Interactions of lithium 2-(2-methoxyethoxy)ethoxide (LiOEEM) with the model dimer ditert-butyl 2-... more Interactions of lithium 2-(2-methoxyethoxy)ethoxide (LiOEEM) with the model dimer ditert-butyl 2-lithio-2,4,4-trimethylglutarate (A) and the living poly(tert-butyl methacrylate) oligomers (B) were studied in tetrahydrofuran-d 8 at 203-273 K using 1 H, 13 C, 7 Li, and 6 Li, 1D and 2D, NMR and ab initio SCF 3-21G and MNDO quantum chemical calculations. LiOEEM is shown to have a strong tendency to self-aggregation, producing dimeric, trimeric, and tetrameric aggregates and competing with its mixed aggregation (or complexation) with A and, in particular, B. When dissolved in THF, LiOEEM as well as its mixtures with A form metastable systems which relax in several days at 258 K into equilibrium. Interaction of LiOEEM with A leads to a system of mixed aggregates LiOEEM 1A1, LiOEEM3A1, and possibly LiOEEM2A1 along with the original components, in relative populations depending on the LiOEEM/A molar ratio, temperature, time, and probably other factors of preparation. Probable structures of these complexes are proposed, and the nature of the prevalent bonding is suggested. Experimental results indicate that LiOEEM is unable to convert A completely at moderate excess (up to 4/1 mol/mol). Interaction of LiOEEM with B leads to quite analogous complexes but with even lower yields. There appear to be traces of uncomplexed B even at the LiOEEM/B ratio of 10 mol/mol. This is suggested to produce at least two different kinds of active growth centers in the corresponding ligated anionic polymerization of tert-butyl methacrylate and, consequently, the observed bimodality of the polymeric product. The difference with respect to methyl methacrylate, where LiOEEM ensures an almost ideal living polymerization, is suggested to be due to the steric hindrance of efficient complexation exerted by the tert-butyl group.
Journal of Polymer Science Part A: Polymer Chemistry, 2004
ABCBA-type pentablock copolymers of methyl methacrylate (MMA), styrene (S), and isobutylene (IB) ... more ABCBA-type pentablock copolymers of methyl methacrylate (MMA), styrene (S), and isobutylene (IB) were prepared by a three-step synthesis, which included atom transfer radical polymerization (ATRP) and cationic polymerization: (1) poly-(methyl methacrylate) (PMMA) with terminal chlorine atoms was prepared by ATRP initiated with an aromatic difunctional initiator bearing two trichloromethyl groups under CuCl/2,2Ј-bipyridine catalysis; (2) PMMA with the same catalyst was used for ATRP of styrene, which produced a poly(S-b-MMA-b-S) triblock copolymer; and (3) IB was polymerized cationically in the presence of the aforementioned triblock copolymer and BCl 3 , and this produced a poly(IB-b-S-b-MMA-b-S-bIB) pentablock copolymer. The reaction temperature, varied from Ϫ78 to Ϫ25°C, significantly affected the IB content in the product; the highest was obtained at Ϫ25°C. The formation of a pentablock copolymer with a narrow molecular weight distribution provided direct evidence of the presence of active chlorine at the ends of the poly(S-b-MMA-b-S) triblock copolymer, capable of the initiation of the cationic polymerization of IB in the presence of BCl 3. A differential scanning calorimetry trace of the pentablock copolymer (20.1 mol % IB) showed the glass-transition temperatures of three segregated domains, that is, polyisobutylene (Ϫ87.4°C), polystyrene (95.6°C), and PMMA (103.7°C) blocks. One glass-transition temperature (104.5°C) was observed for the aforementioned triblock copolymer.
