Solid-state polymerization monomers

Figure 8. Solid-state polymerization of diacetylenes. A crystalline array of monomer units polymerizes through intermediate states to the final crystalline polymer chain.

The solid-state polymerization of diacetylenes is an example of a lattice-controlled solid-state reaction. Polydiacetylenes are synthesized via a 1,4-addition reaction of monomer crystals of the form R-C=C-CeC-R. The polymer backbone has a planar, fully conjugated structure. The electronic structure is essentially one dimensional with a lowest-energy optical transition of typically 16 000 cm-l. The polydiacetylenes are unique among organic polymers in that they may be obtained as large-dimension single crystals. 

Polydiacetylenes are obtained as single crystals by topochemical solid-state polymerization of the monomer single crystal. These compounds have received considerable attention because of their one-dimensionally -conjugated structure. Their unique --electron structures, and therefore superior third-order nonlinear optical properties, have been extensively investigated. 

A kinetic model for single-phase polymerizations— that is, reactions where because of the similarity of structure the polymer grows as a solid-state solution in the monomer crystal without phase separation—has been proposed by Baughman [294] to explain the experimental behavior observed in the temperature- or light-induced polymerization of substimted diacetylenes R—C=C—C=C—R. The basic feature of the model is that the rate constant for nucleation is assumed to depend on the fraction of converted monomer x(f) and is not constant like it is assumed in the Avrami model discussed above. The rate of the solid-state polymerization is given by... 

There are numerous examples of solid state polymerizations. Here we will briefly describe examples based on addition polymers. Generally, the crystalline monomer is irradiated with electrons or some form of high-energy radiation, such as gamma or x-rays. Since many monomers are solids only below room temperature, it is customary to begin irradiation at lower temperatures with the temperature raised only after initial polymerization occurs. (Some reactions are carried to completion at the lower temperature.) After polymerization, the monomer is removed. Table 6.10 contains a list of some of the common monomers that undergo solid-state irradiation polymerization. 

Later, Tieke reported the UV- and y-irradiation polymerization of butadiene derivatives crystallized in perovskite-type layer structures [21,22]. He reported the solid-state polymerization of butadienes containing aminomethyl groups as pendant substituents that form layered perovskite halide salts to yield erythro-diisotactic 1,4-trans polymers. Interestingly, Tieke and his coworker determined the crystal structure of the polymerized compounds of some derivatives by X-ray diffraction [23,24]. From comparative X-ray studies of monomeric and polymeric crystals, a contraction of the lattice constant parallel to the polymer chain direction by approximately 8% is evident. Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, resulting in diisotactic polymer chains. He also referred to the y-radiation polymerization of molecular crystals of the sorbic acid derivatives with a long alkyl chain as the N-substituent [25]. More recently, Schlitter and Beck reported the solid-state polymerization of lithium sorbate [26]. However, the details of topochemical polymerization of 1,3-diene monomers were not revealed until very recently. 

The polymerization proceeds under photo- [49,50],X-ray [51], and y-ray [52] irradiation in the dark in vacuo, in air, or even in water or organic solvent as the dispersant (nonsolvent) for the crystals, similar to the solid-state polymerization of diacetylene compounds [ 12]. The process of topochemical polymerization of 1,3-diene monomers is also independent of the environment surrounding the crystals. Recently, the thermally induced topochemical polymerization of several monomers with a high decomposition and melting point was confirmed [53]. The polymer yield increases as the reaction temperature increases during the thermal polymerization. IR and NMR spectroscopies certified that the polymers obtained from the thermally induced polymerization in the dark have a stereoregular repeating structure identical to those of the photopolymers produced by UV or y-ray irradiation. 

Steinbrunn and Wenz recently reported poly(amide-CD rotaxane)s 53 via a very imaginative approach [90,91]. First, the CD was threaded on to an a,co-amino acid in water to give a pseudorotaxane monomer. An NMR study showed that two CD molecules were threaded per linear molecule for 11-aminoundecanoic acid. The X-ray powder pattern indicated that these rotaxanes stacked like channels in the solid state this provided the basis for solid state polymerization at 200°C to afford polyamide 53 with m/n=2 ... 

Topochemical Polymerization The chiral crystalline environment of a monomer itself can be a source of asymmetric induction in solid-state polymerization [69-72], Prochiral monomers such as 37 give enantiomorphic crystals, one of which can be preferentially formed by recrystallization with a trace amount of optically active compounds. Photoir-... 

Simultaneous evaporation of metal with organic and inorganic substances followed by vapor deposition on a substrate allows the production of composite films containing M nanoparticles stabilized in various dielectric matrices [2, 28]. The use of monomer molecules in this process polymerizing during deposition or as a result of the subsequent reactions yields polymeric nanocomposite films with metal inclusions [2, 3, 28, 37]. The new low-temperature synthesis of polymeric nanocomposite films has been elaborated recently. This synthesis is based on the deposition of M/SC and monomers vapors at temperature 80 K followed by low-temperature solid-state polymerization of obtained films in conditions of frozen thermal movement of molecules (cryochemical synthesis) [2], This synthesis has important features, which will be considered further. 

As mentioned above, the new method of cryochemical synthesis of polymer nanocomposite films has been developed based on co-deposition of M/ SC and monomer vapors at temperature 80K and subsequent low-temperature solid-state polymerization of monomer matrix ([2] and works cited herein). It has been established that a number of monomers (acrylonitrile, formaldehyde, /i-xylylene and its derivatives) polymerize in solid state in absence of thermal movement of molecules owing to own specific supra-molecular structure. When reaction is initiated by y- or UV-radiation the formation of a polymer matrix occurs even at the temperatures close to temperature of liquid helium [66-69]. 

