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Because of the unequal reactivity of the two amine groups of L-lysine hydrochloride, thermal polymerization of this asymmetrical AB2 monomer results in hyperbranched polymers, which contain ?2.5 times more N?-linked linear compared to N?-linked linear structural units. This report discusses the feasibility of three approaches to control polymer architecture during the thermal hyperbranched polymerization of L-lysine hydrochloride. The reactivity of the more reactive ?-NH 2 group was modulated by introducing temporary protective groups that preferentially block the ?-NH2 position. This was achieved by (i) addition of o-vanillin to the polymerization, (ii) copolymerization of N?-benzylidene-L-lysine, and (iii) copolymerization of ?-amino-?-caprolactam. Analysis of the degree of branching (DB) and the average number of branches (ANB) of the obtained polymers did not provide evidence for any significant structural changes. Analysis of the distribution of structural units, in contrast, revealed major structural changes and indicated a rearrangement from N?-linked linear structural units to N ?-linked linear structural units upon addition of o-vanillin and N?-benzylidene-L-lysine. Unlike o-vanillin and N ?-benzylidene-L-lysine, ?-amino-?-caprolactam was found to be rather ineffective in modulating polymer architecture. © 2007 American Chemical Society.

This contribution reports on the synthesis of hyperbranched polylysines via thermal polymerization of L-lysine hydrochloride. Polymerization of L-lysine hydrochloride in the presence of one equivalent KOH at 150°C resulted in polymers with a number-average molecular weight of 4600 g/mol and a polydispersity of 2.6 after 48 h. The rate of polymerization could be significantly enhanced and the polymer molecular weight improved by carrying out the polymerization with 3 mol % of an amidation catalyst. Among the different catalysts that were investigated Zr(OnBu)4 was found to be the most effective. Unequivocal support for the branched architecture of the polymers was obtained from 1H NMR spectroscopy, which allowed the identification and quantification of the four different structural units that constitute the polymer, viz. Nα and Ne linked linear units, dendritic units and terminal units. The structure of the polymers was found to be relatively independent of the reaction conditions. The degree of branching and the average number of branches varied between 0.35-0.45 and 0.15-0.25, respectively. © 2007 Wiley Periodicals, Inc.

A new class of water-soluble, amphiphilic star block copolymers with a large number of arms was prepared by sequential atom transfer radical polymerization (ATRP) of n-butyl methacrylate (BMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA). As the macroinitiator for the ATRP, a 2-bromoisobutyric acid functionalized fourth-generation hyperbranched polyester (Boltorn H40) was used, which allowed the preparation of star polymers that contained on average 20 diblock copolymer arms. The synthetic concept was validated by AFM experiments, which allowed direct visualization of single molecules of the multiarm star block copolymers. DSC and SAXS experiments on bulk samples suggested a microphase-separated structure, in agreement with the core−shell architecture of the polymers. SAXS experiments on aqueous solutions indicated that the star block copolymers can be regarded as unimolecular micelles composed of a PBMA core and a diffuse PPEGMA corona. The ability of the polymers to encapsulate and releas...

This paper reports the synthesis and characterization of a novel class of dendronized polymers which are based on styrene functionalized dendritic L-lysine macromonomers. Free radical polymerization of zeroth and first generation macromonomers afforded dendronized polymers with number-average degrees of polymerization (DP) between 50 and 240, corresponding to number-average molecular weights between 50 × 103 and 240 × 103 g/mol. In contrast, only short oligomers (DP = 5-15) were obtained on polymerization of the second generation dendritic L-lysine macromonomers. Molecularly resolved ATM images allowed first insight into the structure of these peptide dendronized polymers. Depending on the degree of polymerization, either rods, short rods, or globular structures were found. © 2005 American Chemical Society.

