Polymer structure short-chain branches

There are numerous variations on the basic linear structure of polymers. Returning to our example of polyethylene, we find short chain branches and long chain branches, as shown in Figs. 1.2 and 1.3, respectively. The number and type of these branches strongly influences the way that the molecules pack in the solid state, and hence affect the physical properties. Long... 

Short chain branches are frequently introduced into polymers by copolymerization. The chemical structure of the comonomer controls the type and length of the short chain branch. The polymerization catalyst, reaction conditions, and comonomer content in the reaction medium determine the probability of finding a branch at any particular location along a chain. Comonomers, and hence the short chain branches derived from them, can be introduced at random or as blocks. 

We can introduce short chain branching into polymers by three methods copolymerization, "backbiting , and chemical modification. The first two occur during polymerization, while the last requires a secondary chemical reaction. Short chain branches have well defined chemical structures, the nature of which we can accurately determine via analytical methods or know, from the structure of the reactants. 

Willbourn, A. H. Polymethylene and the structure of polyethylene study of short-chain branching, its nature and effects. J. Polymer Sci. 34, 569—597 (1959). 

Due to their multi-sited nature, Ziegler-Natta and chromium catalysts produce structurally heterogeneous ethylene homo- and copolymers. This means that the polymers have broad MWD and broad composition (short-chain branching) distribution (Fig. 9). Catalyst active sites that produce lower molecular weights also have a tendency to incorporate more comonomer... 

High-resolution and NMR, and recently, multidimensional methods have revealed the microstructures of complex polymers. In particular, multidimensional (2D- and 3D-) NMR have proven to be useful techniques to identify small amounts of irregular structures in synthetic polymers. In this entry, specific topics to be covered include the use of solution NMR methods to study polymer stereochemistry/ tacticity, monomer composition and sequence distribution, short-chain branches, and chain-end structure, as these parameters influence the material s mechanical, thermal, optical, and electrical properties. 

As shown earlier, branches with one carbon atom are the most frequent short chain branch (4t 5. 7. 9. 13). In the polymers obtained at low values of P/P it is easy to observe butyl branches, although this structure can be seen also in polymers obtained close to the saturation pressure (7. 22). It is also possible to detect long chain branches in agreement with our earlier results (7. 22). 

The primary structural feature that determines crystallinity is the concentration of short chain branches along the polymer backbone. These short chain branches disrupt the local ordering of the chains and, thus, reduce the degree of crystallinity in the material. The melting point (T, eit) of the polymer is directly related to the amount of short chain branches in the material. Regardless of the supercritical fluid used, exceptionally low solubilities are found at temperatures below Tmeit- At temperatures above the strong... 

LCT (originally developed for di-block copolymers) was found to be particularly useful to explain miscibility of polyolefin blends where the two resins differ in the type and size of short chain branching. The stractural units of a polymer with two carbons in the main chain can be written as PE = (CH,-CH,), PP = [CH,-CH (CHj)], poly-2-butene (P2B) = [CH (CH3)-CH (CHj)], PIB = [CH -C (CHj) ]jj, poly(4,4-dimethyl 1-butene) (PDMB) = [CH -CH (C Hg)], etc. Three structural parameters (ratio of end to interior groups) have been used to distinguish PO structure r, p, and q. Their values for the model macromolecules discussed above are listed in Table 2.7. 

These unusual branched structures give rise to unique polymer properties. For example, the topologies of the polyethylenes vary from linear with moderate branching to hyperbranched structures. For the most highly branched systems, the overall branching number and the distribution of short-chain branches can change very little while the architecture... 

FIGURE 2.3 Schematic representation of linear polymers, short- and long-chain branching (brush) polymers, star-type polymers, and cross-linked polymer chains. The dendrimer structure on the right is an extreme form of highly short-chain branched macromolecules. 

The characterization of the covalent arrangement of the monomers within one polymer chain and within the mixture of polymer chains refers to the secondary structure parameters as well as branching (long and short chain branches). 

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