We apply a molecular theory to predict the structural properties of poly(acrylic acid) macromolec... more We apply a molecular theory to predict the structural properties of poly(acrylic acid) macromolecules grafted via one of their ends to solid surfaces. The theory explicitly incorporates the acid-base equilibrium responsible for the charge regulation of the acrylic groups, as well as the conformations, size, shape, and charge distributions of all the molecular species present. We compare the predictions of the theory with experimental observations presented in the preceding article for the height of the layer as a function of ionic strength for different polymer molecular weight and polymer surface coverage. The calculated heights are found to be in good agreement with the experimental observations. The theory predicts the distribution of charges within the layer. We find that the counterions adsorb to the grafting surface, overcompensating the charge of the polymer. The charge regulation within the polymer layer is determined by the interplay between the bulk pH, the ionic strength, and the density of polymer. The system tends to become uncharged with decreasing ionic strength of the solution and increasing polymer density. In all cases the charge regulation acts in order to minimize the electrostatic repulsions in the system. The local distribution of protons within the polymer layer is predicted to be very different from that of the bulk solution. The local pH within the polymer layer can be tuned by varying the solution ionic strength and the polymer surface coverage; the variation can be large as two pH units, relative to the bulk pH. This large variation of the local pH within a couple of nanometers within the brush can be used in the design of biosensors.
Page 1. Makromol. Chem. 189,951 - 959 (1988) 95 1 The effect of crosslinking on the aggregation o... more Page 1. Makromol. Chem. 189,951 - 959 (1988) 95 1 The effect of crosslinking on the aggregation of syndiotactic poly(methy1 methacrylate) - An investigation using NMR and infrared spectroscopy JiFi SpPvCiZek *, Bohdan Schneider, Jan Stokr, Petr VlZek ...
Metalation of polystyrene, poly(4-methylstyrene), and their block copolymers is described using e... more Metalation of polystyrene, poly(4-methylstyrene), and their block copolymers is described using either superbases (made of organolithium compounds and potassium tert-pentoxide) or secbutyllithium complexed with N,N,N′,N′-tetramethylethylenediamine as metalating agents. Depending on the type and concentration of the metalating agent, different sites in the above polymers were metalated. Generally, only the most acidic and the most accessible hydrogen atoms in polystyrene (aromatic protons) and poly(4-methylstyrene) (methyl group protons) were selectively metalated using the above metalating agents at metalation ratios lower than unity. At higher metalation ratios, nonselective metalation occurred, and also other positions in the polymers (backbone, aromatic protons in poly(4-methylstyrene)) were metalated. To distinguish positions of metalation, functionalization of metalated polymers with chlorotrimethylsilane and subsequent 29 Si NMR analysis of polymer-bonded trimethylsilyl groups were applied.
We show that assemblies comprising anchored polymers with a gradual variation of grafting densiti... more We show that assemblies comprising anchored polymers with a gradual variation of grafting densities on solid substrates can be generated by first covering the substrate with a molecular gradient of the polymerization initiator, followed by polymerization from the substrate-bound initiator centers ("grafting from"). We apply this technique to prepare grafting density gradients of poly(acrylamide) (PAAm) on flat silica substrates. We demonstrate that using the grafting density gradient geometry, the mushroomto-brush transition can be accessed on a single sample. This transition is detected by monitoring the dependence of the thickness of the grafted PAAm in a good solvent using variable angle spectroscopic ellipsometry. Wettability experiments performed on the gradient PAAm substrate provide complementary information about the nature of the mushroom-to-brush transition.
Skip to Main Content. Due to scheduled maintenance access to the Wiley Online Library may be disr... more Skip to Main Content. Due to scheduled maintenance access to the Wiley Online Library may be disrupted as follows: Monday, 6 September - New York 0400 EDT to 0500 EDT; London 0900 BST to 1000 BST; Singapore 1600 to 1700. ...
