- propylene-diene monomer (EPDM) rubber Sulfur deposits... 

Star and miktostar polymers have been synthesized by living cationic polymerization using dienes and trienes as coupling agents and/or multifunctional initiators [Faust and Shaffer, 1997 Hadjichristidis, et al., 1999 Hadjikyriacou and Faust, 2000 Kennedy and Jacob, 1998 Puskas et al., 2001 Sawamoto, 1991]. Multifunctional halides such as hexaiodo-methylmelamines have also been used to obtain star and comb polymers [Ryu and Hirao, 2001 Zhang and Goethals, 2001], Hyperbranched and dendritic polymers have also been studied. 

Traditional Ziegler-Natta and metallocene initiators polymerize a variety of monomers, including ethylene and a-olefins such as propene, 1-butene, 4-methyl-1-pentene, vinylcyclo-hexane, and styrene. 1,1-Disubstituted alkenes such as isobutylene are polymerized by some metallocene initiators, but the reaction proceeds by a cationic polymerization [Baird, 2000]. Polymerizations of styrene, 1,2-disubstituted alkenes, and alkynes are discussed in this section polymerization of 1,3-dienes is discussed in Sec. 8-10. The polymerization of polar monomers is discussed in Sec. 8-12. 

Polymerization of isobutylene, in contrast, is the most characteristic example of all acid-catalyzed hydrocarbon polymerizations. Despite its hindered double bond, isobutylene is extremely reactive under any acidic conditions, which makes it an ideal monomer for cationic polymerization. While other alkenes usually can polymerize by several different propagation mechanisms (cationic, anionic, free radical, coordination), polyisobutylene can be prepared only via cationic polymerization. Acid-catalyzed polymerization of isobutylene is, therefore, the most thoroughly studied case. Other suitable monomers undergoing cationic polymerization are substituted styrene derivatives and conjugated dienes. Superacid-catalyzed alkane selfcondensation (see Section 5.1.2) and polymerization of strained cycloalkanes are also possible.118... 

Conjugated dienes (1,3-butadiene, isoprene) have suitable nucleophilicity to undergo cationic polymerization. There is, however, not much practical interest in these processes since the polymers formed are inferior to those produced by other (free-radical, coordination) polymerizations. A significant characteristic of these polymers is the considerably less than theoretical unsaturation due to cyclization processes.132 A fully cyclized product of isoprene has been synthesized163 by constant potential electrolysis in CH2C12. 

This transition can operate in both anionic and cationic polymerizations. The stereospecific transition state also requires a limited degree of freedom between the propagating polymer and the gegen ion like that with the isotactic polymerization systems. Cis polymerization occurs over a broad middle range of ionieity of the catalysts and there appear to be both anionic and cationic catalysts which produce the cis-diene structure. 

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. 

The reactivity sequence shown above corresponds well to Mayr s [18] model reactions of the electrophilic addition of benzhydryl carbenium ions to substituted alkenes. Table 2 lists the second-order rate constants for the addition of a diarylcarbenium ion to various alkenes and dienes [36]. One alkyl group offers little activation of the double bond a-olefins therefore form only oligomers with isomerized repeat units in low conversions under cationic polymerization conditions. One vinyl group activates the double bond slightly more than alkyl groups do. Table 2 also demon-... 

Termination evidently does not occur as readily in anionic polymerization of thietanes as it does in cationic polymerization. Organo-lithium initiated polymerizations of thietanes lead to living polymers, which have been used to prepare ABA block copolymers of dienes and cyclic sulphides [69, 70]. Since the anionic polymerization of thietanes proceeds via a carbanion, thietEines can initiate vinyl polymerization and their polymerization can be initiated by vinyl monomers. Kinetic parameters of such polymerizations have not yet been reported. 

Cationic Polymerization of 1 7-Dienes. A 500 ml three-necked, round-bottomed flask, equipped with a gas inlet, mechanical stirring bar and septum, was flamed and dried under argon. 

Cationic Polymerization of 1,13-Dienes 11 and 12. By a pro-cedure analogous to that used for 1,7-dienes, the 1,13-dienes were polymerized via cationic initiation. Conversion to 13 90%. Anal. Calcd. for C14H1ft0. C, 67.18, H, 7.24. Found C, 66.94 ... 

Ionic polymerization systems of commercial importance employ mostly batch and continuous solution polymerization processes. Suitable monomers for ionic polymerization include conjugated dienes and vinyl aromatic. Among these, the anionic polymerization of styrene-butadiene (SB) and styrene-isoprene (SI) copolymers and the cationic polymerization of styrene are the most commercially important systems. 

As in the case of cationic polymerization of others diene hydrocarbons oligopiperylene macromolecules synthesized in the presence of studied catalysts contain predominantly rran -units (Table 5.2). Increased content of c s-1,4-units is observed for samples received in the presence of TiCU-Al(i-C4H9)3 and AIC2H5CI2. 

Many references exist for cyclic diene structures formed during certain cationic polymerizations (124-127). The exact mechanism involved in the formation of the ladder-like fused ring structure has yet to be determined. 

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). 

This section focuses on the regio- and stereostructure of CHD polymers, because very few cyclic conjugated diene monomers besides CHD can be polymerized. Although poly(cis,ds-l,3-cycloheptadiene) and poly(cty,cA-l,3-cyclooctadiene) were synthesized by cationic polymerization, the obtained products were found to be oligomers and their detailed structures were not clear. 

A wide variety of transition metal catalysts, such as Ti-, Mo-, W-, Ni-, and Pd-based complexes, have been investigated since the late 1950s for cyclic conjugated diene polymerization. Because this chapter focuses on the coordination polymerization of CHD and its derivatives, cationic polymerization initiated by transition metal compounds will be excluded in this section. 

To our knowledge, titanium complexes were the first catalysts used for cyclic conjugated diene polymerization. Marvel, Hartzell, and Dolgoplosk et al. reported TiCl4/Al(i-Bu)3-catalyzed CHD polymerization, but very little information about microstructure was given.Because the poly(CHD) obtained is soluble in aromatic solvents, its regio- and/or stereostructure is likely not well-controlled. Lefebvre and Dawans also tried to polymerize CHD using TiCU-based catalysts, but the obtained polymer was considered to be a mixture with a cationically polymerized product. ... 

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