Types of homopolymers

With only the monomer. A, polymerization yields a homopolymer 

Polypropylene (PP), for which R = CH3, has been extensively studied by NMR and from the resonances of the pendant methyl groups, five monomer sequences (pentads) can be measured. These are ten possible pentad configurations mmmm, mmmr, rmmr, mmrm, mmrr, rmrm, rmrr, mrrm, mrrr 

These meso and racemic configurations are fixed during the formation of the polymer and cannot be transformed (e.g. m into r, and thereby altering the tacticity) by heat or solution. In many polymers the tacticity does not greatly alter the properties of the polymer, e.g. PMM A or PVC, but where it affects the ability of a polymer to crystallize, e.g. PP, then the difference is very significant. Crystalline syndic tactic or isotactic PP are stiff polymers (softening point 150 °C) whereas atactic PP is a viscous gum. 

In some polymers the end groups, XY are reactive groups, e.g. —NH2, —COOH and the polymer (often low MW) can be considered as a macromer. It is possible to use these groups in the formation of block copolymers and chain extension cross-linking agents. These polymers are called telechelic polymers. 

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In block copolymers [8, 30], long segments of different homopolymers are covalently bonded to each otlier. A large part of syntliesized compounds are di-block copolymers, which consist only of two blocks, one of monomers A and one of monomers B. Tri- and multi-block assemblies of two types of homopolymer segments can be prepared. Systems witli tliree types of blocks are also of interest, since in ternary systems the mechanical properties and tire material functionality may be tuned separately. 

The properties of polymer materials can e greatly extended by blending two or more homopolymers together. Blends may be classified as compatible or incompatible - although this does depend on the dimensions being considered. Compatibility is influenced by the molecular weight of the homopolymers and is enhanced in practice by incorporation of block copolymers and other compatibilizers. The effects of radiation on blends depend on the degree of compatibility and the extent of inter-molecular interaction (physically and chemically) between the different types of homopolymers. 

FIG. 1.7 Some of the microstructures produced by the self-association behavior of diblock copolymer solutions. The figure illustrates the (a) spherical, (b) cylindrical, and (c) lamellar structures (among others) that are possible in such solutions. Each diblock polymer chain consists of strings of white beads (representing one type of homopolymer) and strings of black beads (representing the second type of homopolymer). (Redrawn from A. Yu. Grosberg and A. Khokhlov, Statistical Physics of Macromolecules, AIP Press, New York, 1994.)... 

Table 11. LC copolymers from mesogenic and nonmesogenic monomers which form mesophases different from the type of homopolymer mesophase37 159,39)...

The majority of polymers are immiscible and, in bulk, they phase separate to form domains of varying sizes and shapes, depending on their relative volume fraction. This happens because of the very low entropy of mixing in the case of large polymeric molecules. Therefore, unless there is a large favorable enthalpic contribution, most polymers do not form molecularly miscible systems. The same is true for block copolymers, in which the length of each block exceeds a certain critical value. As mentioned earlier, block copolymers are systems wherein two (or more) different types of homopolymers are linked to each other at the chain end(s) diblock copolymers, represented as (A) -(B) , are systems in which two homopolymers are linked to each other at one end, while triblock copolymers, represented as (A)m-(B) -(C)p, are systems in which one central homopolymer block is linked at either end with two other homopolymers. The values m, n and p, represent the... 

One advantage y-cyclodextrin has demonstrated is its ability to thread many types of homopolymers into its interior cavity due to its larger inner core dimension of... 

When we incorporate termination reactions into the analysis of monomer sequence formation in copolymerization kinetics, we note that in general the development of the copolymer sequence can be prematurely stopped. If one monomer sequence is being formed during propagation and the chain is terminated by disproportionation, then the result is more of a homopolymer than a copolymer. If the chain is terminated by recombination of a similar molecule, then the same type of homopolymer is formed. In fact, termination reactions usually prevent the formation of block copolymers when both monomers are present in the reactor fluid. With the radical trapping mechanism of the FRRPP process that will be discussed in the next chapter, formation of certain block copolymers becomes feasible in statistical-based radical copolymerizations. This is an apparent contradiction in terms, but the FRRPP process has been shown to break new ground in polymerization systems. 

A symmetric binary blend is defined as consisting of two types of homopolymer chains, A and B, which are structurally identical in every respect. Hence, the corresponding single chain structure factors obey the identity (OaW = cobW = )(r). The pair potentials between sites are ... 

The term sequential refers to the time order of polymerization/ " Polymer network I may be formed by simultaneous polymerization and crosslinking, through the use of multifunctional crosslinkers such as divinyl benzene, or polymer I can be crosslinked after its initial preparation, as in the vulcanization of rubber. Monomer II, together with requisite crosslinkers and initiators, is swollen into polymer network I, and polymerized in situ. Table 5.1 summarizes the types of homopolymer networks employed. 

The last main type of copolymer that is usually considered are graft copolymers. They consist of a main homopolymer chain with branches of another type of homopolymer... 

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