Journal of Polymer Science Part A: Polymer Chemistry, 1992
The polymerization of 2-ethylhexyl acrylate (EtHA) initiated with lithium-tert-butoxide (t-BuOLi)... more The polymerization of 2-ethylhexyl acrylate (EtHA) initiated with lithium-tert-butoxide (t-BuOLi) in tetrahydrofuran (THF) and in the temperature range between-60 and 20°C was investigated. The reaction rate is distinctly temperature-dependent and at-60°C is already very low, similarly to the polymerization of methacrylates. Molecular weights of the polymers thus formed, particularly at higher temperatures, are inversely proportional to conversion of the monomer due to the slow initiation reaction. This is documented by the low consumption of alkoxide even at long reaction times, which also depends on the reaction temperature. At higher temperatures the polymerization stops spontaneously, due to the greater extent of autotermination reactions. The weak initiating efficiency of the alkoxide decreases still more with decreasing concentration of the monomer during the polymerization, as confirmed by the concentration dependence of the reaction rate in toluene at-2OOC. The results suggest a negligible initiating effect of alkoxides in complex bases, particularly at lower polymerization temperatures.
After a brief survey of conventional radical polymerization of alkenes, we review their Li + cata... more After a brief survey of conventional radical polymerization of alkenes, we review their Li + catalyzed radical polymerization and their controlled radical polymerization. Emphasis is on homopolymerization, but related copolymerization of less activated monomers is mentioned as well.
The adsorption of block copolymers from a homopolymer melt is studied as a function of matrix mol... more The adsorption of block copolymers from a homopolymer melt is studied as a function of matrix molecular weight using neutron reflectivity and low energy forward recoil spectrometry. The block copolymer is poly(deuterated styrene-block-methylmethacrylate) (dPS-b-PMMA), which ...
Polystyrene and polystyrene-block-poly(methyl methacrylate) densely grafted with poly(ϵ-caprolact... more Polystyrene and polystyrene-block-poly(methyl methacrylate) densely grafted with poly(ϵ-caprolactone) or poly(DL-lactide) in the benzene rings were synthesized by a “grafting-from” method using hydroxylated precursors of the above styrene polymers as macroinitiators of the ring-opening polymerization of appropriate monomers. The poly(ϵ-caprolactone) grafts of the polystyrene-graft-poly(ϵ-caprolactone) copolymer were lengthened with poly(methyl methacrylate) by atom transfer radical polymerization. All the synthesized graft copolymers were
The kinetics of the anionic polymerization of butyl acrylate initiated with the system tertbutyl ... more The kinetics of the anionic polymerization of butyl acrylate initiated with the system tertbutyl 2-lithioisobutyrate (Li-t-BIB)/lithium tert-butoxide (t-BuOLi) in the mixed solvent toluene/tetrahydrofuran was studied at -70 to -40 "C. At -60 "C, the external reaction order is unity with respect to the concentrations of both the monomer and propagating species. Semilogarithmic conversion curves indicate a certain extent of self-temination at higher degrees of conversion. The termination reaction proceeds formally as a first-order reaction with respect to active centers. Kinetic calculations show that while the rate constant of propagation is concentration-independent, the rate constant of termination increases with the initial concentration of the monomer. The temperature dependence of the polymerization was studied for various solvent compositions and various [Li-t-BiB]o/ [t-BuOLiIo mole ratios. In toluene/THF = 9/1 (v/v), the rate constant of self-termination grows more steeply with increasing temperature than the rate constant of propagation so that at -40 "C the limiting monomer conversion is approximately 60%. In toluene/THF = 19/1 (v/v), the temperture dependence of the rate constant of self-termination is less distinct than in the former case and the conversion is 90% even at -20 "C. From the GPC measurement it is seen that the autotermination proceeds in all referred experiments and only the relation between concurrent propagation and termination reactions is responsible for the formation of the polymer with a narrow MWD under suitable conditions.
