Synthesis of amphiphilic macrocyclic graft copolymer consisting of a poly(ethylene oxide) ring and multi-poly(ɛ-caprolactone) lateral chains

Applications of MALDI-TOF-MS in structural characterization of synthetic polymers

In recent years, matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) has been utilized to rapidly and precisely characterize the detailed molecular structures of synthetic polymers. This review summarizes recent progress regarding MALDI-TOF-MS for the characterization of synthetic polymers with a focus on specific important experimental aspects including sample preparation, the choice of matrix, the effects of cationizing agents and solvents, data processing and various applications. Finally, the recent trend of MALDI-TOF-MS development is discussed. We hope this review will be instructive for graduate students and junior users who need to use MALDI-TOF-MS as a necessary characterization technique for new synthetic polymers.

Recent advances in the construction of cyclic grafted polymers and their potential applications

Three main strategies used for the construction of cyclic grafted polymers, “grafting through”, “grafting onto”, and “grafting from”, are summarized.

Conventional Oxyanionic versus Monomer-Activated Anionic Copolymerization of Ethylene Oxide with Glycidyl Ethers: Striking Differences in Reactivity Ratios

Detailed understanding of the monomer distribution in copolymers is essential to tailor their properties. For the first time, we have been able to utilize in situ ¹H NMR spectroscopy to monitor the monomer-activated anionic ring opening copolymerization (AROP) of ethylene oxide (EO) with a glycidyl ether comonomer, namely, ethoxy ethyl glycidyl ether (EEGE). We determine reactivity ratios and draw a direct comparison to conventional oxyanionic ROP. Surprisingly, the respective monomer reactivities differ strongly between the different types of AROP. Under conventional oxyanionic conditions similar monomer reactivities of EO and EEGE are observed, leading to random structures (r_EO = 1.05 ± 0.02, r_EEGE = 0.94 ± 0.02). Addition of a cation complexing agent (18-crown-6) showed no influence on the relative reactivity of EO and EEGE (r_EO = r_EEGE = 1.00 ± 0.02). In striking contrast, monomer-activated AROP produces very different monomer reactivities, affording strongly tapered copolymer structures (r_EO = 8.00 ± 0.16, r_EEGE = 0.125 ± 0.003). These results highlight the importance of understanding reactivity ratios of comonomer pairs under certain polymerization conditions, at the same time demonstrating the ability to generate both random and strongly tapered P(EO-co-EEGE) polyethers by simple one-pot statistical anionic copolymerization. These observations may be generally valid for the copolymerization of EO and glycidyl ethers.

Synthesis, characterization, micellization and application of novel multiblock copolymers with the same compositions but different linkages

Several novel multiblock copolymers, (PEO-b-PS-b-PEO-Diyne)_s, [PEO-b-PS-b-PEO-(OH)₄]_s and (PEO-b-PS-b-PEO-Acetal)_s, with the same compositions but different linkages were constructed, and their micellization and application were studied.

Physiologically relevant, pH-responsive PEG-based block and statistical copolymers with N,N-diisopropylamine units

In order to impart pH-responsiveness within a physiologically-relevant context to PEG-based biomaterials, a new tertiary amine containing repeat unit, N,N-diisopropyl ethanolamine glycidyl ether (DEGE), was developed and incorporated into statistical and block copolymers with ethylene oxide (EO), and allyl glycidyl ether (AGE) via anionic ring-opening polymerization. The reactivity of this novel monomeric building block in copolymerizations with EO was investigated by spectroscopy with observed reactivity ratios of rDEGE = 1.28 ± 0.14 and rEO = 0.82 ± 0.10. It was further demonstrated that DEGE containing copolymers could serve as building blocks for the formation of new pH-responsive materials with a pKa of ca. 9, which allowed macroscopic hydrogels to be prepared from symmetric triblock copolymers PDEGE5.3k-b-PEO20k-b-PDEGE5.3k. The triblock copolymers exhibited clear sol-to-gel transitions in a physiologically-relevant critical gelation range of pH 5.8-6.6 and pH-dependent viscoelastic properties. On the nanometer scale, the preparation of pH-responsive micro- or nanogels was demonstrated by crosslinking P(DEGE-co-AGE) copolymers in miniemulsion droplets stabilized by PEO-b-P(DEGE-co-AGE) diblock terpolymers. These nanoparticles exhibited a reversible pH-dependent swelling profile with a volume phase transition at physiological pH 6.5-7.5.

