The Role of Structural Phenomena in Polymerization - Advances in

30 The Role of Structural Phenomena in Polymerization Downloaded by UNIV OF TEXAS AT AUSTIN on August 24, 2017 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch030

V. A. KARGIN Scientific Committee on High Molecular Compounds, Vavilova St. 32, Moscow B-312, U.S.S.R. Polymerization is influenced by the physical structure and phase of the monomer and polymer. It proceeds in the monomer, and the chemical configuration of the macromolecules formed depends on whether the monomer is a liquid, vapor, or solid at the moment of polymerization. The influence of structural phenomena is evident in the polymerization of acrylic monomer either as liquids or liquid crystals. Supermolecular structures are formed in solid- and liquid-state reactions during and simultaneously with polymerization. Structural effects can be studied by investigating the nucleation effect of the solid phase of the newly formed polymer as a nucleation reaction by itself and as nuclei for a specific supermolecular structure of a polymer. Structural effects are demonstrated also using macromolecular initiators which influence the polymerization kinetics and mechanism.

are accustomed to the notion that during polymerization of monomer molecules are combined into separate macromolecules and that this approximation is almost always sufficient. Actually it is sufficient in most cases in dilute solutions, but if polymerization is accompanied by any phase or structural transitions, including the formation of the polymer as a new phase, one must consider the possibility of structural phenomena affecting the polymerization. The first, and perhaps most vivid, example is the effect of the struc ture of a monomer crystal on polymerization in the solid state. The fact that the configuration of the emerging polymer depends directly on the structure of the solid monomer is no longer doubted, having been proved for many cases. Thus, depending on its phase, acetaldehyde polymerizes

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465 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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to poly (vinyl alcohol) or polyacetal (18); diketene polymerizes to a polyester or a polydiketone (22). A number of cases are known where the crystalline or ordered state of the monomer results in the formation of polymers of regular structure. Long ago it was discovered that the explosive polymerization of solid monomers proceeds often at phase transition points (9, 10, 16). These results have been described (11). The fact that the structure of a solid monomer influences its poly merization substantially now seems obvious. It is not as clear whether structural phenomena can effect polymerization if the monomer is a liquid. It has long been known that ordered regions or "clusters" exist in liquids, and several years ago it was assumed that in some cases these regions in liquid monomers can influence the polymerization. One of the most vivid examples—namely, polymerization in the liquid-crystalline state—was accomplished by Krentzel and co-workers (J, 2, 3). The object of their study was p-methacrylylhydroxybenzoic acid, which forms conventional crystals in the pure state and does not polymerize in the solid state. However, when mixed with alkoxybenzoic acid, it forms liquid crystals of both smectic and nematic forms. Polymerization of p-methacryllylhydroxybenzoic acid in various forms of liquid crystals was com pared with polymerization of the same substance dissolved in dioxane and dimethylformamide ( D M F ) . The rate of polymerization is nearly the same in both solvents and is proportional to the monomer concentration and to the square root of initiator concentration. The molecular weight of the resulting polymer is comparatively small (of the order of dozens of thousands), and at temperatures above 100°C. the monomer does not polymerize completely owing to the establishment of polymerization-depolymerization equilibrium. These features are typical of the radical polymerization of methacrylates. If p-methacrylylhydroxybenzoic acid is mixed with p-cetylhydroxybenzoic acid, a smectic form of liquid crystals results, but if it is mixed with p-nonylhydroxybenzoic acid, the resulting form is nematic above 104 °C. and is smectic below this. This makes it possible to compare the polymerization behavior of the same monomer in solution, where the mutual ordering of its molecules is minimal, in a liquid crystal state with only orientation order (nematic form), and in the liquid-crystal state involving both orientation and coordination order (smectic form). Polymerization in the liquid crystal state involves a major increase in the molecular weight of the polymer, reaching the hundreds of thousands. The thermodynamic equilibrium between the monomer in smectic liquid crystals and the polymer is shifted completely toward for mation of the polymer up to the melting point of the crystal (138°C. ). The polymerization rate does not change with the degree of conversion

Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


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up to 70-75% conversion and accordingly depends little on the initial monomer concentration. This supports the assumption that polymeriza tion proceeds in ordered regions whose lifetime is sufficiently long com pared with the time of chain propagation. When the temperature exceeds the melting point of the liquid crystals (138°C. for the smectic form), the molecular weight drops to 10,000 or 12,000 and the equilibrium monomer concentration increases up to 25%. Thus, a temperature change of a few degrees exerts a considerable effect if it is accompanied by a liquid crystal—liquid phase transformation. The polymerization rate also increases as the monomer molecules in the liquid become more ordered. Figure 1 shows the kinetic curves of polymerization in solution and in liquid crystals under comparable conditions.

10 20 30 40 50 60 Time,minutes Figure 1. Kinetic polymerization curves of n-MAOBA in DMF solution and in liquid crystals. Τ = 110°C.

[M] = I.25M

(1) Initiated polymerization in liquid crystal; [/J = 0.05M (2) Initiated polymerization in DMF so lution. Γ/ ] = 0.05M (3) Thermal polymerization in liquid crystal (4) Thermal polymerization in DMF so lution 0

The ordering may also occur in liquid states especially in those which are able to form liquid crystals. The lower the heat of the phase transition, the higher will be the probability of appearance of heterophasal fluctua tions, whose existence has been proved directly for the acids studied (23). The intermediate region between the liquid and the liquid crystals can be by-passed in ways other than changing the temperature. If an inert solvent is added to the system, the entire interval from a simple


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liquid to a liquid crystal can be covered at a constant temperature. Fig ures 2 and 3 give the dependence of the polymerization rate and intrinsic viscosity of an emerging polymer on the monomer content in a binary solvent [ D M F and p-nonylhydroxybenzoic acid (p-NHBA)] at 90°C. The transition from the liquid to the liquid-crystalline state occurs at a monomer content of 2.3M, but the changes in rate and molecular weight begin long before the phase transition point. Heterophasal fluctuations in the liquid may affect the polymerization quite tangibly.

Figure 2. Dependence of n-MAOBA polymerization rate in binary solvent (DMF + p-NHBA) on initial monomer concentration. Τ = 90°C. [7 ] = 0.0025M 0

In other words, if any structural formations appear in the liquid, they may become a new reaction medium in which the polymerization may change substantially. The part of such a medium may be played, as we have seen, by fluctuational formations of monomer molecules; but there is another possibility. The polymer molecules formed during poly merization are often more unevenly distributed in the monomer than in the solvent. They are much more likely to form structures than monomeric substances. The extreme case is the formation of a polymer which is insoluble in the monomer or in the solvent used for polymerization. In this case the polymer formed is evolved as a new phase, and if poly merization occurs inside or on the surface of the particles of this new phase, structural phenomena will naturally begin to play a major part. Perhaps the most vivid example of such a phenomenon is the bulk poly merization of vinyl chloride, whose structural features were studied recently (β, 7). The peculiarities of this process arise from the fact that



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1.6 2.4 3.2 [Mo],/lf

Figure 3. Dependence of [77] of poly(nMAOBA) on initial monomer concentration. Τ = 90°C. [7 ] = 0.0025M 0

Figure 4. Electron microscope photographs of PVC par ticles at various stages of conversion. Each photograph is 1.25μ wide

the polymer is almost insoluble in the monomer, and the monomer dis solves in the polymer to an extent of 10-15%. During radical polymeriza tion the polymer appears as globular formations 600-800 A. in size. The globular structure of the emerging poly (vinyl chloride) (PVC) is re tained throughout the polymerization until the solid block is formed.