This report presents the results from IUPAC Working Party IV.2.2 of the global trial within the framework of IUPAC Commission IV.2, “Characterization of Commercial Polymers”. The results were compared on the basis of molecular weight obtained by size exclusion chromatography (SEC)using different techniques practiced in participating laboratories, the majority of which were materials suppliers. The practical methodologies used different solvents for the polymers, in particular, benzyl alcohol, 1,1,1,3,3,3 hexafluoropropan-2-ol and tetrahydrofuran; the latter solvent was used after chemical modification of the polyamides, in general with trifluoroacetic anhydride. Eight laboratories participated in the trial. The repeatability for molecular weight in each laboratory was good, whatever technique was used, the relative standard deviation averaged over all laboratories being around 3%. The deviations in distribution of molecular weights with different experimental methodologies were broader, but were reasonably good despite the diversity of methods. The differences in the distribution correspond to a confidence interval of about 30% in molecular weight.

Gel permeation chromatography (GPC) viscometry with universal calibration (UC) has been assessed as a means of characterizing the absolute molecular weight distribution of hydroxyl terminated hyperbranched aliphatic polyesters (HBPs) of aliphatic compounds. To generate a valid UC curve, it is necessary to work under conditions of strict size‐exclusion separation. This requirement was met when both dimethyl formamide (DMF) and tetrahydrofuran (THF) were used as eluants. All the HBPs were highly soluble in DMF at room temperature, but its use may necessitate special columns. Dissolution of the HBPs in THF, on the other hand, which is compatible with a variety of stationary phases, was relatively difficult, requiring prolonged heating above 50°C. As an alternative to direct characterization, the HBP hydroxyl end‐groups were end‐capped with trimethylsilane prior to the GPC measurements. Near quantitative replacement of the hydroxyl groups was achieved and the modified HBPs remained stable for several...

A variety of cobalt(II/III) and iron(II/III) complexes were prepared containing the ligands 4-fluoro-2,2′-bipyridine (fbpy), 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen) and water, and anions hexafluorophosphate, chloride, perchlorate and sulfate. These complexes were characterized by a combination of 19 F , 1 H , 13 C , 15 N NMR and elemental analysis, and used for a mechanistic study of electron transfer in the CoII(bpy)32+–CoIII(fbpy)33+ system.

The kinetics of ultrasonic degradation has been investigated for dilute polystyrene solutions in decalin. The increase in the scission rate constant ( K ) with initial molecular chain length was systematically investigated over 2 decades of molecular weight by using narrow polymer fractions with M = 50 × 10 3 − 8.5 × 10 6 Da. An empirical relation of the form K ∼ ( M - M lim ) 1.9 , with M lim = 30 000, was found for 50 × 10 3 ≤ M ≤ 2.2 × 10 6 . The rate constant becomes independent from M at higher molecular weight. This complex behaviour is rationalized by establishing a parallelism between transient elongational flow and ultrasonic degradation.

Routine MW analysis of polymers is generally performed in solution by Gel Permeation Chromatography. The technique, however, presents increasing difficulties in the ultra-high and ultra-low MW range. Moreover, GPC is not applicable to most fluorinated and»high performance«thermoplastics which require dissolution temperatures close to the polymer melting point. In response to these challenges, polymer scientists have developed new methods of MWD determination. Although these methods have not yet reached the versatility of GPC, they present potential for further development and are interesting as complementary techniques to GPC. For sparingly soluble polymers, melt viscoelastic properties have been used to obtain information on polymer MWD. Flow birefringence, based on the orientation and extension of long flexible molecular chains in a shear field, has been applied to the characterization of ultra-high MW polymers. For low MW polymers and oligomers, the MALDI mass spectrometry technique, with its unique capabilities to resolve individual oligomers within a MWD, proves to be an interesting alternative to conventional GPC characterization.

