Bifunctional repeat units

Fig. 20. Plot of the exponent ras a function of the chain lengths between two branching points. Open symbols results by Colby et al. [118,119] for branched polyesters. The variation of this length was achieved by co-condensation of trifunctional monomers with increasing fraction of bifunctional units. Filled symbols refer to polydicyanurates (N Nj ), anhydride cured phenyl monoglycidyl ether with a small fraction of bisphenol A diglyci-dylether as crosslinker (N=8Nj ) and end-functionalized 3-arm polystyrene stars crosslinked by diisocyanate (N 40 N ). N denotes the number of repeating units between two branching points...

The residue from each diol and each diacid (separately, not together) in the polymer chain is termed a structural unit (or a monomer unit). The repeating unit of the chain consists of two structural units, one each of the diol and diacid. The total number of structural units in any particular system equals the total number of bifunctional monomers initially present. The number-average degree of polymerization X is defined as the average number of structural units per polymer chain. (The symbols P and DP are also employed to signify... 

A quite different approach to the 1,5-benzodiazodne system begins with 1,3-dipolar cycloaddition of a nitrile oxide and an indolenine (78MI51900). The spiroisoxazoline (303) is converted by irradiation in good yield to the 1,5-benzodiazocinone (304). By using TV,TV - alkylenediindoles and bifunctional nitrile oxides, the diazocine ring is incorporated as a repeating unit into a photoconducting polymer. 

The chemistry of furfuryl alcohol polymerization has received much attention over the years. Several recent reviews have been written (5,6,54). Based on the accumulated data, furfuryl alcohol has to be considered a bifunctional monomer in the initial stage and its "normal" reactions give linear chains or oligomers containing essentially two repeating units (15,16) with (16) predominating. 

As the polymerization proceeds, the number of repeating units, n, increases with time. The higher the degree of polymerization is, the higher the molecular weight will be. When monomers with bifunctional groups AA and BB are in exact proportion, the polymer molecules have infinite molecular weight. 

Homopolymerization. In the simplest type of step growth, a bifunctional monomer reacts successively with itself, eventually forming a polymer with a large number of repeating units. The reaction may be an addition, but more commonly is a condensation. Although condensation usually is reversible, its equilibrium is driven toward complete conversion by removal of the small and volatile cast-off molecule ... 

Branched polymers are those in which the repeating units are not linked solely in a linear array, either because at least one of the monomers has functionality greater than 2 or because the polymerization process itself produces branching points in a polymer that is made from exclusively bifunctional monomers. 

Figure 2 Pathways for O antigen assembly, (a) Wzy-dependent pathway. After the initiation reaction, glycosyltransferases extend the O antigen until the completion of the repeating unit, which is translocated across the membrane by Wzz. In the periplasmic side of the membrane nascent units are polymerized by Wzy and the control of the chain length distribution is carried out by Wzz. (b) ABC transporter-dependent pathway. In this pathway the polymer is formed intracellular and terminated by the addition of a termination signal (black square), which also couples the polymer to the Wzt protein. ATP hydrolysis is required for export across the membrane, (c) Synthase-dependent pathway. After initiation, one more adaptor sugar is added (black circle) and the WbbF bifunctional enzyme that is also responsible for its vectorial translocation across the inner membrane extends the rest of the polymer.

It is claimed that ultra-high modulus fibres can be made from those compositions which fall within a defined area when plotted on a triangular composition diagram for the three types of repeat unit. These workers have, in effect, mixed together various combinations of the bifunctional... 

If the alkoxide ion of one molecule displaces the bromide ion of a second molecule, then the reaction is an intermolecular reaction. Inter is Latin for between, so an intermolecular reaction takes place between two molecules. If the product of this reaction subsequently reacts with a third bifunctional molecule (and then a fourth, and so on), a polymer will be formed. A polymer is a large molecule formed by linking together repeating units of small molecules. 

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