Tapered middle block

It is important to control the time at which the second monomer is added to avoid formation of a tapered middle block between the two homoblocks [161]. If the second monomer is added too early, then tapered structures are formed. If the second monomer is added too late, then some chains may terminate and, in addition to block copolymer, homopolymer may be formed. 

An interesting question relating AC diblock and ABC triblock terpolymers is the influence of the B block on the microphase separation between A and C. Annighofer and Gronski [163,164], as well as Hashimoto et al. [165], reported on the morphological properties of ABC triblock terpolymers where B consisted of a random or tapered block of A and C. Kane and Spontak [130] found in their theoretical work that a random A/C middle block can enhance the mixing of the outer blocks due to an increase of the conformational entropy of the middle block. A similar result was obtained for symmetric ABC triblock terpolymers, where B forms either spheres, cylinders, or a lamella between the lamellae of the A and C blocks [166]. Erukhimovich et al. [167] studied the influence of a very short strongly incompatible C block on the ODT of an ABC and ACB block copolymer within the WSL. It was found that in both cases, for certain compositions and certain relative incompatibilities between C and the other two blocks, a stabilization of the disordered phase can occur as compared to the pure AB diblock copolymer. 

In a living system, if Mi is much more reactive than M2 and polymerization is allowed to proceed to completion, the end-product is a tapered block copolymer, in which only the middle section contains units of both monomers, e.g. with anti-1-methylnorbomene (Mi)/syw-7-methylnorbomene (M2), see Section VIII.C.4 also with norbornene (Mi)/cyclooctatetraene (M2), catalysed by 8W (R = Me)360. In the extreme case the cross-propagation reactions may be so slight that the product is indistinguishable from a perfect block copolymer, e.g. with bicyclo[3.2.0]hept-2-ene (Mi)/norbornene (M2) catalysed by 18109,597, or with awft -7-methylnorbornene (Mi) syn-1 -methylnorbomene (M2), catalysed by 7 (R = Me)128. The successive polymerization of the two monomers can be readily followed by NMR. 

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