Cross-linking units

A wide range of polymer networks are constructed in this manner. Poly(vinyltrichloacetate) was used as the coinitiator with styrene, MMA and chloroprene as cross-linking units. Polycarbonates, polystyrene, N-haloge-nated polyamide, polypeptides, and cellulose acetate, suitably functionalized, have been used as a coinitiator... 

The relation (13) should hold regardless of the primary molecular weight distribution, provided only that the cross-linking proceeds between units at random. The number of cross-linked units per primary molecule, which has been called the cross-linking indexequals pyn-For a homogeneous primary polymer — 2/, and the critical value... 

If we regard it as a cross-linked polymer, we require ywj the weight average degree of polymerization if all cross-linkages were severed. This operation would amount to replacing each tetrafunctional unit, or pair of cross-linked units, with two bifunctional units. The extent of reaction p would not be affected. Hence, according to Eq. (VIII-8)... 

The probability that a primary molecule composed of y units possesses i cross-linked units is... 

The relative abundances of cross-linkages and of various primary species in the sol and in the gel fractions may be obtained by extension of this procedure. Since 4>l equals the fraction of the cross-linkages (or of the cross-linked units) occurring in the sol fraction, we can write... 

Fig. 71.—Weight fraction (Wg) of gel and concentrations of cross-linked units in the sol (p ) and in the gel (p") vs. the over-all degree of cross-linking (p) for the rectangular primary distribution described in the text. ...

Occurrence of either step causes the polymer molecule I to be incorporated in the growing chain. The unit of the polymer to which the radical adds becomes a tetrafunctional unit, which is equivalent to a pair of cross-linked units, i.e., to a cross-linkage. 

The kinetic treatment corresponds to that given above for the case of chain transfer with polymer. We have merely to write for the rate of generation of cross-linked units... 

Fig. 91.—(a) Schematic representation of cross-linking as in rubber vulcanization. (b) Incidence of cross-linked units within a given primary molecule. 

Molecular weight per cross-linked unit. Hence also the molecular weight per chain of a perfect network. 

Cross-linking index, i.e., number of cross-linked units per primary molecule in the system as a whole (Chap. IX). 

One of the earliest proton exchange membranes was based on sulfonated polystyrene where divinylbenzene was used as a cross-linking unit for extra stability. Developed by General Electric, this membrane (21) was cheap and easy to manufacture, and it was used for fuel cells in the Gemini space pro-gram.i However, due to the sensitivity of the benzylic hydrogen to radical attack, lifetimes for these membranes under FC operating conditions were quite low. Thus, little work has been carried out on these systems since their inception. 

Mixing different cross-linkers (19b and 19c) yielded systems with a strong gel-weak gel transition, rather than a distinct sol-gel shift. When the concentration of each of the cross-linkers was above the critical percolation threshold, the kinetically slower cross-linker (19c) dictated the gel properties. Upon addition of enough of a competitive binding additive, such as DMAP, to drop the concentration of the active cross-linking units below their individual percolation thresholds but still allowing the total amount of both active cross-linkers (19b and 19c) to be above the percolation threshold results in a gel whose properties are now controlled by the kinetically faster cross-linker (19c). 

Repeat the activity using warm water instead of acetone. When water is used, the starch peanut dissolves and the polystyrene peanut does not. The acetone and polystyrene do not exhibit hydrogen bonding, while starch peanuts and water do exhibit hydrogen bonding. This illustrates the general principle of like dissolves like. Not all the polystyrene dissolves because of the presence of cross-linked units that form the polystyrene. 

A helical structure can be stabilized by introducing ligand units to the amino acid side-chains. On addition of appropriate metal ions the a-helix (Figure 1.3.3A) is formed by use of the metal as a cross-linking unit [5]. Attachment of metal binding sites to the end of well-chosen decapentapeptides and coordination of the random coil peptide to appropriate metal ions leads to induction of an a-helix... 

The strategy is impressively simple the phthalazine derivative 15 can readily be prepared from quinine in one step. Being a divinyl derivative, it can be submitted as a cross-linking unit in the radical polymerization of methyl methacrylate (MMA) or 2-hydroxy methacrylate (HEMA). Thereby, an immobilized (DHQ-PHAL) derivative 16 is obtained, which is suited for the asymmetric dihydroxylation of frantr-stilbene (>99 % ee) and ( )-cinnamic acid methyl ester (>99 % ee. Table 1). The insoluble catalyst can be recovered by simple filtration, and its repeated... 

FA-treated wood shows high ASE values at low WG, e.g., an ASE of 90% is achieved at a WG of 7% (28). Meanwhile, mechanical properties are reduced by the FA treatment (26,28,29). Furthermore, the treatment causes an embrittlement of wood, probably attributable to the short, inflexible, cross-linking unit of the -O-C-O- linkage. 

---