Shish-kebab polymers

The optical and electrochemical properties of porphyrins make these chromophores useful building blocks for the synthesis of electro- and photoactive polymers. Two types of linear polymers have been constructed using the self-assembly approach homo-polymeric assemblies and hetero- or shish kebab polymers. 

Maldotti (96) studied the kinetics of the formation of the pyrazine-bridged Fe(II) porphyrin shish-kebab polymer by means of flash kinetic experiments. Upon irradiation of a deaerated alkaline water/ethanol solution of Fe(III) protoporphyrin IX and pyrazine with a short intense flash of light, the 2 1 Fe(II) porphyrin (pyrazine)2 complex is formed, but it immediately polymerizes with second-order kinetics. This can be monitored in the UV-Vis absorption spectrum, with the disappearance of a band at 550 nm together with the emergence of a new band due to the polymer at 800 nm. The process is accelerated by the addition of LiCl, which augments hydrophobic interactions, and is diminished by the presence of a surfactant. A shish-kebab polymer is also formed upon photoreduction of Fe(III) porphyrins in presence of piperazine or 4,4 -bipyridine ligands (97). 

Collman and eoworkers have reported similar shish kebab polymers of the type lM(OEP))(L-L), (M = Fe. Ru, Os, and L-L = py ., bpy, dabeo) and examined their conduetivities. -Many of the these shish kebab ctKrrdination ixdymers can be doped w ith iodine, w hich drastically increases their conductivity (e.g., rTiooK -... 

Q cm for [Fe"(Pc)(pyz)l2.54Jx)- The conductivity of these doped shish kebab polymers depends on the interaction of the metal d,j orbitals with the n level of the bridging ligand. Collman et al. found that the metal-metal commu-... 

Phthalocyanine-based polymers, such as the shish-kebab polymers (60), are also of considerable interest and significant electrical conductivities of up to ca 0.1 S/cm have been detected for chemically or electrochemically doped materials (187,188). If flexible organic substituents are present on the periphery of the phthalocyanine ring, these materials can also be soluble (at least low molecular weight fractions). 

Li L, Li B, Hood MA, Li CY. Carbon nanotube induced polymer crystallization The formation of nanohybrid shish-kebabs. Polymer 2009 50 953-965. 

Many of the bridged macrocyclic metal complexes [MacM(L)] ( shish-kebab polymers) can be prepared in high yields and high purity by treating the metallomacrocycle MacM with the pure ligand or with... 

Fig. 3a-e. Supermolecular structures of polymers crystallized in various force fields a structure of the shish-kebab type, b structure formed during crystallization in a capillary with a conical inlet and c structure of a polymer crystallized at hydrostatic compression at 4 x 108 Pa... 

Heterogeneous nucleation of polymer crystallization resembling a visualized metaphor compare the way meat is prepared in an oriental way shish - kebab. 

Hanack has reviewed a number of phthalocyanine and tetrabenzopor-phyrin shish-kebab porphyrin polymers (102). 

Usually, synthetic polymers crystallize11 j15 from a melt or a solution in form of folded lamellae. Under specific circumstances it is sometimes also possible to obtain extended chain crystals which is the preferred arrangement in the crystallites of many natural polymers (cellulose, silk). Recently it has been found33 31 that in some cases another crystalline modification can be obtained, the so-called shish-kebabs, which are a sort of hybrid between folded lamellae and extended chain crystals. These shish-kebabs are obtained by shear-induced crystallization, a process in which the polymer crystallizes from solution under the influence of an elongated flow. 

By special processes, like polymerization under shear conditions or flash-spinning of polymer solutions, it is furthermore possible to obtain materials which do not only have this new shish-kebab micro-morphology, but also a new macro-morphology, namely the polymer fibrids.35... 

L. Zhang, T. Tao, C. Li, Formation of polymer/carbon nanotubes nano-hybrid shish-kebab via non-isothermal crystallization, Polymer, vol. 50, pp. 3835-3840, 2009. 

FIGURE 2.17 Crystalline polymer structures formed under applied tension including flow conditions. The center shows the tertiary monofibriUar structure including platelets, the left shows these mono-fibrillar structures bundled together forming a quaternary structure fibril, and the right shows the distorted shish kebab formed with more rapid flow. 

When polymers are crystallized under flow (stirring, extensional, etc.), the ubiquitous morphology [23] is the shish-kebab structure, consisting of central core (shish) surrounded by lamellae (kebabs) attached along the shish. What is the underlying mechanism behind the formation of shish-kebab stmcture ... 

In general, if the chains are oriented by external forces, the melting temperature is increased [28]. Further, it has been known for a long time that complex, row-nucleated structures occur if polymer solutions or melts are crystallized in the presence of flow [29,30]. These so-called shish-kebabs (sketched in Fig. 1.6), consist of a central fiber core, shish, surrounded by lamellar crystalline structures, kebabs, periodically attached along the shish. 

The fiberlike crystalline structures (shish) are highly stable to the point that they can be superheated [31]. Therefore, it is believed that the core of the shish is formed by crystallization of completely stretched polymer chains. The kebabs are believed to be folded-chain lamellar structures. The direction of growth of the kebabs is normal to the shish. The chain ahgnment in the kebabs is believed to be parallel to the shish. Similar structures are obtained by crystalhzation in polymer melt films exposed to orientational deformation [32,33]. These two-dimensional shish-kebabs also consist of a central fiber, shish, and periodically attached linear kebabs, with growth direction normal to the shish. 

Among the numerous challenges faced in understanding the formation and evolution of hierarchical structures in polymer crystallization, we restrict ourselves to explain the essential basic features of folded lamellae. Specihcally, we consider (1) molecular origin of enhanced scattered intensity before any crystallographic features are apparent, (2) spontaneous selection of small lamellar thickness, (3) molecular details of growth front, and (4) formation of shish-kebab structures in the presence of a flow. 

---