Mechanismus der photopolymerisation des styrols, 2. Zur photochemie und photokinetik der intermediären photo-(4π + 2π)-addukte des styrols

The unsensitized photoinitiation of styrene polymerization is accompanied by the formation of some typical oligomeric key substances, which are indicative of two 4π + 2π-photo-intermediates (I), viz. 1-phenyl-1,2,3,8a-tetrahydronaphthalene ( 1 ) and 2-phenyl-1,2,3,8a-tetrahydronaphthalene ( 1 ′). Owing to the very different absorption coefficients of monomer and I, formation and consumption and, therefore, the photo stationary concentration of I exhibits a pronounced dependence on the wavelength λ_e of the light used for irradiation. Under conditions, where the exciting light is not homogeneously but totally absorbed in the system (λ_e = 291 nm, corresponding to the 0–0 band of the lowest ¹L_b←¹A transition in styrene), the intermediates I can be enriched to such an extent, that they are detectable not only by UV-absorption (310?/nmλ/nm ? 360) but also—with the aid of a special high vacuum technique—by ¹H NMR spectroscopy, which gives unambiguous proof for the molecular structure suggested for I. Moreover, the photochemistry, the photokinetics and the photophysical behaviour of I were investigated in detail. In the absence of any fluorescence (φ < 10^?4) of the intermediate the overall quantum yield for consumption was found to be φ_–1 = 1. It is shown, that the photo-retro-Diels-Alder process is the dominating reaction (φ_2M = 0,94) and the 1,3-H-sigmatropic rearrangement does not contribute more than φ_PhT?0,06. Together with the mean natural molar absorption coefficients φ _I = 18400 dm³·mol^?1·cm^?1 for λ = 313 nm and φ _I = 7300 dm³·mol^?1·cm^?1 for λ = 335 nm the photostationary concentration [I]_∞ is calculated to range from [I]_∞ = 8,7.10^?6 mol. dm^?3 at λ_e = 313 nm to [I]_∞ = 1,2.10^?6 mol.dm^?3 at λ_e = 335 nm, which proved to be independent of the intensity of the incident light J₀.     

Polymerization of methyl acrylate photoinitiated by 4-dimethylamino-4′-isopropylbenzophenone

The initial rate of the polymerization of methyl acrylate photoinitiated by 4-dimethylamino-4′-isopropylbenzophenone ( 2 ) varies linearly with monomer concentration and the square-root of light intensity. For small absorbances, the initial polymerization rate, R_p, varies linearly with the square-root of the photoinitiator concentration. For high absorbance values, R_p is independent of the photoinitiator concentration in a well-stirred system and decreases as the photoinitiator concentration increases in an unstirred system. This marked differences between the polymerization kinetics in well-stirred and unstirred systems is discussed. The ratio of the propagation rate constant and the square-root of the termination rate constant, k_p/k, and the quantum yield of photoinitiation were determined and related to the results previously obtained in a study on the photoreactivity of 2 .     

Influence des traitements thermiques à l'état fondu sur les cinétiques de cristallisation du poly(oxyméthylène)

The influence of the temperature of the melt T₁ on the kinetics and the morphology of a semicrystalline polymer (poly(oxymethylene)) was investigated using thermal analysis and optical microscopy. The thermodynamic melting point T and the enthalpy of melting at thermodynamical equilibrium ΔH were determined by extrapolation of the graphs T_f = f(T_c) and ΔH_f = f(T_c); (T = 198°C, ΔH = 251 J/g). For different temperatures of the melt (T₁ = 185°C, 195°C, 205°C), isothermal and non-isothermal crystallizations were analysed using the Avrami and Ozawa equations. Nucleation and spherulitic growth in this polymer were studied by using optical microscopy at elevated temperatures. Using different analyses, we observed initial nucleation followed by spherulite growth with the following influence of the temperature of the melt on the distribution and the number of spherulites: T₁ < T produces many small spherulites; T₁ > T gives rise to few large spherulites.     

