Sung Chul Hong

Preparation of polyisobutene-graft-poly(methyl methacrylate) and polyisobutene-graft-polystyrene with different compositions and side chain architectures through atom transfer radical polymerization (ATRP)

Polyisobutene-graft-poly(methyl methacrylate) and polyisobutene-graft-polystyrene with controlled compositions and side chain architectures were prepared through atom transfer radical polymerization (ATRP). Poly[isobutene-co-(p-methylstyrene)-co-(p-bromomethylstyrene)] (PIB) was used as a macroinitiator in the presence of CuCl or CuBr as a catalyst and dNbpy as a ligand. The compositions were controlled by the conversion of the monomer with polymerization time. The molecular weight distributions of the side chains were controlled through ATRP in the presence/absence of a halogen exchange reaction. DSC and DMA measurements showed that graft copolymers have two glass transition temperatures suggesting microphase separated behavior, which was also confirmed by SAXS measurements. The phase and dynamic mechanical behaviors were strongly affected by the compositions and/or the side chain architectures. The properties of the graft copolymers were controlled in a wide range leading to toughened glassy polymers or elastomers.

Vegetable oil-based polyols for sustainable polyurethanes

Vegetable oils (VOs) are one of the most important bio-renewable resources in the chemical industry due to their biodegradability, universal availability and low price. Using VO-based polyols to substitute petroleum-based polyols is considered a prime and novel route towards preparing sustainable polyurethane (PU). In this context, recent progress on preparing VO-based polyols and PUs is briefly summarized in this article. Especially, thiol-ene reactions allow for the synthesis of VO-based polyols with different functionalities in keeping with efficient click chemistry principles. The studies indicated that VO-based polyols and the resulting PUs can be eco-friendly families of industrially important polymers with a low environmental footprint.

Characteristics of dye-sensitized solar cells with surface-modified multi-walled carbon nanotubes as counter electrodes

A polystyrene-based functional block copolymer is employed as a surface modifier for multi-walled carbon nanotube (MWCNT) paste utilized in the fabrication of a MWCNT counter electrode (CE) in dye-sensitized solar cells (DSSCs). The surface modification of MWCNTs paste improves the dispersibility of MWCNTs, resulting in a facilitated fabrication of electrodes through the screen printing procedure, as evidenced by a lower viscosity and more homogeneous paste, as well as a more uniform MWCNT coating. Upon removing organic compounds from the paste through a thermal treatment procedure, the DSSC with the modified CE exhibits enhanced solar energy conversion efficiency (η) compared with that of the neat MWCNT CE. The behavior stems from an improvement in the overall redox reaction kinetics and the short-circuit current (Jsc) of the DSSC. The DSSC also exhibits an improved η value over an extended storage period, which demonstrates a successful combination of processability, performance, and stability of the DSSC achieved by using an optimum amount of surface modifier for MWCNTs.

Cu catalyst system with phosphorous containing bidendate ligand for living radical polymerization of MMA

The polymerization of methyl methacrylate (MMA) was carried out using CuBr/bidentate phosphorus ligand catalyst systems. MMA polymerization with CuBr/ phosphine-phosphinidene (PP) exhibited high conversion (≈80%) in 5 h at 90°C along with a linear increase of ln([M]0/[M]) versus time, indicating constant concentration of the propagating radicals during the polymerization. The molecular weight of the prepared PMMA tended to increase with conversion, suggesting the living polymerization characteristic of the system. On the other hand, a large difference between the measured and theoretical molecular weight and a broad molecular weight distribution were observed, implicating possible incomplete control over the polymerization. This may have been caused by the low deactivation rate constant (kdeact) of the system. The lowkdeact would result in irreversible generation of radicals instead of reversible activation/deactivation process of ATRP. Polymerizations performed at different ligand to CuBr ratios and different monomer to initiator ratios did not afford better control over the polymerization, suggesting that the controllability of CuBr/phosphorus ligand system for ATRP is inherently limited.

