Living radical polymerization dithiocarbamates

Keywords Controlled Polymerization Living Radical Polymerization Iniferter Chain-End Structure Molecular Weight Control Block Copolymer Dithiocarbamate Disulfide Nitroxide Transition Metal Complex... 

Living radical polymerizations have received considerable attention because they provide a convenient alternative for synthesizing block copolymers, polymers of narrow polydispersity and complex polymer structures (1-5). Because of their ability to initiate living free radical polymerizations, iniferters have been examined extensively after Otsu et al. (6) introduced them in 1982. In particular, dithiocarbamate derivatives have been studied more closely by several researchers. Lambrinos et al (7) have examined the molecular weight evolution during the polymerization of n-butyl acrylate using p-x ylylene bis(A,A-diethyl... 

The Inifer technique enables us to fulfil some requirements of polymer architecture even in some radical processes. An amplified form may be applied, the Iniferter variant, where the radical initiator simultaneously acts as a transfer and terminating agent. Otsu et al. used sulphides and disulphides (tetraethylthiuram disulphide, PhSSPh, Ph2S, PhCH2SSCH2Ph) [96] and carbamates (benzyl-A,A-diethyldithiocarbamate, p-xylylene-A,7V-diethyl-dithiocarbamate) [97] in the photopolymerization of methyl methacrylate and styrene, and phenylazotriphenylmethane in the polymerization of methyl methacrylate [98]. Living radical polymerizations yield polymers with defined end groups or the required block copolymers. 

MAYADUNNE R.T.A., RIZZARDO E., CHIEF ARI J., CHONG Y.K., MOAD G., THANG S.H., Living radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization) using dithiocarbamates as chain transfer agent. Macromolecules, (1999), 32 (21), 6977-80. 

An inverse manner of copolymerization was proposed by Watanabe et First, they polymerized TMC using 4-(chlor-omethyl)benzyl alcohol (CBA) as an initiator and DBU as an organocatalyst. The benzyl chloride group was involved in the incorporation of dithiocarbamate for pseudo-living radical polymerization of vinyl monomers. The authors applied N-isopropylacrylamide, acrylamide glycolic acid, and 2-hydroxyethyl methacrylate as vinyl monomers for the second step of copolymerization (Scheme 93). The resulting block... 

Scheme 93 Preparation of polycarbonates with dithiocarbamate groups for pseudo-living radical polymerization of vinyl monomers.

Among the iniferters used, some compounds containing i, i r-diethyl-dithiocarbamate groups were found to be excellent photoiniferters of living radical polymerization. Otsu et al. [55] summarized these ideas and discussed some characteristics of the living radical polymerization with tetraethylthiuram disulfide (TD), benzyl JV,JV-diethyl-dithiocarbamate (BDC), / -xylene bis(iV,JV-diethyl-dithiocarbamate (XDC), and tetrakis(AT,iV-diethyl-dithiocarbamayl) benzene (DDC) as photoiniferters. 

The chain transfer activity of dithiocarbamate reagents, (11) and (111), prepared by Chiefari [2] and Charmot [3], respectively, was impacted by the substiment selection, (IV) and (V), and was effective in conferring living characteristics to a free radical polymerization. These agents were also used in introducing novel end group functionalities into polymers. 

The polymerizations of St and MMA with dithiocarbamates were found to proceed via a living radical mechanism even in a homogeneous system, i.e., both the yield and the molecular weight of the polymers produced increase with reaction time (Figure 3.1). 

Dithiocarbamatc 16 has been used to prepare low dispersity PMAA ( Mw 1 Mn-1.2).52 Photopolymerization of S in the presence of dithiocarbamate 16 also displays some living characteristics (molecular weights that increase with conversion, ability to make block copolymer). However, 17 appears to behave as a conventional initiator in S polymerization.53 The difference in behavior was attributed to the relatively poor leaving group ability of the 2-carboxyprop-2-yI radical. This hypothesis is supported by MO calculations. Dithiocarbamatc 17 was used to control polymerizations of MMA,54 HEMA54 and NIPAM.5... 

Recently, Lambrinos et al. [101] observed some deviation from the proposed polymerization mechanism in iniferter systems. These authors pointed out the bimolecular termination leading to the deactivation of the iniferter site in the polymerization of n-butyl acrylate initiated by / -xylene bis(iV,iS -die-thyl-dithiocarbamate and claimed that the polymerization was not strictly living. Doi et al. [102] proposed a new two-cx)mponent iniferter system to prevent the deactivation of the iniferter site. In this system, BDC and TD act as an iniferter (or an initiator) and a chain transfer agent and/or a primary radical terminator, respectively. In the polymerization of methylacrylate (MA) with BDC bimolecular termination leading to the deactivation of the iniferter site occurred in preference to chain transfer to BDC and the dithio-carbamayl radical that produce the iniferter site, resulting in a deviation from... 

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