Birefringence measurements are reported in transient and stagnation point elongational flows created by forcing a polymer solution through a narrow contraction. A strongly molecular weight dependent birefringence was observed and attributed to segmental orientation caused by individual chain extension. The birefringence was also found to depend on the position within the flow field, the solvent type, thermodynamic quality and viscosity, the polymer composition, and the type of flow field. Two distinct concentration regimes, corresponding to the dilute and semi-dilute polymer regions, were observed. The transition was marked by a dynamic critical overlap concentration which was lower than that observed under quiescent conditions. In the dilute regime, a laminar flow profile was observed and intermolecular interactions were negligible. The modest local orientation and overall chain deformation however indicate that intramolecular hindrances to extension exist. Transient elongational flows were characterized by a ‘tube-like’ birefringence which had a diameter equal to that of the orifice and extended several orifice diameters into the solution. In contrast, stagnation point elongation flows possessed both the tube-like birefringence and the classical highly localized birefringent ‘lines’. The transition between the tubular and linear birefringent zones is a function of the viscous coupling between the polymer coil and the deforming fluid element. This coupling is itself a complex function of temperature, molecular parameters and the flow conditions. Measurements in stagnation point elongational flows have also indicated that the maximum degree of local orientation does not occur at the stagnation point, and data indicate that chains continue to accelerate until a region very close to the orifice. In transient elongational flow the polymer dynamics follow a coil-to-deformed coil transition with birefringence saturation not observed.

Models for the dynamics of dilute polymer solutions under elongational flow have been examined in the light of new experimental data. In particular birefringence measurements on the polystyrene-decalin and poly(ethylene oxide)—water systems in both stagnation point and transient elongational flows are the most revealing as to the nature of polymer chain uncoiling and fracture. Both the deformation and fracture behaviour have been observed to depend on the type of elongational flow (transient, stagnation point) and the chemistry of the polymer chain. The data on the critical strain rate for chain fracture along with observations of precise midchain scission (as estimated from size exclusion chromatography), a limited overall chain deformation, and the role of accumulated energy (strain) in mechanochemical degradation, tend to imply that a hybrid model of polymer dynamics in elongational flows may be realistic. One such model, a ‘multi-stranded yo-yo with surface energy dissipation and solid like stress response’ has been developed as a general hypothesis to account for the deformation, stretching and fracture behaviour of polymer chains in both transient and stagnation point elongation flows. This modified yo-yo is found to be an energetically favourable alternative to an affinely deforming chain over certain degrees of elongation. It has been shown to qualitatively explain the apparent difference in behaviour of polymer chains in both stagnation point and transient elongational flows. The viscous coupling between the solvent and the polymer chain is also used as a qualitative parameter to account for the deformation and fracture behaviour of polymer chains under transient and stagnation point elongation flows. Based on these analyses, a set of recommendations as to future experiments are presented.

Polymer chains undergo degradation through a variety of mechanisms, including shear action, chemical attack, thermal activation, electromagnetic (γ, UV) and ultrasonic irradiation. In many macromolecular systems, the breaking of bonds does not occur randomly but depends upon the position within the chain and the total chain length. Before the advent of GPC, the distribution of bond scission probability along the chain could only be inferred from the evolution of some molecular weight (MW) average as a function of extent of degradation. Over the past few years, gel permeation chromatography has evolved as a reliable analytical tool to measure not only the different MW averages, but also the molecular weight distribution (MWD) of the degraded samples. Obviously, the possibility of determining change, over the whole MWD, gives more insight into the mechanisms involved than do MW averages. The information content of the GPC traces could, however, be usefully exploited, but only after extensive data treatment. The general procedure starts with the elaboration of a degradation scheme and the integration of the corresponding set of differential kinetics equations. The rate constants were then used as fitting parameters until good agreement was obtained between the calculated MWD and the experimental curves. Case studies taken from the field of mechanochemical degradation are presented to illustrate the possibilities and limitations of the technique. In one instance, during the freezing-and-thawing degradation of polystyrene, it is found that good concordance between calculations and experiments could only be obtained if ternary bond scission is included in the degradation scheme. The formalism developed for single scission kinetics is extended to the case of a concerted multiple fracture process.

Cationic complexes PF4(N-N)+, where N-N = 2,2′-bipyridine (bpy), 4-fluoro-2,2′-bipyridine (fbpy), and 1,10-phenanthroline (phen), were prepared in modest yield and identified by their A2B2 and A2BC fluorine NMR spectra, which are similar to those of the isoelectronic SiF4(N-N) adducts. The ligands bpy, fbpy, and phen may be useful for monitoring reactions of HF and H2O, or other fluorinated compounds, with glass apparatus because any liberated SiF4 is then trapped as SiF4(N-N), which can be readily detected by NMR spectroscopy.

