Cyclization Issues in Silicon-Containing Hyperbranched Polymers

Theoretical descriptions of AB

n

(n ≥ 2) hyperbranched polymerization systems have been known for some time [1], but in them, cyclization is a factor that is generally and largely ignored. However, it is now understood that cyclization is prevalent in polymerizations of this type, and that it can often affect to a significant extent both polydispersity and molecular weights of the polymer products. Since research in hyperbranched polymers has increased dramatically in recent years [2–4], a number of experimental and theoretical studies have focused on the presence and effects of cyclization in these systems (see, for example [5–10] and references cited therein). In essence, intramolecular cyclization of an oligomer in an AB

n

polymerization results in the consumption of the focal A group (see Section 15.3), which converts the oligomer into a B

x

core. Although the newly formed core can continue to grow through the reaction of other A groups with the B groups, this growth is limited, especially if other A groups in the polymerization system are also consumed through similar intramolecular cyclization reactions. Thus, for the control and optimization of the resulting polymer molecular weight, it is necessary to understand these issues and the methods that can be used to avoid excessive amounts of cyclization

This chapter describes cyclization in organosilicon hyperbranched polymer synthesis, and techniques that have been used to minimize its occurrence. The focus is primarily on AB

n

(n ≥ 2) systems, although many of the principles apply to the similar A

2

+ B

3

bimolecular systems that are now gaining more research attention (see Chapter 16) [11]