Journal of Polymer Science Part A: Polymer Chemistry, 2000
The polymerization reactivity of isobutylene/SnCl 4 mixtures in the absence of polar solvent, was... more The polymerization reactivity of isobutylene/SnCl 4 mixtures in the absence of polar solvent, was investigated in a temperature interval from Ϫ78 to 60°C. The mixture is nonreactive below Ϫ20°C but slow polymerization proceeds from Ϫ20 to 20°C with the initial rate r 0 of the order 10 Ϫ5 mol ⅐ l Ϫ1. s Ϫ1. The rate of the process increases with increasing temperature up to ϳ10 Ϫ2 mol ⅐ l Ϫ1 ⅐ s Ϫ1 at 60°C. Logarithmic plots of r 0 and M n versus 1/T exhibit a break in the range from 20 to 35°C. Activation energy is positive with values E ϭ 21.7 Ϯ 4.2 kJ/mol in the temperature interval from Ϫ20 to 35°C and E ϭ 159.5 Ϯ 4.2 kJ/mol in the interval from 35 to 60°C. The values of activation enthalpy difference of molecular weights in these temperature intervals are ⌬H Mn ϭ Ϫ12.7 Ϯ 4.2 kJ/mol and Ϫ38.3 Ϯ 4.2 kJ/mol, respectively. The polymerization proceeds quantitatively, the molecular weights of products are relatively high, M n ϭ 1500-2500 at 35°C and about 600 at 60°C. It is assumed that initiation proceeds via [isobutylene ⅐ SnCl 4 ] charge transfer complex which is thermally excited and gives isobutylene radical-cations. Oxygen inhibits the polymerization from Ϫ20 to 20°C. Possible role of traces of water at temperatures above 20°C is discussed. It was verified by NMR analysis that only low molecular weight polyisobutylenes are formed with high contents of exo-terminal unsaturated structures. In addition to standard unsaturated groups, new structures were detected in the products.
N,N-Dimethylacetamide/lithium chloride (DMAc/LiCl) mixture is a popular solvent system used for c... more N,N-Dimethylacetamide/lithium chloride (DMAc/LiCl) mixture is a popular solvent system used for cellulose dissolution, analysis, and derivatization. However, a pre-treatment (activation) procedure is needed for most celluloses to dissolve readily in DMAc/LiCl. Here, an optimized version of the activation protocol based on solvent exchange to 1,4dioxane was introduced. Its universality was demonstrated by successful activation and dissolution of six different celluloses (AVICEL, Sigmacell, cotton linters, Encell, Lincell, and Whatman paper). Dissolution times varied significantly for different cellulose types and also depended on factors such as the drying method employed or the water removal step inclusion/ omission. Dioxane-activated celluloses were analyzed with a variety of methods. SEC measurements indicated low destructivity of the dioxane activation method. The infrared spectroscopy analysis showed that dioxane remained adsorbed on cellulose even after rigorous drying. In addition, upon dioxane activation, stagnation or a slight increase in the total order index of celluloses was observed. This observation was in accordance with the crystallinity index changes determined by solid-state NMR. Finally, scanning electron microscopy revealed disintegration of AVICEL particles and defibrillation of fibrous celluloses upon dioxane activation; Sigmacell remained apparently unchanged.
Using neutron reflectometry, the adsorption of diblock copolymers from a neutral polystyrene (PS)... more Using neutron reflectometry, the adsorption of diblock copolymers from a neutral polystyrene (PS) matrix is studied as a function of substrate type and non-adsorbing block degree of polymerization. The block copolymer is poly(deutero styrene)-block-poly(methyl methacrylate) and the substrates are silicon oxide, SiOx, and SiOx functionalized with (3-aminopropyl)triethoxysilane (APTES). We have determined the equilibrium volume fraction-depth profiles for such films, and compared them with volume fraction profiles generated by self-consistent mean-field (SCMF) theory and find good agreement between the experimental and theoretical data. SCMF calculations show that the segmental interaction energy between PS matrix chains and APTES is two orders of magnitude stronger than that between PS and SiOx.
