Growing carbocation

In 1982 Higashimura et al. [54] began studies focused on the development of living cationic polymerizations of vinyl monomers. They decided to use IBVE and related alkyl vinyl ethers as monomers because they form the alkoxy-stabilized growing carbocations, along with iodine as the initia-... 

As discussed in the preceding sections of this chapter, the key to living cationic polymerization is to reduce the effect of chain transfer reactions (Scheme 4) because termination is much less important in the cationic polymerization of vinyl monomers. The primary reason for frequent chain transfer reactions of the growing carbocation (1) is the acidity of the /3-H atoms, next to the carbocationic center, where a considerable part of the positive charge is localized. Because of their electron deficiency, the protons can readily be abstracted by monomers, the counteranion (B ), and other basic components of the systems, to induce chain transfer reactions. It is particularly important to note that cationically polymerizable monomers are, by definition, basic or nucleophilic. Namely, they have an electron-rich carbon-carbon double bond that can be effectively poly-... 

Perhaps the most frequently used example is the HI/ZnI2 system, where the iodine in the HI/12 counterpart is now replaced with the mild Lewis acid, zinc iodide [98,99]. A more detailed discussion of the scope and mechanism of the polymerizations by the HB/MtX systems will be given for respective monomers in the later parts of Section IV. It should be noted here that the suitable nucleophilicity of the initiator s anion B- and the mild Lewis acidity of MtX strongly depend on the nature and stability of the growing carbocations or the monomers from which they are derived. 

The actual role of the added nucleophile is still under discussion [41,88,92] (cf., Sections VI.B.2 and VILE.4). They may interact with the growing carbocations and stabilize them through the weak solvating interactions [36]. Another possibility is that the added nucleophiles and the growing carbocations form reversibly onium ions that serve as dormant species, as discussed by Penczek [92] and Matyjaszewski [88] (schemati-... 

Controiied/Living Polymerizations with Added Salts The two approaches discussed above are primarily useful in nonpolar solvents (like toluene and n-hexane) where the interactions of carbocations with nucleophiles are strong and favored. In relatively polar solvents like methylene chloride, these methods often fail to give controlled polymerizations, most likely because the interaction is weaker between the growing carbocations and nucleophiles [whether they are built-in (counteranions) or externally added (esters, etc.)], which facilitates dissociation of the carbocation. The effect of solvent in the latter system, however, is much weaker. 

The mechanistic details and roles of all constituents of the multicomponent initiating systems for new controlled/living carbocationic polymerization are also discussed in Section VI. At this stage it suffices to say that in both the new systems and conventional carbocationic polymerization, monomer is consumed by the repetitive electrophilic addition of growing carbocations whether or not in dynamic equilibrium with either covalent species or onium ions. 

In contrast to p-alkoxystyrenes, styrene lacks an electron-donating, car-bocation-stabilizing substituent, and thus it is much less reactive and forms a much less stable growing carbocation. It has therefore been believed that controlled/living cationic polymerization of styrene would be very difficult. 

Relative to living cationic polymerization, the structure of a-methylsty-rene is both advantageous and disadvantageous. Because of the additional methyl group on the a-carbon, the growing carbocation is tertiary and should be thermodynamically more stable, but it would also be prone to undergo j8-proton elimination (chain transfer) due to the increase in the number of abstractable protons. Another important aspect of this monomer is its low ceiling temperature that requires low temperatures for polymerization. 

Sigwalt proposed, based on calculated transfer constants, that for indene and related isobutene polymerizations, the use of nucleophilic additives does not stabilize the growing carbocation but adjusts the relative rates of initiation and propagation to achieve efficient initiation and narrow MWDs of the polymers [163,164,232]. 

Nucleophiles affect the polymerization in two different ways (cf., Section VII.E.4). They form complexes with Lewis acids reducing their strength, and/or they also form onium ions with the growing carbocations. In both cases they reduce the polymerization rates. 

In the following discussions, we will sometimes employ the term deactivation in the sense that the active growing carbocation is converted into the corresponding dormant or covalent species (e.g., — C +, SnCls - —C—Cl + SnCl4). Accordingly, added nucleophiles and salts are sometimes referred to as deactivators. ... 

All components of controlled systems must be selected with respect to their low basicity in order to decrease the possibility of /3-proton elimination from growing carbocations. 

The equilibrium position between carbocations and dormant species (covalent or onium) should be adjusted to provide convenient rates (i.e., an appropriately low concentration of the growing carbocation at a given time). For a particular monomer, the equilibrium position (and overall rate) depends on the nature and concentrations of the leaving group X in the initiator RX, of the activator (Lewis acid), and of the deactivator... 

Veiy similar conclusions have been drawn for the CFaSOsH/styrene system and for common ion salt effects on monomer reactivity ratios.In the case of initiation by trifluoroacetic acid, bimodal distributions are also obtained and similar conclusions reached. With this conclusion a relatively stable ester might be formed with the growing carbocation, but the experimental evidence indicates that this is not one of species responsible for the production of polymer. 

As can be seen, the attack of the nucleophillic oxygen of the ylide compound to the growing carbocation chain terminates polymerization. A steric hindrance around... 

Consequently, the relatively stable oxonium ions can often be directly observed by nmr spectroscopy, as also can carbocations in nonpolymerizing model systems. With polymerizing systems, the concentration of carbocations is either too small, or the growing species are too reactive. Thus the presence of growing carbocations is mostly indirectly deduced from model reactions, kinetic measurements, conductivity measurements, or addition of cation scavengers. 

In cationic vinyl polymerization reactions eqs. (19)—(21), the propagating carbocation —CH2—CH (R) BA This carbocation is derived from vinyl monomer and an initiator [16—20]. The growing carbocations are highly reactive but unstable and subject to a number of side reactions such as chain transfer and termination, among which chain transfer to monomer is most important. 

The rearranged structure arises from an isomerization of the growing carbocation via a 1,2-hydride migration which competes directly with the propagation step. An increase in the polymerization temperature favors the propagation step, however, so that at temperatures greater than -100 C, the product of the cationic polymerization of 3-methyl-l-butene is in fact a copolymer of 1,2- and 1,3-repeating... 

Since it was considered less difficult to suppress side reaction in the pMOS polymerization due to the stabilization of the growing carbocation, cationic polymerization of pMOS was investigated in detail using iodine. The MWDs of product polymers became unimodal in the polymerization in CCI4 at 0 °C. A nearly linear relationship was observed between the peak molecular weight of the product polymers and monomer conversion, indicative of polymerization mediated by long-lived active species. At -15 °C, the of product polymers increased in almost direct proportion to monomer conversion after the second feed of pMOS. Moreover, block copolymers with IBVE were obtained under similar conditions. 

A combination of a sulfide and a strong protonic add induces living polymerization. In this system, a sulfonium salt is produced from a sulfide and the growing carbocation shortly after a protonic acid initiates polymerization. The sulfonium salts are dormant species, which change reversibly into free ionic species (Scheme 6). This initiating system is available only for VE polymerization. 

---