Polymeric LEDs

Horowitz et al. have prepared bilayer LEDs [130] using unsubstituted sexithio-phene (6T) and substituted derivatives, either substituted with two decyl side-chains, (2D6T), as shown in Fig. 2c or with triisopropylsilyl end-groups, (DPS6T), as in Fig. 2d. Compared to single-layer ITO/6T/A1 devices, the quantiun yield of bilayer LEDs can be increased by three to four orders of magnitude, from 

5 X 10 % for the bilayer device ITO/6T/DPS6T/A1. There are two main effects which contribute to this improvement  

Higher EL efficiency due to narrower tunnelling barrier for electrons 

ITO anode and a Mg/Ag cathode, with an Alq3 layer next to the cathode, as electron-transport layer. The bright yellow emission was observed to originate from the oligothiophene layer, also in the double-layer device. 

The relative ease of inducing alignment of discrete oligomer chains, rather than their polymeric counterparts may yet render the use of oligomer films particularly advantageous in certain applications, such as waveguided or microcavity electroluminescent devices. 

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Early efforts to produce synthetic mbber coupled bulk polymerization with subsequent emulsification (9). Problems controlling the heat generated during bulk polymerization led to the first attempts at emulsion polymerization. In emulsion polymerization hydrophobic monomers are added to water, emulsified by a surfactant into small particles, and polymerized using a water-soluble initiator. The result is a coUoidal suspension of fine particles,... 

Experimental information for polymeric LED is sparse. A key problem for conventional lime-of-flighl studies is that the condition of generating a sheet of carriers whose spatial extension is small compared to the thickness of a = 100 nm film is difficult to meet. On the other hand, thick films fabricated, for instance, by solvent casting rather than by spin coating, may differ regarding their morphology and, concomitantly, the built-in-disorder. On the fundamental side, transport in... 

Our discovery that epoxides can initiate carbocationic polymerization led to the effective direct functionalization of PIBs with hydroxyl groups. Figure 7.18 shows our novel method of direct surface functionalization of SDIBSs using 4-(l,2-oxirane-isopropyl)-styrene, a new inimer. 

PPy was the first conducting polymer to be structurally analyzed. The discovery that a, a -disubstituted pyrrole did not polymerize led to the conclusion that the... 

These efforts coupled with the much earlier work on sodium and lithium initiated polymerizations led to an appreciation of the stereospecificity of the alkyllithium initiators for diene polymerization both industrially and academically. Polymerization of isoprene to a high cis polyisoprene with butyllithium is well known and the details have been well documented 2 Control over polybutadiene structure has also been demonstrated. This report attempts to survey the unique features of anionic polymerization with an emphasis on the chemistry and its commercial applications and is not intended as a comprehensive review. 

The propylene oxide complex not only dissociated into its components but also transformed to either an oligomer or a polymer of propylene oxide when it was allowed to stand in solution. This transformation could be followed by H-NMR techniques with the use of a-deuterated propylene oxide instead of the non-deuterated one. Its rate depended on the nature of solvent and on the temperature. This experimental result implies that the monomer liberated by dessociation of the complex is polymerized by the catalyst, that only a minute fraction of the organozinc component of the complex actually acts as a catalyst for polymerization, and that the rate of propagation is far faster than that of initiation. These implications together with the evidence that coordination of the monomer to the catalyst is a prerequisite for the stereospecific polymerization led us to the detailed studies of the bulk polymerization, that is, the polymerization of propylene oxide in propylene oxide solution. 

The analysis of the reaction serum (the continuous phase without polymer particles) at the end of polymerization led to the conclusion that the molecular weight of the soluble oligomers of styrene and PEO macromonomer varied from 200 to 1100 g mol-1. This indicates that the critical degree of polymerization for precipitation of oligomers in this medium is more than ten styrene units and only one macromonomer unit per copolymer chain. Several reasons for the low molecular weight of the soluble copolymers were proposed, such as the thermodynamic repulsion (or compatibility) between the PEO chain of the macromonomer and the polystyrene macroradical, the occurrence of enhanced termination caused by high radical concentration, and, to a lower extent, a transfer reaction to ethanol [75]. 

Such hydrophilic macromonomers (DPn=7-9) were radically homopolymer-ized and copolymerized with styrene [78] using AIBN as an initiator at 60 °C in deuterated DMSO in order to follow the kinetics directly by NMR analysis. The macromonomer was found to be less reactive than styrene (rM=0.9 for the macromonomer and rs=1.3 for styrene). Polymerization led to amphiphilic graft copolymers with a polystyrene backbone and poly(vinyl alcohol) branches. The hydrophilic macromonomer was also used in emulsion polymerization and copolymerized onto seed polystyrene particles in order to incorporate it at the interface. 

P-Pinene which is a main component of natural turpentine can be polymerized by living cationic isomerization polymerization [82] (Scheme 10) using TiCl3(OfPr) as a Lewis acid in conjunction with rc-Bu4NCl in CH2C12 at -40 °C. When initiator 31 was used, polymerization led to a poly(P-pinene) macromonomer with a methacrylate function at the a end and a chlorine atom at the co chain end [83]. Three macromonomers were prepared with DPn=8,15, and 25 respectively they had narrow MWD (Mw/Mn= 1.13-1.22) and the reported functionality was close to 1 (Fn=0.90-0.96). 

Since the calcium or other metals used so far as electrodes must be rigorously protected from the atmosphere to prevent degradation further applications have to find stable electrode materials. Also, the attainable electric currents and the efficiency of the polymeric LED depends sensitively on the polymer-electrode interface. 

A characteristic nature of the cathodically initiated copolymerisation of isoprene maleate was found [39]. The cathodic polymerization afforded a copolymer with an alter-nting structure with 1,4-isoprene units only and the ratio cis- Altrans-, A = lY chemical polymerization led to a mixture of alternating block and cyclic structures. Stereoregularity has been found in a few studies of the electropolymerization of vinyl compounds. 

The significant contribution by the Nolte and Drenth group in this area was that, for the first time, they demonstrated the existence of a non-racem-ic helical structure for poly(isocyanide)s. Optical resolution using a chiral HPLC technique or asymmetric polymerization led to the isolation of optically active polymers, whose chirality was supposed to be solely due to the main chain helicity. Their effort, in conjunction with that of Novak s and Takahashi s significant contributions to asymmetric polymerization, will be discussed in the next section. Non-asymmetric and asymmetric polymerizations will be described separately in the following sections. 

CHART 10.6 PAB co-polymers for polymeric LED. 2006 by Taylor Francis Group, LLC... 

High levels of local electrical conductivity Emission-free plastics Plastic disks with scratchproof coatings Polymeric LEDs Large-surface illuminated diodes Applications in liquid crystal displays... 

In addition to the considerable advances in the tuning of emission colors of polymeric LEDs, recent attention has focussed on improving the efficiency of devices. In order Corresponding author... 

Aromatic oxadiazole compounds such as 2-(4-biphenyl)-5-(4-teft-butyl-phenyl)-1,3,4-oxadiazole (PBD) have been used as electron transporting materials in organic light emitting diodes (15-17). Multi-layered polymeric LEDs with improved... 

The results just described demonstrate impressively that the transition from homopolymer LPPP 15 to segmented ladder polymers 18 with a partially interrupted conjugative interaction leads to blue LEDs without impairment of the specific characteristics. On the contrary, at least in our example, a very desirable improvement in individual parameters has been established. These results suggested the investigation also of other copolymers formed from oligomeric emitter segments of the PPP type and suitable "spacer" units for their suitability as active components in polymeric LEDs. 

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