Chain polymerization by cationic

CHAIN POLYMERIZATION BY CATIONIC MECHANISM 2.7.1 Mechanism and Kinetics... 

Which of the following could be polymerized by cationic chain polymerization ... 

Cyclic Sulfides. The three-membered cyclic thiiranes can be polymerized cationically, anionically, or by a coordination mechanism. The four-membered cyclic thietanes can be polymerized by cationic and anionic mechanisms, but five-membered rings cannot be polymerized. Polymerization of propylenesulfide initiated with sodium naphthalene yields telechelics with naphthalene groups on both ends, if the living chain is terminated 1-chloromethylnaphthalene (321). 

All systems mentioned sofar are cured by radical, chain polymerization. Other monomers such as epoxides or vinyl ethers may be polymerized by cationic chain polymerization. 

Likewise, p-pinene with its exocyclic vinyl group is readily polymerized by cationic techniques however, the polymers obtained have rather low MWs of<3.4 kg/mol [80, 84-86]. High molecular weight poly(p-pinene) with MW up to 40 kg/mol (PDI 2.2) can be obtained with the H20 /EtAlCl2 system ( H2O indicates adventitious moisture impurities). The polymerizations are carried out in mixtures of methyl chloride/ methylcyclohexane (preferred composition 50 50) at —80°C. Quantitative monomer conversions are reached within 20 min or less. The repeat unit of the poly(p-pinene) is found to consist of a cyclohexene unit in the main chain (Scheme 6), which reflects isomerization polymerization [87]. With AICI3 etherates, e.g., AlCl30Ph2, the polymerization can be performed even at room temperature and low catalyst concentration (2.S-5.5 mM, [p-pinene]o = 0.55 M) to yield polymers with = 9-14 kg/mol... 

The polymerization of cydic disulfides to polydisulfides has been reported in the 1940s and 1950s. In some cases the polymerizations occur spontaneously. Tobolsky et reported that l-oxa-4,5-dithiacycloheptane and l,3-dioxa-6,7-dithiacyclononane are polymerized by cationic initiators such as sulfuric acid or boron trifluoride. Davis and Fettes repotted the anionic polymerization of various cyclic disulfides. In the same period it was also described that cyclic disulfides can copolymetize with vinyl monomers such as styrene and butyl acrylate with AIBN as initiator. That the incorporation of the disulfide was due to copolymerization and not by chain transfer was established by comparing the thermal polymerization of styrene in the presence of dibutyl disulfide and in the presence of l-oxa-4,5-dithiacycloheptane. In the first case, the polymer contained 2 sulfur atoms per macromolecule as a result of transfer reactions and in the second case 4-20 sulfur atoms depending on the ratio of monomers. 

Pathway (1) is mainly encountered in chain polymerization (anionic, cationic, and controlled radical polymerizations) the second one refers essentially to polycondensation and polyaddition. There is no strict distinction between these two sets of techniques, e.g., the difunctional oligoethers used in poly(ether-6-ester)s or poly(ether-6-amide)s can be prepared by ring-opening polymerization and then polycondensed with the other oligomer (Scheme 2) [39]. 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

Butyl mbber, a copolymer of isobutjiene with 0.5—2.5% isoprene to make vulcanization possible, is the most important commercial polymer made by cationic polymerization (see Elastomers, synthetic-butyl rubber). The polymerization is initiated by water in conjunction with AlCl and carried out at low temperature (—90 to —100° C) to prevent chain transfer that limits the molecular weight (1). Another important commercial appHcation of cationic polymerization is the manufacture of polybutenes, low molecular weight copolymers of isobutylene and a smaller amount of other butenes (1) used in adhesives, sealants, lubricants, viscosity improvers, etc. 

B. Formation of MAIs by Cationic Chain Polymerization—Cation Radical Transfer... 

Anionic polymerization is better for vinyl monomers with electron withdrawing groups that stabilize the intermediates. Typical monomers best polymerized by anionic initiators include acrylonitrile, styrene, and butadiene. As with cationic polymerization, a counter ion is present with the propagating chain. The propagation and the termination steps are similar to cationic polymerization. 

Chain growth occurs through a nucleophilic attack of the carbanion on the monomer. As in cationic polymerizations, lower temperatures favor anionic polymerizations by minimizing branching due to chain transfer reactions. 

The initiator can be a radical, an acid, or a base. Historically, as we saw in Section 7.10, radical polymerization was the most common method because it can be carried out with practically any vinyl monomer. Acid-catalyzed (cationic) polymerization, by contrast, is effective only with vinyl monomers that contain an electron-donating group (EDG) capable of stabilizing the chain-carrying carbocation intermediate. Thus, isobutylene (2-methyl-propene) polymerizes rapidly under cationic conditions, but ethylene, vinyl chloride, and acrylonitrile do not. Isobutylene polymerization is carried out commercially at -80 °C, using BF3 and a small amount of water to generate BF3OH- H+ catalyst. The product is used in the manufacture of truck and bicycle inner tubes. 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

A special case of the internal stabilization of a cationic chain end is the intramolecular solvation of the cationic centre. This can proceed with the assistance of suitable substituents at the polymeric backbone which possess donor ability (for instance methoxy groups 109)). This stabilization can lead to an increase in molecular weight and to a decrease in non-uniformity of the products. The two effects named above were obtained during the transition from vinyl ethers U0) to the cis-l,2-dimethoxy ethylene (DME)1U). An intramolecular stabilization is discussed for the case of vinyl ether polymerization by assuming a six-membered cyclic oxonium ion 2) as well as for the case of cationic polymerization of oxygen heterocycles112). Contrary to normal vinyl ethers, DME can form 5- and 7-membe red cyclic intermediates beside 6-membered ringsIl2). 

Thus, the preferred intramolecular stabilization of the cationic chain end by the formation of 5- and 3-membered cycles, which is possible for DME but impossible for vinyl ethers, can explain the characteristics of the cationic polymerization of DME in solvents of low polarity. 

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). 

---