Cationic polymerization of cyclic

Cationic polymerization of cyclic acetals generally involves equilibrium between monomer and polymer. The equilibrium nature of the cationic polymerization of 2 was ascertained by depolymerization experiments Methylene chloride solutions of the polymer ([P]0 = 1.76 and 1.71 base-mol/1) containing a catalytic amount of boron trifluoride etherate were allowed to stand for several days at 0 °C to give 2 which was in equilibrium with its polymer. The equilibrium concentrations ([M]e = 0.47 and 0.46 mol/1) were in excellent agreement with that found in the polymerization experiments under the same conditions. The thermodynamic parameters for the polymerization of 1 were evaluated from the temperature dependence of the equilibrium monomer concentrations between -20 and 30 °C. 

L-ascorbic acid and, 25 751 catalytic esterification of, 10 482 in cationic polymerization of cyclic siloxanes, 22 560 cellulose as, 11 266 a-chiral and homologated, 13 669 control methods for, 26 687-690 derived from halogen fluorides,... 

Protic acids, in cationic polymerization of cyclic siloxanes, 22 560 PROTO (-)-Protoemetine, 2 84, 85 Protonated ozone, 7 7 774-775 Protonated pyridines, 27 100-101 Protonation, 75 653-654... 

Studies on the cationic polymerization of cyclic ethers, cyclic formals, lactones and other heterocyclic compounds have proliferated so greatly in the last few years that a detailed review of the evidence concerning participation of oxonium and analogous ions in these reactions cannot be given here. Suffice it to say that there is firm evidence for a few, and circumstantial evidence for many such systems, that the reactive species are indeed ions and there appears to be no evidence to the contrary. A few systems will be discussed in sub-sections 3.2 and 4.4. 

Carbon-13 NMR Studies on the Cationic Polymerization of Cyclic Ethers... 

Propagation in the cationic polymerization of cyclic ethers is generally considered as proceeding via a tertiary oxonium ion, for example, for the polymerization of 3,3-bis(chlor-omethyl)oxetane (R = CH2C1)... 

The commercial importance of polysiloxanes (silicone polymers) was discussed in Sec. 2-12f. The higher-MW polysiloxanes are synthesized by anionic or cationic polymerizations of cyclic siloxanes [Bostick, 1969 Kendrick et al., 1989 Noll, 1968 Saam, 1989 Wright, 1984]. The most commonly encountered polymerizations are those of the cyclic trimers and tetra-... 

It is generally agreed that propagation in the cationic polymerization of cyclic ethers occurs after nucleophilic attack by the monomer oxygen atom (equation 3). Therefore, many authors attempt to explain their copolymerization data by noting that the more basic monomer has the higher reactivity with the active chain end. The order of basicity which has been established (36, 38) is ... 

LPEI is obtained by cationic polymerization of cyclic iminoethers as oxazoline and oxazine derivatives producing a linear low-molecular compound with a high crystallinity [6-20]. 

Table 2. Oligomer formation during the cationic polymerization of cyclic ethers... 

The initiation mechanism for cationic polymerization of cyclic ethers, vinyl amines, and alkoxy styrenes has been investigated by A. Ledwith. He used stable cations, like tropylium or triphenylmethyl cations with stable anions, like SbCl6, and distinguished between three initiation reactions cation additions, hydride abstraction, and electron transfer. One of the typical examples of cationic polymerization, in which the propagating species is the oxonium ion, is the polymerization of tetra-hydrofuran. P. and M. P. Dreyfuss studied this polymerization with the triethyloxonium salts of various counterions and established an order of... 

N-Benzyl pyridinum salts were found to initiate thermally cationic polymerization of cyclic and vinyl monomers. The representative monomers with the following structures were successfully polymerized ... 

It is well known that cyano derivatives of anthracene form charge transfer (CT) complexes with certain aromatic compounds. It was reported [67] that the radical cations formed upon irradiation of these complexes played an important role in initiation of cationic polymerization of cyclic ethers. Pyridinium salts were also found [68] to form CT complexes with hexamethyl benzene and trimethoxy benzene which result in the formation of a new absorption band at longer wavelengths where both donor and acceptor molecules have no absorption. This way the light sensitivity of the pyridinium salts may be extended towards the visible range. According to the results obtained from the... 

