Polymerization controlled/living cationic

PS/PIB/PS block copolymers can be made by controlled-living cationic polymerization. In this polymerization process, the propagating chains are in equilibrium with the dormant species. A suit-... 

Figure 6.4 Catalyst Systems for Controlled-Living Cationic Polymerization (4,36)...

Figure 16 Controlled/living cationic polymerizations of alkyl vinyl ethers with the HI/I2 initiating systems. (From Ref. 58.)...

In light of the knowledge currently available, this chapter aims to discuss the general principles, methodologies, and scope of controlled/ living cationic polymerizations and more specific examples for representative classes of monomers. The next chapter will be devoted to the controlled syntheses of new functional polymers using these systems. 

Since its discovery for vinyl ethers and isobutene in the 1980s, the scope of controlled/living cationic polymerization has been expanded rapidly in terms of monomers and initiating systems. Figure 18 shows a partial list of representative monomers for which controlled/living cationic polymerizations are available. They cover virtually all classes of cationically polymerizable vinyl compounds, such as vinyl ethers, isobutene, styrene and its derivatives, and A/-vinylcarbazole. A rough estimate indicates that the total number of monomers for controlled/living cationic polymerization... 

Figure 18 Vinyl monomers for which controlled/living cationic polymerizations are feasible at the end of 1994.

Yes controlled/living cationic polymerization is feasible ( ) controlled/living polymerizations based on this method are not yet reported. b See Section IV.B for the three general methods for initiator design. 

Since the discovery of the first controlled/living cationic polymerization of isobutyl vinyl ether [IBVE CH2=CH OCH2CH(CH3)2 ] with the HI/... 

The HB/MtX initiating systems with stronger Lewis acids than zinc halides induce very rapid or almost instantaneous polymerizations of alkyl vinyl ethers and are not suited for controlled/living cationic polymerizations (Section IV.B.2). These initiating systems include ... 

For these initiating systems, externally added nucleophiles are necessary to induce controlled/living cationic polymerizations of vinyl ethers [36,64]. Table 2 A lists nucleophiles (Lewis bases) that are effective for such purposes and includes esters (carboxylates and carbonates) [100,101,130-133], ethers (linear and cyclic) [102-104,137-140], methyl-pyridines [140], and phosphines [21,141]. CF3S03H-initiated polymerizations, sulfides are also effective [37,38,134,135]. 

An important advantage of the use of such added nucleophiles is that it allows controlled/living cationic polymerization of alkyl vinyl ethers to proceed at +50 to +70°C [101,103], relatively high temperatures at which conventional cationic polymerizations fail to produce polymers but result in ill-defined oligomers only, due to frequent chain transfer and other side reactions. Recently, initiators with functionalized pendant groups [137] and multifunctional initiators [ 138—140] have been developed for the living cationic polymerizations with added nucleophiles. 

Propenyl Ethers and Unsaturated Cyclic Ethers Propenyl ethers (CH3—CH=CH—OR R = ethyl, isobutyl, etc. cis- and trans-isomers) and 3,4-dihydrofuran are linear and cyclic a,/3-unsaturated ethers, that can be regarded as / -substituted vinyl ether derivatives. For these monomers a few controlled/living cationic polymerizations have been reported. The HI/I2 system is generally effective for both linear and cyclic monomers [181,182,183], whereas a recent study by Nuyken indicates that the IBVE-HI adduct coupled with nBu4NC104 is suited for 3,4-dihydrofuran (see Section V.A.4) [184]. A variety of mono- and bifunctional propenyl ethers can readily be prepared by the ruthenium complex-catalyzed isomerization of corresponding allyl ethers [185]. 

In 1986 Faust and Kennedy reported the first example of controlled/living cationic polymerization of isobutene, which was initiated by a cumyl ace-... 

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. 

For this class of styrenic monomers, controlled/living cationic polymerizations have been reported for p-methylstyrene, p-ferf-butylstyrene, and 2,4,6-trimethylstyrene (Figure 22). Structurally, these monomers lie between styrene and p-alkoxystyrenes, and the moderately electron-donat-... 

Figure 24 Typical multifunctional initiators for controlled/living cationic polymerizations. See also Table 3 for references with isobutene. ...

As an extension of the H0S02CF3/SMe2 system (SMe2 as an added nucleophile) [37], it was reported that Me3Si0S02CF3 initiates controlled/ living cationic polymerization of IBVE in the presence of acetone (for initiation) and SMe2 [134,244]. 

Most controlled/living cationic systems provide well-defined polymers of relatively low molecular weight, typically in the range M = 5,000 to 20,000. The degrees of polymerization are equal to the ratio of concentrations of the reacted monomer to that of the introduced initiator (DP = d[M]/[I]0), and the polydispersities remain relatively low, MJMn < 1.2. The addition of new portions of monomer (sometimes called incremental monomer addition, IMA) [5] leads to the expected increase of molecular weights, and the addition of another comonomer results in block copolymers, whereas the addition of terminating reagents provides end-functionalized macromolecules (cf. Chapter 5). 

Polymerization of vinyl ethers (VE) has been the subject of a considerable amount of theoretical studies. These monomers can be polymerized through radical initiation but the reaction is very slow and leads only to oligomers. Cationic polymerization initiated by a wide variety of Lewis acids is much more efficient and definitely preferred for homopolymer synthesis. Detailed theoretical aspects, and particularly recent developments concerning the controlled/living cationic polymerization of these monomers, have been discussed as well in previous exhaustive review [1,13,98,99] as in the present book (Chapters 4 and 5), and they will no longer be considered here. 

Controlled/Living Cationic Polymerization of Vinyl Monomers Based on Cation Stabilization... 

One of the major drawbacks of the controlled/living cationic polymerization based on stabilization of the carbocationic intermediates is slow propagation. Because the concentration of the active propagating species is very low because of the equdibrium between active species and dormant species, overall polymerization reactions are much slower than those without the equilibrium. Another important drawback of the controlled/living polymerization is the use of additives such as Lewis bases. Such additives remain in polymer products and are generally rather difhcult to remove from the polymer products. 

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