Gudrun Schmidt-Naake

Polyanion-polycation complex formation as a function of the position of the functional groups

Abstract A method consisting of the combination of turbidimetry and conductometry was investigated to detect polyanion-polycation complex formation. We used ‘strong acid’ polyelectrolytes varying in charge density and ‘strong basic’ polyelectrolytes varying in the length of the spacer groups and in the accessibility of the quaternary ammonium function. Our systematic investigations have shown a predominantly 1:1 stoichiometry at the turbidimetric as well as the electrochemical titration endpoints. Deviations were observed when the individual components were significantly less soluble, i.e. when the polymers had units with long hydrophobic spacer groups and/or with quaternary ammonium functions that are sterically less accessible. Colloid stability as well as the type of turbidity curve are discussed on the basis of a two-step mechanism of symplex formation. The results are also compared with our earlier investigations. We previously found a general trend that a 1:1 stoichiometry could only be achieved with ‘strong’ polyelectrolyte components. Independently of the molar ratio of the cationic to anionic functional groups at the titration endpoint, the stability of the colloids of the symplex system was found to depend also on the molar mass, the charge density and the hydrophobicity of the comonomer units.

Study on polymer blends of poly(styrene-co-acrylonitrile) and poly(styrene-co-maleic anhydride)

Abstract Polymer blends of poly(styrene- co -acrylonitrile) and poly(styrene- co -maleic anhydride) have been investigated by differential scanning calorimetry and light scattering measurements. Depending on the copolymer compositions it is possible to obtain miscible and immiscible blends and blends with lower critical solution temperature behaviour. In this way a miscibility channel can be designed. Fourier transform infrared spectroscopy suggests that the miscibility is caused chiefly by intramolecular interactions, and intermolecular interactions can be partially neglected.

Controlled radical copolymerization of styrene and 4‐vinylpyridine

In this paper the copolymerization of styrene and 4-vinylpyridine at 125°C inthe presence of benzoyl peroxide (BPO) and 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) on the one hand and in the presence of a TEMPO-terminated polystyrene on the other is described. The molecular weights of the synthesized copolymers and block copolymers were found to increase with increasing conversion. Various nitroxides were used to prepare poly-[styrene-co-(4-vinylpyridine)] copolymers with BPO as the initiator. We found the use of 4-Oxo-TEMPO yielded high polymerization rates and high molecular weights. However, the polymerization in the presence of 4-NH 2 -TEMPO exhibits a slow rate and leads to copolymers with broad polydispersities. The influence of the other investigated nitroxides (4-OH- and 4-ACETAMIDO-TEMPO) on the copolymerization range between these both limits. The TEMPO-mediated copolymerizations of styrene and 4-vinylpyridine are slower than the comparable autopolymerization. However, the addition of an initiator which decomposes as the reaction temperature, such as dicumyl peroxide, leads to a considerable acceleration. By means of the Kelen-Tudos method, the monomer reactivity ratios r S (styrene) and r 4-VPy (4-vinylpyridine) were determined. The calculation for the TEMPO-mediated copolymerization at 125°C results in r S = 0.73 ± 0.09 and R 4-VPy = 0.96 ± 0.15. The values for the spontaneous copolymerization at this temperatures are r S = 0.58 ± 0.04 and r 4-VPy = 0.91 ± 0.05.

Rate enhancement of the N-oxyl-controlled free radical copolymerization of styrene with N-vinylcarbazole