Hyperbranched PEG by random copolymerization of ethylene oxide and glycidol

The synthesis of hyperbranched poly(ethylene glycol) (hbPEG) in one step was realized by random copolymerization of ethylene oxide and glycidol, leading to a biocompatible, amorphous material with multiple hydroxyl functionalities. A series of copolymers with moderate polydispersity ($\overline {M} _{{\rm w}} /\overline {M} _{{\rm n}} $ < 1.8) was obtained with varying glycidol content (3-40 mol-%) and molecular weights up to 49 800 g mol(-1) . The randomly branched structure of the copolymers was confirmed by (1) H and (13) C NMR spectroscopy and thermal analysis (differential scanning calorimetry). MTS assay demonstrated low cell toxicity of the hyperbranched PEG, comparable to the highly established linear PEG.

Reactivity ratios, and mechanistic insight for anionic ring-opening copolymerization of epoxides

Reactivity ratios were evaluated for anionic ring-opening copolymerizations of ethylene oxide (EO) with either allyl glycidyl ether (AGE) or ethylene glycol vinyl glycidyl ether (EGVGE) using a benzyl alkoxide initiator. The chemical shift for the benzylic protons of the initiator, as measured by (1)H NMR spectroscopy, were observed to be sensitive to the sequence of the first two monomers added to the initiator during polymer growth. Using a simple kinetic model for initiation and the first propagation step, reactivity ratios for the copolymerization of AGE and EGVGE with EO could be determined by analysis of the (1)H NMR spectroscopy for the resulting copolymer. For the copolymerization between EO and AGE, the reactivity ratios were determined to be r(AGE) = 1.31 ± 0.26 and r(EO) = 0.54 ± 0.03, while for EO and EGVGE, the reactivity ratios were r(EGVGE) = 3.50 ± 0.90 and r(EO) = 0.32 ± 0.10. These ratios were consistent with the compositional drift observed in the copolymerization between EO and EGVGE, with EGVGE being consumed early in the copolymerization. These experimental results, combined with density functional calculations, allowed a mechanism for oxyanionic ring-opening polymerization that begins with coordination of the Lewis-basic epoxide to the cation to be proposed. The calculated transition-state energies agree qualitatively with the observed relative rates for polymerization.

Synthesis, rapid responsive thickening, and self-assembly of brush copolymer poly(ethylene oxide)-graft-poly(N,N-dimethylaminoethyl methacrylate) in aqueous solutions

Double hydrophilic brush copolymer poly(ethylene oxide)-graft-poly(N,N-dimethylaminoethyl methacrylate) (PEO-g-PDMAEMA) was successfully prepared via atom transfer radical polymerization (ATRP). We investigated the pH/thermoresponsive behaviors of PEO-g-PDMAEMA brush-shaped copolymer concentrated aqueous solutions by rheology. The observed LCST strongly decreased with increasing pH of the solutions, which was lower than that of linear block copolymer for different pH, indicating rapid thermoresponsiveness of the brush PDMAEMA chains. An unexpected shear thickening behavior was observed and could be tuned by the pH, resulting from the mobile nature and tractive force of the densely grafted hydrophobic chains of PDMAEMA at high pH. Self-assembly of the brush copolymer in a different pH and ionic strength environment was studied by transmission electron microscopy. A wormlike cylinder structure was formed at low pH. Fractals were observed for the brush copolymer aqueous solution in the presence of NaCl. The results showed that by adjusting the pH and NaCl concentration of the dispersions fractal aggregates with different topology were obtained. The observations reported here can supply a better understanding of the molecular self-assembling nature and be used to develop responsive materials with better performance.