Junpeng Zhao

Thermo-Induced Aggregation Behavior of Poly(ethylene oxide)-b-poly(N-isopropylacrylamide) Block Copolymers in the Presence of Cationic Surfactants

Low-molecular-weight cationic surfactants, dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB), were introduced to dilute aqueous solutions of thermosensitive poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) block copolymers at concentrations (C(s)) either lower or higher than the critical micelle concentrations (cmc) of the surfactants. The copolymer/surfactant mixtures were investigated by dynamic and static light scattering at different temperatures. At temperature lower than the aggregation temperature (T(agg)), the disaggregation of the copolymers from the loose associations was observed upon addition of the surfactants(.) The thermo-induced aggregation behavior was found to be profoundly influenced with the cooperation of cationic surfactants in terms of T(agg) and the structural characteristics of the aggregates formed at high temperature. In general, T(agg) was increased together with the decrease in the size and molecular weight of the aggregates. These were attributed to the copolymer/surfactant interactions and the electrostatic repulsion coming from the ionic head groups of the surfactants within the mixed aggregates. These changes were much more pronounced at higher C(s). CTAB, which has a longer hydrophobic tail, displayed higher influences compared to DTAB. The formation of vesicles, by one of the copolymers, was suppressed in the presence of CTAB. At the higher CTAB concentration, only small mixed aggregates with very low mass were observed even at the highest temperature investigated.

Polymerization of 5-alkyl δ-lactones catalyzed by diphenyl phosphate and their sequential organocatalytic polymerization with monosubstituted epoxides

Organocatalytic ring-opening polymerization (ROP) reactions of three renewable 5-alkyl δ-lactones, namely δ-hexalactone (HL), δ-nonalactone (NL) and δ-decalactone (DL), using diphenyl phosphate (DPP) were investigated. Room temperature, together with a relatively high monomer concentration (≥3 M), was demonstrated to be suitable for achieving a living ROP behavior, a high conversion of the lactone, a controlled molecular weight and a low dispersity of the polyester. HL, containing a 5-methyl substituent, showed a much higher reactivity (polymerization rate) and a slightly higher equilibrium conversion than the compounds with longer alkyl substituents (NL and DL). The effectiveness of DPP-catalyzed ROP of 5-alkyl δ-lactones facilitated the one-pot performance following the t-BuP4-promoted ROP of monosubstituted epoxides. It has been shown in an earlier study that substituted polyethers acted as “slow initiators” for non-substituted lactones. However, efficient initiations were observed in the present study as substituted lactones were polymerized from the substituted polyethers. Therefore, this reinforces the previously developed “catalyst switch” strategy, making it a more versatile tool for the synthesis of well-defined polyether–polyester block copolymers from a large variety of epoxide and lactone monomers.

Organocatalytic copolymerization of mixed type monomers

Triggered by environmental concerns and the rising demands for metal-free polymers in e.g. bio-related and microelectronic applications, studies on organocatalytic polymerization have been launched and developed unprecedentedly during the last 15 years. A wide range of organic molecules are now available in polymer chemists’ toolbox to choose from as catalysts for polymerization of (hetero)cyclic and polar vinyl monomers. Apart from the intrinsic merits such as lower toxicity and better solubility compared with (transition) metal catalysts/initiators, organocatalysts have also shown, in many cases, excellence to achieve high polymerization rates and/or good control (selectivity). In addition, particular natures and catalytic/activating mechanisms of organocatalysts have led to new opportunities for rational design and efficient synthesis of macromolecular architectures, i.e. chain structures, topological structures and functionalities. This mini-review is specially themed on pathways to construct copolymer chain structures by organocatalytic copolymerization of mixed type monomers (comonomers bearing different polymerizing moieties) and will be sectioned by different comonomer combinations, including cyclic monoesters of different sizes, cyclic monoesters and lactides, cyclic esters and cyclic carbonates or epoxides, heterocycles and vinyl monomers.

