Molecular weight with living radical polymerization

As discussed in Section 7.3, conventional free radical polymerization is a widely used technique that is relatively easy to employ. However, it does have its limitations. It is often difficult to obtain predetermined polymer architectures with precise and narrow molecular weight distributions. Transition metal-mediated living radical polymerization is a recently developed method that has been developed to overcome these limitations [53, 54]. It permits the synthesis of polymers with varied architectures (for example, blocks, stars, and combs) and with predetermined end groups (e.g., rotaxanes, biomolecules, and dyes). 

As with other forms of living radical polymerization, the degree of polymerization and the molecular weight can be estimated from the concentration of monomer and reagents as shown in eqs. 13 and 14 respectively. 183... 

A half-metallocene iron iodide carbonyl complex Fe(Cp)I(CO)2 was found to induce the living radical polymerization of methyl acrylate and f-bulyl acrylate with an iodide initiator (CH3)2C(C02Et)I and Al(Oi- Pr)3 to provide controlled molecular weights and rather low molecular weight distributions (Mw/Mn < 1.2) [79]. The living character of the polymerization was further tested with the synthesis of the PMA-fc-PS and PtBuA-fi-PS block copolymers. The procedure efficiently provided the desired block copolymers, albeit with low molecular weights. 

The resulting polymers can further induce the radical polymerization of second monomers to give block copolymers. The polymerization with the alkoxyamines has developed to the recent living radical polymerization providing polymers with well-controlled molecular weight and molecular weight distribution, as will be described in Sect 6.1. 

It was confirmed that the resulting polymers obtained from the St polymerization with 13 induced further photopolymerization of MMA to produce a block copolymer, and the yield and molecular weight increased as a function of the polymerization time, similar to the results for the polymerization of MMA with 13, indicating that this block copolymerization also proceeds via a living radical polymerization mechanism [64]. Similar results were also obtained for the photoblock copolymerization of VAc. Thus, various kinds of two- or three-component block copolymers were prepared [157,158]. 

The living radical polymerization process is also valid for the polymerization of water-soluble monomers. The polymerization of sodium styrenesulfonate in aqueous ethylene glycol (80%) in the presence of TEMPO using potassium per-sulfate/sodium bisulfite as the initiator at 125 °C gave a water-soluble polymer with well-controlled molecular weight and its distribution [207]. 

It was found that the adduct 59 also induces living radical polymerization similar to 56, but the adduct 60 does not [215]. In the polymerization of St with 60, the molecular weight did not increase with conversion, and a broad molecular weight distribution, i.e., Mw/Mn of 1.5-2.2 was observed. The half-fife time was determined to be 5-10 min at 123 °C for 59, while that of 60 is much longer (ca. 150 min). The dissociation properties of the alkoxiamines used determined the nature of the polymerization with 59 and 60. 

As is expected from these results, it is very difficult to control the polymerization of monomers other than St, e.g., that of MMA, because of the too small dissociation energy of the chain end of poly(MMA). In fact, the polymerization of MMA in the presence of TEMPO yielded the polymer with constant Mn irrespective of conversion, and the Mw/Mn values are similar to those of conventional polymerizations [216]. The disproportionation of the propagating radical and TEMPO would also make the living radical polymerization of MMA difficult. In contrast, the controlled polymerization of MA, whose propagating radical is a secondary carbon radical,has recentlybeen reported [217]. Poly(MA) with a narrow molecular weight distribution and block copolymers were obtained. 

Cobalt complexes are used for the living radical polymerization of acrylates to give a high molecular weight polymer with a narrow molecular weight distribution (Mw/Mn 1.2) (Eq. 71), whereas the complex is applied to the introduction of an unsaturated group into the methacrylate polymers with a high efficiency via a reaction mechanism illustrated in Eq. (72) [27,28,267,268]. 

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