Chain transfer stereoselective polymerization

The free-radical nature of ATRP is well established through a number of studies [Matyjaszewski et al., 2001], The effects of inhibitors and retarders, solvents, and chain-transfer agents are the same in ATRP as in conventional radical polymerization. The regio-selectivity, stereoselectivity, and copolymerization behaviors are also the same. 

Each of the fluorinated catalysts has an optimum temperature-range for stereoselective polymerization. At the lower temperatures, the rate of propagation and yield of polymer decrease dramatically. At higher temperatures, the molecular weight of the polymer produced becomes lower, presumably because chain transfer or termination processes increase in importance. At still higher temperatures, the stereoregulation is lost, and the low-d.p. polymer produced has a mixed, anomeric configuration. 

In conclusion, in site-controlled stereoselective polymerizations, it is accepted and proved that the site chirality is unable to select directly between the two enantiofaces of the inserting monomer. Instead, it is accepted and proved that the site chirality can force a chiral orientation of the growing chain, which in turn is able to select between the two enantiofaces of the inserting monomer. Thus, the growing chain acts as a messenger to transfer the chiral information from the catalytic site to the monomer.172... 

Historically, heterogeneous polymerization catalysts have been the workhorse of the polymer industry. Although these catalysts offer many important advantages over their homogeneous counterparts in commercial production, they also have a significant number of drawbacks. For example, hetereogeneous catalysts typically have multiple active sites, each of which has its own rate constants for monomer enchainment, stereoselectivity, comonomer incorporation, and chain transfer. Therefore a substantial amount of empirical optimization of these catalysts is necessary before polymers of relatively uniform molecular weights, composition, and stereochemistry can be produced. 

However, all synthetic approaches involving ATRP rely on a metal catalyst. Full metal-free and thus greener approaches to block copolymers were realized by the combination of Upase ROP with nitroxide-mediated living free radical polymerization [44]. With this system it was also possible to successfully perform a one-pot chemoenzymatic cascade polymerization from a mixture containing a dual initiator, CL and styrene (Fig. 12). Moreover, it was shown that this approach is compatible with the stereoselective polymerization of 4-methylcaprolactone for the synthesis of chiral block copolymers. A metal-free synthesis of block copolymers using a radical chain transfer agent as a dual initiator in enzymatic ROP to yield poly(CL-f -styrene) was also reported recently [119]. 

Isotactidty of poly(POx) chains corresponding to the crystalline fraction is explained by steric constraints and orientation of the complexed monomer, which induces stereoselectivity in the nudeophilic attack by the chain end. It was demonstrated that monomer insertion proceeds by attack at the carbon atom of the epoxide ring where it is deaved with inversion of the carbon configuration. This necessitates an attack of the complexed monomer by the nudeophile from the back, which requires the partidpation of two adjacent aluminum atoms and chain transfer from one aluminum atom to the other one at each monomer addition. The coordination polymerization mechanism proposed by Vandenberg for the trialkylaluminum/water system is shown in Scheme 23. 

Stereocontrolled polymerization requires elimination of the intermolecular transfer (segmental exchange) between the homochiral chains containing repeating units of opposite configuration. Otherwise, even if initially stereoselective polymerization proceeds giving the homochiral chains. 

---