Beng H. Tan

Poly(ethylene glycol) Conjugated Poly(lactide)-Based Polyelectrolytes: Synthesis and Formation of Stable Self-Assemblies Induced by Stereocomplexation

A series of pH-responsive amphiphilic poly(N,N-dimethylaminoethyl methacrylate)-block-poly(D-lactic acid)-block-poly(N,N-dimethylaminoethyl methacrylate) conjugated with poly(ethylene glycol) (D-PDLA-D@PEG) and D-PLLA-D@PEG copolymers were synthesized using a combination of ring-opening polymerization and atom-transfer radical polymerization followed by sequential quaternization of PDMAEMA chains and azide-alkyne click reaction with alkyne-end PEG. Gel permeation chromatography, nuclear magnetic resonance, and Fourier transform infrared spectroscopy results demonstrate the successful synthesis of the copolymers, and the conjugated PEG percentages in the copolymers can be tuned by the feeding ratios in the quaternization reaction. Conjugating PEG onto the PDMAEMA segments also successfully facilitated the D-PDLA-D@PEG, D-PLLA-D@PEG, and its corresponding 1:1 D/L mixtures to be dissolved directly in aqueous solution at the desired concentration range without using any organic solvents unlike the copolymers without PEG conjugation (D-PDLA-D and D-PLLA-D). We demonstrate control over micellar size, charge, and stability via three different preparation pathways, i.e., solution pH, percentages of PEG conjugation in the copolymers, and formation of PLA stereocomplex in micellar core. Static and dynamic light scattering studies demonstrate that the size of the core-shell micelles increases when the solution pH is reduced due to the protonation of the PDMAEMA segments that caused the osmotic pressure within the micelle to increase until the micelles reached a maximum size. It is interesting to note that the micelles formed by 1:1 D/L mixtures have larger swelling ratios as well as aggregation number and hydrodynamic radius that do not change significantly with pH and dilution, respectively, as compared to micelles formed from individual D or L forms of the copolymers. The enhanced stability of the pH-responsive micelles prepared by direct dissolution of the 1:1 D/L mixtures of the PEG conjugated PLA-based polyelectrolytes in aqueous medium is attributed to the stereocomplex formation between PLLA and PDLA in the micellar core as confirmed by wide-angle X-ray scattering measurements.

Synthesis and self-assembly of brush-type poly[poly(ethylene glycol)methyl ether methacrylate]-block-poly(pentafluorostyrene) amphiphilic diblock copolymers in aqueous solution.

Well-defined fluorinated brush-like amphiphilic diblock copolymers of poly[poly(ethylene glycol)methyl ether methacrylate] (P(PEGMA)) and poly(pentafluorostyrene) (PPFS) have been successfully synthesized via atom transfer radical polymerization (ATRP). The self-assembly behavior of these polymers in aqueous solutions was studied using (1)H NMR, fluorescence spectrometry, static and dynamic light scattering and transmission electron microscopy techniques. The micellar structure comprised of PPFS as the core and brush-like (hydrophobic main chain and hydrophilic branches) polymers as the coronas. The hydrodynamic radius (R(h)) of the micelles in aqueous solution was in the nanometer range, independent of the polymer concentration, consistent with a closed association model. Diblock copolymers with a longer P(PEGMA) block formed micelles with smaller R(h) and lower aggregation numbers consistent with an improved solubilization of the core. The micelles possessed a thick hydration layer as verified by the ratio of the radius of gyration, R(g) to the hydrodynamic radius, R(h). The aggregation number and ratio of R(g) to R(h) were observed to increase with temperature (20-50 degrees C), while the R(h) of the micelle decreased slightly over the same temperature range. An increase in temperature induced the brush-like PEG segments in the corona to dehydrate and shrink while forming micelles with larger aggregation numbers.