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The number of globules does not change during this process, although each globule enlarges as polymerization proceeds. Figure 4 shows elec tron microscope photographs of P V C particles at various degree of conversion—i. e. less than 1% to several score percent of conversion. Neither new small globules nor very large ones appear during the polymerization. The size distribution curves of the emerging globules, shown in Figure 5 for various stages of conversion, remain unchanged during polymerization, shifting only along the ordinate axis. Figure 6 shows the relationship between the amount of polymer formed and the cube of its average particle diameter. The linear nature of this depend ence indicates that the increase in the amount of polymer is not related to any change in the number of particles but only to an increase in thensize. In other words, polymerization occurs practically entirely in the particles already formed. The entire picture comes close to that of emulsion polymerization except that the nuclei of the new phase of the emerging polymer are the medium in which polymerization proceeds.


y

0.05

0.10 0.15 0.20 0.25 0 3 0

Figure 5. Size distribution curves of emerging globules at various stages of conversion during polymerization The picture described is the simplest one and is observed only at high rates when PVC globules cannot aggregate. However, if the rate of temperature lowering or the initiator content are decreased, secondary processes occur, caused by aggregation of the PVC globule. On combin-



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Relationship between the amount of polymer formed and the cube of its average particle diameter

ing into an aggregate, individual particles do not merge like the drops of a liquid but remain separated with interfaces between them. Figure 7 shows electron microscopic photographs of such aggregates ( direct image and replica of a fracture). However, this aggregation does not alter the polymerization picture described above. With an increasing degree of conversion, the size of the primary particles ( and simultaneously the size of the aggregates) increases. However, their number does not change. Again polymerization proceeds inside the globular structures of PVC.

Figure 7. Electron microscope photographs of PVC globular aggregates. Each photograph is 1.25μ wide



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Polymerization in an emerging polymer may also occur when the polymer is soluble in the reaction medium and does not evolve as a separate phase. Sufficiently large structures may arise in solutions for polymerization processes to proceed within them—e.g., polymerization on polymeric initiators. Anionic processes of this type were studied in detail by Plate (15, 20). The initiator used was polystyrene containing lithium, with a molecular weight of 4000-6000; lithium was introduced by its reaction with a complex of butyllithium-tetramethyldiamine. The polymerization rate of styrene in Decalin was studied for such a polymeric catalyst and compared with the polymerization rate in the presence of an equivalent amount of pure butyllithium. In the presence of the polymeric initiator the rate in polar medium was higher (Figure 8), the activation energy decreased slightly (from 8.6 to 6.8 cal./mole), and the

Time, minutes Figure 8. Polymerization of styrene onto PLPS + BuLi mixture and onto pure BuLi at 25°C. in Decalin with tetramethyldiamine. [ M ] = 0.9M; [polystyrene'] — 3.9% of monomer. Catalyst concentration, corresponding to each pair of curves from left to right: (1) (2) (3) (4)

1.4 X 6.5 X 4.8 X 2.1 X

10~ M lO-'M 2

JO-'M

10-*M


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steric factor changed considerably (from 2 Χ 10 to 3 Χ 10 ). Evidently the polymeric chains growing on a polymeric catalyst form comparatively large coils in which further polymerization develops. Inside these coils the monomer concentration is higher than the average concentration in volume. By varying the solvent and polymeric catalyst the rate of poly merization can be either increased or decreased (as in the case of nonpolar medium), owing to the different distribution of the monomer between the solvent and the polymer coil. Downloaded by UNIV OF TEXAS AT AUSTIN on August 24, 2017 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch030

