1,3-Dioxolane living polymer

Block Copolymers. Several methods have already been used for the synthesis of block copolymers. The most conventional method, that is, the addition of a second monomer to a living polymer, does not produce the same spectacular results as in anionic polymerization. Chain transfer to polymer limits the utility of this method. A recent example was afforded by Penczek et al. (136). The addition of the 1,3-dioxolane to the living bifunctional poly(l,3-dioxepane) leads to the formation of a block copolymer, but before the second monomer polymerizes completely, the transacetalization process (transfer to polymer) leads to the conversion of the internal homoblock to a more or less (depending on time) statistical copolymer. Thus, competition of homopropagation and transacetalization is not in favor of formation of the block copolymers with pure homoblocks, at least when the second block, being built on the already existing homoblock, is formed more slowly than the parent homoblock that is reshuffled by transacetalization. 

Schulz studies of cationic trioxepane polymerization demonstrated that the system rapidly approaches a state in which the living polymers cease to grow while the concentrations of the three monomers, trioxepane, dioxolane, and formaldehyde approach some stationary values as illustrated by Fig. 12. 

Monofunctionalized PTHF is thus obtained by end capping living polymers with monofunctional initiators. Bifunctional initiators must be used to synthesize bifunctional telechelics. Yamashita has described the synthesis of bis(dioxolan-2-ylium) cations (60) and their use as initiators for the polymerization of THF. Bi- and tri-functional initiators (61)-(63) were synthesized by Penczek and co-workers. Chains reportedly grow independently at all sites, and copolymers can be formed if 10-20% of THF is replaced by methyl oxirane. ... 

When 1,3-dioxolane was added to the solution of living (nontermi-nated) poly(l,3-dioxepane) or vice versa, further polymerization ensued and the increase of molecular weight indicated that polymerization of added monomer proceeded exclusively on living active species of the former monomer. The isolated copolymer was analyzed by l3C NMR spectroscopy and it was found that, instead of a block copolymer, the copolymer with nearly statistical distribution of DXL and DXP units was formed practically from the beginning of the process. This is a clear indication that chain transfer to polymer leads to branched oxonium ions, which participate in further reactions with a rate comparable to the rate of propagation. 

In the polymerization of 1,3-dioxolane and tetrahydrofuran it has been shown additionally that concentration of active centers is constant throughout the polymerization (both by direct determination and from analysis of polymerization kinetics). In some other polymerizations, believed to proceed as living processes, only the moderate molecular weights regions (M < 105) were studied thus, for example, no very high molecular weight polymers were obtained in the polymerization of oxazolines. 

All the approaches described have been used to prepare functional polymers by cationic ring-opening polymerization. From this point of view, groups of monomers that have been investigated most are cyclic ethers (tetrahydrofuran), cyclic acetals (1,3-dioxolane), cyclic imines (N-f-butylaziridine), and oxazolines, i.e., these monomers for which the living conditions can be approached. 

Although several telechelic polymers of 1,3-dioxolane have been prepared by cationic polymerization, their application is limited due to their susceptibility to acid-catalyzed hydrolysis and/or depolymerization. By termination of living mono- and difunctional poly(l,3-dioxolane) with amines or phosphines, polymers containing one or two stable ionic (ammonium, phosphonium) end groups has been prepared [129,274],... 

Of the cationically-polymerizable cyclic acetals, 1-3-dioxolan has received the most attention. By means of an ion-trapping technique, quantitative measurements of the concentration of active centres have been made and correlated with initiator incorporation to confirm the living characteristics of the linear polymers under certain conditions. The oxycarbenium ion nature of these propagating centres has been verified by C-n.m.r. spectroscopy. Methyl substitution alters the polymerizability of the homologous 1,3-dioxepanes significantly and steric hindrance in the initiator can influence both the homopolymerization and copolymerization of cyclic acetals. ... 

It was reported that the rate of living cationic polymerization of IBVE with Al-based initiating systems was enhanced if the basicity of an added base was reduced. Thus, a weaker base was examined in the polymerization using SnCU and FeCU. An alternative weaker base, ethyl chloroacetate, realized very fast polymerization with SnCU in toluene at -78 °C, being completed within 2 s (determined using a high-resolution digital video camera).Moreover, FeCU induced faster polymerization with 1,3-dioxolane, a weaker base than 1,4-dioxane, which was completed in 2-3 s in toluene at 0 °C. ° In both cases, product polymers had very narrow MWD (Mw/Mn< 1.1), irrespective of the monomer conversion. 

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