Monomers heterocyclic

Heterocyclic Monomers.—Reviews of the polymerization of tetrahydrofuran (THF) were published. Rate constants of propagation of THF on macroesters and macroions were measured. In the polar solvent nitromethane, where macroesters are not important, it was shown that k and k t are identical within experimental error, and are not influenced by the nature of the counterion. It was postulated that the active centres are so highly solvated by monomer that free ions and ion-pairs are indistinguishable in terms of reactivity. 

Kubisa and Penczek measured the ratio of secondary and tertiary oxonium ions in the polymerization of dioxolane by CF3SO3H and found that it varied with both the concentration of reagents and with conversion. The use of the kinetic isotope effect to determine the structure of active centres in the polymerization of heterocycles was described. Bucquoye and Goethals discuss the mechan- D. J. Sikkema and H. AngadOaur, Makmmol. Chern-, 1980,181,22S9. 

Publications dealing with oxygen heterocycles containing 3 or 4 oxygen atoms include the kinetics and mechanism of acid-initiated trioxane polymerization and its behaviour towards impurities, and the copolymerization of 1,3,6,9-tetraoxacycloundecane with styrene.  

Several studies deal with the cationic polymerization and oligomerization of epoxides. Particularly interesting is the oligomerization of styrene oxide, which is characterized by hydride shift and back-biting reactions to form cyclic oligomers. 

The stability of ions and ion-pairs in the polymerizations of e-caprolactone was studied by varying the solvent polarity, while Yamashita describes the CFjSOjH-initiated polymerization as a living system, which degrades by back-biting reactions. 

We shall discuss separately the polymerization of heterocyclic monomers and vinylic monomers since the mechanism of the cationic polymerization is quite different for these two types of monomers. In contrast to some heterocyclic monomers olefinic and vinylic monomers do not produce living cationic species. Therefore, the methods for synthesizing macromonomers differ greatly. 

To initiate efficiently, rapidly and quantitatively the polymerization of these heterocycles it was searched for cationic species which add to the monomer and are associated to stable counterions. Suitable initiators are oxocarbenium salts (I), stabilized car-benium salts (II) and trifluorosulfonic derivatives (III) (triflic esters or anhydrides)42 44  

Since efficient initiators are available and the ring-opening polymerization of oxolane yields living species, the basic principles of macromonomer synthesis that have been developed above should also apply to the following case  

This conformation is mainly caused by non-bonding interactions between hydrogen atoms in the eclipsed conformation. 

a nmlecule of a heterocycle assumes a conformation, in which the sum of all possible interactions, which are a source of strain, is minimized. 

In Table 1 the ring strains of some heterocyd arrf their cycloalkane analogs are listed. The values of the rir strain were calculated as the difference between the calculated and measured enthalpies of formation of the correspondir cyclic compounds. 

The monomers most comprehensively studied in cationic polymerization are the non-planar tetrahydrofuran and 1,3-dioxolane. HowevCT, 5-membered rings do not assume such favorable conformations like the chair conformation of 6-membered rings. Consequently, the energetic barrier (pseudorotation barrier) between the con- 

The deviation from planarity is characterized by torsional angles. This angle for tetrahydrofuran is 21° whaeas for tetrahydrothiophene a value of 42° is reported For the 5-membered cyclic acetal, 2,2-dimethyl-l,3-dioxolane, the value of the torsional angle is 20—25°, sinular to that in tetrahydrofuran .  

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Polyheterocycles. Heterocychc monomers such as pyrrole and thiophene form hiUy conjugated polymers (4) with the potential for doped conductivity when polymerization occurs in the 2, 5 positions as shown in equation 6. The heterocycle monomers can be polymerized by an oxidative coupling mechanism, which can be initiated by either chemical or electrochemical means. Similar methods have been used to synthesize poly(p-phenylenes). 

The use of other heterocyclic rings in displacement polymerization has been recently reported. Table 3 shows the new dihalo heterocyclic monomers used for synthesis of poly(aryl ethers). 

Pencek, S., Kubisa, P. and Matyjaszewski, K. Cationic Ring-Opening Polymerization of Heterocyclic Monomers. Vol. 37, pp. 1 —149. 

Dreyfuss, P., Adaway, T., and Kennedy, J. P., Polymerization and grafting of heterocyclic monomers from reactive mono-and macrohaUdes, Appl. Polym. Symp.. 30, 183-192, 1977. 

As far as the polymerisation of heterocyclic monomers is concerned, the situation is qualitatively similar, but quantitatively different. As a model for the active species in oxonium polymerisations, Jones and Plesch [10] took Et30+PF6 and found its K in methylene dichloride at 0 °C to be 8.3 x 10"6 M however, in the presence of an excess of diethyl ether it was approximately doubled, to about 1.7 x 10 5 M. This effect was shown to be due to solvation of the cation by the ether. Therefore, in a polymerising solution of a cyclic ether or formal in methylene dichloride or similar solvents, in which the oxonium ion is solvated by monomer, the ion-pair dissociation equilibrium takes the form... 