Interactions of lithium 2-(2-methoxyethoxy)ethoxide (LiOEEM) with the model dimer ditert-butyl 2-... more Interactions of lithium 2-(2-methoxyethoxy)ethoxide (LiOEEM) with the model dimer ditert-butyl 2-lithio-2,4,4-trimethylglutarate (A) and the living poly(tert-butyl methacrylate) oligomers (B) were studied in tetrahydrofuran-d 8 at 203-273 K using 1 H, 13 C, 7 Li, and 6 Li, 1D and 2D, NMR and ab initio SCF 3-21G and MNDO quantum chemical calculations. LiOEEM is shown to have a strong tendency to self-aggregation, producing dimeric, trimeric, and tetrameric aggregates and competing with its mixed aggregation (or complexation) with A and, in particular, B. When dissolved in THF, LiOEEM as well as its mixtures with A form metastable systems which relax in several days at 258 K into equilibrium. Interaction of LiOEEM with A leads to a system of mixed aggregates LiOEEM 1A1, LiOEEM3A1, and possibly LiOEEM2A1 along with the original components, in relative populations depending on the LiOEEM/A molar ratio, temperature, time, and probably other factors of preparation. Probable structures of these complexes are proposed, and the nature of the prevalent bonding is suggested. Experimental results indicate that LiOEEM is unable to convert A completely at moderate excess (up to 4/1 mol/mol). Interaction of LiOEEM with B leads to quite analogous complexes but with even lower yields. There appear to be traces of uncomplexed B even at the LiOEEM/B ratio of 10 mol/mol. This is suggested to produce at least two different kinds of active growth centers in the corresponding ligated anionic polymerization of tert-butyl methacrylate and, consequently, the observed bimodality of the polymeric product. The difference with respect to methyl methacrylate, where LiOEEM ensures an almost ideal living polymerization, is suggested to be due to the steric hindrance of efficient complexation exerted by the tert-butyl group.
Journal of Polymer Science Part A: Polymer Chemistry, 2004
ABCBA-type pentablock copolymers of methyl methacrylate (MMA), styrene (S), and isobutylene (IB) ... more ABCBA-type pentablock copolymers of methyl methacrylate (MMA), styrene (S), and isobutylene (IB) were prepared by a three-step synthesis, which included atom transfer radical polymerization (ATRP) and cationic polymerization: (1) poly-(methyl methacrylate) (PMMA) with terminal chlorine atoms was prepared by ATRP initiated with an aromatic difunctional initiator bearing two trichloromethyl groups under CuCl/2,2Ј-bipyridine catalysis; (2) PMMA with the same catalyst was used for ATRP of styrene, which produced a poly(S-b-MMA-b-S) triblock copolymer; and (3) IB was polymerized cationically in the presence of the aforementioned triblock copolymer and BCl 3 , and this produced a poly(IB-b-S-b-MMA-b-S-bIB) pentablock copolymer. The reaction temperature, varied from Ϫ78 to Ϫ25°C, significantly affected the IB content in the product; the highest was obtained at Ϫ25°C. The formation of a pentablock copolymer with a narrow molecular weight distribution provided direct evidence of the presence of active chlorine at the ends of the poly(S-b-MMA-b-S) triblock copolymer, capable of the initiation of the cationic polymerization of IB in the presence of BCl 3. A differential scanning calorimetry trace of the pentablock copolymer (20.1 mol % IB) showed the glass-transition temperatures of three segregated domains, that is, polyisobutylene (Ϫ87.4°C), polystyrene (95.6°C), and PMMA (103.7°C) blocks. One glass-transition temperature (104.5°C) was observed for the aforementioned triblock copolymer.
