Homopolymer chains

Two other forms of copolymers that can be prepared under certain conditions are called block copolymers and graft copolymers. Block copolymers are those in which different blocks of identical monomer units alternate with each other graft copolymers are those in which homopolymer branches of one monomer unit are "grafted" onto a homopolymer chain of another monomer unit. 

Graft copolymer (Section 31.3) A copolymer in which homopolymer branches of one monomer unit are grafted onto a homopolymer chain of another monomer unit. 

The classical representation of a homopolymer chain, in which the end groups are disregarded and only one monomer residue is considered, allows no possibility for structural variation. However, possibilities for stercoscqucnce isomerism arise as soon as the monomer residue is considered in relation to its neighbors and the substituents X and Y are different. The chains have tacticity (Section 4,2.1). Experimental methods for tacticity determination are summarized in 4.2.2 and the tacticity of some common polymers is considered in 4.2.3. 

Triblock copolymers, as shown in Fig. 5.8 d), comprise a central homopolymer block of one type, the ends of which are attached to homopolymer chains of another type. As with other block copolymers, the components of triblocks may be compatible or incompatible, which will strongly influence their properties. Of particular interest are triblocks with incompatible sequences, the middle block of which is rubbery, and the end blocks of which are glassy and form the minor phase. When such polymers phase-segregate, it is possible for the end blocks of a single molecule to be incorporated into separate domains. Thus, a number of rubbery mid-block chains connect the glassy phases to one another. These materials display rubber-like properties, with the glassy domains acting as physical crosslinks. Examples of such materials are polystyrene/isoprene/polystyrene and polystyrene/polybutadiene/polystyrene triblock copolymers. 

Polymers that are built from the repetition of identical "repeat units" are called "homopolymers" (from ancient Greek "d/iog — same).2 Linear homopolymer chains are obtained upon linking chemically identical units exclusively at both ends. However, repeat units are not always symmetrical in the chain direction. Depending on the orientation of the repeat unit, different microstructural... 

Noguchi, H. Yoshikawa, K., Morphological variation in a collapsed single homopolymer chain, J. Chem. Phys. 1998,109, 5070-5077... 

Liang, H. J. Chen, H. N., First-order transition of a homopolymer chain with Lennard-Jones potential, J. Chem. Phys. 2000,113, 4469 1471... 

Generally the graft copolymers are made up of a main homopolymers chain with branches of another... 

If one is able to control precisely the formation of end-functionalized polymers carrying groups of the proper reactivity and philicity, this scheme should open efficient and diversified new ways of access to interesting block copolymers. The basic problem remains accordingly a catalytic one, i.e. the quantitative end-functionali-zation of growing homopolymer chains by efficient termination or transfer reactions that is fortunately a rapidly improving field. 

Polyester copolymers (or copolyesters) are those polyesters synthesized from more than one glycol and/or more than one dibasic acid. The copolyester chain is less regular than the homopolymer chain and therefore has a reduced tendency to crystallize. Such copolyesters are thus predominately amorphous and have high clarity and toughness (see Chapters 6 and 7). 

Similar to the branches in copolymer stars and miktoarms, the grafted chains in brushes can be of different chemical compositions. Brown et al. [223] studied the microphase separation of grafted mixtures of homopolymer chains composed of immiscible A and B units and also [224] of diblock AB copolymers. In the former case, the brushes expand laterally and then experience lateral microphase separation. In the latter case, however, monomers segregate vertically to the surface forming a three layer structure. 

Statistical co-crystallization of different constitutional repeating units, which may either belong to the same copolymer chains (copolymer isomorphism) or originate from different homopolymer chains (homopolymer isomorphism). 

Most of the biochemical studies on polysaccharide synthesis to date have been concerned with the formation of homopolymers even when it is known that the synthesis of the homopolymer chain occurs in vivo as part of a heteropolysaccharide (4-6). Cytochemical investigations have made no such distinctions and the polymers located by these studies have nearly always been sites at which heteropolymers were present and where deposition in the wall occurred. The bulk of the polysaccharides that occur in the wall, with the exception of cellulose and callose, are heteropolymers. Generally the polysaccharides of the hemicelluloses and pectins are composed of poly-... 

Isomorphism in macromolecular systems may be defined as the statistical substitution, within a single crystalline phase, between monomer units differing either in chemical structure or in conformation or in configuration. The distribution of the different monomer units needs not to be totally random. As we shall see, there are examples of isomorphous systems consisting of a mixture of different homopolymer chains. In these cases the randomness is confined to the macromolecules... 

The free energy of the corona contains contributions from the deformation energy (cf. eqn 3.21) and from the entropy of mixing of homopolymer chains and A blocks in the micellar corona (de Gennes 1978 Meier 1969 Noolandi and Hong 1982). Summing these two terms gives (Leibler et al. 1983)... 

The phase behaviour of a binary blend of a block copolymer and a homo-polymer is primarily governed by the length of the homopolymer chain compared to the copolymer. Experiments by the groups of Hashimoto and Winey have led to the identification of three regimes, depending on the degree of polymerization of the homopolymer A, NAti, and that of the same component of the copolymer, NAc. 

Addition of A homopolymer to type IV micelles was found to lead to a shift in the regions of stability of cylindrical and spherical morphologies towards more symmetric compositions compared to the neat copolymer. For sufficiently long homopolymers, the structure formed by asymmetric copolymer chains was anticipated to be copolymer micelles in a matrix of unstretched homopolymer chains. In contrast to the experimental results of Winey et al. (1991c) (who observed... 

Fig-1 Schematic representation of the CDSD chemical modification of a homopolymer chain a initial conformation formed by a homopolymer globule b chemical modification (surface coloring ) c resulting copolymer. Modified chain segments are shown in gray... 

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