Polymerization by Cationic Mechanism

In these chain addition reactions, the active species is cationic in nature, initiated by strong acids, either of the protic or Lewis variety [21,104-111], Since most of these ionic polymerizations are carried out in nonaqueous solvents with low dielectric constants [109], it is unlikely that the active species is a free ion, analogous to a free radical. A multiplicity of active species may be involved as propagating species as shown below by the spectrum of cationic species, one or more of which may be involved as active propagating species, especially in more polar solvents [112], Unfortunately, very little information 

Chain-transfer to monomer is also an integral step in the cationic polymerization of isobutylene, and it is this reaction which controls the molecular weight [116]  

This mechanism has been supported by infrared and NMR spectroscopic data which provided evidence for the presence of the polymer end groups proposed by this mechanism [105,106,117-119], i.e.. 

Furthermore, the use of the tracer complex BF3-D20 showed [118] that the polymer contained deuterium while the initiator became converted to BF3 H2O. 

The foregoing simplified mechanism is amenable to kinetic analysis, using the steady-state method, as in the case of the free radical mechanism. Using the notation HA to designate the acid initiator, we can write 

---

Heterocyclic compounds, which polymerize by cationic mechanism, contain one or more heteroaroms within a ring. Depending on the nature of heteroatoms and their arrangement, these monomers belong to the different classes of organic compounds. 

Cyclic ethers of various ring size polymerize by cationic mechanism, including oxiranes (3-membered), oxetanes (4-membered), oxolanes (5-membered), oxepanes (7-membered), and larger rings. Six-membered cyclic ethers do not polymerize because of the thermodynamic restrictions. Oxiranes may be also polymerized by anionic mechanism, whereas for other groups the cationic polymerization is the only mechanism of polymerization. 

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. 

The cyclic sulfides that can be polymerized by cationic mechanism include 3-membered rings, thiiranes, and 4-membered rings, thietanes. Five-membered rings do not undergo polymerization, although the 6-membered sulfur analog of 1,3,5-trioxane, namely 1,3,5-trithiane, has been reported to polymerize [155], Polymerization of thiiranes and thietanes is practically irreversible. 

Six-membered rings can also be polymerized by cationic mechanism and it is believed that the driving force for polymerization of this essentially unstrained monomer comes from conversion of imino ether group into an amide group, which has more resonance stabilization [175] ... 

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). 

It is apparent that the bridged ring monomers in which the double bond is made more reactive by steric strain are comparable in reactivity with ethylene, whereas, as would be expected, the aliphatic dienes are more like a-olefins in reactivity. Baldwin and van Strate have critically discussed this work [244], drawing attention to the possibilities of E/P reactivity ratios being changed by the presence of the diene and of diene polymerization by cationic mechanisms. They identify as... 

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

Cationic polymerizations can take place by either one of three paths stepwise additions, intermediate-type additions, and concerted additions. When phthalaldehyde is polymerized by cationic mechanism or by y-irradiation at -78 °C, the products consist of dioxyphthalan units ... 

Polymers from Novel Monomers.—Novel compounds that have been polymerized by cationic mechanisms usually incorporate the reactive functional groups of more conventional monomers, and several recently developed monomers can be classified as analogs of vinyl ethers, vinyl carbazole, p-methoxystyrene, or previously studied oxygen heterocycles. 

Cyclic ethers polymerize by ionic mechanism. Three-membered cyclic ethers (oxiranes) polymerize by both cationic and anionic mechanisms. Four-membered and higher cyclic ethers polymerize by cationic mechanism only (although examples of anionic polymerization are occasionally mentioned in the literature). Thus, cationic mechanism is a general mechanism of cyclic ether polymerization. 

Also, 3,3-bis(hydroxymethyl)oxetane (BHMO) polymerizes by cationic mechanism. Every unit in BHMO polymers contains two HO groups therefore, the probability of branching should be enhanced. Poly(BHMO) is, however, insoluble in common organic solvents, thus the structure of polymer cannot be easily determined. Copolymerization of BHMO with EOX leads to soluble polymers with molecular weights similar to those of poly(EOX), that is, 2-3 x 10 with a considerably higher fraction of branched units as shown by NMR. 

---