We apply a molecular theory to predict the structural properties of poly(acrylic acid) macromolec... more We apply a molecular theory to predict the structural properties of poly(acrylic acid) macromolecules grafted via one of their ends to solid surfaces. The theory explicitly incorporates the acid-base equilibrium responsible for the charge regulation of the acrylic groups, as well as the conformations, size, shape, and charge distributions of all the molecular species present. We compare the predictions of the theory with experimental observations presented in the preceding article for the height of the layer as a function of ionic strength for different polymer molecular weight and polymer surface coverage. The calculated heights are found to be in good agreement with the experimental observations. The theory predicts the distribution of charges within the layer. We find that the counterions adsorb to the grafting surface, overcompensating the charge of the polymer. The charge regulation within the polymer layer is determined by the interplay between the bulk pH, the ionic strength, and the density of polymer. The system tends to become uncharged with decreasing ionic strength of the solution and increasing polymer density. In all cases the charge regulation acts in order to minimize the electrostatic repulsions in the system. The local distribution of protons within the polymer layer is predicted to be very different from that of the bulk solution. The local pH within the polymer layer can be tuned by varying the solution ionic strength and the polymer surface coverage; the variation can be large as two pH units, relative to the bulk pH. This large variation of the local pH within a couple of nanometers within the brush can be used in the design of biosensors.
Page 1. Makromol. Chem. 189,951 - 959 (1988) 95 1 The effect of crosslinking on the aggregation o... more Page 1. Makromol. Chem. 189,951 - 959 (1988) 95 1 The effect of crosslinking on the aggregation of syndiotactic poly(methy1 methacrylate) - An investigation using NMR and infrared spectroscopy JiFi SpPvCiZek *, Bohdan Schneider, Jan Stokr, Petr VlZek ...
Metalation of polystyrene, poly(4-methylstyrene), and their block copolymers is described using e... more Metalation of polystyrene, poly(4-methylstyrene), and their block copolymers is described using either superbases (made of organolithium compounds and potassium tert-pentoxide) or secbutyllithium complexed with N,N,N′,N′-tetramethylethylenediamine as metalating agents. Depending on the type and concentration of the metalating agent, different sites in the above polymers were metalated. Generally, only the most acidic and the most accessible hydrogen atoms in polystyrene (aromatic protons) and poly(4-methylstyrene) (methyl group protons) were selectively metalated using the above metalating agents at metalation ratios lower than unity. At higher metalation ratios, nonselective metalation occurred, and also other positions in the polymers (backbone, aromatic protons in poly(4-methylstyrene)) were metalated. To distinguish positions of metalation, functionalization of metalated polymers with chlorotrimethylsilane and subsequent 29 Si NMR analysis of polymer-bonded trimethylsilyl groups were applied.
We show that assemblies comprising anchored polymers with a gradual variation of grafting densiti... more We show that assemblies comprising anchored polymers with a gradual variation of grafting densities on solid substrates can be generated by first covering the substrate with a molecular gradient of the polymerization initiator, followed by polymerization from the substrate-bound initiator centers ("grafting from"). We apply this technique to prepare grafting density gradients of poly(acrylamide) (PAAm) on flat silica substrates. We demonstrate that using the grafting density gradient geometry, the mushroomto-brush transition can be accessed on a single sample. This transition is detected by monitoring the dependence of the thickness of the grafted PAAm in a good solvent using variable angle spectroscopic ellipsometry. Wettability experiments performed on the gradient PAAm substrate provide complementary information about the nature of the mushroom-to-brush transition.
Skip to Main Content. Due to scheduled maintenance access to the Wiley Online Library may be disr... more Skip to Main Content. Due to scheduled maintenance access to the Wiley Online Library may be disrupted as follows: Monday, 6 September - New York 0400 EDT to 0500 EDT; London 0900 BST to 1000 BST; Singapore 1600 to 1700. ...
