Low Number-average Molecular Weight Research Articles

Degradation Behavior of Porous Copolyester Microparticles in the Light of Dynamic Changes in Their Morphology

AbstractThe pore structures of microparticulate drug carriers are important diffusion pathways, which are not a static property but rather may be changing in the case of degradable matrix polymers such as poly[(rac‐lactide)‐co‐glycolide] (PLGA). In this study, the mutual impacts of dynamic changes in microparticle porosity and polymer degradation were analyzed for PLGA with different molecular weights and end groups as well as PLGA‐based triblock copolymers. In selected cases, particularly for PLGA with hydrophilic end groups and low initial number average molecular weight of 5 kDa, pore opening/pore closing phenomena were detected during incubation in phosphate buffer at 37 °C. Initially, pore closing was induced by water‐induced plasticization and the reduction of interfacial tension. The pattern of molecular weight decrease and mass loss suggested that pore closing did not result in undesired autocatalytic acceleration of degradation or delayed mass loss due to trapped acidic degradation products.

Hydrolytic degradation and thermal properties of linear 1-arm and 2-arm poly( dl-lactic acid)s: Effects of coinitiator-induced molecular structural difference

“Linear” 1-arm and 2-arm poly( dl-lactide) [i.e., poly( dl-lactic acid), or PDLLA] polymers with relatively low number-average molecular weights ( M n in the range 0.2–6 × 10 4 g mol −1) were synthesized using ring-opening polymerization of dl-lactide initiated with tin(II) 2-ethylhexanoate (i.e., stannous octoate) and coinitiators of dl-lactic acid and ethylene glycol (these PDLLA polymers are hereafter abbreviated as 1-DL and 2-DL, respectively). Their glass-transition properties were monitored by differential scanning calorimetry, and their hydrolytic degradation was investigated using gravimetry and gel permeation chromatography. The results of the present study indicate that the coinitiator-induced molecular structural difference of the terminal groups, the chain directional change, the incorporated coinitiator moiety as an impurity in the middle of the molecule, and the molecular weight each affect both the hydrolytic degradation behavior and rate, and the glass-transition properties of the “linear” 1-DLs and 2-DLs. The glass-transition temperature ( T g) values were higher for the 2-DLs than for the 1-DLs, indicating low chain mobility and a strong inter-chain interaction of 2-arm PDLLA. However, the coinitiator-induced molecular structural difference did not produce a difference in the excess free volume of the end groups between the 1-DLs and 2-DLs, despite the difference produced in the terminal groups. On the other hand, although the hydrolytic degradation of the 1-DLs and 2-DLs proceeds via bulk erosion, significant surface erosion also occurs in the 2-DLs. This should have caused a larger weight loss and lower decrease rate of M n of the 2-DLs compared to those of the 1-DLs. Moreover, the results of the present study indicate that in 2-arm PDLLA selective chain cleavage at the terminal ester groups or second ester groups from the chain terminals, which are induced by two terminal hydroxyl groups, is the significant hydrolytic degradation route. However, the random cleavage of ester groups, irrespective of their position, is the main hydrolytic degradation route.

New poly (urethane-methacrylate)s obtained by adjusting the structure of the polyols moieties: synthesis, transparent, thermal and mechanical properties

A series of novel cross-linked poly (urethane-methacrylate)s (PUAs) were synthesized in a three-step process: Four kinds of low number average molecular weight aromatic polyesters (PEs) with terminal hydroxyl groups were obtained by phthalic anhydride (PA) reacted with 1,2-propanediol (PDO), 1,4-butanediol (BDO), 1,6-hexanediol (HDO), and diethylene glycol (DEG), respectively. Then, a series of urethane-methacrylate macromonomers(UMAs), which have double bonds at the end of the chain, were prepared from isophorone diisocyanate (IPDI), the PEs obtained in first step, poly(ethylene glycol) 600 (PEG600) and s-hydroxyethyl methacrylate(HEMA). Dibutytin dilaurate (DBTL) was used as catalyst. Finally, a series of thermosetting PUAs were synthesized by the polymerization of novel functional UMAs using 2,2'-azobis(isobutyronitrile) (AIBN) as catalyst. The compositions of the PEs and UMAs were determined by 1H-NMR and fourier transform infrared (FT-IR). The properties of PUAs were measured by dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), wide-angle X-ray diffraction (WAXD), water uptake and optical properties testing, and mechanical performance measurements. The results revealed that the mechanical and degradation properties of PUAs were influenced by the kinds of PE or polyether polyols in the PUAs remarkably. The obtained PUAs present the good thermal and mechanical properties, low water uptake (below 1%) and high transparence (above 90%). The diversity of properties suggests that these PUAs can be used in a wide range of excellent optical polymer applications.

