Novel catalyst system for the synthesis of poly(alkylene oxide) with controlled molecular weight

An organoaluminum compound with bulky phenoxide groups as a Lewis acid can accelerate much the living polymerization of alkylene oxide initiated with aluminum porphyrin by the coordinative activation of the monomer. This concept can be extented to the polymerization initiated with aluminum Schiff base and tetraazaannulene complexes.     

One-step synthesis of low polydispersity, biotinylated poly(N-isopropylacrylamide) by ATRP

Low polydispersity poly(N-isopropylacrylamide) with a biotin end-group was obtained in one step from a biotinylated initiator for atom transfer radical polymerization and interacted with streptavidin to generate the thermosensitive polymer-protein conjugate.     

Catalyst-transfer polycondensation. mechanism of Ni-catalyzed chain-growth polymerization leading to well-defined poly(3-hexylthiophene)

We studied the mechanism of the chain-growth polymerization of 2-bromo-5-chloromagnesio-3-hexylthiophene (1) with Ni(dppp)Cl2 [dppp = 1,3-bis(diphenylphosphino)propane], in which head-to-tail poly(3-hexylthiophene) (HT-P3HT) with a low polydispersity is obtained and the M(n) is controlled by the feed ratio of the monomer to the Ni catalyst. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra showed that the HT-P3HT uniformly had a hydrogen atom at one end of each molecule and a bromine atom at the other. The reaction of the polymer with aryl Grignard reagent gave HT-P3HT with aryl groups at both ends, which indicates that the H-end was derived from the propagating Ni complex. The degree of polymerization and the absolute molecular weight of the polymer could be evaluated from the 1H NMR spectra of the Ar/Ar-ended HT-P3HT, and it was found that one Ni catalyst molecule forms one polymer chain. Furthermore, by reaction of 1 with 50 mol % Ni(dppp)Cl2, the chain initiator was found to be a bithiophene-Ni complex, formed by a coupling reaction of 1 followed by insertion of the Ni(0) catalyst into the C-Br bond of the dimer. On the basis of these results, we propose that this chain-growth polymerization involves the coupling reaction of 1 with the polymer via the Ni catalyst, which is transferred intramolecularly to the terminal C-Br bond of the elongated molecule. We call this mechanism "catalyst-transfer polycondensation".     

Time of flight secondary ion mass spectrometric determination of molecular weight distributions of low polydispersity poly(dimethyl siloxane) with polyatomic primary ions

This work reports a comparison of oligomer and fragment ion intensities resulting from primary ion bombardment with several primary ion sources (Bi_n⁺, C₆₀⁺, and Cs⁺) at various energies in secondary ion mass spectrometry (SIMS). Although the use of polyatomic primary ions are of great interest due to increased secondary ion efficiency and yield, we demonstrate that monatomic primary ions result in increased oligomer ion yield for polymers prepared as submonolayer films on silver substrates. The enhancement of oligomer secondary ion yield with monatomic ions is evidence that monatomic primary ions have a shallower sampling depth than polyatomic ions, resulting from a collision cascade that is less energetic at the sample surface. The results are also consistent with a lower degree of fragmentation of the resultant secondary ions, which is observed when evaluating the fragmentation data and the spectral data.     

Synthesis of high molecular weight polystyrene and poly(methyl methacrylate) with low polydispersity by [Cp2ZrCl2] catalyzed aqueous polymerization

An early transition metal metallocene compound, Cp₂ZrCl₂, with an anionic surfactant, sodium n‐dodecyl sulfate (SDS) as emulsifier has been found to be an effective catalyst for polymerization of monomers like styrene and methylmethacrylate in aqueous medium. The molecular weights of the polymers have been found to be very high with low molecular weight distribution. The added surfactant has been found to play the dual role of stabilizer of the cation as well as an emulsifying agent for the monomer. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3797–3803, 2005     

Synthesis of poly(N‐isopropylacrylamide) with a low molecular weight and a low polydispersity index by single‐electron transfer living radical polymerization