Luminescent characteristics of tris(2,2′-bipyridine)-ruthenium(II) adsorbed in a silk fibroin membrane

The emission characteristics of the excited state of tris(2,2′-bipyridine)ruthenium(II), Ru(bpy), adsorbed on a silk fibroin membrane is studied. The life time of the excited state of the adsorbed Ru(bpy) is unusually longer (1 000 ns) than that of Ru(bpy) in aqueous solution (598 ns). The excited state of the adsorbed Ru(bpy)₃²⁺ is quenched by oxygen when dipped in methanol, but not in water. The quenching by oxygen in methanol follows the Stern-Volmer equation. The luminescent characteristics of the adsorbed Ru complex is discussed in terms of binding in hydrophobic domains composed of tyrosine residues of the protein.     

Instantaneous shape and relative orientation of model chains as a function of their intermolecular separation

Ensembles of self avoiding five-way cubic lattice chains consisting of 50 segments were generated for athermal and θ-conditions, and the instantaneous shape and the orientation of each chain were characterized by evaluation of the squared three axes L, L, and L of its equivalent ellipsoid and their orientation in space; the variation of these quantities within allowed pair configurations (relative to their values in isolated chains) was studied as a function of the separation R between the two centers of gravity of the two members of all isolated chain pairs, which could be formed from the ensemble. For small R (when R is smaller than the average radius of gyration of isolated chains 〈s²〉^1/2₀) L, the squared longest axis of the equivalent ellipsoid, is strongly expanded for athermal as well as for θ-conditions, while the squared two shortes axes, L and L, are much less enlarged or even decreased in this range, thus emphasizing the prolate character of the ellipsoids on approaching R = 0. Furthermore the angle between the two L₃-axes of the two chains forming a compatible pair is remarkably increased at vanishing separation (by ca. 10° in athermal systems and by ca. 7° in θ-systems). Thus, chains forming compatible pairs at R = 0 not only have to increase their dimensions (as already described earlier) but also have to change their instantaneous shape (towards a still higher anisotropy of the equivalent ellipsoids) and their relative orientation (the longest axes L₃ tend towards orthogonal orientation) in order to relieve the thermodynamic stress which they impose on each other. By integration of the effects over all separations the concentration dependence of all quantities was determined in the limit of zero concentration. Most quantities are subject to an appreciable, non-zero concentration dependence. Contrary to most theoretical predictions this is also true for θ-conditions.     

Stereokontrolle. IV. Konstitutionseinflüsse bei radikalischen Polymerisationen

A compensation effect exists between the quantities (ΔH ? ΔH) and (ΔS ? ΔS) in the free radical polymerization of a monomer in different solvents ΔH, ΔH, ΔS, and ΔS are the activation enthalpies and entropies, resp. for the formation of isotactic and syndiotactic dyads. The quantities ΔΔH and T₀ are by definition independent of the temperature of polymerization and other polymerization conditions and thus a pair of constants characteristic for each monomer. A linear relationship between ΔΔH and T₀ has been found for acrylic and vinyl monomers each. Both true activation and conformational effects seem to be responsible for the stereocontrol in free radical polymerizations.     

α,ω-dichloropoly(2-methylpropene) as an inifer: A modification to the Kennedy-Smith mechanism