UV-cross-linked block copolymers for initiator-free, controlled in situ gelation of electrolytes in dye-sensitized solar cells

In this work, a block copolymer as an initiator- and monomer-free UV-cross-linked component for the controlled in situ gelation of electrolytes in dye-sensitized solar cells (DSSCs) is proposed. This block copolymer consists of one block of p-hydroxystyrene units and the other block with poly(ethylene oxide-co-propylene oxide) side chains. This block copolymer is successfully prepared by means of nitroxide-mediated polymerization, imidization reactions and successive hydrolysis reactions. When a homogeneous mixture of this copolymer and a typical liquid electrolyte (LE) is irradiated with UV, the cross-linking is efficient and controlled. The electrolyte in the DSSCs transforms in situ to a gel polymer electrolyte when exposed to UV, affording DSSCs with a relatively high initial solar-to-electricity energy conversion efficiency (η) and improved long-term stability that is significantly greater than that of the DSSCs with LE. These results demonstrate that the electrolyte system containing the UV-cross-linked block copolymer improves the long-term performance of DSSCs and contributes to the development of efficient and stable DSSCs.

Dispersion controlled platinum/multi-walled carbon nanotube hybrid for counter electrodes of dye-sensitized solar cells

AbstractPlatinum (Pt) nanoparticles are incorporated in dispersion controlled multi-walled carbon nanotubes (CNTs) for counter electrodes (CEs) in dye-sensitized solar cells (DSSCs). The dispersion of CNTs is controlled by a styrenic block copolymer as a surface modifier, facilitating the fabrication of the CE on F-doped SnO glass (FTO glass) by a screen printing method. By employing sufficiently large amounts of CNTs in a paste and using a stepwise thermal treatment procedure in which organic compounds are successfully removed from the paste while minimising damage to the CNTs, a conductive and uniform CNT coating can be screen-printed onto FTO glass. Pt precursors in the pastes transform to discrete Pt nanoparticles under the thermal treatment conditions, thereby successfully producing a Pt/CNT composite. Incorporation of the small amount of Pt nanoparticles into the CNTs results in significant improvements in the performances of the DSSCs. The DSSC with the Pt/CNT CE also exhibits very stable solar-to-electricity energy conversion efficiency (η) values for extended light soaking times that are superior to those of DSSC with the standard Pt CE. The results demonstrate the successful combination of DSSC processability, performance and stability enabled by employing Pt/CNT hybrid CEs.

Study on Immobilized Metallocene and Single-Site Catalysts for the Preparation of Ultra-High Molecular Weight Polyethylene at Various Polymerization Conditions

Ultra-high molecular weight polyethylene (UHMWPE) possesses advantages over conventional polyolefins such as excellent mechanical properties. Recent progresses in transition-metal complexes have led to the discovery of highly active catalysts for the preparation of UHMWPE. In this study, Ti with bis(phenoxy-imine) ligand (FI catalyst) and Me2Si(C5Me4)(N-tBu)TiCl2 (CGC) were immobilized on silica and tested for the preparation of UHMWPE. Results revealed that soluble FI catalyst and CGC can produce polyethylene having relatively high molecular weight above 106 g/mol, which also can be successfully immobilized on carriers for better adaptability to production processes.

Synthesis of Bis(indolyl)methanes Using Hyper-Cross-Linked Polyaromatic Spheres Decorated with Bromomethyl Groups as Efficient and Recyclable Catalysts

Highly uniform and hyper-cross-linked polyphenanthrene and polypyrene microspheres were synthesized by Friedel–Crafts bromomethylation of phenanthrene (Phn) and pyrene (Py) in the presence of zinc bromide as a catalyst, followed by self-polymerization of bromomethylated Phn and Py. The resultant 3-D carbon microspheres consisting of micro-, meso-, and macropores bear peripheral unreacted bromomethyl groups, which are directly utilized as catalysts to efficiently promote the electrophilic substitution reaction of indoles with aldehydes to yield a variety of bis(indolyl)methanes. The important features of this catalysis are easy catalyst synthesis, high product yields, environmental benignity, short reaction time, broad substrate scope, use of nontoxic solvents, and recyclability.

