Cationic initiators, ring-opening polymerization

Fig. 9. Initiation of epoxy cure. Irradiation of a triaryl sulfonium salt produces a radical cation that reacts with an organic substrate RH to produce a cation capable of releasing a proton. The proton initiates ring-opening polymerization. X = BF , PFg, AsFg, and SgFg. ...

Marks reported ring-opening polymerization of methylenecyclopropane and methylenecyclobutane promoted by metallocenes of Zr and Lu. Cationic zirconocene complex initiates ring-opening polymerization of methylenecyclobutane to give a polymer having the =CH2 substituent on every fourth carbon of the polyethylene chain (Eq. 40) [154, 155]. Polymerization... 

A cationic -allylpalladium complex with a diimine ligand, [(7r-allyl)Pd(4-Me-C6H4-N=CH-CH=N-C6H4-4-Me)](BF4) initiates ring-opening polymerization of 2-phenyl- 1-methylenecyclopropane at 80 °C (Eq. 41) [158, 159]. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

One product is poly(2-ethyl-2-oxa2oline) (PEOX). It is prepared by the ring-opening polymerization of 2-ethyl-2-oxazoline (19) with a cationic initiator (48) (eq. 6). 

The cationic ring opening polymerization of oxolane (THF) or of N-substituted aziridines can be initiated by oxocarbenium salts [42]. The methacrylic ester unsaturation is insensitive to cationic sites, and polyoxolanes (poly-THF) macromonomers are obtained in good yields. 

This polymeric oxocarbenium salt readily initiates the cationic ring opening polymerization of oxolane to produce a polystyrene-polyTHF block copolymer. Molecular weight control is provided, polydispersity is narrow and compositional heterogeneity is small59). 

Quite often in the ring-opening polymerization, the polymer is only the kinetic product and later is transformed to thermodynamically stable cycles. The cationic polymerization of ethylene oxide leads to a mixture of poly(ethylene oxide) and 1,4-dioxane. In the presence of a cationic initiator poly(ethylene oxide) can be almost quantitatively transformed to this cyclic dimer. On the other hand, anionic polymerization is not accompanied by cyclization due to the lower affinity of the alkoxide anion towards linear ethers only strained (and more electrophilic) monomers can react with the anion. 

The cationic ring opening polymerization of e-caprolactone, CL, and 8-valerolactone, VL, was investigated using n-Bu0H/HCl-Et20 as the initiation system [56]. It was observed that narrow molecular weight distribution samples were obtained. These results were combined with those previously... 

The bifunctional initiator 4-hydroxy-bulyl-2-bromoisobulyralc, HBBIB, promoted the ATRP of styrene as well as the cationic ring opening polymerization of THF [134], In the presence of Cu/CuBr2/PMDETA styrene was polymerized through the bromoisobutyrate function of HBBIB, to give PS chains end-functionalized with hydroxyl groups, PS-OH. The in situ... 

Cationic guar gum, 4 724t Cationic hydroxyethylcellulose, 5 455-456 Cationic initiators, 14 265-273 controlled initiation and, 14 268-269 direct initiation and, 14 270 initiating systems for, 14 266-268 photoinitiation and, 14 270 ring-opening polymerization and,... 

The living cationic ring opening polymerization (CROP) of 2-oxazolines was first reported in the 1960s [61, 62]. The polymerization can be initiated by an electrophile such as benzyl halides, acetyl halides, and tosylate or triflate derivatives. The typical polymerization mechanism for 2-alkyl-2-oxazoline initiated by methyl tosylate is shown in Scheme 6. 

Paulus RM, Becer CR, Hoogenboom R et al. (2008) Acetyl halide initiator screening for the cationic ring opening polymerization of 2-ethyl-2-oxazoline. Macromol Chem Phys 209 794-800... 

Becer CR, Paulus RM, Hoppener S et al. (2008) Synthesis of poly(2-ethyl-2-oxazoline)-h-poly(styrene) copolymers via a dual initiator route combining cationic ring opening polymerization and atom transfer radical polymerization. Macromolecules 41 5210-5215... 

Ring-opening polymerizations are generally initiated by the same types of ionic initiators previously described for the cationic and anionic polymerizations of monomers with carbon-carbon and carbon-oxygen double bonds (Chap. 5). Most cationic ring-opening polymerizations involve the formation and propagation of oxonium ion centers. Reaction... 

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