The single‐electron transfer living radical polymerization (SET‐LRP) method in the presence of chain transfer agent was used to synthesize poly(N‐isopropylacrylamide) [poly(NIPAM)] with a low molecular weight and a low polydispersity index. This was achieved using Cu(I)/2,2′‐bipyridine as the catalyst, 2‐bromopropionyl bromide as the initiator, 2‐mercaptoethanol as the chain transfer agent (TH), and N,N‐dimethylformamide (DMF) as the solvent at 90 °C. The copper nanoparticles with diameters of 16 ± 3 nm were obtained in situ by the disproportionation of Cu(I) to Cu(0) and Cu(II) species in DMF at 22 °C for 24 h. The molecular weights of poly(NIPAM) produced were significantly higher than the theoretical values, and the polydispersities were less than 1.18. The chain transfer constant (C_tr) was found to be 0.051. Although the kinetic analysis of SET‐LRP in the presence of TH corroborated the characteristics of controlled/living polymerization with pseudo‐first‐order kinetic behavior, the polymerization also exhibited a retardation period (k > k_tr). The influence of molecular weight on lower critical solution temperature (LCST) was investigated by refractometry. Our experimental results explicitly elucidate that the LCST values increase slightly with decreasing molecular weight. Reversibility of solubility and collapse in response to temperature well correlated with increased molecular weight of poly(NIPAM). © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Novel synthesis of high molecular weight polymers with extremely low polydispersity by the unique initiation properties of organo-lanthanide complexes

Organolanthanide(III) complexes such as |(C₅Me5₅)₂SmH|₂ and (C₅Me₅)₂SmMe(THF) were found to initiate the living polymerizations of methyl methacrylate (MMA) to give high molecular weight polymers (M _n > 500 × 10³) with extremely low polydispersity (M _w/M_n = 1.04). The syndiotacticity increased up to 95.2% by lowering the temperature to −95°C. The molecular structure of the 1:2 adduct of (C₅Me₅)₂SmH with MMA determined by X-ray method indicates that this intermediate assumes the 8 membered ring conformation where the Sm atom is bound to MMA in an enolate form and the ester of penultimate MMA is coordinated to the metal. Based upon these results, an anionic coordination mechanism has been proposed for the present reaction. Organolanthanide(II) complexes also exhibit high activity and proceed the living polymerizations. Organolanthanide(III) complexes also initiate the living polymerizations of lactones such as ϵ-caprolactone and δ-valerolactone. The stoichiometric reactions indicate that real active species assumes the alkoxylanthanide(III) form.     

Melting behaviour of low molecular weight poly (ethylene-oxide) fractions

Summary Melting temperatureT _m and enthalpy of fusion have been measured, by DSC, for folded chain crystals of low molecular weight poly(ethylene-oxide) fractions ranging from 3000 to 10000. These crystals are formed by molecules folded a small integer number,n, of times and show unusual thermal stability on heating. The rates of chain unfolding during isothermal crystal growth and subsequent heating were measured and a reliable stability criterion could be defined for folded chain polymer crystals.Extending the theoretical treatment ofFlory andVrij to folded chain crystals and usingT _m data, a reasonable estimate was derived for the respective surface free energy contributions of chain ends and chain folds. The results suggest considerable hydrogen bonding between OH end groups, with a bonding energy of 3.1 Kcal/mole, when the crystal surface contains only chain ends. Hydrogen bonding is essentially destroyed by chain folding. Further analysis leads to an estimate of the contour length of cilia, associated with chain ends and to that of chain folds containing, on average, 2.8 and 3.5 monomer units respectively. Chain folds must thus be sharp involving adjacent re-entry. Finally, an analytical expression is derived showing the separate dependence ofT _m on chain length andn, parameters which determine the crystal thicknessL. Critical comparison of this relationship to that commonly used for determining surface free energies from linearT _m vs 1/L plots shows that the latter only applies accurately to chains of infinite length and to crystals of thickness larger than a critical valueL*.