Journal of Polymer Science Part A: Polymer Chemistry, 1992
The polymerization of 2-ethylhexyl acrylate (EtHA) initiated with lithium-tert-butoxide (t-BuOLi)... more The polymerization of 2-ethylhexyl acrylate (EtHA) initiated with lithium-tert-butoxide (t-BuOLi) in tetrahydrofuran (THF) and in the temperature range between-60 and 20°C was investigated. The reaction rate is distinctly temperature-dependent and at-60°C is already very low, similarly to the polymerization of methacrylates. Molecular weights of the polymers thus formed, particularly at higher temperatures, are inversely proportional to conversion of the monomer due to the slow initiation reaction. This is documented by the low consumption of alkoxide even at long reaction times, which also depends on the reaction temperature. At higher temperatures the polymerization stops spontaneously, due to the greater extent of autotermination reactions. The weak initiating efficiency of the alkoxide decreases still more with decreasing concentration of the monomer during the polymerization, as confirmed by the concentration dependence of the reaction rate in toluene at-2OOC. The results suggest a negligible initiating effect of alkoxides in complex bases, particularly at lower polymerization temperatures.
After a brief survey of conventional radical polymerization of alkenes, we review their Li + cata... more After a brief survey of conventional radical polymerization of alkenes, we review their Li + catalyzed radical polymerization and their controlled radical polymerization. Emphasis is on homopolymerization, but related copolymerization of less activated monomers is mentioned as well.
The adsorption of block copolymers from a homopolymer melt is studied as a function of matrix mol... more The adsorption of block copolymers from a homopolymer melt is studied as a function of matrix molecular weight using neutron reflectivity and low energy forward recoil spectrometry. The block copolymer is poly(deuterated styrene-block-methylmethacrylate) (dPS-b-PMMA), which ...
Polystyrene and polystyrene-block-poly(methyl methacrylate) densely grafted with poly(ϵ-caprolact... more Polystyrene and polystyrene-block-poly(methyl methacrylate) densely grafted with poly(ϵ-caprolactone) or poly(DL-lactide) in the benzene rings were synthesized by a “grafting-from” method using hydroxylated precursors of the above styrene polymers as macroinitiators of the ring-opening polymerization of appropriate monomers. The poly(ϵ-caprolactone) grafts of the polystyrene-graft-poly(ϵ-caprolactone) copolymer were lengthened with poly(methyl methacrylate) by atom transfer radical polymerization. All the synthesized graft copolymers were
The kinetics of the anionic polymerization of butyl acrylate initiated with the system tertbutyl ... more The kinetics of the anionic polymerization of butyl acrylate initiated with the system tertbutyl 2-lithioisobutyrate (Li-t-BIB)/lithium tert-butoxide (t-BuOLi) in the mixed solvent toluene/tetrahydrofuran was studied at -70 to -40 "C. At -60 "C, the external reaction order is unity with respect to the concentrations of both the monomer and propagating species. Semilogarithmic conversion curves indicate a certain extent of self-temination at higher degrees of conversion. The termination reaction proceeds formally as a first-order reaction with respect to active centers. Kinetic calculations show that while the rate constant of propagation is concentration-independent, the rate constant of termination increases with the initial concentration of the monomer. The temperature dependence of the polymerization was studied for various solvent compositions and various [Li-t-BiB]o/ [t-BuOLiIo mole ratios. In toluene/THF = 9/1 (v/v), the rate constant of self-termination grows more steeply with increasing temperature than the rate constant of propagation so that at -40 "C the limiting monomer conversion is approximately 60%. In toluene/THF = 19/1 (v/v), the temperture dependence of the rate constant of self-termination is less distinct than in the former case and the conversion is 90% even at -20 "C. From the GPC measurement it is seen that the autotermination proceeds in all referred experiments and only the relation between concurrent propagation and termination reactions is responsible for the formation of the polymer with a narrow MWD under suitable conditions.