Experimental details are given of attempts to enumerate the binary ionogenic equilibria (B.I.E.) of 1-chloro-1-methylethylbenzene ( 1 )/BCl₃, 1,4-bis(1-chloro-1-methylethyl)benzene ( 2 )/BCl₃ and 1,3,5-tris(1-chloro-1-methylethyl)benzene ( 3 )/BCl₃ in CH₂Cl₂. Due to chemical reaction (dimerisation or polymerisation) no experimental values for the B.I.E. constants could be obtained. A Born-Haber cycle is constructed to estimate the relative sequence of the overall B.I.E. constants. A similar treatment for 2-chloro-2methylpropane as a thermodynamic model for α,ω-dichloropoly(2-methylpropene) ( 4 ) suggests that the overall B.I.E. constant for these polymers is somewhat smaller than those for 1 and 2 but greater than that for 3 . Using 2 /BCl₃ as initiator for the polymerisation of 2-methylpropene (IB) it is shown, that the degree of polymerisation of 4 can be controlled within the limits 10 < DP < 100. It is shown that 4 can also act as an initiator for the polymerisation of IB, that these polymerisations involve only free ion propagation and, from a kinetic analysis of these polymerisations, that: (k)²/k = 12 1 · mol^?1 · s^?1, k = 1,2 · 10^?3 l · mol^?1 · s^?1, k [P] = 1,7 · 10^?3 s^?1, and k/(k K) = 102. The same analysis demonstrates that the self-ionisation of BCl₃ can be neglected in terms of any influence on the molar mass of the products. Experiments are also described which show that 2-chloro-2-methylpropane is not suitable as a substitute initiator for IB, but that 2-chloro-2,4,4-trimethylpentane is a useful model for 4 as an initiator for the polymerisation of IB.     

The thermodynamics of polymerisation of aldehydes and cyclic ethers, 4.1,3-dioxocane and 1,3,6-trioxocane

The equilibrium between gaseous monomer (g) and amorphous polymer (c) has been studied for 1,3-dioxocane and 1,3,6-trioxocane between 100 and 137°C. From the equilibrium pressures of monomer, the ΔH and ΔS values have been calculated. Thermodynamic data for the vaporisation of each monomer have also been measured so that values of ΔH and ΔS for the polymerisations could be calculated (l: liquid phase). For 1,3-dioxocane: For 1,3,6-trioxocane:      

Thermodynamics of depolymerization of biotechnological poly[(R)-3-hydroxybutyrate] with formation of (R,R,R)-4,8,12-trimethyl-1,5,9-trioxadodeca-2,6,10-trione and of its polymerization with ring-opening to highly isotactic poly[(R)-3-hydroxybutyrate] from 0 to 500 K at standard pressure

In adiabatic vacuum and dynamic calorimeters the temperature dependence of the heat capacity C of (R,R,R-4,8,12-trimethyl-1,5,9-trioxadodeca-2,6,10-trione a twelve-membered cyclic trilactone), biotechnological poly[(R)-3-hydroxybutyrate] and highly isotactic poly[(R)-3-hydroxybutyrate] was studied between 5 K and 500 K; temperatures and enthalpies of melting of the above mentioned substances were measured. In a calorimeter with a static bomb and an isothermal shield the energy of combustion of the same substances was measured. From the results the thermodynamic functions C (T), H⁰(T) ? H⁰ (0), S⁰ (T), G⁰ (T) ? H⁰ (0) were calculated in the range of 0 K to 500 K and thermochemical parameters ΔH, ΔH, ΔS, ΔG were estimated at T = 298,15 K and standard pressure. The thermodynamic parameters of depolymerization of the biotechnological polymer to the 12-membered trilactone ΔH, ΔS ΔG and of the polymerization of the monomer formed in the highly isotactic poly[(R)-3-hydroxybutyrate] ΔH, ΔS, ΔG were calculated for 0 K to 500 K.     

Studies on formaldehyde resins, 25. Kinetic study on the hydroxymethylation of melamine by means of HPLC

The kinetics of hydroxymethylation of melamine (2,4,6-triamino-1,3,5-triazine) with formaldehyde to form N-(hydroxymethyl)melamine (2,4-diamino-6-hydroxymethylamino-1,3,5-triazine) was investigated in aqueous hydrogen phosphate/phosphate buffer solutions in the range of pH 10,51 to 12,20 at 20°C, determining unreacted melamine by means of HPLC. As a result, this reaction is subject to a general base catalysis, and the second order rate constant k is expressed by k = k′ + k_A ? [A^?] + k[A^?]²/[HA] (or k [A^?][OH^?]), where HA and A^? denote acid and basic constituents of the buffer, and k′ is the rate constant in unbuffered media. This rate equation is compared with those reported previously.     