Study on the controlled preparation of polyolefin based block or graft copolymers

Department of Nano Science and Technology, Sejong University, 98 Gunja-dong, Gwanjin-gu, Seoul 143-747, Korea (south)(Received December 4, 2012, Revised December 12, 2012, Accepted December 17, 2012)요 약：폴리올레핀은 광범위한 분야에서 이용되는 범용성 고분자로 물성이 우수하고 가격경쟁력이 높기 때문에 오랜 시간 동안 산업적 요구에 따라 발전하여 왔다. 그러나 폴리올레핀은 비극성 재료로서 다른 물질과의 상호 작용이 부족하기 때문에 그 용도가 제한되고 있다. 따라서 폴리올레핀 사슬에 극성기를 도입함으로써 그 응용 분야를 확장하기 위한 노력이 계속되고 있다. 폴리올레핀에 기능성을 부여하기 위하여 블록 공중합체 및 그라프트 공중합체로 대표되는 분절 공중합체를 합성할 수 있으며, 이러한 공중합체는 폴리올레핀 고유의 물성 손실을 최소화함과 동시에 기능성을 부여할 수 있다는 점에서 주목 받고 있다. 또한 리빙 라디칼 중합법을 이용하면 잘 제어된 구조와 조성을 가지는 공중합체를 제조할 수 있으며, 다양한 중합공정에 적용될 수 있다. 이에 따라, 본 리뷰에서는 리빙 라디칼 중합법을 이용한 폴리올레핀 기반 블록 또는 그라프트 공중합체의 제조 예들에 대하여 정리해 보았다. ABSTRACTPolyolefin is one of the most important commodity polymers having excellent physical properties and cost competitiveness, which has continuously broadened their market in response to a heavy demand from industry. However, the lack of polarity in polyolefin has limited its applications, especially where interactions with other materials are important. In view of the above, the incorporation of polar functional groups in polyolefin has been widely attempted. Especially, the preparations of segmented modified polyolefin copolymers, such as block and graft copolymers have been extensively investigated, since the loss of the original properties of polyolefin can be minimized while the polar segments can endow interactions with other materials. Living radical polymerization (LRP) method can be one of the most attractive synthetic tools for the preparation of the modified polyolefin block or graft copolymers. In this review, progress on the preparation of the polyolefin based block or graft copolymers through LRP technique is briefly summarized.Keywordspolyolefin, polar functionalization, copolymer, block, graft, living radical polymerization

Implications of passivated conductive fillers on dielectric behavior of nanocomposites

AbstractIn order to fabricate a flexible composite of large dielectric constant, the preparation of dielectric fillers employing single-walled carbon nanotubes (SWNT) passivated with alkyl pyrenes having different alkyl chain lengths and their mixing with a polymer matrix were investigated. When the lengths of alkyl chains (n) of alkyl pyrene for SWNT passivation were varied from 0 to 4, dielectric constant of the composite were almost identical (K < 25). However, further increase regarding the length of alkyl chains in alkyl pyrene (n=12) dramatically increased the dielectric constant of the epoxy composite (K > 200) without loss of flexibility, despite the concentration of passivated SWNTs being as low as 0.04 wt% in the composite. Especially, highly suppressed dielectric loss was observed for the passivated SWNT/epoxy composite with alkyl pyrene containing an alkyl chain length of 12, which made it an ideal candidate for the embedded flexible capacitor prerequisite to flexible electronics. These dielectric behaviors were attributed to an effective debundling and partially aligned dispersion of SWNTs in the matrix, when alkyl chains with optimum length were employed in the alkyl pyrenes.