Rheology and birefringence measurements showed conclusively that quasi-full chain extension could be achieved in stagnant extensional flow above some critical strain rate of the order of the reciprocal of the chain characteristic time. Degradation in this highly expanded state could be rationalized with a model of chain scission through frictional loading in which the fracture event is controlled by the force balance between polymer solvent viscous friction and the breaking strength of the bond under consideration. In contrast to the process of chain extension, which could be observed only under rather restrictive conditions of flow, polymer degradation is ubiquitous and is readily detected in situations as diverse as convergent flow, ultrasonic irradiation, solution crystallization, turbulence and flow across porous media. Although some extensional flow component was always present in these types of degradation, the residence time in the high strain-rate region is so short that only a limited degree of coil expansion could be expected. Degradation in transient elongational flow indicated a behaviour definitely distinct from the degradation of fully extended chains under stagnant conditions. In particular, the following experimental facts cannot be explained by the model of fully extended frictionally loaded chains: dotef ≊ M−1 (where dote is the critical strain rate for chain fracture); dotef ≊ ηs−0.25 (where ηs, is the solvent viscosity); degradation yield is a fairly unique function of average fluid velocity or entrance pressure drop and is almost independent of differences in elongational strain rate introduced by nozzle geometry. These facts lead us to the conclusion that the fundamental parameter which governs the rate of degradation of partly deformed coils is not the magnitude of the local stress, but rather the deformation energy accumulated by the coil up to the moment of rupture.

The crack-heating behaviour of γ-irradiated poly (styrene-co-acrylonitrile) (SAN) was investigated as a function of radiation dose over the range 0–800 Mrad. The predominant response of SAN to γ-irradiation is crosslinking as evidenced by the radiation yields Gx(crosslink) = 0.077 and Gs (main chain scission) = 0.055. The presence of crosslinks considerably increased the time required for complete reheal of compact tension (CT) specimens. Fracture mechanics testing on non-irradiated samples showed that the energy release rate, G1, increased linearly with the square root of the rehealing time, tp, until the original value of the energy release rate, G10, of the virgin material was restored. In contrast, for crosslinked samples, the initial increase in energy release rate, Gl, was followed by a plateau region at a value of Glp which was less than Glo. A further recovery was observed over a long time scale, some minutes to one hour after the first plateau (Glp) was reached. The ratio of the plateau value and the original fracture toughness ( G lp G lo ) was found to vary with the square root of the average molecular weight between cross-links, M x . This phenomenon may be correctly explained in terms of the average length of free chains formed during the fracture process. A simple double-layer diffusion model describes well the existence of the two recovery regions, it is not yet sufficient, however, since the form of the transition from the first to the second recovery mechanism is not adequately represented by this model.

A recently developed theory permits the determination of peak broadening in size-exclusion chromatography by combining the simultaneous signals obtained from two different detectors, one being sensitive to the mass concentration and the other to the molecular weight of the solute eluted. Based on this methodology, a new experimental technique employing polymer-bound chromophores and a standard UV detector was developed to provide information normally obtained with an absolute detector. The advantages of the method are its simplicity and the utilization of a single cell for measurements, which relieves much of the detection geometry problems inherent to other techniques. Fractions of polystyrene labelled with azobenzene groups and synthesized by radical polymerization were used. The experimental broadening parameters are critically evaluated with respect to the possible sources of error and compared with results obtained from quasi-monodisperse polymer fractions.

Narrow polydispersity polystyrene with a molecular weight of 1.03 x 10 6 has been functionalized with anthracene groups using the metallocene technique. Side reactions such as chain coupling and bond scission can be limited by working in THF at -75 °C. Gel permeation chromatography combined with on-line viscometric, refractive index and fluorescence detection allows us to verify that degradation did not exceed 1 bond scission/ 180000 monomer units. The degree of grafting increases with the reaction time but remains modest (< 5% after 7 h) at low temperature.