Star-shaped polylactic acid-based triazine dendrimers: the catalyst type and time factors influence on polylactic acid molecular weight

Nowadays, extensive environmental problems due to the use of synthetic polymers have provoked efforts for their replacement with biopolymers as the most important research priorities. Polylactic acid (PLA) is one of the well-known biodegradable biopolymers. In this work, for the first time, PLA was modified with hydrophilic triazine-based dendrimers to obtain PLA with improved hydrophilic properties. In this regard, at first, dl-lactic acid (DLLA) was polymerized through the melt polycondensation to obtain poly-dl-lactic acid (PDLLA). The effects of reaction time and catalyst types on the molecular weight of the PDLLA were investigated. Also, the thermal behavior of PDLLAs with different molecular weights was evaluated using differential scanning calorimetry (DSC) technique. The obtained results from the DSC analysis showed that the PDLLA with higher molecular weight has a higher glass transition temperature (T_g) and melting point (T_m). In the following, various generations (G) of the triazine-based dendrimers were synthesized. To increase the hydrophilicity of the prepared PDLLA, chemical modification of PDLLA with the different generations of triazine-based dendrimer (G₁, G_1.5 and G₂) was performed. Due to the modification of PDLLA with dendrimers, the number of functional groups and hydrophilicity of PDLLA increased. Based on the obtained results, it is expected that the prepared systems could be a good and promising candidate for the production of biocompatible plastics with more hydrolytic degradation ability.

     

Study of specific interactions in inclusion complexes of amine-terminated PAMAM dendrimer/flavonoids by experimental and computational methods

Polyamidoamine (PAMAM) dendrimers have a multifunctional structure, able to encapsulate molecules for pharmacological applications. We evaluated the specific interaction that govern the encapsulating and affinity of one group of natural and synthetic flavonoids into the G₅-PAMAM dendrimers. The complexation and capture percent of one flavonoid series into G₅-PAMAM dendrimers, under neutral and acid pH conditions, were studied through UV–Vis spectroscopy. Additionally, only three of the flavonoids (two synthetic and one natural) were studied by high-performance liquid chromatography (HPLC) and molecular dynamic (MD) simulation, at neutral pH to calculate the affinity constants (K_d) and binding free energies (ΔG_b). From spectroscopic results, we observed that the encapsulation was much more rapid at low pH than at neutral pH, which was attributed to a greater number of cavities inside the dendrimer. The MD simulations suggested that the more compact molecular structure at neutral pH reduces the capture kinetics. Finally, the relative binding free energies calculated using MD simulations showed the same tendency as the experimental data for the three complexes. These affinities appear to be due to a complex balance of different contributions, which cannot be attributed to hydrogen bonds or charge–charge interactions alone. Nevertheless, we suggest that a protocol including UV–Vis, HPLC, and MD simulation can be a powerful predictive tool to determine the affinity of drug binding to nanocarriers.     

Thermal and crystallinity property studies of poly (L‐lactic acid) blended with oligomers of 3‐hydroxybutyrate or dendrimers of hydroxyalkanoic acids

Poly (L ‐lactic acid) (PLA) is a biodegradable polymer with slow crystallization rate. Oligomers of 3‐hydroxybutyrate (OHB) and dendrimers of hydroxyalkanoic acids with different molecular weights were blended with PLA in a hope to improve the crystallization ability and thermal stability of PLA, respectively. Four thermally‐degraded PHB products oligomers termed OHB‐1, OHB‐2, OHB‐3, and OHB‐4 with various number average molecular weights (M_n) of 4000, 7400, 14,000, and 83,000, respectively, were blended with PLA. The lower cold‐crystallization temperature (T_cc) and higher heat of cold crystallization (ΔH_cc) for blend of PLA/OHB‐1 suggested that thermally‐degraded OHB‐1 formed suitable crystal size during the cooling process and then acted as nucleation agents for PLA in the subsequent heating process. On the other hand, for the blending systems of PLA/dendrimers of hydroxyalkanoic acids, no obvious change on the thermal properties was observed compared with pure PLA except an improved PLA thermal stability possibly resulted from the crosslinking effects of the dendrimers © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

树枝状聚赖氨酸的合成

合成了一种具有精确分子结构的树枝状聚赖氨酸，并用^1H—NMR＆^13C-NMR和ESI-MS和MAL—DI-TOFMS等手段对所得产物结构进行了表征。通过不同条件实验的方法对树枝状聚赖氨酸的合成条件进行了优化。结果表明，反应温度、切割时间和溶剂对产率的影响明显。优化后的反应条件是原料配比为Ni-Boc-赖氨酸：N己二胺=2：1．0；冰水浴下反应2小时后温度升高至25℃继续反应24小时；保护基团切割时间为1．5小时。在优化的条件下，产物最高收率为80．2％。采用DMF作溶剂，效果优于DCM。     

