Sequencing of Homopolymers

This chapter is concerned with the precise sequence in which monomer units exist in a homopolymer. This information is quite different from that covered in methods for the determination of the ratio in which monomer units occur. Sequencing in copolymers is more complicated and is dealt with separately in Chapter 7. 

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Anionic polymerizations are particularly useful for synthesizing block copolymers. These macromolecules contain long sequences of homopolymers Joined together by covalent bonds. The simplest vinyl-type block copolymer is a two segment molecule illustrated by structure (9-2). This species is called an AB block copolymer, because it is composed of a poly-A block joined to a long sequence of B units. Other common block copolymer structures are shown as (9-3)-(9-6). 

In homopolymers all tire constituents (monomers) are identical, and hence tire interactions between tire monomers and between tire monomers and tire solvent have the same functional fonn. To describe tire shapes of a homopolymer (in the limit of large molecular weight) it is sufficient to model tire chain as a sequence of connected beads. Such a model can be used to describe tire shapes tliat a chain can adopt in various solvent conditions. A measure of shape is tire dimension of tire chain as a function of the degree of polymerization, N. If N is large tlien tire precise chemical details do not affect tire way tire size scales witli N [10]. In such a description a homopolymer is characterized in tenns of a single parameter tliat essentially characterizes tire effective interaction between tire beads, which is obtained by integrating over tire solvent coordinates. 

In tire simple version of tire lattice representation of proteins tire polypeptide chain is modelled as a sequence of connected beads. The beads are confined to tire sites of a suitable lattice. Most of tire studies have used tire cubic lattice. To satisfy tire excluded volume condition only one bead is allowed to occupy a lattice site. If all tire beads are identical we have a homopolymer model the characteristics of which on lattices have been extensively studied. 

Grafting provides a convenient means for modifying the properties of numerous polymers. It is often required that a polymer possess a number of properties. Such diverse properties may not be easily achieved by the synthesis of homopolymers alone but can be achieved through the formation of copolymers or even terpoly-mers. The formation of graft copolymer with sufficiently long polymeric sequences of diverse chemical composition opens the way to afford speciality polymeric materials. 

Polymers are classified according to their chemical structures into homopolymers, copolymers, block copolymers, and graft copolymers. In a graft copolymer, sequences of one monomer are grafted onto a backbone of the other monomer and can be represented as follows ... 

Random copolymers usually exhibit properties which are intermediate between those of the specihc homopolymers. The fact that graft copolymers contain long sequences of two different monomer units indicates that it should be possible to select polymer combinations to give highly specihc properties which are characteristics of the homopolymers involved. 

In analysis of homopolymers the critical interpretation problems are calibration of retention time for molecular weight and allowance for the imperfect re >lution of the GPC. In copolymer analysis these interpretation problems remain but are ven added dimensions by the simultaneous presence of molecular weight distribution, copolymer composition distribution and monomer sequence length distribution. Since, the GPC usu y separates on the basis of "molecular size" in solution and not on the basB of any one of these particular properties, this means that at any retention time there can be distributions of all three. The usual GPC chromatogram then represents a r onse to the concentration of some avera of e h of these properties at each retention time. 

Diblock copolymers, as illustrated in Fig. 5.8 c), comprise homopolymer sequences of the two monomers linked together. The homopolymer blocks may be either compatible or incompatible, depending on their chemical structure. If the sequences are compatible, they will mix to form a material with characteristics similar to those of a blend of the two homopolymers. On the other hand, if the blocks are incompatible, they will tend to segregate from one another to form distinct phases. Each phase will display properties characteristic of the homopolymer, modified by the constraints placed on them by having one end attached... 

A less obvious explanation is that the observed residual structure is not due to attractive interactions, but rather to repulsive ones. The steric repulsion between atoms forced to partially overlap is a dominant, if not the dominant, force in all of chemistry. These highly local interactions are known to be important in polymer conformations (Flory, 1969 Ramachandran and Sasisekharan, 1968). For homopolymers or simple alternating polymers, they can often be safely neglected by assuming they confer no net directionality to the chain. Polypeptide chains, however, are chiral and support specific sequences of 20 differently shaped... 

Instead of the familiar sequence of morphologies, a broad multiphase window centred at relatively high concentrations (ca. 50-70% block copolymer) truncates the ordered lamellar regime. At higher epoxy concentrations wormlike micelles and eventually vesicles at the lowest compositions are observed. Worm-like micelles are found over a broad composition range (Fig. 67). This morphology is rare in block copolymer/homopolymer blends [202] but is commonly encountered in the case of surfactant solutions [203] and mixtures of block copolymers with water and other low molecular weight diluents [204,205]. 

An A-B diblock copolymer is a polymer consisting of a sequence of A-type monomers chemically joined to a sequence of B-type monomers. Even a small amount of incompatibility (difference in interactions) between monomers A and monomers B can induce phase transitions. However, A-homopolymer and B-homopolymer are chemically joined in a diblock therefore a system of diblocks cannot undergo a macroscopic phase separation. Instead a number of order-disorder phase transitions take place in the system between the isotropic phase and spatially ordered phases in which A-rich and B-rich domains, of the size of a diblock copolymer, are periodically arranged in lamellar, hexagonal, body-centered cubic (bcc), and the double gyroid structures. The covalent bond joining the blocks rests at the interface between A-rich and B-rich domains. 

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