Macromolecules backbone

Up to now we have considered the CP film as an amorphous, homogeneous and isotropic medium. However, CPs are intrinsically anisotropic since the 7r-electrons are delocalized along the macromolecule backbone. An anisotropic optical response, typical of oriented samples, is extremely important both for fundamental science (e.g., comparison with theoretical predictions) and for technological reasons (polarized emission is recommended in displays [55,56]). The orientation process for conjugated polymers is very difficult and several approaches have been used to remove the typical random-coil conformation of CP and to induce the extended aligned conformation. It is not the aim of this article to review all orientational techniques used with CPs. However, it should be mentioned that very high degrees of orientation have been achieved... 

Many investigators have studied dissolved solutions of poly(organosiloxanes) with different side groups at silicon atoms in the macromolecule backbones [32 - 35], These works show results of the studies of the effect of the side groups origin, their disposition and the influence of hydrodynamic and conformation parameters of macromolecules. 

Liquid crystal forming polymers with flexible spacers in the side-chain were developed as a logical consequence of the analysis of side-chain crystallization. With sufficiently long side-chains, starting with about 10 chain atoms, the crystallization behavior becomes increasingly similar to low molecular mass molecules, i.e. the macromolecule backbone becomes only a connecting backbone with little consequence for the thermodynamics and kinetics of crystallization Detailed reviews of... 

The presence of multiple bonds in the macromolecule backbone chain greatly reduces its resistance to external effects. Introduction of heteroatoms (nitrogen, oxygen, sulfur) impairs polymer resistance to chemical agents. 

The visuahzation of hundreds or thousands of connected atoms, which are found in biological macromolecules, is no longer reasonable with the molecular models described above because too much detail would be shown. First of aU the models become vague if there are more than a few himdied atoms. This problem can be solved with some simplified models, which serve primarily to represent the secondary structure of the protein or nucleic acid backbone [201]. (Compare the balls and sticks model (Figure 2-124a) and the backbone representation (Figure 2-124b) of lysozyme.)... 

To understand the function of a protein at the molecular level, it is important to know its three-dimensional stmcture. The diversity in protein stmcture, as in many other macromolecules, results from the flexibiUty of rotation about single bonds between atoms. Each peptide unit is planar, ie, oJ = 180°, and has two rotational degrees of freedom, specified by the torsion angles ( ) and /, along the polypeptide backbone. The number of torsion angles associated with the side chains, R, varies from residue to residue. The allowed conformations of a protein are those that avoid atomic coUisions between nonbonded atoms. 

Copolymer macromolecules are composed of a single backbone having simple grafts attached to it, i.e., the macromolecules are of the comb-like type. No further grafting of grafted chains is contemplated (4). 

Polyethylene s simplicity of structure has made it one of the most thoroughly studied polymeric materials. With an estimated demand of close to 109 billion pounds in 2000 of the homopolymer and various copolymers of polyethylene,24 it is by far the world s highest volume synthetic macromolecule. Therefore, it is still pertinent to study its structure-property relationships, thermal behavior, morphology, and effects of adding branches and functional groups to the polymer backbone. 

The main chain of dendronized polymers, due to die large size of the mon-odendrons, is usually forced to take a stretched shape thus the whole molecule exists as a rigid rod architecture both in solution and in the solid state.32d Depending on the backbone stiffness, the degree of monodendron coverage, and the size of die monodendron, the architecture of these macromolecules is no longer a sphere but a cylinder this dictates die properties of the dendronized polymers. 

Two types of well defined branched polymers are acessible anionically star-shaped polymers and comb-like polymers87 88). Such macromolecules are used to investigate the effect of branching on the properties, 4n solution as well as in the the bulk. Starshaped macromolecules contain a known number of identical chains which are linked at one end to a central nodule. The size of the latter should be small with respect to the overall molecular dimensions. Comb-like polymers comprise a linear backbone of given length fitted with a known number of randomly distributed branches of well defined size. They are similar to graft copolymers, except that backbone and branches are of identical chemical nature and do not exhibit repulsions. 

Although the name polyurethane might be taken as implying that these materials contain urethane groups (—NHCOO—) in the backbone of the macromolecule, for those polyurethanes in major commercial use this is not tme. For such materials the initial macromolecule tends to be a polyester or polyether it is the crosslinks that involve the formation of a polyurethane stmcture. 

Root-mean-square end-to-end distance, which effectively takes account of the average distance between the first and the last segment in the macromolecule, and is always less that the so-called contour length of the polymer. This latter is the actual distance from the beginning to the end of the macromolecule travelling along the covalent bonds of the molecule s backbone. Radius of gyration, which is the root-mean-square distance of the ele-... 

This review has shown that the analogy between P=C and C=C bonds can indeed be extended to polymer chemistry. Two of the most common uses for C=C bonds in polymer science have successfully been applied to P=C bonds. In particular, the addition polymerization of phosphaalkenes affords functional poly(methylenephosphine)s the first examples of macromolecules with alternating phosphorus and carbon atoms. The chemical functionality of the phosphine center may lead to applications in areas such as polymer-supported catalysis. In addition, the first n-conjugated phosphorus analogs of poly(p-phenylenevinylene) have been prepared. Comparison of the electronic properties of the polymers with molecular model compounds is consistent with some degree of n-conjugation in the polymer backbone. 

In the same scheme, moreover, it is evident that, besides phosphazene homopolymers, the substitution of the chlorines with two (or more) different substituents leads to the preparation of substituent phosphazene copolymers [263] containing different homosubstituted and heterosubstituted monomeric units. Moreover, the cationic polymerization of phosphoranimines [215-217] produces polymers with hving reactive ends (vide supra) from which the preparation of chain phosphazene copolymers (block copolymers) [220,223,225, 229,232-235,239, 240] formed by different polymeric backbones linked together in a unique macromolecule could be obtained. 

In fact, considering the basic structure of these materials (vide supra), it can be immediately realized that the basic features of poly(organophosphazenes) are the result of two main contributions. The first one is fixed and is basically related to the intrinsic properties of the -P=N- inorganic backbone, while the second is variable and mostly connected to the chemical and physical characteristics of the phosphorus substituent groups. Skeletal properties in phos-phazene macromolecules intrinsically due to the polymer chain are briefly summarized below. 

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