Facile Layer-by-Layer Self-Assembly toward Enantiomeric Poly(lactide) Stereocomplex Coated Magnetite Nanocarrier for Highly Tunable Drug Deliveries

A highly tunable nanoparticle (NP) system with multifunctionalities was developed as drug nanocarrier via a facile layer-by-layer (LbL) stereocomplex (SC) self-assembly of enantiomeric poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) in solution using silica-coated magnetite (Fe3O4@SiO2) as template. The poly(lactide) (PLA) SC coated NPs (Fe3O4@SiO2@-SC) were further endowed with different stimuli-responsiveness by controlling the outermost layer coatings with respective pH-sensitive poly(lactic acid)-poly(2-dimethylaminoethyl methacrylate) (PLA-D) and temperature-sensitive poly(lactic acid)-poly(N-isopropylacrylamide) (PLA-N) diblock copolymers to yield Fe3O4@SiO2@SC-D and Fe3O4@SiO2@SC-N NPs, respectively, while the superparamagnetic properties of Fe3O4 were maintained. TEM images show a clearly resolved core-shell structure with a silica layer and sequential PLA SC co/polymer coating layers in the respective NPs. The well-designed NPs possess a size distribution in a range of 220-270 nm and high magnetization of 70.8-72.1 emu/g [Fe3O4]. More importantly, a drug release study from the as-constructed stimuli-responsive NPs exhibited sustained release profiles and the rates of release can be tuned by variation of external environments. Further cytotoxicity and cell culture studies revealed that PLA SC coated NPs possessed good cell biocompatibility and the doxorubicin (DOX)-loaded NPs showed enhanced drug delivery efficiency toward MCF-7 cancer cells. Together with the strong magnetic sensitivity, the developed hybrid NPs demonstrate a great potential of control over the drug release at a targeted site. The developed coating method can be further optimized to finely tune the nanocarrier size and operating range of pHs and temperatures for in vivo applications.

Stable Dispersions of Hybrid Nanoparticles Induced by Stereocomplexation between Enantiomeric Poly(lactide) Star Polymers

We report the formation and characterization of stable dispersions of hybrid nanoparticles in solution formed via stereocomplexation of enantiomeric poly(lactide) hybrid star polymers. The hybrid starlike polymers, having polyhedral oligomeric silsesquioxane (POSS) nanocages as the core and either poly(L-lactide) (PLLA) or poly(D-lactide) (PDLA) as the arms, are synthesized via ring-opening polymerization of lactide using octafunctional POSS as the macroinitiator. In the solid state, differential scanning calorimetry and wide-angle X-ray scattering measurements confirmed the formation of the stereocomplex in the mixture of POSS-star-PLLA and POSS-star-PDLA (50:50, wt %). In a solution of the same mixture in tetrahydrofuran (THF), sterocomplexation leads to formation of hybrid nanaoparticles. Detailed accounts of the nanoparticle formation and influence of aging and concentration have been presented. It was observed that at low concentration the stereocomplexed nanaoparticles remain stable over 45 days and are not sensitive to dilution, suggesting the formation of a stable hybrid nanoparticle dispersion in solution. In contrast, the aggregates of the individual POSS-star-PLLA or POSS-star-PDLA in THF, formed via weak solvophobic interactions, tended to disintegrate into smaller aggregates on dilution. Exploiting the PLLA-PDLA stereocomplexation with an appropriate molecular design can be a versatile route to develop stable organic/inorganic hybrid nanoparticle dispersions.