Kinetics of the anionic polymerization of ϵ-caprolactone with K+ · (dibenzo-18-crown-6 ether) counterion. Propagation via macroions and macroion pairs

Following our earlier work on the polymerization of lactones involving crowned cations, kinetics of the anionic polymerization of ?-caprolactone (?CL) with K⁺ · (dibenzo-18-crown-6 ether) (K⁺DB18C6) counterion was studied calorimetrically in THF solution in the temperature range from 0 to 20°C. Dissociation constants of CH₃(CH₂)₅O^?K⁺DB18C6, modelling the active centers, were determined conductometrically: K_D (20°C) = 7,7 · 10^?5 mol · dm^?3, ΔH = 9,3 ± 0,2 kJ · mol^?1, ΔS = ?47 ± 2J · mol^?1 · K^?1. From kinetic measurements and from measurements of the dissociation constant of CH₃(CH₂)₅O^? K⁺DB18C6, rate constants of propagation via macroions and via macroion pairs were determined. Activation parameters for propagation via these species are equal to: ΔH = 39,2 ± 0,2 kJ · mol^?1, ΔS = ?63 ± 1 J · mol^?1 · K^?1, ΔH = 13,7 ± 0,1 kJ · mol^?1, ΔS = ?185 ± 2 J · mol^?1 · K^?1. At 20°C, k = 3,50 · 10² dm³ · mol^?1 · s^?1 and k = 5,2 dm³ · mol^?1 · s^?1. Due to the large difference of ΔH^≠ for propagation via macroions and macroion pairs (vide supra), the isokinetic point (k = k) would appear at ?65°C.     

Study by 19F nuclear magnetic resonance spectroscopy of the cationic polymerization of tetrahydrofuran initiated by PF5

By means of ¹⁹F-NMR spectroscopy it was possible to detect and estimate the hexafluorophosphate ion (PF) in the THF polymerization, initiated by PF₅ in methylene dichloride at 25°C. The observed variations with time of [PFPF], [PF₅·THF], and [PPF] were analysed kinetically, and by means of the resulting equations, the rate constants for initiation, propagation and termination were evaluated. Comparison of our k_p value with those of others indicates that our method is sound, and we conclude that it should be applicable to other similar systems.     

Berechnung der ungestörten dimensionen von vinylpolymeren mit unterschiedlichen seitenketten. Poly(propyläthylen), poly(äthyläthylen), polystyrol

The unperturbed dimensions 〈r〉/nl² and their temperature coefficients have been evaluated for poly(propylethylene) (poly-n-pentene-1), poly(ethylethylene) (poly-n-butene-1), and polystyrene with the rotational isomeric state model of FLORY. The calculated values of 〈r〉/nl² for atactic and isotactic chains are in good agreement with the experimental data reported in the literature. The values of the model parameters asked for good agreement change in a meaningfull way with the length of the side chain. The measured temperature coefficients, however, are described satisfactorally by the FLORY model for atactic polypentene and polybutene only.     

Static and dynamic light scattering studies of dextran from leuconostoc mesenteroides in the dilute region

Dextrans with mass-average molar masses 8.10⁴ g/mol ? M?_w ? 10⁸ g/mol have been studied in water by means of static and dynamic light scattering at different temperatures. Static light scattering (SLS) yields the z-average mean-square radius of gyration, 〈S²〉_z and the second virial coefficient A₂. It is found that the dependences of 〈S²〉 and A₂ on the mass-average molar mass, M?_w, can be well described by the power laws 〈S²〉 = K_S · M? and A₂ = K_A · M?. The exponent v_s is independent of the temperature T, while v_A decreases as T is raised. Dynamic light scattering (DLS) yields the apparent diffusion coefficient, D_app (q,c), and the hydrodynamic radius, R_h, where q is the wave vector and c the polymer concentration. For small q, a plot of D_app (q, c) versus q² · 〈S²〉_z gives a straight line. The intercept represents the centre-of-mass translational diffusion coefficient. Its dependence on the concentration, c, can be well simulated by the relation $ D_{\rm z} (c) = D_{{\rm z,}0} \left[ {1 + k_{{\rm D},2} \cdot c} \right] $. Here k_{D, 2} is the second hydrodynamic virial coefficient and D_z,0 the z-average of the translational diffusion coefficient at infinite dilution. The analysis of the k_D,2 -data shows that dextran molecules behave rather as interpenetrable than as non-interpenetrable spheres. The density p = (3 M?_w)/(N_A · 4 π R) proves to be a measure for the degree of penetration; p decreases with increasing M?_w, indicating that penetration becomes easier at higher molar masses.     

