Cyclic acetal polymerization Initiators

In contrast to the fact that cyclic acetals can be polymerized only by cationic initiators, lactones undergo polymerization both cationically and anionically, and therefore a wide variety of initiators including coordinated catalysts can be used. In this section, the polymerization of bicyclic lactones is described, although only a limited number of papers on this subject have been published. 

Cyclic ethers, cyclic acetals, and some vinyl compounds can be polymerized by cationic processes. Photoinitiation of these polymerizations by ultraviolet light are known (12,106). Some extension of direct photolytic sensitization of cationic processes to visible wavelengths is obtained by the use of colored diazonium salts as initiators. For example, Schlesinger (11a) used diazonium salts substituted in the para position with electron withdrawing groups, but sensitivity was limited to the blue to green regions of the spectrum. 

A few papers have examined the range of Lewis bases that are effective for isobutene controlled/living polymerization [91,149,156]. One proposal is that effective nucleophiles (electron donors) should have relatively high donor numbers (DN > 26) [149]. Another screening shows that triethylamine is exceedingly effective for isobutene [156]. Pratap et al., also reported recently the use of cyclic amides (lactams l-methyl-2-pyr-rolidone) [158] and cyclic amines [152] for the dicumyl acetate/BCL initiating system. 

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],... 

Esters of very strong protonic acids (trifluoromethanesulfonic, fluoro-sulfonic, perchloric), however, are sufficiently strong alkylating agents to initiate the polymerization of even weakly nucleophilic monomers (cyclic acetals, ethers) [2-6], Also their anhydrides (e.g., triflic anhydride) are efficient initiators. This last compound is especially interesting, because in the polymerization of cyclic ethers it leads to macromolecules with two identical growing chain ends (difunctional initiator) [30] ... 

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. 

Thus, in sequential polymerization of two different cyclic acetals, 1,3-dioxolane and 1,3-dioxepane, the sequential polymerization (i.e., polymerization of added second monomer initiated by active species of the first monomer polymerization) may be easily achieved as evidenced by increase of molecular weight [130]. The isolated polymer is not a block copolymer, however, having nearly statistical distribution of both types... 

The utility of onium ions depends not only on the value of K and related rate constants of the equilibration involved in initiation but also on the rate constant of the subsequent propagation. Thus, in spite of the much higher nucleophilicity of ethws, cyclic acetals can be polymerized by trialkyloxonium salts, because of the... 

Polymerization of Cyclic Acetals Initiated by Protonic Adds... 

U Polymerization cX Cyclic Acetals Initiated by Preformed or Ifidden Carbocations... 

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. 

Cyclic acetals are known to produce long lived species provided proper Initiators are used ( 7)- Nevertheless, their polymerization Is more complex than that of THF backbiting causes the formation of small cycles and transacetallzatlon Is Important ( ) their active sites are tertiary cyclic oxonlum groups like THF - but In addition are also In equilibrium with alkoxycarbenlum species. The enhanced reactivity of the latter could lead to Interesting synthetic applications. We have Intestlgated the polymerization of... 

Onium Ions. Trialkyloxonium ions (R O A ) became the conventional initiators for the cationic ring-opening polymerization of all classes of heterocycles (cyclic acetals, ethers, sulfides, lactones, phosphates, and amines). They are prepared by two methods developed by Meerwin (38) and Olah (39). Another more general and convenient synthesis method was recently developed by Penczek et aL (40) ... 

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). 

In the cyclic acetal-photosensitized polymerization of styrene and methyl methacrylate, the conversion was found to increase with an increase in the number of cyclic acetal groups in the initiating molecule. Asakura et have described in detail the homo- and co-polymerization of )S-allyloxypropionaldehyde by direct photoexcitation. Although no mechanism was postulated it was certainly free radical in nature. Triallylidene sorbital (7) has also been found to polymerize by direct photoexcitation by the radical mechanism shown in Scheme 2>P ... 

Trioxane is unique among the cyclic acetals because it is used commercially to form polyoxymethylene, a polymer that is very much like the one obtained by cationic polymerization of formaldehyde. Sc ne questions still exist about the exact mechanism of initiation in trioxane polymerizations. It is uncertain, for instance, whether a cocatalyst is required with strong Lewis acids like BF3 or TICU. 

Many heterocyclic compounds can be polymerized by ring opening under certain conditions with ionic initiators, to produce linear macromolecules. Amongst these are cyclic ethers, cyclic sulfides, cyclic acetals, cyclic esters (lactones), cyclic... 

Like THF, cyclic acetals (e.g., 1,3-dioxolane and 1,3,5-trioxane) are polymerizable only with cationic initiators. The ring-opening polymerization of 1,3,5-trioxane (cyclic trimer of formaldehyde) leads to polyoxymethylenes (see Example 3.24), which have the same chain structure as polyformaldehyde (see Example 3.22). They are thermally unstable unless the semiacetal hydroxy end groups have been protected in a suitable way (see Example 5.7). Like the cyclic ethers, the polymerization of 1,3,5-trioxane proceeds via the addition of an initiator cation to a ring oxygen atom, with the formation of an oxonium ion which is transformed to... 

Gancheva et al. employed LCCC to investigate the cationic polymerization of a cyclic acetal initiated by mono and bifunctional initiators [38, 39]. Termination with 9-anthryhnethyl improved the detection of the end functionality and the separation into functionality fractions. Mengerink et al. separated and identified the... 

The polymerization section consists of a series of continuously stirred tank reactors (24,25). The solvent (25) (usually cyclohexane) and monomers are fed into the reactor and an initiator as well as a randomizer [usually tetrahydrofuran (THF)] is added. The function of the randomizer is to make sure that no blocks are formed from a single monomer (Hall, Oxolanyl Cyclic Acetals as Anionic Polymerization Modifiers 68). This is because the reactivity ratios of the monomers are not ideal. Usually the reaction temperature is kept less than 110°C to prevent deactivation of the growing chain ends (25). Once polsrmerization is complete, alcohol is added to terminate the polymerization reaction and the polymer solution is transferred to holding tanks to be blended to increase imiformity. Subsequent steps of washing and filtering remove all the unreacted monomers, THF, and other chemicals. 

Cyclic Acetals. A number of cyclic acetals of varying ring size have been polymerized with cationic initiators (315) trioxane and dioxolane are the most important. Polymerization of trioxane in the presence of a transfer agent such as dimethyl formal or an anhydride yields polyoxymethylene with ether and ester end groups, respectively (316). 

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