Design of polyhedral oligomeric silsesquioxane (POSS) based thermo-responsive amphiphilic hybrid copolymers for thermally denatured protein protection applications

A series of thermo-responsive amphiphilic hybrid copolymers with a random brush-like structure were synthesized by copolymerizing hydrophilic poly(ethylene glycol)methacrylate (PEGMA) and hydrophobic polyhedral oligomeric silsesquioxane methacrylate (POSSMA) together with temperature sensitive poly(propylene glycol)methacrylate (PPGMA) via atom transfer radical polymerization (ATRP). The resulting poly(PEGMA–PPGMA–POSSMA) (PEPS) hybrid copolymers show a lower critical solution temperature (LCST) in the range of 31–33 °C. Static and dynamic light scattering (SLS and DLS) studies show that micellar structures created by the PEPS copolymer in aqueous media were core-shell structures and possessed a thick hydration layer. The presence of a small amount of POSS (3.1 wt%) in the PEPS copolymers lowered the CMC of the micelles at room temperature by one order of magnitude compared to samples without POSSMA (PEP). Incorporation of POSSMA also enhanced the stability of the formed micelles, i.e. PEPS containing 6.7 wt% POSS exhibited a constant hydrodynamic radius, Rh (∼65 nm), and an aggregation number, Nagg (∼350), when the temperature was varied from 20 to 70 °C while PEP without POSS showed a large increase in both Rh and Nagg values. On the other hand, the change of Rg as the temperature increases could be attributed to the PPG brush adopting a more extended and compact conformation below and above LCST respectively. The thermo-responsiveness of the PPG brush in PEPS hybrid micelles was also exploited to mimic the natural GroEL–GroES chaperone functionalities for renaturation of thermally denatured proteins. Above LCST of PPG, the chaperone-like system comes into effect with hydrophobic PPG domains on the micelle surface, providing spontaneous capture and protection of the unfolded proteins, thus inhibiting the undesired protein aggregation at elevated temperatures. Upon cooling, PPG returns to its hydrophilic state, thereby inducing the release of the bound unfolded proteins. The renaturation process of the detached proteins is spontaneously accomplished by the presence of PEG and OH-groups in the micelle corona. The working mechanism and thermal denaturation protection effect were also investigated by DLS, SLS and circular dichroism (CD) spectroscopy. In the presence of PEPS hybrid micelles, the protection efficiencies for GFP, lipase and lysozyme that can be achieved during the heat-induced denaturation process are 81.4%, 89.3% and 88.7%, respectively. Cell culture and cytotoxicity studies revealed that the PEPS hybrid micelles could be effectively internalized by C6 glioma cells and possess good cell biocompatibility. These interesting findings open up new opportunities to exploit PEPS hybrid copolymers as artificial chaperones for protecting unfolded proteins from toxic aggregation at high temperatures.

A semi-empirical approach for modeling charged soft microgel particles

We describe a unified semi-empirical approach for predicting the viscosity of dilute and concentrated hard and soft sphere systems. A variable specific volume, k was introduced to convert the mass concentration to effective volume fraction. With increasing particle concentration, the concentration of free counter-ions in the solution can be large enough to induce an osmotic de-swelling of soft particles, resulting in the particle shrinkage. The viscosity data for four different microgel systems at different neutralization degree showed excellent agreement with the modified Krieger–Dougherty model.

Tuning self-assembly of hybrid PLA-P(MA-POSS) block copolymers in solution via stereocomplexation