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In all cases cited, the formation of structures during polymerization resulted in the polymerization reaction's proceeding subsequently within these newly appearing structural formations. The influence of the struc tural processes was reduced to the appearance of a new polymerization medium, but the formation of polymeric structures during polymerization may result in other effects; the most essential is a change in the thermo dynamic conditions of polymerization. The change in free energy is taken into account during the conversion of monomer molecules into separate macromolecules. However, if poly merization results directly in polymeric structures rather than separate molecules, the total free energy change includes the change in free energy from structural formation as well. This value may be of essential impor tance, primarily during the formation of polymers with conjugated double bonds and in ring-opening polymerization as well as in the formation of crystalline polymers. We know of several cases involving the formation of large crystalline structures during polymerization. One example is the growth of crystal line nascent polyethylene on an aluminum wire with a slightly chlorinated surface during the polymerization of ethylene from a benzene solution in the presence of titanium trichloride (5). Figure 9 is a general view of the emerging polyethylene and electron microscope photographs of its individual fibrils. No matter how effective such photographs might be, unfortunately they cannot supply the answer to the question of whether these structures form during the polymerization process itself or are a result of the regrouping of individual chains formed during polymeriza tion. Meanwhile the answer to this question is decisive if we are inter ested in the influence of structure formation on polymerization. To be certain that no secondary regrouping of finished macromolecules occurs, the entire polymerization test must be carried out at temperatures below the glass transition temperature of the forming polymer—i.e., under con ditions where the polymeric molecules are immobile. So far only one set of data exists, obtained at the Moscow University, by polymerizing solid monomers directly in the electron microscope, where polymerization was initiated by the electron beam itself (4,8,19). Under these conditions it could be shown for a number of substituted methacrylates and for



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Figure 9.

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Emerging polyethylene and its individual fibrils

sodium acrylate that at temperatures below the melting point of the monomers and below the softening point of the forming polymers, ordered supermolecular polymeric strictures form directly during polymerization. It is curious that conditions favoring polymerization also result in the formation of more perfect structures. Thus, for phenylmethacrylamide, polymerization with perceptible yields of the polymer, occurs at tem peratures 5 ° - 1 0 ° below the melting point of the monomer crystals. Under the same conditions more perfect structures appear. A comparison of the formation of structures from solution (where the conditions would seem to be the most favorable ) and during polymerization revealed that the most ordered structures appear during the chemical reactions of formation of the polymeric molecules themselves. All the data suggest that simultaneous growth of clusters of chains is preferable to growth of isolated chains. If these assumptions are true, polymerization in all such cases must be connected closely with evolution of the polymer as a new phase. Hence, the effect of nucleus action should also be observed. This suggests the possibility of existence of polymerization nuclei, similar to the nuclei of crystallization. Such effects were first demonstrated a few years ago in pyridine polymerization. The last example of structural influence on polymerization is the action of already formed polymeric chains on the formation of new poly meric molecules. During recent years we have been investigating the matrix (templet) polymerization of 4-vinylpyridine on polymeric acids (12,13,14). Recently we discovered another peculiar case where the pre formed polymer and growing chains participate in the formation of the polymeric substance (17). Ryan and Fleischer, Jr. (21) found that if isotactic and syndiotactic poly (methyl methacrylate ) (PMMA) are mixed, they form a stereocomplex. It might have been supposed that the course of polymerization of methyl methacrylate in the presence of preformed stereoisomers of



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PMMA added previously would depend on their structure. Indeed, when polymerization was carried out in the presence of isotactic PMMA, it influenced the formation of new macromolecules, affecting the rate of polymerization, the molecular weights of the polymer formed, and the structure of its molecules. It is natural to assume that these effects are caused by the appearance of a stereocomplex during polymerization be tween the polymer added beforehand and the growing macroradical. It is also characteristic that the ratio of polymerization rates in the presence and in the absence of the polymer is independent of the concentrations of monomer and polymer added, depending only on their ratio,. Viscosity investigations revealed that these solutions are highly crosslinked. All these tests were run in D M F as solvent. In such a system the templet polymerization cannot be pictured as alignment of the monomer molecules to the polymer molecule serving as the matrix. The interaction between them is comparatively weak, and D M F is a better solvent with respect to monomer than dead polymer. Evidently, comparatively large growing radicals, enriched in the configuration which favors the forma tion of the stereocomplex, attach themselves to the matrix molecules. Further growth is caused by the matrix molecules. Here we are dealing with a new peculiar type of matrix process. Perhaps it will also furnish new possibilities for stereoregulation in radical polymerization. These examples of the phenomena which influence structural factors of the polymerization process should always be taken into account. Eventually, they may reveal new possibilities in polymerization phenomena. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)

