Junpo He

Porous graphene-based materials by thermolytic cracking

Porous graphene monolith and porous composites of graphene oxide (GO) and silica were prepared by thermolytic cracking of graphene-coated, or GO/silica-coated polystyrene (PS) spheres. The spheres were synthesized through in situprecipitation polymerization of styrene using GO and poly(vinylpyrrolidone) as stabilizing agents. During polymerization, GO adsorbed to the surface of the PS particle due to its amphiphilicity as well as spontaneous grafting on GO basal plane. The GO-coated PS spheres were either reduced by hydrazine to graphene-coated PS spheres, or underwent a sol–gel reaction with tetraethyloxysilane (TEOS). These materials were finally subjected to thermolytic cracking in a thermogravimetry instrument or in a furnace under nitrogen up to 550 or 700 °C, resulting in graphene-based porous materials in which the pores are surrounded by graphene or GO/silica walls. The factors affecting the specific surface area were discussed. The method may serve as a new approach to fabricate 3D graphene-based porous materials.

Rate enhancement of nitroxide-mediated living free-radical polymerization by continuous addition of initiator

A new technique, which involves the continuous addition of a small amount of radical initiators, was developed with the assistance of computer simulation to increase the rate of nitroxide-mediated living free-radical polymerization. Using this method, the polymerization rate of styrene in the presence of 4-hydroxy-2,2,6,6-tetramethyl piperidinyl-1-oxy was increased more than 3-fold compared to that with one-batch addition of initiator, while the molecular weight and distribution remain the same, respectively, at higher monomer conversion.

Monte Carlo simulation on rate enhancement of nitroxide-mediated living free-radical polymerization

Four rate enhancing cases of nitroxide-mediated living free-radical polymerizations are simulated by the Monte Carlo method to investigate the effects of parameters on kinetics and molecular weight distribution of the resulting polymers. In all cases the equilibrium between growing and dormant chains shifts in favor of the growing chains under corresponding reaction conditions. The polymerization rates are therefore increased substantially without much loss in control of molecular weight and distribution of the products. The optimization of rate-enhancement in living free-radical polymerization in also discussed.

Redox chemistry between graphene oxide and mercaptan

We report here redox reactions between graphene oxide (GO) and mercaptans, which reduces GO to reduced graphene oxide (RGO) and oxidizes mercaptans into disulfides. The reduction processes of GO using various mercaptans as the reducing agents are investigated through XPS, TGA, FT-IR, Raman and EA analysis. The degree of reduction of RGO depends on molecular structure of mercaptans and is controlled by the reaction time. The redox reaction is also employed to oxidize mercaptans into disulfides in medium to high yields under moderate conditions. The mechanism of the redox reaction may involve nucleophilic ring opening of oxirane on GO by alkylthio moiety, followed by addition of another alkylthio group, leaving the resulting disulfide. The reduction of the hydroxy group could be more complex, involving both radical and anionic processes.

Easy synthesis of dendrimer-like polymers through a divergent iterative “end-grafting” method

We report here an easy method for the synthesis of dendrimer-like polymers with high branching functionality (1 → 8). The synthetic process involves iterative grafting reactions in a divergent way. A multi-functional core containing short segments of polyisoprene (PI), either as a star-like block copolymer of isoprene and styrene or as a linear triblock copolymer of isoprene, styrene and isoprene (coded G1), is epoxidized on the double bonds and grafted with a living block copolymer, polyisoprene-b-polystyrenyllithium (PI-b-PSLi), again with a short PI segment, through the ring-opening reaction of oxirane by polymeric anions. The resulting graft polymer, G2, possesses a definite number of PI segments at the periphery. These PI segments are further epoxidized followed by the ring-opening addition of PI-b-PSLi, affording G3. Repeating the process leads to the synthesis of a dendrimer-like polystyrene up to 5th generation with a polydispersity lower than 1.21, as measured by SEC. A feature of the process is the easily accessible high chain density in the final product, although defects exist due to steric hindrance in the reactions of high generations. The solution properties of the dendritic products are investigated using viscometry and dynamic and static laser light scattering on the molecular conformation. The results support a compact globular conformation model for the dendrimer-like products. In addition, the chain density of the products from the star-like core is higher than that of products from a linear triblock core. AFM results show that the dendritic products adopt flattened conformations and tend to form lateral sphere-like aggregates on mica substrate.

Synthesis of ABB′ and ABC star copolymers via a combination of NMRP and ROP reactions

We report here a facile method to prepare ABB′ and ABC type miktoarm star copolymers by a combination of nitroxide-mediated radical polymerization (NMRP) and ring opening polymerization (ROP). The first two arms were prepared by sequential NMRP of the same or different monomers, e.g. styrene and tert-butyl methacrylate (tBMA). The hydroxyl group was introduced between the two blocks by one-step radical addition to N-(2-hydroxyethyl)maleimide. The attached in-chain hydroxyl group was used to initiate the ring opening polymerization of L-lactide in the presence of DBU, forming the third block of poly(L-lactide). Thermal properties, microphase separation and solution self-assembly of the miktoarm star copolymers were investigated using TGA, DSC and TEM. It was found that the star copolymer underwent microphase separation to form two phases with a lamellar structure in the bulk state. In addition, hollow spheres were formed from ABC copolymer solution when a mixed solvent was used.