Synthesis and characterization of a novel versatile poly(amic acid) derived from ethylenediaminetetraacetic dianhydride

The polycondensation reaction between ethylenediaminetetraacetic dianhydride and 1,4-diaminobutane in various aprotic polar solvents is being exploited to create a novel linear aliphatic polymer poly(amic acid) (PAA). PAA samples were characterized by Fourier transform infrared, Proton nuclear magnetic resonance and Carbon nuclear magnetic resonance spectroscopy resulting in the identification of characteristic absorption bands and thereby verifying successful synthesis. Gel permeation chromatography confirmed narrow molecular weight distributions with polydispersity indices ranging from 1.2 to 2.2 and reported low number average molecular weights ranging from 4000 to 6000 g mol −1. X-ray photoelectron spectroscopy and energy dispersive X-ray analysis showed the presence of nitrogen on the surface and also found nitrogen to be homogenously distributed throughout the bulk of the PAA samples.

Influence of polydispersity of poly(lactic acid) on particle formation by rapid expansion of supercritical CO 2 solutions

Poly(lactic acid) (PLA) particles were generated by rapid expansion of supercritical PLA/CO 2 solutions (RESS). Two different PLA samples, one with high (PDI = 2.4) and the other one with low (PDI = 1.4) polydispersity but similar number average molecular weight, were compared. After micronization, the polymers were analysed by rapid-scan infrared spectroscopy, scanning electron microscopy, size-exclusion chromatography, differential scanning calorimetry, and NMR spectroscopy. Our investigation reveals that the polydispersity of the polymers strongly affects the size but not the shape of the particles. We found larger particles (∼730 nm) for the PLA with high polydispersity than for the PLA with low polydispersity (∼270 nm). In both cases, spherical particles were formed. Moreover, our results clearly show that PLA with high polydispersity is less suitable for RESS processing because the low-molecular weight chains are depleted over time and process conditions are thus not constant.

Synthesis and characterization of excellent transparency poly(urethane-methacrylate)

A series of novel crosslinked poly(urethane–methacrylate) (PUA) was synthesized. PUA was polymerized in a three-step process: The low number average molecular weight unsaturated polyester (UPE) containing hydroxyl groups was synthesized by 1,2-propanediol (PG), 2-butyl-2-ethyl-1,3-propanediol (BEPD) and maleic anhydride (MA). A series of prepolymers, which had double bonds at the end of the chain, was prepared from isophorone diisocyanate (IPDI), UPE, and β-hydroxyethyl methacrylate (HEMA) in the presence of dibutytin dilaurate (DBTL) as catalyst. The novel functional prepolymer was initiated by 2,2-azobis-isobutyronitrile(AIBN) to form thermosetting materials. The structure of the thermosetting materials was characterized by Fourier Transform Infrared (FTIR). Mechanical, thermal, and optical properties were tested. The results showed that the advanced PUA had good thermal and mechanical properties and high transparence. The throughput of PUA was above 90%, thus the PUA was possible to be used as excellent optical material. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Synthesis and characterization of fluorene end-labeled polymers prepared by nitroxide-mediated polymerization

The fluorene-based alkoxyamine 9-(2,2,6,6-tetramethyl-4-hydroxypiperidinyloxy)fluorene was prepared and investigated as a unimolecular initiator in the nitroxide-mediated polymerization (NMP) of styrene. Reactions performed in either bulk or ethereal solvents at 125°C generated polymers possessing low polydispersity indices (PDIs) and number average molecular weight (Mn) values close to those anticipated based on monomer-to-initiator ratios. A linear relationship between monomer conversion and Mn values was observed by analysis of reaction aliquots with 1H NMR and gel permeation chromatography (GPC), while PDI values remained low throughout. Analysis of the polymers by UV–vis spectroscopy indicated that approximately 80% of the polymer chains were labeled with the fluorene chromophore, while fluorescence spectroscopy was used to verify that the fluorene was polymer-bound.

