Macromonomer RAFT polymerization

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. 

Macromonomer RAFT polymerization is most effective with methacrylate... 

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... 

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. 

Shinoda, H., and Matyjaszewski, K. (2001). Improving the structural control of graft copol5mers. Copolymerization of poly(dimethylsiloxane) macromonomer with methyl methacrylate using RAFT polymerization. Macromolecular Rapid Communications, 22(14) 1176-1181. 

Patton D, Advincula, R. A versatile synthetic route to macromonomers via RAFT polymerization. Macromolecules 2006 39 8674-8683. 

A series of OEtOxMA macromonomers was prepared with different oligo(2-ethyl-2-oxazo-line) chain length. Subsequent controlled-radical polymerization of the macromonomers with the RAFT polymerization technique, using CBDB and AIBN, resulted in the formation of well-defined comb POEtOxMAs (Scheme 22.4) (Weber et al, 2009b). The comb copolymers all exhibited a temperature-induced phase transition in aqueous solution at 0.5 wt% polymer... 

Scheme 22.4 Schematic representation of the synthesis of poly(oligo[2-ethyl-2-oxazoline] methacrylate) by subsequent cationic ring-opening polymerization of 2-ethyl-2-oxazoline followed by end capping with triethylammonium methacrylate and RAFT polymerization of the macromonomer.

An alternative approach to synthesis of star-shaped polymers or nanoparticles is the use of macromonomers. The Heise group prepared PBLG macromonomers with a styrene endgroup by NCA polymerization initiated with 4-vinylbenzylamine. The macromonomers were then copolymerized with divinylbenzene by free-radical or RAFT polymerization (see Fig. 22). Finally, the peptide block was deprotected to give PGA blocks and resulted in pH-responsive water-soluble nanoparticles [132]. 

Audouin F, Knoop RJI, Huang J, Heise A (2010) Star polymers by cross-linking of linear poly (benzyl-L-glutamate) macromonomers via free-radical and RAFT polymerization, a simple route toward peptide-stabilized nanoparticles. J Polym Sci A Polym Chem 48 4602-4610... 

RAFT Polymerization with Macromonomer RAFT Agents... 

The reactions associated with RAFT equilibria are in addition to those that occur normally during conventional radical polymerization (i.e. initiation, propagation and termination). The RAFT agent is a transfer agent. Since radicals are neither formed nor destroyed as a consequence of the RAFT process, termination is not directly suppressed. Retention of the macromonomer end group in the polymeric product is responsible for the living character of RAFT polymerization and renders the process potentially suitable for synthesizing block copolymers and end functional polymers. 

The relatively narrow molecular weight distributions of macromonomers formed by high temperature polymerization of acrylates and other mono-substituted monomers can also be attributed to the RAFT mechanism (Scheme 6.5). Similarities between the chemistry of RAFT polymerization and that seen in formation and reaction of mid-chain radicals formed during polymerization of acrylates were highlighted in a recent pubhcation (refer to Scheme 6.6). 

These and Reemtsma described the analysis of humic adds, Deery et discussed the application of the technique for polysaccharides, and Toy et al used SEC-ESI-MS to study the initial stages of the RAFT polymerization. To overcome the difficulties of ESI-MS, Prokai and Simonsick " added sodium cations to the mobile phase to fadlitate ionization. To simplify the resulting ESI spectra, the number of components entering the ion source was reduced. Combining SEC with electrospray detection, the elution curves of polyethylene oxides (PEOs) were calibrated. The chemical composition distribution (CCD) of acrylic macromonomers was profiled across the MMD. The analysis of PEOs by SEC-ESI-MS with respect to chemical composition and oligomer distribution was discussed by Simonsidc. In a similar approach, aliphatic polyesters, phenolic resins, methyl methacrylate macromonomers, and polysulfides have been analyzed. ... 

As in the case of PS (Section 8.2.1) polymers formed by living radical polymerization (NMP, ATRP, RAFT) have thermally unstable labile chain ends. Although PMMA can be prepared by NMP, it is made difficult by the incidence of cross disproportionation.42 Thermal elimination, possibly by a homolysis-cross disproportionation mechanism, provides a route to narrow polydispersity macromonomers.43 Chemistries for end group replacement have been devised in the case of polymers formed by NMP (Section 9.3.6), ATRP (Section 9.4) and RAFT (Section 9.5.3). 

Polymerizations of methacrylic monomers in the presence of methacrylic macromonomers under monomer-starved conditions display many of the characteristics of living polymerization (Scheme 9.36). These systems involve RAFT (Section 9.5.2). However, RAFT with appropriate thiocarbonylthio compounds is the most well known process of this class (Section 9.5.3). It is also the most versatile having been shown to be compatible with most monomer types and a very wide range of reaction conditions.382... 

Transfer constants of the macromonomers arc typically low (-0.5, Section 6.2.3.4) 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 polymerization380 395 where rates of termination are lowered by compartmentalization effects. A one-pot process where macromonomers were made by catalytic chain transfer was developed.380" 95 Molecular weights up to 28000 that increase linearly with conversion as predicted by eq. 16, dispersities that decrease with conversion down to MJM< 1.3 and block purities >90% can be achieved.311 1 395 Surfactant-frcc emulsion polymerizations were made possible by use of a MAA macromonomer as the initial RAFT agent to create self-stabilizing lattices . 

One of the major advantages of radical polymerization over most other forms of polymerization, (anionic, cationic, coordination) is that statistical copolymers can be prepared from a very wide range of monomer types that can contain various unprotected functionalities. Radical copolymerization and the factors that influence copolymer structure have been discussed in Chapter 7. Copolymerization of macromonomers by NMP, ATRP and RAFT is discussed in Section 9.10.1. 

The grafting through approach involves copolymerization of macromonomers. NMP, ATRP and RAFT have each been used in this context. The polymerizations are subject to the same constraints as conventional radical polymerizations that involve macromonomers (Section 7.6.5). However, living radical copolymerization offers greater product uniformity and the possibility of blocks, gradients and other architectures. 

There have been several studies on the use of RAFT to form polymer brushes by polymerization or copolymerization of macromonomers 348-350. 

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