End-linking reactions

As is clear from the earlier discussions of pre-gel intramolecular reaction, such reaction in principle always occurs in random polymerisations, although its amount may be reduced by using reactants of higher molar mass, lower functionalities, and stiffer chain structures. Thus, the use of end-linking reactions to produce model networks (for example(35) and references quoted... 

In the case of moderately crosslinked and topologically restricted networks, we obtain results which may be used to explain the low (non-affine) values of molecular deformation observed experimentally (Fig. 14). Values of a close to 1 indicate that the elastic properties of networks prepared by the end-linking reaction are very... 

Variable number of chains emanating from different cross-linking nodes - Use of pure cross-linker of constant functionality - Preservation of reactive ends of elastic chains - Correct stoichiometry -Very high yield in end-linking reaction High... 

Iwata, K. Ohtsuki, T., On Catenate Network Formation by End-Linking Reactions. J. Polym. Set, Part B Polym. Phys. 1993, 31(4), 441-446. 

In this type of material, two networks are formed, either simultaneously or sequentially, in such a way as to interpenetrate one another. The networks thus communicate with one another through interchain physical forces and entanglements, rather than through covalent bonds. A particularly simple example is the simultaneous formation of two PDMS networks, one by a condensation end-linking reaction and the other by an addition end-linking reaction, with the two types of chains mixed at the molecular level. - ... 

The kinetics of decay of double bonds initially present in the material have also been studied in detail. Dole and co-workers (61) reported that the decay of terminal vinyl or vinylene groups follows first-order kinetics until the formation of vinylene groups, mentioned immediately above, becomes significant. The exact mechanism of reaction of the vinyl end groups remains unclear however, the participation of these groups in end-linking reactions (reaction 12) has been confirmed by Randall and co-workers (62-64) and Horii and co-workers (65,66). 

There has been much activity in recent years relating theoretical and experimental values of the static shear moduli of polymer networks using model systems based on end-linking reactions essentially non-linear polymerisations . Almost all the work has assumed that, with stoichiometric reaction mixtures, chemically perfect networks are formed at complete reaction. The number of elastic chains per unit volume is taken to be defined precisely by the molar masses of the monomers and/or prepolymers... 

The end-linking reactions described above can also be used to make networks having unusual chain-length distributions [98-102]. Those having a bimodal distribution are of particular interest with regard to their ultimate properties, and are discussed in the following section. 

Fig. 19. Results of calculation on the mole fraction for PPS star unit of State 1 defined in Figure 18. Data obtained with/0i=0.5, /qj=0.1, 0.667 and 1 are displayed. The weight fractions of the first three oligomers in the end-link reaction determined by SEC measurement were added for comparison.

Postpolymerization end-linking reactions of poly(isoprenyl)lithium with DVB have been used to prepare viscosity improvers for motor oils, as shown in Scheme 3 [37]. With a DVB/PILi molar ratio of 3, relatively efficient link-... 

A typical end-linking reaction producing a tetrafunctional network is given by ... 

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