Monomers with Bulky Side Substituents

Since the pioneering works of the Russian researchers, many attempts have been made to prepare new monomers, diamines and dianhydrides, with bulky pendent groups for novel processable polyimides. Table 5 shows some of these monomers. Monomers with small side substituents, like methyl, methoxy, tri-fluoromethyl or halogen, have not been included. In this respect, it is worth noting that many monomers derived from biphenyl, containing small side substituents, have been discovered and used in the synthesis of soluble polyimides in the last few years. They are not going to be discussed in depth in this chapter as they are considered in other parts of this book. 

Introduction of proper pendant groups in monomer unit of addition polynor-bornene is the way to get polynorbornenes with desired properties. However, the appearance of substituents in norbornene molecules resulted in a decrease of their activity as monomers [220, 223]. In the case of ROMP, this effect is softened to some extent by substantial thermodynamic driving force in the process appearing as a result of opening highly strained bicyclic norbornene skeleton (see Table 1). AP is not such a thermodynamically favorable process. Therefore, in this case the introduction of side substituents, especially bulky or functional groups, led to a dramatic decrease in activity of norbornene derivatives [224-227]. Probably, this fact is one of the reasons for the limited number of publications devoted to AP of silyl substituted norbornenes. 

If the metal (M) has very bulky substituents, the second insertion step is impossible. With smaller substituents, the monomer always approaches the reactive metal-carbon bond from the same side, the customary situation if M is part of a disturbed crystal lattice, leading to isotactic polymerization (see Fig. 5). 

Cl-symmetric catalysts such as 9 and 10 bear a Cp ring substituent pointing in the same direction as one of the chlorine atoms on the Zr center. In the active catalyst species, the chlorine is replaced by the growing polymer chain or a n-complex-bonded olefin. The coordination site on the same side as the Cp substituent (site B) is more sterically hindered than the other coordination site (site A). After insertion of a monomer coordinating at site B, the polymer chain is walked back to less sterically hindered site A by the next monomer insertion (alternating/chain migratory mechanism). But in some cases, the polymer chain can back skip to site A before the next monomer insertion (site epimerization). Norbornene, as a bulky olefin, can be rr-bonded only at site (see Chapter 2 for a further discussion of the alternating and site epimerization mechanisms with catalyst 9). 

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