Anionic polymerization weak links

There is a substantial literature on the thermal and photochemical degradation of PS and it is well established that polymer properties are sensitive to the manner in which a particular sample of PS is prepared. For example, it has been reported that PS prepared by anionic polymerization shows enhanced stability with respect to that prepared by a radical mechanism.2 10 This has often been attributed to the presence of "weak links" in the latter polymers. However, the precise nature of the "weak links" remains the subject of some controversy. T he situation is further confused by all PS prepared by radical mechanisms often being considered as a class without reference to the particular polymerization conditions employed in their preparation. In many cases the polymers are "commercial samples" with details of the method of preparation incomplete or unstated. 

Water-soluble biodegradable polycarboxylates with an acetal or ketal weak link were inventions of Monsanto scientists in the course of their search for biodegradable deteigent polymers. However, the polymers were prevented by economics from reaching commercial status. The polymers are based on the anionic or cationic polymerization of glyoxylic esters at low temperature (molecular weight is inversely proportional to the polymerization temperature) and subsequent hydrolysis to the salt form of the polyacid, which is a hemiacetal (R = H) or ketal (R = CH3) if methylglyoxylic acid is used, and stable under basic conditions. 

PS produced by anionic polymerization contains fewer weak links than PS prepared using free radical chemistry [44] ... 

Figure 34 A polymer-supported metallocene catalyst (51) with a weakly coordinating anion, [B(C6F5)4] , produced from lightly cross-linked, chloromethylated polystyrene beads for olefin polymerization. (Adapted from ref. 75.)...

Reaction (9-23) could be used to produce an ABA copolymer of styrene and methyl methacrylate even though the latter anion is too weakly basic to initiate the polymerization of styrene. If the presence ofa linking group, like the—(—CH2—) in the above reaction, is undesirable a coupling agent like I2 can be used. 

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