Radical polymerization of N-vinylpyrrolidone in the presence of syndiotactic poly(methacrylic acid) templates

Radical polymerization of N-vinylpyrrolidone along poly(methacrylic acid) templates of high syndiotatic content was followed dilatometrically in dimethylformamide, which was used as solvent. The effects of template concentration, template molar mass, and temperature on polymerization rate and average molar mass of the formed polyvinylpyrrolidone (PVP) were examined. Template concentrations were varied around the critical concentration for homogeneous segmental distribution, C^*. Below this concentration, template coils can act as separate microreactors wherein growing PVP radicals exhibit maximum rate enhancement, i. e., relative rate ν_R = ν_R,max. In the free solution, blank polymerization occurs, i. e., ν_R = 1. Consequently, ν_R can be approximated by the equation ν_R = ?ν_R,max + (1 ? ?), where ? represents the volume fraction occupied by template coils. The slight increase in UR and PVP molar mass with the template chain length is supposed to be caused by the influence of translational diffusion on the termination step. Over the investigated temperature range of 50–70°C, the activation energy and entropy were almost identical for blank and template polymerization. An expected decrease of ΔE^≠ and ΔS^≠ in template systems is supposed to be compensated by the effects of desolvation of the template macromolecules during the propagation step.     

Radical polymerization of methyl methacrylate in the presence of stereoregular poly(methyl methacrylate). III. Influence of temperature

Methyl methacrylate (MMA) was polymerized by radical initiation at 0, 25, 50, 75 and 100°C in DMF in the presence of preformed isotactic PMMA (iMA) or preformed syndiotactic PMMA (sMA) with different M?_v and also without preformed PMMA (“blank” polymerizations). From the tacticities of the formed polymers it is concluded that blank polymerization does not conform to simple Bernoulli statistics, but follows at least first-order Markov statistics. The formation of long syndiotactic sequences in the presence of iMA and long isotactic sequences in the presence of sMA denotes still higher-order Markov statistics. The stereoregulating action is improved by higher M?_v of the preformed polymer (matrix) and lower reaction temperature. These influences can be explained by assuming an equilibrium between polymer growth on the matrix and in the “free” solution. For polymerizations in the presence of iMA or sMA below 300°K, the differences in activation enthalpies (ΔH_s/i? – ΔH_i/s?) are practically equal to that for the blank polymerization, ca. 900 cal/mole, whereas the differences in activation entropies (ΔS_s/i? – ΔS_i/s?) differ considerably. (ΔS_s/i? – ΔS_i/s?) values are highly negative in the presence of iMA and highly positive in the presence of sMA. From these results it is concluded that the isotactic and syndiotactic polymer matrices exert a steric influence on the monomer addition process, thus promoting so-called stereospecific replica polymerization.     

Stereospecific bulk polymerization of methyl methacrylate initiated by its tactic polymers

Methyl methacrylate (MMA) was polymerized by radical initiation at 90°C by its own preformed tactic polymers, i.e., conventional (c-PMMA), isotactic (i-PMMA), and syndiotactic (s-PMMA) PMMA, and also by a preformed 1:1 stereocomplex of i-PMMA and s-PMMA. The collected polymers were separated into two fractions by extraction with boiling acetone and characterized by 60 MHz NMR spectra and viscometry. Higher polymerization rates were obtained in the presence of stereo-regular PMMA than in the presence of c-PMMA. Moreover, it appeared that i-PMMA promoted the formation of s-PMMA, and conversely s-PMMA the formation of i-PMMA, especially in the initial stages of the reaction. A higher M?_v of the preformed polymer yielded a higher rate and a higher stereospecificity of the polymerization. No polymerization took place in the absence of performed PMMA. The results support a replica mechanism proposed by Szwarc, in which polymerization is preceded by a specific arrangement of monomeric units along the polymeric chain into a distinctive pattern. Such arrangement coupled with a strong tendency of the isotactic and syndiotactic species to associate may lead to the present stereospecific replica polymerization. This association is demonstrated by rapid gelation during polymerization and by lowering of reduced viscosity in very dilute mixtures of i-PMMA and s-PMMA in MMA.     