The solid-state polymerizations of several racemic optically-resolved amino acid NCAs were investigated by Kanazawa et al. [203,204], who demonstrated, by kinetic and crystallographic investigations, that the rate of polymerization of these systems depends upon the packing arrangement of the monomers. 

A low degree of tacticity is obtained because these monomer-orienting forces are quite weak. Thus, polymer stereoregularity is achieved only with certain suitable monomers and at low temperatures. Increased tacticity can be achieved in some cases by using monomer-orienting forces other than the catalyst or the polymer end groups, but these have rather limited utility (canal complexes, solid state polymerizations, etc.). Simple polymerization systems fall outside the scope of this review and are not discussed further. 

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

The solid state polymerization of hexamethylcyclotrisiloxane has been investigated over the temperature range —196° to 60°C. The rates of polymerization have been related to the presence of ion scavengers, H20, NH3, in the monomer and to the size of the crystals. Using large crystals dried over sodium, G values of polymerization of 11 X 103 were obtained at 50°C. This is five times larger than previously reported values. The reaction is concluded to be surface initiated and to be terminated at a crystal face or at a defect. 

The first reported solid-state polymerization of this monomer was that of Lawton, Grubb, and Balwit (6) in 1956. Subsequent studies involving irradiation initiation have been reported by Burlant and Taylor (I), and Trofimova et al. (15) recently Prut et al. (12) reported the results of a study of the solid polymerization in which they used SnCi as the initiator. 

Pofy-l-vinyluradl (poly-VUr, 10) was also obtained by a free-radical polymerization6). In this case, it should be noted that the formation of substituted dihydrouradl rings occurred via a cydopolymerization mechanism11). y-Ray induced solid-state polymerization of the monomer (9) in high concentration and at low temperature excluded cydopolymerization completely12). Poly-VUr was also prepared by a free-radical polymerization of 2-ethoxy-4-l-vinyl-pyrimidone (ii)8) or 4-ethoxy-l-vinyl-2-pyrimidone (13)iy> followed by acid hydrolysis of the resulting polymers (12) or (14) (Scheme 2). 

A great majority of polymerizations are simultaneously affected by many physical and chemical factors, and their course is the result of a superposition of these effects. Only in rare cases does one of these factors dominate and the polymerization is formally simplified. In topochemical polymerizations, the growth of macromolecules is governed by forces in the crystal lattice of the monomer. Solid-state polymerization of trioxane (trioxacyclohexane) is a typical example of topochemical polymerization. 

It is known that in a solid state polymerization occurs along the certain axis of the monomer crystal and extended chain crystals are formed [146]. In the polymerization from liquid phase, on the other hand, lamellar crystals are formed. This indicates that back-biting reaction proceeds not at random but in a specific manner, forced by the nature of crystals. The model of growth of lamellar crystals of polyoxymethylene is known [147]. According to this model, the subsequent layers are formed on the surface of the crystal by growth of folded chain as shown schematically below ... 

One approach to the determination of intrinsic properties, which has been utilized since the earliest interest in conjugated polymers, is to study the properties of related oligomers, as in the preceding paper ( ). It is, however, also possible to study model macromolecules, the polydiacetylene. The existence of solid state polymerization in diacetylene monomers has a long history (22, 23, 2 ), but it was not thoroughly studied until... 

Figure 1. Solid-state polymerization of diacetylenes shown schematically (left) an array of monomer molecules in the crystal lattice frightj the resulting polydiacetylene chain.

Fig. 6.8-19 shows the reaction scheme as well as the Raman spectra of the a-DSP monomer and its polymer at 100 K in the spectral region between 1100 and 1700 cm . The significant difference between the spectra leads to the conclusion that n-DSP has undergone a chemical transformation. Fig. 6.8-20 exhibits phonon spectra of a-DSP, recorded during polymerization. Clearly, the phonon spectrum of the partially polymerized material results from superposition of monomer and polymer bands. As the reaction proceeds, the intensity of the monomer peaks decreases while the intensity of the polymer bands increases. The authors concluded that the solid state polymerization of a-DSP proceeds by a heterogeneous mechanism throughout the entire conversion. 

Fig. 1. Schematical representation of the solid state polymerization reaction at room temperature. The partially polymerized monomer crystal contains long polymer filaments...

The theory was used to calculate kinetic curves for the polymerization of PTS deducing the ratio cJCp from the known conversion dependence of the lattice parameters. Time conversion curves normalized with respect to the time necessary to reach 50 percent conversion can be calculated for different values of the lattice mismatch using the crystal strain theory. For PTS a satisfactory fit of the experimental data of the thermal and y-ray polymerization can be obtained. However, further studies of the kinetics of the solid-state polymerization of PTS and other monomers provided results which cannot be explained by the theory. 

It appears that the reaction mechanism and the intermediates involved in the solid-state polymerization of diacetylenes are reasonably well understood. However, experimental results obtained with special monomers should not be generalized. It is not possible to design a monomer with desired properties. Inspection of Table 1 shows that on the basis of the crystallographic data and the monomer packing the absolute reactivity and the polymerization kinetics caimot be quantitatively predicted, e.g. it is not possible, to date, to explain why certain diacetylenes can be polymerized thermally whereas others with equal packing are thermally inactive. A more realistic kinetic model should include the various energy transport processes and the complex side group motions which are connected to the reaction. 

A somewhat unique situation has been studied in the free radical polymerization in urea 70) and thiourea (77) clathrates. Several monomers have been found for which all conditions for solid state polymerization to a stereoregular equilibrium crystal outlined above are fulfilled. The radicals are protected from termination by neighboring radicals by the urea or thiourea walls of the canals. The proper approach of the monomer molecules is achieved in thiourea by stacking the monomer... 

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