Doping of poly(phthalidylidenearylene)s. Interaction of iodine with poly(3,3′-phthalidylidene-4,4′-biphenylylene)

Interactions of iodine with films prepared from poly(3,3′-phthalidylidene-4,4′-biphenylylene) (PPB) were studied by IR, far-IR and UV spectroscopy. The carbonyl group of the lactone cycle was found to play a main role in the complexation of iodine with PPB. Iodine incorporated into the polymer transforms into polyiodide ions ( I, n = 1 ? 3). Various polyiodides, along with complexed iodine (I₂^comp), may exist in the polymer at a time, and can be transformed into each other, depending on ambient conditions. The process of doping is almost completely reversible. After removal of iodine from the polymer by heating, the latter restores its starting properties.     

Chirality induction in cyclopolymerization, 5. Template effect of chiral acyclic 1,2-glycols on the cyclo-copolymerizations of (S)-1,2-propanediyl and (2S,3S)-2,3-butanediyl bis(4-vinylbenzoate)s with styrene

The copolymerizations of (S)-1,2-propanediyl and (2S,3S)-2,3-butanediyl bis(4-vinyl-benzoate)s ( 2a and 2b ) (M₁) with styrene (M₂) were carried out using 2,2′-azoisobutyronitrile in toluene at 60 °C. For the cyclocopolymer 3 , the extent of cyclization of 3b is higher than that of 3a . The monomer reactivity ratios are r₁ = 8.34 and r₂ = 0.27 for 2a and r₁ = 6.82 and r₂ = 0.19 for 2b . The specific rotation ([α] in deg · dm⁻¹ · g⁻¹ · cm³ c = 1.0 g · dL⁻¹; CHCl₃) changes from +230 to +176 for 3a and from +319 to +150 for 3b . After removing the chiral templates, the resulting poly[(methyl 4-vinylbenzoate)-co-styrene]s showed a specific rotation ([α] in deg · dm⁻¹ · g⁻¹ · cm³ c = 1.0 g · dL⁻¹; CHCl₃) of −2 for 4a and of −8 for 4b , indicating that the (2S,3S)-2,3-butanediyl template shows a higher ability to cause asymmetric induction by comparison with the (S)-1,2-propanediyl one. A split Cotton effect due to the positive chirality was observed in the CD spectrum of 2 and the reversed one for 4 . According to the CD exciton chirality method, the clockwise-twist of two 4-vinylbenzoate for 2 transmitted its chirality to 4 in which two 4-vinylbenzoyl groups are twisted counterclockwise, i. e., an (R,R)-configuration of vicinal (methyl 4-vinylbenzoate) units in the main chain.     

Effect of molecular weight distribution of the original polymer on separation characteristics in successive solutional fractionation

An attempt has been made to clarify the effect of molecular weight distribution (MWD) in the original polymer on the separation characteristics in successive solutional fractionation (SSF) and comparisons were made with successive precipitational fractionation (SPF). Polymers with Schulz-Zimm distribution and Wesslau distribution of the degree of polymerization (ratio of the weight- to number-average degree of polymerization, X?/X? = 2 to 5) were brought into solution, and cooled to cause precipitation according to the modified simulative procedure of Kamide and Sugamiya, based on the Flory-Huggins's theory of dilute solutions of polymers. No double-peak distributions were observed in the fractions obtained by SSF under the conditions which give double-peak distributions in the case of SPF. The minimum value of X?_w in the fractions obtained in a given run is almost the same for SSF and SPF, the minimum increasing with decreasing X?/X? (the suffix zero denoting the original polymer). The ratio X?_w/X?_n in the fractions is not greatly influenced by X?/X? except for the first few fractions. The standard deviation σ′ of the MWD in the fractions decreases with increasing X?/X? in an initial stage, while this situation is reversed in the later stages. In general SSF furnishes sharp fractions regardless of X?/X?.     