Macromolecular metal complexes as catalysts with improved stability

Polymer-bound tertiary amine–copper complexes and polymer-bound phosphite–rhodium complexes were studied as catalysts for oxidative coupling of phenols and hydroformylation of alkenes, respectively. The activity and stability of these catalysts could be tuned or optimized by adapting the structure of the ligands and by changing the distance between adjacent ligands along the polymer chains. The latter effect has been described in terms of strain in the intermediate chain segments in the copper complexes or enhancement of the effective local ligand concentration around the rhodium complexes. So-called immobilized homogeneous catalysts were obtained by end-grafting of both types of macromolecular catalysts on to inert and insoluble silica particles. These immobilized polymeric catalysts could easily be separated and recovered. Under proper conditions a grafted polymerbound imidazole–copper complex and a new type of polymer-bound triphenyl–phosphite–rhodium complex showed excellent stability in continuous processes.     

Effects of poly(2-vinylpyridine) as a template for the radical polymerization of methacrylic acid—III. Influence of monomer concentration

The template polymerization of methacrylic acid along atactic poly (2-vinylpyridine) was studied calorimetrically at various monomer concentrations from 0.1 to 9.0 M in DMF; the concentration of the P2VP template was fixed at 0.4 base M. Azobisisobutyronitrile and bis(p-tert-butylcyclohexyl)peroxydicarbonate were used as initiators at concentrations of 0.1 and 0.01 M, respectively. The temperature was 30°. The results could be well described by a generalized kinetic model in which the effect of preferential adsorption of monomer by the template, this adsorption being assumed to be of the Langmuir type, has been taken into account, i.e. the model incorporates propagation along the template not only by picking-up monomer from the surrounding solution (mechanism II) but also by zipping-up pre-adsorbed monomer (mechanism I).     

Temperature dependence of the conformation of isotactic polystyrene in toluene

In order to obtain information about a possible helix–coil transition of isotactic polystyrene (i-PS) at 80°C in toluene, as has been reported in other solvents, solution properties were examined at temperatures between 10 and 110°C. Use was made of viscometry, high-resolution nuclear magnetic resonance, infrared spectroscopy, calorimetry, and light scattering. No distinct transition was found at 80°C but rather a second-order transition between 62 and 65°C. A similar transition occurred in toluene solutions of atactic polystyrene. The transition may be attributed to a sudden change in the mobility of the phenyl side-group of the polymer. From this study it is concluded that i-PS has a helical conformation in toluene, the mean helix length decreasing smoothly with increasing temperature.     

Die oxydative Kopplung von 2,6-Xylenol durch polymere Cu-Komplexe. 1. Mitt. Kinetische Untersuchung der Katalyse de Cu(II)-Komplexes von partiell aminiertem Polystyrol

Ohne Zusammenfassung     

Kinetik der oxidativen Dimerisation von Äthynylbenzol mit den Cu-Komplexen von 2-Phenylpyridin und von einem polymeren Pyridin-Derivat als Katalysatoren

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Crystallization of isotactic polystyrene from dilute solutions

The overall rate of crystallization of isotactic polystyrene from dilute solutions, 1% by weight, in trans-decalin and benzyl alcohol was studied as a function of temperature using dilatometry. These solvents were chosen because the dissolution temperatures of crystalline isotactic polystyrene are practically the same in both solvents. The overall rate of crystallization as a function of crystallization temperature showed a maximum in both solvents at about 50°C. At lower crystallization temperatures the rate of crystallization is much lower. The overall rate of crystallization of isotactic polystyrene in benzyl alcohol is far larger than in trans-decalin at the same undercooling throughout the temperature range, which is in apparent contradiction to present crystallization theories. At very large undercooling (T_c lower than about 0°C) the solutions of isotactic polystyrene in both solvents quickly become “rigid” gels. Surface replicas of freeze-etched gels indicate that a fringed micelle type of crystallization takes place at these low temperatures. The transition from folded chain crystallization to fringed micelle crystallization may be due to a stiffening of the polymer chain below about 50°C, with a reduced rotational mobility of the phenyl groups on the chain. If very dilute solutions, below 0.5% by weight, are crystallized at these low temperatures no gels were formed but fibrous crystals are produced which could be observed under the polarizing microscope.     

Die oxidative Kopplung von 2,6-Xylenol mit polymeren Komplexen von Cu. 2. Mitt. Physikochemische Untersuchung an den Cu(II)-Komplexen mit partiell dimethylaminomethyliertem Polystyrol

Ohne Zusammenfassung