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Résumé La températureT _m et l'enthalpie de fusion ont été mesurées sur des cristaux à chaines repliées de fractions de polyoxyéthyléne, de masse moléculaire variant entre 3000 et 10000. Ces cristaux sont constitués par des molécules repliées un nombren entier de fois et ils ont une stabilité thermique inhabituelle. La vitesse de dépliement des chaines a été mesurée lots de la croissance isotherme des cristaux et du chauffage consécutif et l'on a pu définir un critère de stabilité pour des cristaux à chaines repliées.En étendant la théorie deFlory etVrij aux cristaux à chaines repliées et en utilisant les valeurs deT _m on a pu estimer les contributions respectives des bouts de chaine et des repliements à l'énergie libre superficielle. Les résultats suggèrent un couplage important, par liaison hydrogène entre les groupes terminaux OH lorsque la surface des cristaux est constituée entièrement de bouts de chaine, l'énergie de couplage étant de 3,1 Kcal/mole. Ce couplage disparait pratiquement par le rephement des chaines. L'analyse des résultats permet aussi d'estimer la longueur des bouts de chaines et celle des repliements qui émergent du réseau cristallin: ils sont formés, respectivement de 2,8 et 3,5 unités de monomère, en moyenne. Les repliements sont serrés et relient des positions adjacentes du réseau. On déduit finalement une expression deT _m en fonction de la longueur des chaines et den, paramètres qui déterminent l'épaisseurL du cristal. La comparaison critique de cette relation avec celle généralement utilisée pour déterminer l'énergie libre superficielle, impliquant une variation linéaire deT _m avec 1/L, montre que cette dernière ne s'applique en toute rigueur qu'aux chaines de longueur infinie et aux cristaux d'épaisseur supérieure à une valeur critiqueL*.

     

Temperature-sensitive swelling of poly(N-isopropylacrylamide) brushes with low molecular weight and grafting density

Temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAm) brushes with different molecular weights M(n) and grafting densities σ were prepared by the "grafting-to" method. Changes in their physicochemical properties according to temperature were investigated with the help of in situ spectroscopic ellipsometry and in situ attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Brush criteria indicate a transition between a brush conformation below the lower critical solution temperature (LCST) and an intermediate to mushroom conformation above the LCST. By in situ ellipsometry distinct changes in the brush layer parameters (wet thickness, refractive index, buffer content) were observed. A broadening of the temperature region with maximum deswelling occurred with decreasing grafting density. The brush layer properties were independent of the grafting density below the LCST, but showed a virtually monotonic behavior above the LCST. The midtemperature ?(half) of the deswelling process increased with increasing grafting density. Thus grafting density-dependent design parameters for such functional films were presented. For the first time, ATR-FTIR spectroscopy was used to monitor segment density and hydrogen bonding changes of these very thin PNIPAAm brushes as a function of temperature based on significant variations of the methyl stretching, Amide I, as well as Amide II bands with respect to intensity and wavenumber position. No dependence on M(n) and σ in the wavenumber shift of these bands above the LCST was found. The temperature profile of these band intensities and thus segment density was found to be rather step-like, exceeding temperatures around the LCST, while the respective profile of their wavenumber positions suggested continuous structural and hydration processes. Remaining buffer amounts and residual intermolecular segment/water interaction in the collapsed brushes above the LCST could be confirmed by both in situ methods.     

An efficient solid‐state polycondensation method for synthesizing stereocomplexed poly(lactic acid)s with high molecular weight

Simultaneous solid‐state polycondensation (SSP) of the powdery prepolymers of poly(L ‐lactic acid) (PLLA) and poly(D ‐lactic acid) (PDLA) can produce entire stereocomplexed poly(lactic acid)s (sc‐PLA) with high molecular weight and can be an alternative synthetic route to sc‐PLA. Ordinary melt polycondensations of L ‐ and D ‐lactic acids gave the PLLA and PDLA prepolymers having medium molecular weight which were pulverized for blending in 1:1 ratio. The resultant powder blends were then subjected to SSP at 130–160 °C for 30 h under a reduced pressure of 0.5 Torr. Some of the products thus obtained attained a molecular weight (M_w) as high as 200 kDa, consisting of stereoblock copolymer of PLLA and PDLA. A small amount of the stereocomplex should be formed in the boundaries of the partially melted PLLA and PDLA where the hetero‐chain connection is induced to generate the blocky components. The resultant SSP products showed predominant stereocomplexation after their melt‐processing in the presence of the stereoblock components in spite of containing a small amount of racemic sequences in the homo‐chiral PLLA and PDLA chains. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3714–3722, 2008     

Control of molecular weight distribution in polycondensation polymers 2. Poly(ether ketone) synthesis

Synthesis of aromatic poly(ether ketone) (3) with a narrow molecular weight distribution (M_w/M_n) was investigated via polycondensation. M_ns of 3 could be controlled varying the feed ratio of monomer (1) and initiator (2) maintaining relatively narrow M_w/M_ns (<1.3). The kinetics of polycondensation obeyed a first-order relationship between polycondensation time and -(1/[2]₀) ln([1]/[1]₀), and the rate of polycondensation was estimated as 2.57 mol⁻¹ L h⁻¹. MALDI-TOF mass analysis of 3 indicated that polycondensation should proceed via chain growth manner to give 3 having an initiator unit in each chain end.     