Crystallization behavior of linear 1-arm and 2-arm poly(l-lactide)s: Effects of coinitiators

Linear 1-arm and 2-arm poly(l-lactide) [i.e., poly(l-lactic acid) (PLLA)] polymers having relatively low number-average molecular weights (Mn) (≤5×104gmol−1) were synthesized by ring-opening polymerization of l-lactide initiated with tin(II) 2-ethylhexanoate (i.e., stannous octoate) and coinitiators of l-lactic acid, 1-dodecanol (i.e., lauryl alcohol), and ethylene glycol (these PLLA polymers are abbreviated as LA, DN, and EG, respectively). For Mn below 1.5×104gmol−1, non-isothermal crystallization during heating and isothermal spherulite growth were disturbed in linear 2-arm PLLA (EG) compared to those in linear 1-arm PLLA (LA and DN). This finding indicates that the chain directional change, the incorporation of the coinitiator moiety as an impurity in the middle of the molecule, and their mixed effect disturbed the crystallization of linear 2-arm PLLA compared to that of linear 1-arm PLLA, in which the chain direction is unvaried and the coinitiator moiety is incorporated in the chain terminal. Also, the finding strongly suggests that the reported low crystallizability of multi-arm PLLA (arm number≥3) compared to that of linear 1-arm PLLA is caused not only by the presence of branching points but also by the chain directional change, the incorporation of the coinitiator moiety in the middle of the molecule, and their mixed effect. The effects of the chain directional change and the position of the incorporated coinitiator moiety on the crystallization and physical properties of linear 1-arm and 2-arm PLLA decreased with an increase in Mn.

A new chromium(III)/ p-tert-butylcalix[4]arene/alkylaluminum catalyst system for the preparation of low molecular weight polyethylene

A new chromium(III)/ p-tert-butylcalix[4]arene/alkylaluminum catalyst system was applied to assist the ethylene polymerization. The catalyst system was found to be very active for the ethylene polymerization, and gave strictly linear polyethylene, with high crystallinity and low number average molecular weight ( M n) ranging from 910 to 9740, without using molecular hydrogen as a molecular weight reducer. The effects of polymerization parameters such as temperature, ethylene pressure, chromium catalyst concentration, type of alkylaluminum compounds, and Al/Cr molar ratio on the catalytic activity and properties of the polyethylene were investigated. The formation of low molecular weight polyethylene using the chromium(III)/ p-tert-butylcalix[4]arene/alkylaluminum catalyst system was explained by the coordination effect of structurally hindered and less electron-donating p-tert-butylcalix[4]arene ( LH 4) anion ligand.

Mechanical properties of ethylene-vinyl acetate/polystyrene blends studied byin situ polymerization

This study examined ethylene–vinyl acetate (EVA)-toughened polystyrene (PS). EVA is well-known to be incompatible with PS; thus, the PS graft to the EVA backbone (EVA-g-PS) was used as a compatibilizer and provided good adhesion at the interface of PS and EVA. In addition, the mechanical properties and impact resistance of the PS matrix were obviously improved by EVA-g-PS and by EVA itself. Meanwhile, differential scanning calorimetry results showed that the grafted PS chain influenced the crystallization of EVA; for example, the melting temperature, the crystallization temperature, and the percentage crystallinity related to EVA were reduced. Moreover, the addition of 10% EVA increased the impact strength by a factor of five but reduced the modulus by the same factor. Additionally, a lower number-average molecular weight EVA delayed phase inversion and resulted in poor mechanical properties. A fracture surface photograph revealed that the major mechanism of EVA-toughened PS was craze and local matrix deformation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 699–705, 2003

