Block copolymer prepolymer formation

These polyazoesters can be used to produce block copolymers in a two-step procedure. In the first step a fraction of the azo group is decomposed in presence of a first monomer. Thus, an azo group-containing prepolymer is produced. Decomposition of the rest of the azo groups in the presence of a second monomer results in the formation of block copolymers. Bulk, solution, and precipitation polymerization can be used as polymerization techniques. The efficiency in the preparation of the azo group-containing-prepolymer can be as high as 0,8 if bulk polymerization is carried out in such a way that short blocks are obtained. In the second step, typical values for the efficiency are 0.3-0.4. 

Figure 5.10 Formation of block copolymers from condensation polymers ( ) from two prepolymers with mutually reactive terminal end groups (II) by the use of a coupling agent...

It was also shown that functional zinc alkoxides are also effective initiators for the ring-opening polymerization of -caprolactone imder very mild conditions via a polymerization mechanism described for aluminum alkoxides (345). Tin octoate, Sn(0(0)CCH(C2H5)C4H9)2, is another type of initiator to synthesize telechelics based on PCL. In particular, when it is used in conjunction with hydroxyl functional compounds or prepolymers, telechelics, linear and star-shaped block copolymers, or networks can be obtained via corresponding alkyl octoate formation (346-354). More recently, novel end-chain and mid-chain telechelics functionalized with photoactive groups of PCL were synthesized (355). 

The formation of step copolymers that are forming blocks is best illustrated through considering polyurethanes that are formed by prepolymers. This was discussed earlier in outline for the polyurethane co-polyureas. In the following example, a polyester-urethane is formed by reacting an —OH-terminated polyester with an -NCO-terminated polyurethane. Each of these is formed by the appropriate stepwise polymerization and then reacted to give a larger -NCO-terminated prepolymer ... 

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