We demonstrate the formation of stable hybrid nanoparticles in solution through self-assembly driven by the stereocomplexation between enantiomeric poly(lactide) (PLA) chains in organic–inorganic hybrid diblock copolymers. The well-defined hybrid diblock copolymers (PLLA-b-P(MA-POSS) and PDLA-b-P(MA-POSS)) are synthesized via atom transfer radical polymerization of methacrylisobutyl POSS (MA-POSS) using either PLLA or PDLA as a macroinitiator. The structure of the block copolymers is confirmed by 1H NMR and GPC. The mixture of PLLA-b-P(MA-POSS) and PDLA-b-P(MA-POSS) in THF solution resulted in the formation of self-assembled nanoparticles as confirmed by the light scattering data. It is further verified that the only driving force for self-assembly in THF solution is the ‘stereocomplexation’ between the PLLA and PDLA blocks as no aggregation could be observed in THF solutions of homochiral polylactides at low concentration (∼1 mg mL−1). A solution mixture of 50:50 weight% of PLLA-b-P(MA-POSS) and PDLA-b-P(MA-POSS) for all the samples yields the best stereocomplexation results in terms of particles size and aggregation numbers. For a given composition, the size of the stereocomplexed hybrid nanoparticles, however, decreases with the increasing length of the (MA-POSS) block in the copolymer. As an example, for a 50:50 weight% mixture, the mean hydrodynamic radius Rh and apparent aggregation number Nagg of the particles decreased from 220 nm and 890, respectively, to 72 nm and 290, when the degree of polymerization of P(MA-POSS) increased from ∼2 to 11–12. It is assumed that the bulky POSS nanocages of P(MA-POSS) in the PLA-b-P(MA-POSS) block copolymer sterically hinder the formation of larger nanoparticles by block copolymers with longer P(MA-POSS) blocks. The stereocomplexed nanoparticles remain stable over 30 days and are not sensitive to dilution, suggesting the formation of stable hybrid nanoparticles dispersion. In contrast, the homopolymer mixture of PLLA and PDLA turned cloudy and was no longer stable after 12 days due to the formation of larger macroscopic aggregates. This interesting finding opens up new opportunities to tune the size and stability of the stereocomplexed nanoparticles in solution by manipulating the block length of the inorganic P(MA-POSS) segment.

Barnacle repellent nanostructured surfaces formed by the self-assembly of amphiphilic block copolymers.

Well-defined nanostructured surface with domains of dimension ∼20 nm was formed by the self-assembly of brush-type amphiphilic block copolymers of poly[poly(ethylene glycol)methyl ether methacrylate]-block-poly(2,3,4,5,6-pentafluorostyrene) (P(PEGMA)-b-PPFS) which demonstrate promise in discouraging barnacle settlement as proven using laboratory settlement assays and marine field tests.

Synthesis and Self‐Assembly of pH‐Responsive Amphiphilic Poly(dimethylaminoethyl methacrylate)‐block‐Poly(pentafluorostyrene) Block Copolymer in Aqueous Solution

We report the synthesis of a novel pH-responsive amphiphilic block copolymer poly(dimethylaminoethyl methacrylate)-block-poly(pentafluorostyrene) (PDMAEMA-b-PPFS) using RAFT-mediated living radical polymerization. Copolymer micelle formation, in aqueous solution, was investigated using fluorescence spectroscopy, static and dynamic light scattering (SLS and DLS), and transmission electron microscopy (TEM). DLS and SLS measurements revealed that the diblock copolymers form spherical micelles with large aggregation numbers, N(agg)  ≈ 30 where the dense PPFS core is surrounded by dangling PDMAEMA chains as the micelle corona. The hydrodynamic radii, R(h) of these micelles is large, at pH 2-5 as the protonated PDMAEMA segments swell the micelle corona. Above pH 5, the PDMAEMA segments are gradually deprotonated, resulting in a lower osmotic pressure and enhanced hydrophobicity within the micelle, thus decreasing the R(h) . However, the radius of gyration, R(g) remains independent of pH as the dense PPFS cores predominate.

DNA Polyplexes Formed Using PEGylated Biodegradable Hyperbranched Polymers

A novel PEGylated biodegradable hyperbranched PEG-b-PDMAEMA has been synthesized. The low toxicity, small molecular weight PDMAEMA chains were crosslinked using a biodegradable disulfide-based dimethacrylate (DSDMA) agent to yield higher molecular weight hyperbranched polymers. PEG chains were linked onto the polymer surface, masking the positive charge (as shown by Zeta potential measurements) and reducing the toxicity of the polymer. The hyperbranched structures were also cleaved under reducing conditions and analyzed, confirming the expected component structures. The hyperbranched polymer was mixed with DNA and efficient binding was shown to occur through electrostatic interactions. The hyperbranched structures could be reduced easily, generating lower toxicity oligomer chains.