Revealing the Cytotoxicity of Residues of Phosphazene Catalysts Used for the Synthesis of Poly(ethylene oxide)

We herein report a case study on the toxicity of residual catalyst in metal-free polymer. Eight-arm star-like poly(ethylene oxide)s were successfully synthesized via phosphazene-catalyzed ring-opening polymerization of ethylene oxide using sucrose as an octahydroxy initiator. The products were subjected to MTT assay using human cancer cell lines (MDA-MB-231 and A2780). Comparison between the crude and purified products clearly revealed that the residual phosphazenium salts were considerably cytotoxic, regardless of the anionic species, and that the cytotoxicity of more bulky t-BuP4 salt was higher than that of t-BuP2 salt. Such results have therefore put forward the necessity for removal of the catalyst residues from PEO-based polymers synthesized through phosphazene catalysis for biorelated applications and for the development of less or nontoxic organocatalysts for such polymers.

Polymerization Using Phosphazene Bases

In the recent rise of metal-free polymerization techniques, organic phosphazene superbases have shown their remarkable strength as promoter/catalyst for the anionic polymerization of various types of monomers. Generally, the complexation of phosphazene base with the counterion (proton or lithium cation) significantly improves the nucleophilicity of the initiator/chain end resulting in highly enhanced polymerization rates, as compared with conventional metal-based initiating systems. In this chapter, the general features of phosphazene-promoted/catalyzed polymerizations and the applications in macromolecular engineering (synthesis of functionalized polymers, block copolymers, and macromolecular architectures) are discussed with challenges and perspectives being pointed out.

Betulin-Constituted Multiblock Amphiphiles for Broad-Spectrum Protein Resistance

Multiblock-like amphiphilic polyurethanes constituted by poly(ethylene oxide) and biosourced betulin are designed for antifouling and synthesized by a convenient organocatalytic route comprising tandem chain-growth and step-growth polymerizations. The doping density of betulin (DB) in the polymer chain structure is readily varied by a mixed-initiator strategy. The spin-coated polymer films exhibit unique nanophase separation and protein resistance behaviors. Higher DB leads to enhanced surface hydrophobicity and, unexpectedly, improved protein resistance. It is found that the surface holds molecular-level heterogeneity when DB is substantially high due to restricted phase separation; therefore, broad-spectrum protein resistance is achieved despite considerable surface hydrophobicity. As DB decreases, the distance between adjacent betulin units increases so that hydrophobic nanodomains are formed, which provide enough landing areas for relatively small-sized proteins to adsorb on the surface.

Three-Dimensional Bacterial Behavior Near Dynamic Surfaces Formed by Degradable Polymers

Understanding the behavior of bacteria near biodegradable surfaces is critical for the development of biomedical and antibiofouling materials. By using digital holographic microscopy (DHM), we investigated the three-dimensional (3D) behavior of Escherichia coli and Pseudomonas sp. in lipase-containing aquatic environments near dynamic surfaces constructed by biodegradable poly(ε-caprolactone) (PCL)-based polymers in real time. As the enzymatic degradation rate increases, the percentage of near-surface subdiffusive bacteria and consequently, the irreversible adhesion decreases. Atomic force microscopy (AFM) measurements reveal that the adhesion force between bacteria and the surfaces decreases with an increasing degradation rate. In addition, the degradation products elicit a negative chemotactic response in E. coli, further driving them away from the dynamic surfaces through more frequent tumbling motion. Our study clearly demonstrates that bacterial adhesion can be reduced on dynamic surfaces formed by degradable polymers.

Schlenk Techniques for Anionic Polymerization

Anionic polymerization-high vacuum techniques (HVTs) are doubtlessly the most prominent and reliable experimental tools to prepare polymer samples with well-defined and, in many cases, complex macromolecular architectures. Due to the high demands for time and skilled technical personnel, HVTs are currently used in only a few research laboratories worldwide. Instead, most researchers in this filed are attracted to more facile Schlenk techniques. The basic principle of this technique followed in all laboratories is substantially the same, i.e. the use of alternate vacuum and inert gas atmosphere in glass apparatus for the purification/charging of monomer, solvents, additives, and for the manipulation of air-sensitive compounds such as alkyl metal initiators, organometallic or organic catalysts. However, it is executed quite differently in each research group in terms of the structure of Schlenk apparatus (manifolds, connections, purification/storage flasks, reactors, etc.), the use of small supplementary devices (soft tubing, cannulas, stopcocks, etc.) and experimental procedures. The operational methods are partly purpose-oriented while also featured by a high flexibility, which makes it impossible to describe in detail each specific one. In this chapter we will briefly exemplify the application of Schlenk techniques for anionic polymerization by describing the performance of a few experiments from our own work.