Structure and properties of macromolecules with side dendrons based on <Emphasis Type="Italic">L</Emphasis>-aspartic acid

The molecular characteristics of cylindrical first-third generation dendrimers carrying dendrons based on L-aspartic acid are compared. It is shown that both the generation number of side dendrons and their structure determine the conformational, hydrodynamic, optical, and dipole characteristics of dendrite macromolecules. It is found that the studied class of cylindrical dendrimers is distinguished by a marked amount of intramolecular hydrogen bonds between side dendrite substituents. These bonds ensure the unusual combination of high kinetic rigidity and a compact conformation (low equilibrium rigidity) of macromolecules. The rupture of hydrogen bonds leads to an enlargement of molecular coils (an increase in equilibrium rigidity) that is accompanied by a sharp drop in kinetic rigidity.     

Study of the glycolysis of PET by oligoesters

A series of oligoesters was synthesized via the transesterification of dimethyl isophthalate with neopentyl glycol or tetraethylene glycol and the esterification of adipic acid with neopentyl glycol or tetraethylene glycol under diol/diester or diol/diacid molar ratios sufficient to limit molecular weight increasing.These oligoesters were used to depolymerize poly(ethylene terephthalate) (PET) in the presence of zinc acetate to yield new types of glycolysates. The oligoesters and the glycolysates are characterized by ¹H NMR, size exclusion chromatography (SEC), differential scanning calorimetry (DSC), thermogravimetric analyses and matrix assisted laser desorption/ionisation time of flight mass spectrometry (MALDI-TOF MS). These analyses revealed the structure of the different glycolysates.     

Dynamics in complex systems: Dendrimer-polymer blends in electric and mechanical fields

An investigation was carried out on the molecular dynamics of blends composed of poly(amidoamine) (PAMAM) dendrimers with ethylenediamine core and amino surface groups (generations 0 and 3) and three linear polymers: poly(propylene oxide) - PPO and two block copolymers, poly(propylene oxide)/poly(ethylene oxide) - PPO/PEO with different mole ratios: 29/6 (amorphous) and 10/31 (crystalline). The results were generated over a broad range of frequency and temperature by Dielectric Relaxation Spectroscopy (DRS) and Dynamic Mechanical Spectroscopy (DMS). Dielectric spectra of dendrimers in the PPO matrix reveal a decrease in the time scale of normal and segmental relaxation with increasing dendrimer concentration. In the amorphous blends with 29PPO/6PEO matrix, no effect of concentration on the time scale of normal and segmental processes was observed. But in the crystalline blends with 10PPO/31PEO matrix, relaxation time increases with increasing dendrimer concentration. Results acquired by DRS and DMS were contrasted and the obtained relaxation times were found to be in excellent agreement. A detailed analysis of the effect of generation and concentration of dendrimers, hydrophilicity and morphology of the polymer matrix and temperature on the molecular origin, the shape of the relaxation spectra, the dielectric relaxation strength and the frequency location for the maximum loss in dendrimer-polymer blends is provided.     

Preparation, MALDI-TOF analysis, and micelle-like behavior of alkyl-modified poly(propylene imine) dendrimers

Summary MALDI-TOF mass spectral analysis was used to characterize the novel unimolecular micelles that result from functionalization of the chain ends of poly(propylene imine) dendrimers with C₁₀ alkanoyl or C₁₂ alkyl groups. Received: 7 July 1999/Accepted: 2 September 1999     

Peptide dendrimers as artificial enzymes, receptors, and drug-delivery agents

The dendritic architecture applied to peptides provides a practical entry into globular macromolecules resembling proteins. A modular design was chosen using a divergent synthesis on solid support alternating proteinogenic alpha-amino acids with branching diamino acids, producing peptide dendrimers with a molecular weight of 3-5 kDa. Initial studies focused on models for hydrolases and produced esterase peptide dendrimers featuring histidine as the key catalytic residue. Variations of amino acid composition and the branching diamino acid led to enantioselective catalysts. Rate accelerations of k(cat)/k(uncat) = 90,000 were obtained when the design was changed to monomeric peptide dendrimers alternating two amino acids with the branching unit. A combinatorial approach was developed allowing for the preparation of large libraries (>60,000 members), which were screened for B12 binding and catalytic activity. The peptide dendrimers were also investigated for drug delivery. Glycopeptide dendrimers conjugated to colchicine selectively inhibited the proliferation of targeted cells, whereas colchicine alone displayed high toxicity.     