Thus the reaction mechanisms for olefins and for the heterocyclic monomers are essentially different in all respects, and hence there is really no rational basis for comparing their kp values. 

Neither for olefins nor for heterocyclic monomers do we yet have a sufficiently extensive body of activation energies of the kp-s to make a detailed discussion profitable. It is worth noting, however, that for the cationic (as opposed to the pseudo-cationic) polymerisation of olefins in solvents of DC greater than about 10, it is likely that a reduction of the temperature does not affect the rate except through its effect on k p, since these reactions are mainly carried by free ions only. 

For cationic polymerisation of olefins in solvents of DC appreciably less than ca. 10 and for those of heterocyclic monomers in all solvents of DC up to perhaps 15-20, this is not so. For such systems the polymerisations are probably at least dieidic (free ions and ion-pairs) and a lowering of the temperature will increase the DC of the ion pairs. Thus in such systems the change of temperature affects not only k p and k"p, but also the relative abundance of the different types of chain-carriers therefore the proper interpretation of the apparent activation energies is difficult and by no means obvious. 

An example of the first type of study is the cationic pol erization of alkenes and heterocyclic monomers in the presence of 2-alWlfurans. As discussed above, electrophilic substitution at C5 is quite facile with these compounds and one can therefore prepare monofunctional oligomers bearing a furanic end-group. By a judicious choice of experimental conditions this transfer reaction will predominate over all other chain-breaking events and virtually all the chains will have the same terminal structure, i.e. a 5-oligomer-2-al lfuran. Structure 32 illustrates this principle with isobutyl vinyl ether oligomers capped by 2-methylfuran ... 

A change of architecture is another route that enables diversification of the properties of aliphatic polyesters. This review will focus on star-shaped, graft, macrocyclic, and crosslinked aliphatic polyesters. It must be noted that the ROP of lactones has been combined with several other polymerization mechanisms such as ROP of other heterocyclic monomers, ionic polymerization, ROMP, and radical polymerization. Nevertheless, this review will not cover these examples and will focus on polymers exclusively made up of poly(lactone)s. 

Penczek S, Cypryk M, Duda A, Kubisa P, Slomkowski S (2007) Living ring-opening polymerizations of heterocyclic monomers. Prog Polym Sci 32 247-282... 

Living Ring-Opening Polymerization of Heterocyclic Monomers with Aluminum Porphyrin-Organoaluminum Compound Systems... 

These and related heterocyclic monomers are usually highly polar and strongly nucleophilic conpounds. During polymerization chains containing heteroatoms are formed and they can, as well as monomers themselves, interact with components of ionic growing species. The interaction of the macroion-pairs with the elements of the chains has well been documented for the polymerization of ethylene oxide [Z]. 

In conclusion, it has been shown that use of cryptates for the anionic polymerization of heterocyclic monomers leatis to a tremendous increase of the rates of polymerization. There are two main causes to the higher reaction rates observed with cryptates. The first one is a suppression of the association between ion pairs in the non polar media, and the second one is the possibility of ion pairs dissociation into free ions in ethereal solvents like THP or THF. By this way, it has been possible to make detailed studies of the propagation reaction for propylene sulfide, ethylene oxide, and cycloslloxanes. 

The preparation of polymers from heterocyclic monomers that contain polymerizable functional groups undoubtedly constitutes the most common method of incorporating heterocycles into polymeric materials. Polymer-forming reactions are of two possible types addition reactions and condensation reactions. Addition monomers in general contain a site of unsaturation, i.e. a double or triple bond, through which polymerization occurs by successive single bond formation from one monomer to the next. With condensation monomers a bond is formed between two monomers with concomitant elimination of a... 

Several polymerizable oxazole- and isoxazole-functional monomers have been reported in the literature (70MI11100). The polymerizabilities of several alkenyl heterocyclic monomers, including oxazoles (83) and 3-isopropenyl-5-methylisoxazole (84) have been thoroughly studied (75MI11102). These heterocyclic monomers were found to be more... 

Unsaturated tetrahydropyran derivatives have received only cursory attention in the literature as heterocyclic monomers. 2,3-Dihydropyran and several of its substituted derivatives apparently undergo cationic polymerization in a manner typical of vinyl ethers (72MI11103), while tetrahydropyranyl esters of methacrylic acid (123) are fairly typical free radically polymerizable monomers (Scheme 35) (74MI11105). The THP group was used in this study as a protecting group for the acid functionality, and it was found that deprotection of polymers (124) could be accomplished under extremely mild conditions. 

Simpler alkenylmorpholines, such as 4-vinylmorpholine (148), would seem to be interesting heterocyclic monomers. While monomer (148) is unstable, substituted enamines such as 1-morpholino-l-butene (149) have been incorporated into copolymers with acrylonitrile. Copolymers containing up to 30% of units derived from monomer (148), however, may be prepared according to Scheme 45 (70MI11102). 

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