Polymerization of cyclic acetals

In connection with studies on the ring-opening polymerization of cyclic acetals, we have undertaken investigations on the polymerization of bicyclic acetals, bicyclic oxalactone, and bicyclic oxalactam, which yield polysaccharide analogs, macrocyclic oligoesters, and a hydrophilic polyamide, respectively, some of which can be expected to be useful as novel speciality polymers. The monomers employed in the studies were prepared via synthetic routes presented in Scheme 1, starting from 3,4-dihydro-2H-pyran-2-carbaldehyde (acrolein dimer) I. 

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. 

Penczek, S. and P. Kubisa, Progress in Polymerization of Cyclic Acetals, Chap. 5 in Ring-Opening Polymerization, T. Saegusa and E. Goethals, eds., American Chemical Society, Washington, DC, 1977. 

Formally similar scheme may also operate for other initiating systems. Thus, for polymerization of cyclic acetals initiated with triphenyl-methylium (trityl) salts, the cationation of the monomer is a fast, reversible reaction [7], The next reaction, however, due to the steric hindrance, is very slow and consequently active species are formed by a parallel path involving hydride transfer (cf., Section II.A.2) [8,9],... 

On the other hand, when highly stabilized carbenium ion results from unimolecular ring opening, the carbenium active species may be present in the system in detectable concentrations. Indeed, in the polymerization of cyclic acetals —O -Ct + active species exist in equilibrium with tertiary oxonium ion active species (cf., Section lI.B.6.b.). 

The system for which the reactivity of carbenium and onium active species have been quantitatively compared is the polymerization of cyclic acetals [76] ... 

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. 

Thus, even in the least favorable case of polymerization of cyclic acetals, the counterions that do not interact with growing species may be selected, which indicates that, in general, the proper choice of counterion eliminates the possibility of termination by recombination with counterion. 

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 ... 

In contrast to the previously discussed case of THF polymerization, where chain transfer to polymer is slow as compared to propagation, in the polymerization of cyclic acetals, chain transfer to polymer is fast as compared to propagation and the polymerization is dominated by reactions involving polymer chains. Polymerization of the two best studied monomers of this group, 1,3-dioxolane and 1,3,5-trioxane, shows certain specific features. Thus both systems will be discussed separately in the following sections, with special emphasis on the consequences of the chain transfer to polymer. 

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... 

Intermolecular chain transfer to polymer leads also to the exchange of segments between macromolecules (scrambling). This may effectively preclude the isolation of block copolymers. This phenomenon is especially pronounced in the polymerization of cyclic acetals. 

In the polymerization of cyclic acetals there has been a long lasting discussion about the nature of growing species and it seems to be at present only partially resolved. 

Polymerization of Cyclic Acetals Initiated by Protonic Adds... 

It cannot be excluded that in the polymerization of cyclic acetals which are less nudeo-philic than 1,3-dioxolane, propagation on alkoxycarbenium ions can also have its share in building up the macromolecules (the polymerization of 1,3,5-trioxane, the least nucleojMlic acetal, may proceed this way). 

In Tables 10-12, the values of kp are listed for a number of heterocyclic monomers these values were determined either on the bases of analytically determined concentrations of active species or with an assumption that the concentration of the growing species is equal to the initial concentration of initiator. This assumption led to a number of incorrect values of kp listed in the literature, particularly for the polymerization of cyclic acetals. A comprehensive critical treatment was published on this subject , some of the reasons for the low efficiency of initiation are discussed in Sect. 3. 

Both reactions, namely tte intermolecular process (131) and the intramolecular one (132) can be either reversible or irreversible (termination). In the case of reversible reactions true chain transfer takes place vdien the rate constant of the backward reaction (kj ) becomes comparable with the rate constant of M-opaptun. This applies to the polymerization of cyclic acetals where the product of chain traiKfer is equally active in propaption. 

Apparently, the polymerization of cyclic acetals, or at least of 1,3-dioxolane, constitutes another and highly special case where the originally formed active species (e.g. cationated monomer and/or cationated cyclic oligomers) are converted at the early stage of polymerization into the polymeric cations which are still active, e.g. ... 

Apparently, the polymerization of cyclic acetals, notably the pol)mierization of 1,3,6,9-tetraoxacycloundecane studied by Schulz oceeds according to Scheme (165) with kinetic enhancement. This may be the reason that, at the early stages of polymerization, predominantly cyclic dimer, trimer, tetramer, etc., are observed although propagation proceeds on the linear growing species. 

Probably the first reference to the polymerization of cyclic acetals (formals) was by Hill and Carothers [143]. Since then a number of groups of research workers have shown interest in these monomers, particularly in l,3-dioxolan(I). l,3-dioxepan(II) has been studied to a much lesser extent [144, 145], while 1,3 dioxan(III) does not appear to polymerize, but merely forms crystalline dimer and trimer. 

The polymer has a relatively low ceiling temperature (119 °C), but this is the highest of all the formaldehyde polymers. The lack of success at polymerizing other monomers was due to the low ceiling temperatures (e.g. —39 C for acetaldehyde) (Odian, 1991). Aldol condensation can be a side reaction competing with polymerization for these monomers. There are other routes, such as cationic ring-opening polymerization of cyclic acetals, to achieve the same polymer (Penczek and Kubisa, 1989). 

In these two systems eventually macromolecules are formed by one kind of active species winning early enough in competition with the other species. However, it has been observed in this laboratory, particularly in the cationic polymerization, especially in the polymerization of cyclic acetals (J[6) and orthoesters (J 7), that two or more chemically different kinds of active species may coexist throughout the whole polymerization process, their proportions may depend (cyclic acetals) on the monomer conversion. Thus, in the polymerization of cyclic acetals the carbenium-oxonium equilibria have to be taken into account (H) ... 

Researches previously attempted to treat the mechanism of the polymerization of cyclic acetals as being similar to the mechanism of tetrahydrofuran (THF) polymerization. The above data show an essential difference between the cationic polymerization mechanisms of cyclic ethers and cyclic acetals. 

In papers55,56 the authors studied the effect of water on the polymerization of cyclic acetals. It was concluded that the inhibiting action of water during the induction period is caused by the direct chemical interaction of water with the active centers. 

Recent review articles on the following topics were published the controversy concerning the cationic ring-opening polymerization of cyclic acetals (213), photoinitiators for cationic polymerization (21A), living polymerization and selective dimerization (215). raacroraonomers (216), and functional polymers and sequential copolymers by carbocationic polymerization (217). 

The polymerization of cyclic acetals initiated with protonic acids can also be described in terms of the end-to-end cyclization scheme 7,8,18,19). 

The general thermodynamics of polymerization of cyclic acetals and the influence of substitution are discussed in Chapt. 2 of this volume (Thermodynamics). It may suffice to state here, that the monomers used to date for polymer synthesis are mostly derivatives of 1,2-glycols or 1,4-glycols and formaldehyde (i.e. 5- and 7-membered formals). 6-membered formals (1,3-dioxane and its derivatives) are nonpolymeri-zable due to the thermodynamic restrictions. 

The structure of active species in the homogenous polymerization of cyclic acetals (e.g., DXL) was discussed in detail in Adv. Pol. Sci. 37, Sect. 4.1.4. In short, the problem may be stated as follows. The polymerization of cyclic acetals involves an equilibrium between oxonium and carbenium (carboxonium) ions ... 

Such a process is thus equivalent to kinetic termination. In the homogenous cationic polymerization of cyclic acetals (e.g. DXL), chain transfer to polymer proceeds efficiently, because polymer is more basic than monomer. In this case, however, the polymeric oxonium ions remain active and may further propagate ... 

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