Amerik, U . B., Krentsel, V. Α., J. Polymer Sci., Pt. C, 16, 1383 (1967). Amerik, U. B., Konstantinov, Ν . I., Krentsel, Β. Α., Vysokomol. Soyed. A9, 2236 (1967). Amerik, U . B., Konstantinov, I. I., Krentsel, Β. Α., J. Polymer Sci., Pt. C, 23, 231 (1968). Azori, M . , Plate, Ν . Α., Kargin, V. Α., Proc. Symp. Radiation Chem., 2nd Tihany, Budapest, 547 (1967). Bort, D. N., Minsker, K. S., Okladnov, Ν. Α., Starkman, B. P., Kargin, V. Α., Dokl. Akad. Nauk SSSR 145, 787 (1962). Bort, D. N., Rylov, Ε . E . , Okladnov, Ν. Α., Starkman, E . P., Kargin, V. Α., Vysokomol. Soyed. 7, 50 (1965). Bort, D. N., Rylov, Ε. E . , Kargin, V. Α., Vysokomol. Soyed. A9, 303 (1967). Kargin, V. Α., Azori, M . , Platé, Ν. Α., Bandurian, S., Dokl. Akad. Nauk SSSR 154, 1157 (1964). Kargin, V. Α., Kabanov, V. Α., Zubov, V. P., Vysokomol Soyed. 1, 265 (1959). Kargin, V. Α., Kabanov, V. Α., Papisov, I. M . , J. Polymer Sci., Pt. C, 4, 767 (1964).



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(11) Kargin, V . Α., Kabanov, V . Α., Zh. Vsesoyuzn. Khim. Obshch. Mendeleyeva 9 (6), 602 (1964). (12) Kargin, V . Α., Kabanov, V . Α., Kargina, Ο. V . , Dokl. Akad. Nauk. SSSR 161, 1131 (1965). (13) Kargina, Ο. V., Ulyanova, V . Α., Kabanov, V . Α., Patrikeyeva, T . , Kargin, V. Α., Vysokomol. Soyed. A9 (1967). (14) Kargina, Ο. V . , Kabanov, V . Α., Kargin, V . Α., Symp. Macromol. Chem. Brussels, 1967, Preprint 1-119. (15) Jampolskaya, M . , Platé, Ν. Α., Kargin, V. Α., Vysokomolek. Soyed. 102, 152 (1968). (16) Miayama, H . , Kamashi, M . , J. Polymer Sci., Pt. B, 9, 4651 (1963). (17) Orlova, Ο. V . , Kargin, V . Α., Krentsel, Β. Α., Amerik, U . B., Dokl. Akad. Nauk. SSSR. (18) Pauissov, I. M . , Pisarenko, Τ. Α., Panasenko, Α. Α., Kabanov, V . Α., Kargin, V. Α., Dokl. Akad. Nauk. SSSR 156, 669 (1964). (19) Plate, Ν. Α., Azori, M . , Kargin, V. Α., Vysokomolek. Soyed. 8, 764, 1562, 1966. (20) Plate, Ν. Α., Jampolskaya, Μ. Α., Davydova, S. L . , Kargin, V. Α., Symp. Macromol. Chem., Brussels, 1967, Preprint. (21) Ryan, C . F., Fleischer, P. C., Jr., J. Phys. Chem. 69, 3384 (1965). (22) Shreiner, E . S., Zubov, V . P., Kavanov, V . Α., Kargin, V. Α., Dokl. Akad. Nauk. SSSR 156, 396 (1964). (23) Tsvetkov, V . N . , Ryumtsev, Ε . M . , Dokl. Akad. Nauk. SSSR 176, 382 (1967). R E C E I V E D April 1, 1968.


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