Preparation of poly(ethylene glycol)-modified poly(amidoamine) dendrimers with a shell of hydrophobic amino acid residues and their function as a nanocontainer

We studied the use of poly(ethylene glycol) (PEG)-modified dendrimers as a nanocapsule with a biocompatible surface. We designed PEG-modified dendrimers having a shell of hydrophobic amino acid residues in the peripheral moiety of the dendrimer to increase their encapsulation ability. Subsequently, l-phenylalanine or γ-benzyl-l-glutamate residues were introduced to all chain ends of the poly(amidoamine) G4 dendrimers. Furthermore, PEG (MW 2000) chains were attached to the amino acid residues. These hydrophobic amino acid residues rendered the PEG-modified dendrimers as more compact. After binding of Rose Bengal (RB) guest molecules to dendrimers, an assay using the Klotz plot showed that the hydrophobic amino acid layer slightly affected the guest site number, but significantly increased intrinsic binding of the dendrimers to guest molecules. The PEG-modified dendrimers with the hydrophobic amino acid layer were better able to retain guest molecules than the dendrimer without the layer: they are therefore useful for drug delivery.     

Poly(amidoamine) Dendrimers Bearing Electron-Donating Chromophores: Fluorescence and Electrochemical Properties

Summary Poly(amidoamine) dendrimers from the zeroth generation (G 0) to the fifth generation (G 5) bearing electron-donating chromophores, i.e. N, N-dimethylphenylamine (PA), carbazole (Cz) and pyrene (Py) chromophores have been synthesized. The macromolecular reactions of chromophoric modifications of PAMAM dendrimers were monitored by fluorescence spectroscopy. The reliability of fluorescence monitoring was confirmed by NMR spectroscopy. Compared with widely applied environment-sensitive fluorescence probe techniques, a distinct feature of the fluorescence monitoring was that it directly reflected the conversion of chemical bonds. A comparative study revealed that the fluorescence quenching of small molecular Py by the high generation dendrimers of PAMAM-PA was a static quenching process. The high generation dendrimers of PAMAM-Py possessed relatively compact Py shells as compared with the same generation PAMAM-PA. Electrochemical studies revealed an architectural transition occurring from an open and flexible structure to a closely packed globular structure between G 2 and G 3. The electron-donating ability of PAMAM-PA was affected by dendritic architectures. Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Analysis of ethoxylated fatty amines. Comparison of methods for the determination of molecular weight

Specific lengths of the fatty and polyoxyethylene chains of ethoxylated fatty amines are critical to their performance in specific applications, and thus the ability to characterize these surfactants accurately is crucial. Normal-phase high-performance liquid chromatography (HPLC) and matrixassisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry methods were developed to determine with accuracy the molecular weight and degree of ethoxylation of ethoxylated fatty amines. Ethoxylated fatty amines were analyzed using these methods, and comparison was made to molecular weight determinations using proton nuclear magnetic resonance (NMR), neutralization equivalent weight, and hydroxyl value methods. Molecular weight results from normalphase HPLC analyses were in very good agreement with MALDI-TOF results, typically varying less than one ethylene oxide unit. A reversed-phase HPLC method was developed to determine concentrations of polyethylene glycols (PEG) and fatty homologs. PEG interfered with molecular weight determinations by NMR, neutralization equivalent weight, and hydroxyl value methods. PEG caused no interference with molecular weight determinations by normal-phase HPLC and MALDI-TOF methods.     

Bamboo biochar-catalytic degradation of lignin under microwave heating

Abstract

Lignin biochar-catalytic depolymerization using biochar Fe-600, Fe-800, Ni-600, Ni-800 catalysts under microwave-heating (180?°C for 30?min) was explored in an ethanol/formic acid (1:1) media. Non-catalyst depolymerization was also studied and compared with the biochar-catalysts results. Characteristics of the bio-char catalysts were analyze by BET, XRD, and FT-IR. GPC, FT-IR, and MALDI-TOF MS spectrometry were also used to characterize the depolymerization products. The experimental results showed that the S_BET, V_t, and V_mec and average pore diameter of the biochars are considerably dependent on the preparation temperature and type of cation (Ni²⁺ or Fe³⁺). The maximum yield of bio-oil product was obtained as 85?wt% with the addition of biochar Ni-600 and the total amount of oligomers or monomers with a molecular weight of 164 to 446 reaches 80.4%.     

