Macromonomer RAFT

Macromonomer RAFT polymerization is most effective with methacrylate 

4) and it is necessary to use starved feed conditions to achieve low dispersities and to make block copolymers. Best results have been achieved using emulsion polymerization where rates of termination are lowered by compartmentalization effects. A one-pot process where macromonomers were made by catalytic chain transfer was dcvclopcd. Molecular weights up to 28000 that increase linearly with conversion as predicted by eq. 16, dispersities that decrease with conversion down to M J M 3 and block purities 90% can be achieved. Surfactant-free emulsion polymerizations were made possible by use of a MAA macromonomer as the initial RAFT agent to create self-stabilizing lattices . 

Although the temi RAFT (an acronym for Reversible Addition-Fragmentation chain Transfer) is sometimes used in a more general sense, it was coined to describe, and is most closely as.socialed with, the reaction when it involves thiocarbonylthio compounds. RAFT polymerization, involving the use of xanthates, is also sometimes called MADIX (Macromolccular Design by 

Interchange of Xanthate). The process has been reviewed by Rizzardo ef aL, Chiefari and Rizzardo, Bamer-Kowollik et al., McCormick et and Moad et n/.  

In 1988 a paper by Zard and coworkers reported that xanthates were a convenient source of alkyl radicals by reversible addition-fragmentation and used the chemistry for the synthesis of a nionoadduct to monomer (a maieiniide), Many applications of the chemistry in organic synthesis have now been described in papers and reviews by the Zard group, 

---

Chain transfer to methacrylate and similar maeromonomers has been discussed in Section 6.2.3.4. The first papers on the use of this process to achieve some of the characteristics of living polymerization appeared in 1995.380 The structure of macromonomer RAFT agents (163) is shown in Figure 9.3. An idealized reaction scheme for the case of a MMA terminated macromonomer is shown in Scheme 9.36. 

Macromonomer RAFT polymerization is most effective with methacrylate monomers (Table 9.9).With monosubstituted monomers (e.g. S, acrylates) graft copolymerization, is a significant side reaction which can be mitigated but not eliminated by the use of higher reaction temperatures. 

Table 9.9 Block Copolymers Prepared by Macromonomer RAFT Polymerization under Starved-Feed Conditions.380"595...

A novel approach to RAFT emulsion polymerization has recently been reported.461529 In a first step, a water-soluble monomer (AA) was polymerized in the aqueous phase to a low degree of polymerization to form a macro RAFT agent. A hydrophobic monomer (BA) was then added under controlled feed to give amphiphilic oligomers that form micelles. These constitute a RAFT-containing seed. Continued controlled feed of hydrophobic monomer may be used to continue the emulsion polymerization. The process appears directly analogous to the self-stabilizing lattices approach previously used in macromonomer RAFT polymerization (Section 9.5.2). Both processes allow emulsion polymerization without added surfactant. 

RAFT Polymerization with Macromonomer RAFT Agents... 

Scheme 6.4 Mechanism for reversible addition-fragmentation chain transfer (RAFT) with macromonomer RAFT agents.

The chain activation-deactivation mechanism in thiocarbonylthio RAFT polymerization (Scheme 6.7) is directly analogous to that in macromonomer RAFT polymerization (Scheme 4.4). However, rate constants for radical addition to the thiocarbonylthio double bond are typically several orders of magnitude higher than those to analogous carbon arbon double bonds and... 

---