Poly(styrene-co-N-vinylcarbazole) copolymers with controlled molecular weights and narrow polydispersities were synthesized by N-oxyl-controlled free radical copolymerization using benzoyl peroxide as initiator and 2,2,6,6-tetramethylpiperidine-N-oxyl as terminating agent. To improve the low polymerization rates, the radical initiator dicumyl peroxide (DCP) was introduced and its effects on the polymerization rate, the molecular weight and the polydispersity were studied. It was demonstrated that the introduction of DCP leads to a significant enhancement of the polymerization rate and that copolymers with high N-vinylcarbazole contents as well as N-vinylcarbazole homopolymer can be obtained. The enhancement of the polymerization rate corresponds to a decrease in the number of chain breaking reactions which leads to polymers with lower polydispersities compared with those polymerized at the same monomer conversion without DCP. This could be proved via comparative chain-extension experiments with styrene leading to poly(styrene-co-N-vinylcarbazole)-block-poly(styrene) diblock copolymers. Poly(Styrol-co-N-Vinylcarbazol)-Copolymere mit kontrollierter Molmasse und enger Polydispersitat wurden durch N-Oxyl-kontrollierte radikalische Polymerisation hergestellt. Als Initiator wurde Dibenzoylperoxid und als Terminator 2,2,6,6-Tetramethylpiperidin-N-oxyl verwendet. Um die geringen Polymerisationsgeschwindigkeiten zu beschleunigen, wurde der Einflus eines zusatzlich hinzugefugten Initiators, Dicumylperoxid (DCP), auf die Polymerisationsgeschwindigkeit, die Molmassen und die Polydispersitat der hergestellten Polymere diskutiert. Mit Hilfe von DCP kann die Polymerisationsgeschwindigkeit signifikant erhoht werden, so das auch Copolymere mit sehr hohem N-Vinylcarbazol-Gehalt bis hin zum Homo-N-Vinylcarbazol zuganglich werden. Die Beschleunigung der Polymerisationsgeschwindigkeit reduziert gleichzeitig die Anzahl der auftretenden Abbruchreaktionen, so das die mit DCP synthetisierten Copolymeren eine geringere Polydispersitat besitzen als bei gleichem Monomerumsatz entnommene Produkte, die ohne DCP synthetisiert worden sind. Dies konnte mit Hilfe von Kettenverlangerungsexperimenten verifiziert werden, bei denen Styrol an unterschiedlich hergestellte Poly(Styrol-co-N-Vinylcarbazol)-Copolymere polymerisiert worden ist.

Acetic Anhydride – Accelerating Agent for Nitroxide-Controlled Free-Radical Copolymerization of Styrene and Acrylonitrile

The influence of acetic anhydride on the controlled radical copolymerization of styrene and acrylonitrile was studied. If benzoyl peroxide/2,2,6,6-tetramethylpiperidine-N-oxyl (BPO/TEMPO) are used the addition of acetic anhydride up to a molar ratio of additive/TEMPO = 2 : 1 results in a twelve times higher rate of polymerization. The molecular weights are increased and the molecular weight distributions are slightly broadened. A further increase of this molar ratio did not accelerate the polymerization rate further, but broadened the molecular weight distributions. Contrarily, the influence of acetic anhydride on the PS-TEMPO controlled radical copolymerization is less pronounced. By means of UV-VIS spectroscopy the reduction of the concentration of free TEMPO through reaction with acetic anhydride at 125°C in ethylbenzene was demonstrated.

Combination of Mechanochemical Degradation of Polymers with Controlled Free‐Radical Polymerization

The result of ultrasound on polymer solutions is the breakage of macromolecular C-C bonds due to cavitation. The fact that termination reactions of mechanoradicals as disproportionation and combination are suppressed in the presence of radical scavengers makes the following method possible. Thus the use of nitroxides acting as chain-terminating agents allows the creation of macroinitiators which can be used in controlled free-radical polymerization. In this work, we investigate the mechanochemical degration of poly(methyl methacrylate) (PMMA) in the presence of OH-TEMPO and the application of the irradiated polymers as macroinitiators in a controlled radical polymerization. The content of OH-TEMPO terminated chains in the degraded product is determined by a computer-aided procedure on the basis of molecular weight distributions.

Synthesis of polystyrene‐block‐poly(styrene‐co‐acrylonitrile) block copolymers and thermoanalytical studies of nitroxide‐terminated poly(styrene‐co‐acrylonitrile) copolymers

The synthesis of polystyrene-block-poly(styrene-co-acrylonitrile) block copolymers by conversion of polystyrene terminated with 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) are described. We have found a considerable acceleration in polymerization rate and increasing of molecular weights due to the donor acceptor interaction of styrene and acrylonitrile. Analysis of the block copolymer composition by means of 1H NMR spectroscopy allowed to determine the copolymerization parameters. The comparison with the parameters of the free radical copolymerization shows that there is no influence of TEMPO on the composition. The polydispersity index of the copolymer subchain in the block copolymers decreases rapidly with increasing time of polymerization, depending on the polymerization rate of the copolymerization. The thermal stability of nitroxide-terminated poly(styrene-co-acrylonitrile) copolymers was investigated by means of thermogravimetry and pyrolysis gas chromatography coupled with mass spectrometry. At temperatures above 200°C the splitting of the nitroxyl polymer bond takes place, indicating a weak degradation of the polymer. However, there is no effect of the terminator on the decomposition pattern of the copolymer at 500°C.