Polypentamethylnadimides obtained by Diels-Alder reaction

The reactions between benzylbromide or α,α′-dibromo-meta-xylene (DBMX) and sodium 1,2,3,4,5-pentamethylcycopenta-1,3-dienide (CpMe₅^?, Na^⊕) give only the expected pentamathylcyclopentadiene derivatives with a good yields. These products lead to endo isomer adducts via a Diels-Alder reaction with maleimide.New low molecular weight polypentamethylnadimides were obtained in this way from the Diels-Alder polymerization of bispentamethylcyclopentadiene monomers and commercial or synthetic bismaleimides. The polyadducts were characterised by size exclusion chromatography (SEC), matrix assisted laser desorption ionization-time of flight (MALDI-TOF), nuclear magnetic resonance (NMR) and thermogravimetric analyses (TGA).The low molecular weight was attributed to methyl steric hindrance. For the lowest molecular weight products, two different endcapped oligomers were observed.     

Blending and characterizations of microbial poly(3‐hydroxybutyrate) with dendrimers

Blending of microbial polyester poly(3‐hydroxybutyrate) (PHB) with various dendritic polyester oligomers or dendrimers was achieved by solution casting to improve the film forming ability of PHB. Films of the blends were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron micrograph (SEM), and Fourier transform infrared spectroscopy (FTIR). It was revealed that there were mainly two types of interactions in the blending system: the plasticizing or lubricating effect of the low melting spherical dendrimers molecules improved the polymer chain mobility through the suppression of PHB crystallization in the blends; The dendrimers also functioned as crosslinking agents or antiplasticizing agents via weak hydrogen bonding to enhance the overall intermolecular interactions which decrease the chain mobility and thus cause the increase of glass transition temperature (T_g) of PHB. TGA results concluded that incorporating the dendrimers could retard the thermal decomposition of PHB and enhanced its thermal stability accordingly. With the above blend processes, the so‐obtained PHB possessed better film forming ability and even patterned surface structures. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:3782–3790, 2006     

Developing High Field MRI Contrast Agents by Tuning the Rotational Dynamics: Bisaqua GdAAZTA-based Dendrimers

Monomeric and dimeric AAZTA-based bifunctional chelators (AAZTA=6-amino-6-methylperhydro-1,4-diazepine tetraacetic acid) were attached to different generations (G0, G1 and G2) of ethylenediamine-cored PAMAM dendrimers (PAMAM=polyamidoamine) to obtain a series of six dendrimeric systems with 4 to 32 chelates at the periphery. These Gd^III-loaded dendrimers have molecular weight ranging from 3.5 to 25 kDa, thus allowing a systematic investigation on the changes in relaxivity (r₁) with the variation of the rotational dynamics following the increase in molecular size. Variable-temperature ¹⁷O NMR (on the dimeric building block Gd₂ L2 ) and ¹H Nuclear Magnetic Relaxation Dispersion measurements at different temperatures indicate that the water exchange lifetime (τ_M∼90 ns) of the two inner sphere water molecules does not represent a limiting factor to the relaxivity of the systems. The r₁ values at 1.5 T (60 MHz) and 298 K increases from 10.2 mM⁻¹ s⁻¹ for the monomer Gd L1 to 31.4 mM⁻¹ s⁻¹ for the dendrimer Gd₃₂ G2-32 (+308 %). However, the relaxivity (per Gd) does not show a linear dependence on the molecular mass, but rather the enhancement tends to attenuate markedly for larger systems. This effect has been attributed to the growing decrease in correlation between local rotational motions and global molecular tumbling.     

Antitumor efficacy of doxorubicin encapsulated within PEGylated poly(amidoamine) dendrimers

We report here a general approach to using poly(amidoamine) (PAMAM) dendrimers modified with polyethylene glycol (PEG) as a platform to encapsulate an anticancer drug doxorubicin (DOX) for in vitro cancer therapy applications. In this approach, PEGylated PAMAM dendrimers were synthesized by conjugating monomethoxypolyethylene glycol with carboxylic acid end group (mPEG‐COOH) onto the surface of generation 5 amine‐terminated PAMAM dendrimer (G5.NH₂), followed by acetylation of the remaining dendrimer terminal amines. By varying the molar ratios of mPEG‐COOH/G5.NH₂, G5.NHAc‐mPEG_n (n = 5, 10, 20, and 40, respectively) with different PEGylation degrees were obtained. We show that the PEGylated dendrimers are able to encapsulate DOX with approximately similar loading capacity regardless of the PEGylation degree. The formed dendrimer/DOX complexes are water soluble and stable. In vitro release studies show that DOX complexed with the PEGylated dendrimers can be released in a sustained manner. Further cell viability assay in conjunction with cell morphology observation demonstrates that the G5.NHAc‐mPEG_n/DOX complexes display effective antitumor activity, and the DOX molecules encapsulated within complexes can be internalized into the cell nucleus, similar to the free DOX drug. Findings from this study suggest that PEGylated dendrimers may be used as a general drug carrier to encapsulate various hydrophobic drugs for different therapeutic applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40358.     