Synthesis of high molecular weight poly(ether ketone)s by polycondensation of activated bis(aryl chloride)s with bisphenolates

The ability to achieve high molecular weight poly(ether ketone)s from the polycondensation of bis(aryl chloride)s with bis(phenolate)s has been consistently demonstrated. The polymerizations presented here help to delineate for specific bis(aryl chloride)/bisphenolate pairs the reaction conditions required to obtain high molecular weight polymers. Polycondensation of 1,3-bis(4-chlorobenzoyl)-5-tert-butylbenzene ( 6 ) and 2,2′-bis(4-chlorobenzoyl)-biphenyl ( 15 ) with various bisphenolates as well as of 2,2′-bis(4-hydroxyphenoxy)biphenyl ( 33 ) with 4,4′-dichlorobenzophenone ( 41 ) and 1,3-bis(4-chlorobenzoyl)benzene ( 43 ) were used as representative model systems to select reaction conditions that led to high molecular weight polymers. © 1995 John Wiley & Sons, Inc.     

Layer-by-layer polyelectrolyte coating of low molecular weight poly(lactic acid) nanoparticles

Low molecular weight (M(w)) poly(L-lactic acid) (PLA) nanoparticles were coated with polyelectrolytes (PEs) by layer-by-layer (LbL) technique using a filtration approach. Poly(allylamine hydrochloride) and poly(sodium 4-styrenesulfonate) were applied as PEs in coating. LbL coating is aimed to use in producing (nano)particulate drug delivery systems with improved biocompatibility and sustained or targeted release of drug substances. Nanoparticles of rapidly biodegradable polymers, like the low M(w) PLA, open up a possibility to control the release of the encapsulated substance by the coating, but set challenges to the coating process due to increased aggregation tendency and degradation rate of the polymer. When the core PLA nanoparticles were prepared by nanoprecipitation, surface properties of the nanoparticles were affected by solvent selection. Successful LbL coating of the PLA nanoparticles was obtained only with chloroform, but not with dichloromethane as the solvent during nanoprecipitation. Reason for this was found to be the more charged surface of the nanoparticles prepared with chloroform compared to the nanoparticles prepared with dichloromethane.     

N-substituted poly(p-phenylene terephthalamide)

N-substituted poly(p-phenylene terephthalamide)s (PPTA), such as N-alkylated, N-aralkylated, and N-carboxymethylated poly(p-phenylene terephthalamide), were synthesized from PPTA and the corresponding halides by the polymer reaction via the metalation reaction in a solution of sodium methylsulfinylcarbanion in dimethyl sulfoxide at low temperature. The introduction of various substitutional groups into the amide groups of PPTA increased their solubilities, but decreased their thermal stabilities compared with PPTA. The effects of various substitutional groups on the thermal properties and the solubilities are discussed. Liquid crystal formation was noticed for PPTA substituted with bulky groups such as 9-anthrylmethyl group.     

Chemical structures assigned for the low molecular weight fractions from degradation of poly(styrene sulfides)

A non-destructive analysis is performed of thermally unstable sulfur-styrene reaction products, combining preparative size exclusion chromatography (P-SEC), ultraviolet-visible (UV-vis) and nuclear magnetic resonance (NMR). The crystallizing compounds are identified as be 2,4-diphenylthiophanes with short sulfur bridge (x ∼ 2) contrary to earlier suggestions which were based on destructive analysis. A new cyclic structure (styrene polysulfide x = 1 up to 8) was assigned to the amorphous species containing a single styrene repeating unit. Comparison with the amorphous fractions suggests that the rigid ring of styrene repeating units in adjacent sequences is the characteristic feature for the crystallisability. The melting and crystallization behaviour of this crystalline component was observed, by optical microscopy (OM) and differential scanning calorimetry (DSC), to be step-wise as well as broad, due to the variation in the length of the sulfur bridge.