Chemical and gelling profile of ice-ice infected carrageenan from Kappaphycus striatum (Schmitz) Doty “sacol” strain (Solieriaceae, Gigartinales, Rhodophyta)

The occurrence of ice-ice disease in farmedKappaphycus spp. has tremendous implication on thecarrageenan industry in the Philippines. To elucidate the effect of thisdiseaseon the carrageenan quality, the chemical profile of carrageenan extracted fromhealthy and diseased Kappaphycus striatum“sacol” strain, was investigated using 13C and1H NMR, FT-IR and GPC methods of analysis. The native carrageenanextracts were composed predominantly of kappa carrageenan with trace amounts ofiota and 6-O-methylated carrageenan. Structural evidence indicatesdepolymerization of carrageenan from the ice-ice infectedportion of the thallus (whitish and soft part) lowered levels of iota andmethylconstituents. This finding was correlated to its low number average molecularweight (30 kDa) value. Accordingly, appreciable decreases incarrageenan yield, gel strength and viscosity and increase in syneresis indexwere noted. On the other hand, archetype quality of kappa-carrageenan wasobserved from both healthy and uninfected portion of the diseased thalli.

Biosynthesis of PHB tercopolymer by Bacillus cereus UW85.

The study was attempted to determine the ability of a Gram-positive Bacillus cereus UW85 strain to biosynthesize poly (3-hydroxybutyrate) copolymers when epsilon-caprolactone, or epsilon-caprolactone and glucose, were used as carbon sources. Bacillus cereus was grown for 24 h under nitrogen-limited conditions in a mineral salts medium. Growth was monitored by measurement of turbidity. Glucose level was determined by the colorimetric anthrone The epsilon-caprolactone concentration was determined by gas chromatography. The bacterial biopolymers were extracted with chloroform in a Soxhlet extractor and then characterized by nuclear magnetic resonance and gel permeation chromatography. When epsilon-caprolactone was used as a carbon substrate, the bacterial strain produced tercopolymer with 3-hydroxybutyrate, 3-hydroxyvalerate and 6-hydroxyhexanoate units. However, when caprolactone and glucose were supplied together, only homopolymer of poly (3-hydroxybutyrate) was produced. All tercopolymers isolated from B. cereus UW85 cells were obtained with yields up to 9% (w/w) and low number-average molecular weight compared with the homopolymer PHB. Bacillus cereus UW85 produced tercopolymer with a low molecular weight from one substrate (epsilon-caprolactone) used as a carbon source. The results are significant for the potential future application of Bacillus biopolymers to bioplastics production.

Poly 1,3-butylene adipate Reoplex® as high molecular weight plasticizer for PVC-based cling films—microstructure and number-average molecular weight studied by 1H and 13C NMR

Reoplex® 346, a low number-average molecular weight (M̄n) poly 1,3-butylene adipate—frequently used as plasticizer in PVC-based cling-films—was studied by 1H and quantitative 13C NMR. It was found that Reoplex® results from the linear polyesterification of adipic acid (A) by an excess of butane 1,3-diol (B) with a (A/B) molar ratio of 0.88 giving a polyester having a number-average degree of polymerization (DPn) and an absolute M̄n of 15.7 and 1600g/mol, respectively. As primary alcohol function is more reactive than secondary alcohol, Reoplex® chain-ends are made up of 28.6% of primary alcohol and 71.4% of secondary alcohol.

A new generation of highly efficient expander products and correlation between their chemical composition and the performance of the lead–acid battery

A family of eight lignosulphonates produced by Borregaard LignoTech (Norway) is investigated with the aim to determine their efficiency as expanders in the negative plates of SLI lead–acid batteries. The lignosulphonates are introduced during preparation of the paste from which the negative active mass is formed. All battery tests were performed employing the DIN43539-2 test protocol. Besides the battery tests, all lignosulphonates were characterized with regard to their chemical composition (i.e., the content of various functional groups such as –COOH, Ar–OH, –O·CH 3 and organic sulphur in the structure of the lignins), average molecular weight and purity. It has been found that the lignins UP-393, M, D, Vaniperse-A (VS-A) and UP-414 have a very strong beneficial effect as expanders on the performance of the negative plates. It has also been established that the cycle life of the batteries is limited by the conversion of the energetic NAM structure into skeleton one. As a result of this conversion, large shapeless lead crystals were formed with small surface area and large macropores in-between. The beneficial effect of the expander has been found to correlate positively with low number average molecular weight of the lignins and their derivatives, high –COOH content, low –O·CH 3 and organic S content, optimum Ar–OH content and high lignin purity. Expanders with different chemical composition should be selected for different battery type applications.