The partial specific volume of lysozyme chloride in water under pressure

The apparent specific volume ν of lysozyme chloride dissolved in pure water has been measured from 1 to 4000 bar using the Adams-Gibson-Anderson method of trapped mercury. A parametric expression for ν as a function of solute weight fraction X₂ is given for the various pressures at which the experiments have been performed. This parametric expression is valid when the densities of the solutions at atmospheric pressure and their compression at pressure P can be considered to be linear functions of the solute weight fraction X₂. The relation between ν and the partial specific ν ₂ is discussed and it is shown that within the range of concentrations considered and given the precision of the experiments under pressure, the two quantities are indistinguishable.     

Ion pairs,cations and anions in ternary solutions of bis(polystyryl)magnesium and magnesium bromide in tetrahydrofuran

A conductometric method is developed for the study of the ternary solutions in tetrahydrofuran of bis(polystyryl)magnesium (PStMgPSt) containing various proportions r of magnesium bromide. The concentration of bis(polystyryl)magnesium varies from 10^?4 to 2.10^?3 mol.dm^?3, the ratio r from O to 17,4 and the temperature from +20°C down to ?60°C. The addition of MgBr₂ provokes the replacement of the ion pairs PStMgPSt by PStMgBr, the dismutation constant being at 20°C K_dism = 42 ± 2 with a molar enthalpy of ? 5 ± 2 kJ.mol^?1. The dismutation is chiefly ruled by a conjugated steric effect that lowers the stability of PStMgPSt. In this concentration range, aggregation of styryl ion pairs is negligible. Moreover for a given r the degree of dissociation of the polymer is independent of its concentration. This is due to the complete predominance in this concentration range of the triple anions Mg(PSt), MgBr(PSt) and MgBr₂PSt^?, approximately in the proportions of the ion pairs f_pp, f_bp and f_bb as calculated from the dismutation constant. The triple anions MgBr are still completely negligible, as are also the triple cations. The single cations MgPSt⁺ and MgBr⁺ are approximately in the proportion f_pp. K_dism/f_bp, the latter are ones becoming predominant above r = 3.     

Iodine-poly(vinyl alcohol) interactions, 2. Anomalous slow quenching of the chromophore after extraction of free iodine with carbon tetrachloride

An equilibrium blue colored solution containing poly(vinyl alcohol) (PVA)-iodine-boric acid was prepared at 5°C. With increasing PVA concentration, at the same concentration of iodine, the absorption band of the chromophore I (λ_max = 650 nm, band D) linearly increased and the intensities of the bands for both I^? (λ_max = 226 nm, band A) and I (λ_max = 290 nm, band B) decreased. The band due to another iodide species (λ_max = 355 nm, band C), tentatively assigned to I · I₂, remained unchanged. Three solutions with different PVA concentration were then extracted with the same volume of carbon tetrachloride to remove I₂ present in the system. It was found that the chomophore due to I gradually decays with repeated extractions. After one extraction the change of the absorbance of I with time was measured at 5°C. In the system with a high PVA concentration the chromophore recovers the equilibrium within three days without losing much intensity, while in the systems with lower PVA concentration recovering of equilibrium takes more than four days with a considerable loss of chromophore. In the latter case, free I₂ extracted is supplied by the decomposition of polyiodide ions to I^?. Analysis of the rate of re-equilibration of iodine species revealed two reaction processes: one is a reaction involving free iodine species in an aqueous environment and the other is a slow reaction involving the polyiodide ions bound in a PVA cage.