TEMPO-controlled radical suspension polymerization of poly(styrene)-block-poly(styrene-co-acrylonitrile) and poly(styrene)-block-poly(styrene-co-butyl methacrylate)

Suspension polymerization expands the study of controlled radical polymerization to high conversions and is known as a method to synthesize polymers with high molecular weights. The radical block copolymerizations of styrene (S) and acrylonitrile (AN) or butyl methacrylate (BUMA) controlled by 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) was performed in an oil/water pressure reactor system at a temperature of 125°C. TEMPO-terminated styrene homopolymer was employed as macroinitiator. The systems were examined by varying the composition of the monomer mixture at a constant reaction time, as well as by varying the reaction time for a characteristic monomer composition to get all of the possible conversion range. The solubility effects of acrylonitrile in the suspension medium were considered. Furthermore, the yield of the reaction was improved through initiator addition by taking control of the reaction. The polymerizations could proceed under control up to a conversion of 80–90%. By using the copolymerization equations, the solubility of pure acrylonitrile in the suspension medium could be calculated and was found to be 8 wt.-%.

N-oxyl mediated free radical donor-acceptor co- and terpolymerization of styrene, cyclic maleimide monomers and n-butyl methacrylate

The N-oxyl mediated donor-acceptor copolymerization of styrene as a donor and the maleimides N-phenylmaleimide (NPI), N-benzylmaleimide (BMI), and N-cyclohexylmaleimide (CMI) as acceptor monomers is studied, using 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) as reversible capping agent and benzoyl peroxide (BPO) as initiator. These copolymerizations are characterized by high polymerization rates and very short induction periods. In bulk, a control of the molecular weights and the polydispersity is only possible for the weakest acceptor CMI. For the stronger acceptors BMI and NPI the polymerizations have to be performed in solution (anisol) to realize a linear increase of the molecular weight with conversion and polydispersities below Mw/Mn 1.55. The polymerization rates (NPI > BMI > CMI), the induction periods (CMI > BMI > NPI) and the total number of polymer chains (NPI > BMI > CMI) of the TEMPO mediated free radical donor-acceptor copolymerizations are a function of the acceptor strength of the maleimide monomers. As the main reason for the observed high reactivity, an “acceptor depending self-initiation”, proceeding via a Diels-Alder reaction mechanism, is discussed. Additionally, the concept of the “acceptor depending self-initiation” is used for the rate of acceleration of S/BuMA copolymerizations leading to poly(S-co-BuMA-co-maleimide) terpolymers with well-controlled molecular weights and narrow molecular weight distributions.

Rate acceleration of N-oxyl-mediated free radical random copolymerization of styrene and n-butyl methacrylate

The N-oxyl-mediated free radical bulk copolymerization of styrene and n-butyl methacrylate was studied at 130°C using dibenzoyl peroxide (BPO) and 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) as well as polystyrene-2,2,6,6-tetramethylpiperidine-N-oxyl adducts (PS-TEMPO). The main focus was to describe a rate acceleration by additionally added radical initiator dicumyl peroxide (DCP), since the polymerization rates become rather slow with increasing butyl methacrylate content. The effects of DCP on the polymerization rates and the molar masses were studied and compared with the accelerating effects of camphersulfonic acid (CSA) and acetic anhydride (Ac 2 O). It was demonstrated that for an equimolar composition of the monomers, DCP allows a significant rate acceleration up to a factor of 9, depending on the polymerization conditions, without causing any appreciable increase in the polydispersity of the copolymers. In comparison with not accelerated copolymerizations, even the relative portion of dead chains can be reduced. The accelerating effects of CSA (factor 4) and Ac 2 O (factor 2) are distinctly smaller.