Vinyl ethers, living cationic polymerization

Diffiuex investigated a synthesis of cyclic poly(vinyl ether) using cationic polymerization [26,28]. The reaction process is depicted in Fig. 9. They studied on the living cationic polymerization of 2-chloroethyl vinyl ether (CEVE) initiated with the HI adduct of 4-(vinylbenzyloxy)butyl vinyl ether prepared by reacting chloromethyl styrene with sodium salt of 4-hydroxy-butyl vinyl ether in THF at 80 °C. By the cationic polymerization of CEVE, o /o-hetcrofunclional linear polymer precursor of cyclic poly(CEVE) was produced. The MWDs of the polymers were unimodal and very narrow (< 1.2),... 

In general, styrene and its substituted derivatives are less reactive than vinyl ethers in cationic polymerization, although the reactivity depends considerably on the nature of the substituents. This in turn requires some care in synthesizing block copolymers of styrene derivatives by sequential living cationic polymerization. For example, styrene-methyl... 

Polymerization of Vinyl and Cyclic Ethers Living Cationic Polymerizations... 

The observation in 1949 (4) that isobutyl vinyl ether (IBVE) can be polymerized with stereoregularity ushered in the stereochemical study of polymers, eventually leading to the development of stereoregular polypropylene. In fact, vinyl ethers were key monomers in the early polymer Hterature. Eor example, ethyl vinyl ether (EVE) was first polymerized in the presence of iodine in 1878 and the overall polymerization was systematically studied during the 1920s (5). There has been much academic interest in living cationic polymerization of vinyl ethers and in the unusual compatibiUty of poly(MVE) with polystyrene. 

It is to be noted that N-vinylcarbazole (NVC) undergoes also living cationic polymerization with hydrogen iodide at —40 °C in toluene or at —78 °C in methylene chloride and that in this case no assistance of iodine as an activator is necessary 10d). NVC forms a more stable carbocation than vinyl ethers, and the living propagation proceeds by insertion between the strongly interacting NVC-cation and the nucleophilic iodide anion. 

A recent contribution reported by Aoshima and Tsukuda showed the aerobic oxidation of alcohols such as benzyl alcohol catalyzed by gold nanoclusters. These stable and durable clusters of less than 4 nm were prepared using thermosensitive vinyl ether star polymers previously obtained by living cationic polymerization. 

M. Sawamoto, T. Enoki, and T. Higashimura, End-functionalized polymers by living cationic polymerization. 1. Mono- and bifunctional poly(vinyl ethers) with terminal malonate or carboxyl groups, Macromolecules, 20(l) l-6, January 1987. 

The first synthesis of star polymers with a microgel core was reported by Sa-wamoto et al. for poly(isobutyl vinyl ether) (poly(IBVE)) [3,4]. In the first step, living cationic polymerization of IBVE was carried out with the HI/ZnI2 initiating system in toluene at -40 °C. Subsequent coupling of the living ends was performed with the various divinyl ethers 1-4. 

Well-defined macromonomers of poly(BVE), poly(IBVE), and poly(EVE) with co-methacrylate end group [91] were prepared by living cationic polymerization of the corresponding monomers initiated by trifluoromethanesulfonic add in CH2C12 at -30 °C in the presence of thiolane as a Lewis base. After complete conversion, the polymers were quenched with 37 in the presence of 2,6-lu-tidine or with 41 to produce macromonomers with Mn up to 10,000 g mol-1, with narrow MWD, bearing one polymerizable methacrylate function per molecule. The same polymers were also quenched with 38 in the presence of 2,6-lutidine to give poly(vinyl ether)s with an allylic terminal group. 

A very interesting variant of this kind of initiation, leading to living cationic polymerizations of vinyl ethers, is the dissociation [233]... 

In a series of papers (see for example, ref. 250), Higashimura and Sawamo-to have described the living cationic polymerization of vinyl ethers at low... 

Cationic polymerizations are not only important commercial processes, but, in some cases, are attractive laboratory techniques for preparing well-defined polymers and copolymers. Polyacetal, poly(tetramethyl-ene glycol), poly(e-caprolactam), polyaziridine, polysiloxanes, as well as butyl rubber, poly(N-vinyl carbazol), polyindenes, and poly(vinyl ether)s are synthesized commercially by cationic polymerizations. Some of these important polymers can only be prepared cationically. Living cationic polymerizations recently have been developed in which polymers with controlled molecular weights and narrow polydispersity can be prepared. 

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-... 

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

Figure 17 MWDs of poly(isobutyl vinyl ether) to illustrate the three general methods for living cationic polymerizations at -15° C [M]0 = 0.38 Af [HCI]0 = [ZnCl2]0 = rSnCL(]0 = 5.0 mA/ conversion = ca.100%. Initiating systems and reaction conditions (a) HCl/ZnCl2, in toluene without additive (b) HCl/SnCU, in toluene without additive (c) HCl/SnCLt, in toluene with added tetrahydrofuran (100 mM) (d) HCl/SnCU, in CH2C12 without additive (e) HCl/SnCU, in CH2C12 with added Bu4NCl (2.0 mM). (From Refs. 73 and 105.)...

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... 

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

Figure 20 Protonic acid/Lewis acid (HB/MtX ) initiating systems for living cationic polymerizations of vinyl ethers. See Section V.A.l for references.

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. 

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