Organic–inorganic rod–coil block copolymers comprising substituted polyacetylene and poly(dimethylsiloxane) segments

In this article, we report the synthesis of rod–coil diblock copolymers comprising substituted polyacetylene and poly(dimethylsiloxane) (PDMS) segments through a precursor approach based on living polymerization. The precursor was prepared by sequential anionic vinyl polymerization of a designed template monomer, 2,3-di(n-hexylphenyl)-1,3-butadiene (n-HD), and the anionic ring-opening polymerization of a cyclic monomer, hexamethylcyclotrisiloxane (D3). Block copolymers with well-defined molecular weights were obtained as demonstrated by gel permeation chromatography (GPC), nuclear magnetic resonance (NMR) spectroscopy, and matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF MS). The polybutadiene segment of the precursor block copolymer was transformed into a substituted polyacetylene segment, or polyene, by dehydrogenation in the presence of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The self-assembly behavior of the resulting rod–coil diblock copolymers in dilute solution was investigated by using dynamic light scattering (DLS) and transmission electron microscopy (TEM). It was found that the block copolymers showed a strong tendency to form vesicular aggregates. In some cases intermediate morphologies were observed, such as large size particles and vesicles with loosely packed thick walls. These morphologies developed further into regular vesicles through orientational assembly of the polyacetylene segments, as indicated by the blue shift in UV-Vis and fluorescence spectra.

Probing the RAFT Process Using a Model Reaction between Alkoxyamine and Dithioester

A small-molecular model reaction was designed to probe the reversible addition–fragmentation chain transfer (RAFT) process. In this reaction, alkoxyamine releases radicals that react in situ with dithioester through the RAFT process, generating new radicals through the fragmentation of the intermediate radical. The new radicals can be trapped by free 2,2,6,6-tetramethyl-piperidinyl-N-oxyl radicals (TEMPO) from homolysis of alkoxyamine. The overall reaction is the crossover of the leaving groups between alkoxyamine and dithioester. The advantage of this model as a probe of the RAFT process is that it does not involve polymerization-related elementary reactions such as initiation, propagation, and chain length dependent termination. The kinetics of the model reaction were measured using high-performance liquid chromatography, and then fitted by Monte Carlo simulation to estimate rate coefficients. The obtained rate coefficients of addition for various dithioesters fell into a narrow range of 107–108 L mol–1 s–1, whereas the rate coefficient of fragmentation was model-dependent. It was also found that a significant fraction of the dithioester was consumed by an unspecified additional mechanism. A tentative explanation is proposed in which the intermediate radical undergoes a secondary RAFT reaction with dithioesters, forming a secondary intermediate that serves as a radical reservoir.

A Facile Synthesis of Cleavable Block Copolymers via Tandem Polymerizations of NMRP and ATRP

A functionalized compound, 4‐(2‐bromoisobutyryl)‐2,2,6,6‐tetra‐methylpiperidine‐1‐oxyl (Br‐TEMPO), was synthesized and used to synthesize block copolymers through tandem nitroxide‐mediated radical polymerization (NMRP) and atom transfer radical polymerization (ATRP). First, Br‐TEMPO was used to mediate the polymerization of styrene. The kinetics of polymerization proved a typical “living” nature of the reaction and the effectiveness in the mediation of polymerization of Br‐TEMPO. Then the PS‐Br macroinitiator was used to initiate atom transfer radical polymerization (ATRP). A series of acrylates were initiated by PS‐Br macroinitiators in typical ATRP processes at various conditions. The controlled polymerization of ATRP was also confirmed by molecular weight and kinetic analysis. Several cleavable block copolymers of PS‐b‐P(t‐BA), PS‐b‐P(n‐BA), and PS‐b‐PMA, with different molecular weights, were synthesized via this strategy. Relatively low polydispersities (<1.5) were observed and the molecular weights were in agreement with the theoretical ones. Hydrolysis of PS‐b‐P(t‐BA) was carried out, giving amphiphilic block copolymer PS‐b‐PAA without the cleavage of C‐ON bond or ester bond. All the block copolymers have two Tgs as demonstrated by DSC. A typical cleavable block copolymer of PS‐b‐PMA was cleaved by adding phenylhydrazine at 120°C to produce homopolymers in situ.

Synthesis of a [60]fullerene-end-capped polyacetylene derivative – a “rod-sphere” molecule from a “coil-sphere” precursor

Herein, we report the synthesis of a hybrid material composed of a head-to-head substituted polyacetylene end-capped with [60]fullerene through anionic polymerization. The synthesis was carried out in three successive steps: anionic polymerization of 2,3-diphenyl-1,3-butadiene, anionic addition of the resulting living chains towards C60, and dehydrogenation of the polymer segment in the presence of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Since the precursor polymer is a coil-like polybutadiene derivative and the dehydrogenated polymer is a rod-like conjugated segment, the molecular hybrids underwent a change from a “coil-sphere” to “rod-sphere” conformation before and after dehydrogenation. The structure and properties of the molecular hybrids were investigated via1H NMR, FTIR, GPC, FLS, UV and TGA. The self-assembly behaviors of both molecular hybrids in THF were studied using DLS and TEM. Moreover, we also observed an obvious optical limiting property of the hybrid product, which indicates that it may potentially be utilized as a functional opto-electronic material.