Isolation and Characterization of Low and High Molecular Weight Acidic Compounds from Cerro Negro Extraheavy Crude Oil. Role of These Acids in the Interfacial Properties of the Crude Oil Emulsions

Two acidic samples, A and B, were isolated from Cerro Negro extraheavy crude oil. Sample A has a relatively low number average molecular weight (Mn = 480 Da), low aromaticity (fa = 15.2%), and low γ (6.8 m Nm-1, toluene−0.1 N HCl), whereas for B, these values were Mn = 1200 Da, fa = 20%, and γ = 14.8 m Nm-1. These values could be compared to those for Cerro Negro asphaltenes under the same conditions (Mn = 5000, in toluene by VPO, fa = 47%, γ = 20.9 m Nm-1).

Photochemistry and photopolymerization activity of monomers and copolymers of 2-substituted amidoanthraquinone and acryloxyanthraquinone with methyl methacrylate

The photophysical and photoinduced polymerization activities of the monomers 2-acrylamidoanthraquinone (2-AAAQ) and 2-acryl-oxyanthraquinone (2-AOAQ) were determined in solution, together with the corresponding properties of their copolymers with methyl methacrylate (MMA). Copolymers of 2-acrylamidoanthraquinone with a narrow polydispersity (1.8–2.5) were obtained with chromophore contents in the range 8.49–9.92 mol.% and a number average molecular weight dhich decreased from 89 937 to 35 624 g mol −1 depending on the irradiation time. Copolymers of 2-acryloxyanthraquinone with a broader polydispersity (2.9–5.6) were obtained with similar chromophore contents, but a lower number average molecular weight which decreased from 69 446 to 20 678 g mol −1 over an irradiation period of 840 min. Overall, low fluorescence quantum yields were obtained, with the values for the copolymers being higher than those for the monomers. This suggests a high rate of deactivation of the lowest excited singlet state via either internal conversion or intersystem crossing to the lowest excited triplet state. The latter appears to be highly favourable in the case of the 2-acryloxyanthraquinone copolymer where the phosphorescence quantum yields were enhanced markedly by two orders of magnitude (10 2). The photoreduction quantum yields in acetonitrile, although low, were enhanced in the presence of a tertiary amine. Higher values were obtained for the 2-acryloxyanthraquinone monomer and copolymer. Similar effects were observed in the photoinduced polymerization of methyl methacrylate. Higher rates were observed for the 2-acryloxyanthraquinone monomer and copolymer, and the rates for the two copolymers were higher than those of the corresponding monomers. These two notable effects are associated with the higher activity of the lowest excited triplet nπ ∗ state of the 2-acryloxyanthraquinone chromophore, and the greater rigidity of the chromophores in the copolymers which enhances the intersystem crossing rates and triplet state activity via hydrogen atom abstraction or electron transfer processes.

Instrumental effects in the analysis of polymers of wide polydispersity by MALDI mass spectrometry

Matrix-assisted laser desorption/ionization (MALDI) time-of-flight (tof) mass spectrometry (MS) has been reported to give low number average and weight average molecular weight values for polymers of wide polydispersity. Spectra of wide polydisperse polymers more closely resemble the distribution observed by size exclusion chromatography when obtained in the reflectron mode. However, the highest mass molecules in the distribution are not observed. These high-mass ions can be observed when the more abundant lower mass ions are deflected from reaching the detector. In addition, a mixture of three narrow distribution polymer standards selected to provide a wide mass distribution do not show discrimination against the high-mass molecules. These results are interpreted to suggest that the difficulty in observing accurate molecular weight distributions by MALDI-TOF-MS is at least in part attributable to instrumental limitations rather than ionization limitations.