The effect of iron oxide nanoparticles on the mechanical relaxation of magnetic polymer hybrid films composed of a polystyrene matrix,a fractional calculus approach

A magnetic polymer hybrid film (MPHF) with a thickness of ~80 μm, composed of iron oxide nanoparticles (IONPs) in a polystyrene (PS) matrix, was successfully prepared. Its structure and morphology were analyzed by HRTEM, XRD, and FTIR. The optical and magnetic behaviors were studied by UV–Vis spectroscopy and VSM, respectively. The main relaxation of the MPHF was characterized by dynamic mechanical analysis (DMA), and the molecular mobility was analyzed by a fractional Zener model (FZM). Results obtained by DMA reveal the mechanical manifestation of the α -relaxation for both, PS and MPHF, and how this process is modified by IONPs into MPHFs. Good agreement between experimental DMA spectra and the theoretical results calculated from the FZM was obtained. Fractional parameters a and b characterize the molecular mobility at low and high temperatures, respectively. These results show that at low temperatures ( a parameter), molecular mobility is slightly affected by the presence of IONPs, while at high temperatures ( b parameter), molecular mobility is affected in a greater degree. IONPs decrease the molecular mobility of PS matrix; this effect is more pronounced at temperatures above the glass transition temperature. These results validate the effect of IONPs on PS matrix considering future applications of the MPHFs. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47840.     

An innovative approach for the chemical structural characterization of poly(styrene 4-vinylpyridine) copolymers by matrix-assisted laser desorption/ionization time of flight mass spectrometry

Poly(styrene-co-4-vinylpyridine) random copolymers with different molar composition were synthesized by nitroxide-mediated controlled-radical polymerization using 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO) as a mediator. We record the matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) spectra under various conditions, and we find (at last) that they show mostly intact ions [using 2(-4-hydroxyphenylazo-)benzoic acid as MALDI matrix]. Spectra are highly resolved, and thus they allow for the determination of all end-groups, even some less-abundant ones. Spectra are dominated by intact “dormant” copolymer chains terminated with TIPNO at one end and with (4-Bromo-phenyl)ethyl group (starting fragment) at the other one. Applying the mass analysis of copolymers (MACO) statistical model to the spectra, we show that the MACO/MALDI-TOF mass spectrometry (MS) analysis can be successfully applied to copolymers having a difference between the mass of the comonomers as small as 1 g mol⁻¹ (the styrene and 4-vinylpyridine units are 104.15 and 105.15 g/mol, respectively), which results in overlapping isotopic patterns. The results are accurate: chemical composition evaluated by means of MS agrees with that calculated by ¹H-nuclear magnetic resonance, for all copolymers investigated. This analytical method allows to extract detailed information on the composition of the copolymer samples and their structure. Glass transition temperatures of copolymers were also determined by differential scanning calorimetry. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46976.     

Investigating the UV-curing performance for polyacrylated polymer in dendritic and regular conformation

In this study, PDSC was employed to monitor the UV curing process for dendritic and regular monomers with various acrylate sites. Experimental results reveal that even with only a small increase in molecular weight for regular monomers caused a large increase in viscosity. A dendritic monomer with the same number of sites has a much lower viscosity. The conversion of hydroxyl groups into acrylic groups in the dendritic monomer slightly reduces the viscosity by destroying the hydrogen bonds. The curing conversion and curing rate increased with the number of acrylate sites to maxima at five. The acceleration of the double bond reaction within a dense group of multiacrylate sites is responsible for the initial rise, but the steric effect of the branches, hindering the simultaneous free-radical propagation, causes a decline as the number of sites increases in the curing of dendrimers. The autocatalyzed reaction model was then applied to simulate the curing results from PDSC. Dendrimers with 10–19 acrylate sites were found to have lower rate constant k and smaller autocatalyzed order m than the traditional ones. Finally, the heterogeneity of the cross-linking density of dendrimers generally causes dendritic monomers to have weaker hardness and lower T _d values than the regular ones. Nevertheless, dendrimers with 19 acrylate sites yield a film having low processing viscosity, satisfactory hardness (6H), reasonable T _d (290 °C), and a superior refractive index (1.5).