Distinguishing Between Radical, Cationic, and Anionic Polymerizations

Another approach to block copolymer is the mutual termination of a living cationic polymer with a living anionic polymer, but the efficiency of coupling is generally low because of proton transfer from the carbocation to the carbanion, resulting in two homopolymers instead of the block copolymer [Richards et al., 1978]. 

Another approach to block copolymers involves changing the type of propagating center part way through the synthesis via a transformation reaction [Burgess et al., 1977 Richards, 1980 Souel et al., 1977 Tung et al., 1985]. For example, after completion of the living anionic polymerization of monomer A, the carbanion centers are transformed into carbocations 

Anionic propagating centers are transformed into radical centers in the presence of a second monomer that undergoes radical propagation, by reaction with trimethyl lead chloride 

The potential of transformation reactions for synthesizing a wider range of block copolymers has not been realized because either the reactions are not quantitiative or deterimental side reactions occur. Thus coupling of two propagating carbanions by one phosgene competes with the 1 1 transformation in Eq. 5-123. The anionic-to-radical transformation in Eq. 5-124 involves the formation of trimethyllead radical, which initiates homopolymerization of monomer B. 

5-5 DISTINGUISHING BETWEEN RADICAL, CATIONIC, AND ANIONIC POLYMERIZATIONS 

There is sometimes a question as to whether a particular initiator or initiator system initiates polymerization by radical, cationic, or anionic means. Such a question can easily arise, for example, in polymerizations initiated by ionizing radiation. The mode of initiation of a particular initiator can be distinguished by a consideration of its characteristics compared to those of known radical, cationic, and anionic initiators  

---

As noted above, all chain-reaction polymerizations involve essentially the same number of steps. The main distinguishing feature between chain-reaction polymerizations, however, is by the initiation mechanism, which may be a free-radical, ionic (cationic or anionic), or coordinatioa The time between initiation and termination of a given chain is typically from a few tenths of a second to a few seconds. During this time thousands or tens of thousands of monomers add to the growing chain. 

---