Thermoreversible Morphology Transition of Block Copolymers.

Thermoreversible morphological transition of a poly(styrene-block-isoprene) (SI) block copolymer was studied by small-angle X-ray scattering and transmission electron microscopy (TEM). Although block copolymers are theoretically predicted to exhibit a reversible morphological change between lamellae and disordered state via cylinders and spheres with temperature, this thermoreversibility in morphology transition has not been experimentally confirmed yet. In order to study this thermoreversible transition, the SI diblock copolymer may have to have a biased composition and sufficient mobility. To meet these requirements, we used the SI with the weight fraction of polystyrene (PS) 0.16 and with a relative low number-average molecular weight 8.2×104. Heat treatments of the specimen were carefully conducted. Moreover, the ultrathin sectioning was performed by cryo-ultramicrotomy at -85°C for TEM studies. The cylindrical and spherical microdomains of PS which are embedded in the polyisoprene matrix were thermoreversibly observed at 150 and 200°C, respectively.

Synthesis and Structure-Activity Relationships of Poly[(3,4-dihydro-2H-pyran)-alt-(maleic acid)] and Its Derivatives as Antitumor Agents

Poly[3,4-dihydro-2H-pyran)-alt-(maleic acid)] and its derivatives were synthesized by hydrolyses or substitution reactions of the alternating copolymers obtained by radical copolymerizations of 3,4-dihydro-2H-pyran de rivatives with maleic anhydride The polymers have different substituents in the 2-position of tetrahydropyran rings in copolymer backbone, such as, hydroxyl, methoxy, ethoxy, acetoxy, carboxyl, methoxycarbonyl, carbamoyl, for myl, hydroxymethyl, acetoxymethyl, tosyloxymethyl, and mercaptomethyl groups. The copolymers obtained by solution copolymerizations were found to have low number-average molecular weights (Mn < 2,000), while bulk copoly merizations gave copolymers with higher Mn (4,000 - 8,000). ID50 values of the copolymers on normal and tumor cells (3LL and B16) in vitro and the in creased life span against B16 melanoma in vivo were measured and they dis played cytotoxicities and antitumor activities comparable to or superior to that of DIVEMA, an alternating copolymer of divinyl ether-maleic anhydride (1:2). Especially high cytotoxicities against tumor and normal cells were found by the copolymers containing methoxy, acetoxy or formyl groups in vitro and high antitumor activities were found by the copolymers having hydroxy or acetoxy groups in vivo with the increased life spans of 140 and 147%, respectively.

In vivo characteristics of low molecular weight copoly( L-lactic acid/glycolic acid) formulations with controlled release of luteinizing hormone-releasing hormone agonist

A copolymer of L- lactic acid (LA) and glycolic acid (GA), which has a relatively low number-average molecular weight ( M ̄ n=1600–3300 ) was synthesized by means of direct copolycondensation without catalysts. The luteinizing hormone-releasing hormone agonist, des-Gly 10-[Leu 6]-LH-RH ethylamide monoacetate (LH-RH agonist), was incorporated by the melt-pressing technique in copoly(LA/GA) to obtain a fine cylindrical formulation. This solid formulation was implanted subcutaneously in the back of male adult Wistar strain rats. The in vivo degradation of copoly(LA/GA) formulation without LH-RH agonist showed a marked acceleration with increasing GA component in the copolymer, and the resulting maximal value was obtained at 30 70 mol% LA/GA composition; for example, the copolymer with M ̄ n=2300 went up to 100% in the 6th week from the implantation. The period required for 100% in vivo degradation increased linearly with increasing M̄ n of the copolymer. From this linear relation it was found that the period required for 100% biodegradation per 1000 g mol increase of M ̄ n corresponds to approximately 6 weeks. The serum LH-RH agonist level in rats in which the copoly( 70 30 mol% LA/GA, M ̄ tn=2900 ) formulation was implanted, which was degraded up to 100% after 16 weeks implantation, was kept constant during the first 10 weeks and then decreased linearly until its level could no longer be detected in the 15th week from the implantation. This finding shows that LH-RH agonist is constantly released in vivo from the biodegradable formulation over a long period.