UV irradiation of the resulting prepolymers caused a-scission, and benzoyl and polymer bound electron donating radicals are formed in the same manner as described for the low-molar mass analogues. Electron donating polymeric radicals thus formed may conveniently be oxidized to polymeric carboeations to promote cationic polymerization of cyclic ethers. It was demonstrated that irradiation of benzoin terminated polymers in conjuction with pyridinium salts as oxidants in the presence of cyclohexene oxide makes it possible to synthesize block copolymers of monomers with different chemical natures [75] (Scheme 19). 

Another novel class of silicon-based initiators has been described [44]. Compounds containing Si—H bond in the presence of platinum catalysts (Ptl2, H->PtBr6, PtCl2(C6HsCN)2) are effective initiators of cationic polymerization of cyclic ethers. 

Thus, chain transfer to polymer does not influence the number average DP , it may however alter the molecular weight distribution. If the reversible chain transfer to polymer described in Eq. (78) occurred frequently, it would lead to statistical distribution, i.e., MJM = 2. The other consequence is that if the two originally present chains are different, the repetition of reaction sequence will lead to segmental exchange (so called scrambling ). Both effects are clearly detectable, for example, in the cationic polymerization of cyclic acetals as it will be discussed in Section III.B. 

Termination by Reactions of More Reactive Species Existing in Equilibrium with Stable Onium Species As already discussed, in the systems, in which unimolecular ring-opening of cyclic onium ion leads to highly stabilized carbocationic species, a concentration of the latter species in equilibrium with onium ions may be significant. This is, for example, the case of cationic polymerization of cyclic acetals, where carboxonium ions exist in equilibrium with their oxonium counterpart ... 

The most characteristic feature of the cationic polymerization of cyclic acetals, however, is an excessive participation of the polymer chain in the polymerization processes. This is exemplified by the results of attempted synthesis of block copolymer containing segments of poly(l,3-dioxolane, DXL) and poly(l,3-dioxepane, DXP) [130]. 

Indeed, it was shown mat cyclic oligomers are always formed in the cationic polymerization of cyclic acetals and their distribution agrees well with the thermodynamic distribution calculated on the basis of Jacobson-Stockmayer theory (with exception of small rings with n = 2-4) [89]. 

The preceding discussion shows that in the cationic polymerization of cyclic acetals chain transfer to polymer can not be avoided. If however polymerization is carried out at high initial monomer concentration (preferentially in bulk) the content of cyclic fraction may be limited to a few percent. As the cyclic fraction is composed mainly of medium-size rings, the high molecular weight polymer may be separated from cyclic fraction by fractionation. 

If R = R (bifunctional polymers), reaction (119) does not affect the functionality but leads to the broadening of the molecular weight distribution, which is occurring anyway, due to the reversibility of propagation. Thus, several bifunctional polymers of 1,3-dioxolane were prepared and used, for example, to form the networks containing degradable and hydrolyzable polyacetal blocks (cf., Section IV.B). Reaction (119), however, may effectively prohibit the preparation of monofunctional polymers, e.g., macromonomers. Indeed, two recent attempts to prepare macromonomers by cationic polymerization of cyclic acetals led to nearly statistical... 

Cationic polymerization of cyclic carbonates, e.g., ethylene carbonate resembles cationic polymerization of lactones. The postulated polymerization scheme is shown by Eq. (153) (for initiation with CF3SO3CH3) [201,202] ... 

It has been found that decarboxylation may be completely eliminated if cationic polymerization of cyclic carbonates is initiated with alkyl iodide or bromide. It is believed that polymerization proceeds with the participation of covalent active species favoring propagation over side reactions leading to C02 elimination [204]. It is interesting to note, that BF3-initiated polymerization of some cyclic carbonates leads to high molecular weight polymers (M > 10s) [205]. 

Cationic polymerization of cyclic compounds containing trivalent phosphorus atom has also been reported. Monomers include cyclic phoshites (a) [223,224], phosphonites (b) [225-227], and deoxophosphones (c) [228] ... 

Cationic polymerization of cyclic silaethers, octamethyl-l,4-dioxa-2,3,5,6-tetrasilacyclohexane [Eq. (178)] and octamethyl-l-oxa-2,3,4,5-tet-rasilacyclopentane [Eq. (179)] was also reported [256] ... 

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