Journal of Polymer Science Part A: Polymer Chemistry, 2000
The polymerization reactivity of isobutylene/SnCl 4 mixtures in the absence of polar solvent, was... more The polymerization reactivity of isobutylene/SnCl 4 mixtures in the absence of polar solvent, was investigated in a temperature interval from Ϫ78 to 60°C. The mixture is nonreactive below Ϫ20°C but slow polymerization proceeds from Ϫ20 to 20°C with the initial rate r 0 of the order 10 Ϫ5 mol ⅐ l Ϫ1. s Ϫ1. The rate of the process increases with increasing temperature up to ϳ10 Ϫ2 mol ⅐ l Ϫ1 ⅐ s Ϫ1 at 60°C. Logarithmic plots of r 0 and M n versus 1/T exhibit a break in the range from 20 to 35°C. Activation energy is positive with values E ϭ 21.7 Ϯ 4.2 kJ/mol in the temperature interval from Ϫ20 to 35°C and E ϭ 159.5 Ϯ 4.2 kJ/mol in the interval from 35 to 60°C. The values of activation enthalpy difference of molecular weights in these temperature intervals are ⌬H Mn ϭ Ϫ12.7 Ϯ 4.2 kJ/mol and Ϫ38.3 Ϯ 4.2 kJ/mol, respectively. The polymerization proceeds quantitatively, the molecular weights of products are relatively high, M n ϭ 1500-2500 at 35°C and about 600 at 60°C. It is assumed that initiation proceeds via [isobutylene ⅐ SnCl 4 ] charge transfer complex which is thermally excited and gives isobutylene radical-cations. Oxygen inhibits the polymerization from Ϫ20 to 20°C. Possible role of traces of water at temperatures above 20°C is discussed. It was verified by NMR analysis that only low molecular weight polyisobutylenes are formed with high contents of exo-terminal unsaturated structures. In addition to standard unsaturated groups, new structures were detected in the products.
N,N-Dimethylacetamide/lithium chloride (DMAc/LiCl) mixture is a popular solvent system used for c... more N,N-Dimethylacetamide/lithium chloride (DMAc/LiCl) mixture is a popular solvent system used for cellulose dissolution, analysis, and derivatization. However, a pre-treatment (activation) procedure is needed for most celluloses to dissolve readily in DMAc/LiCl. Here, an optimized version of the activation protocol based on solvent exchange to 1,4dioxane was introduced. Its universality was demonstrated by successful activation and dissolution of six different celluloses (AVICEL, Sigmacell, cotton linters, Encell, Lincell, and Whatman paper). Dissolution times varied significantly for different cellulose types and also depended on factors such as the drying method employed or the water removal step inclusion/ omission. Dioxane-activated celluloses were analyzed with a variety of methods. SEC measurements indicated low destructivity of the dioxane activation method. The infrared spectroscopy analysis showed that dioxane remained adsorbed on cellulose even after rigorous drying. In addition, upon dioxane activation, stagnation or a slight increase in the total order index of celluloses was observed. This observation was in accordance with the crystallinity index changes determined by solid-state NMR. Finally, scanning electron microscopy revealed disintegration of AVICEL particles and defibrillation of fibrous celluloses upon dioxane activation; Sigmacell remained apparently unchanged.
Using neutron reflectometry, the adsorption of diblock copolymers from a neutral polystyrene (PS)... more Using neutron reflectometry, the adsorption of diblock copolymers from a neutral polystyrene (PS) matrix is studied as a function of substrate type and non-adsorbing block degree of polymerization. The block copolymer is poly(deutero styrene)-block-poly(methyl methacrylate) and the substrates are silicon oxide, SiOx, and SiOx functionalized with (3-aminopropyl)triethoxysilane (APTES). We have determined the equilibrium volume fraction-depth profiles for such films, and compared them with volume fraction profiles generated by self-consistent mean-field (SCMF) theory and find good agreement between the experimental and theoretical data. SCMF calculations show that the segmental interaction energy between PS matrix chains and APTES is two orders of magnitude stronger than that between PS and SiOx.
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Papers by Petr Vlček