Two xanthate end-functional poly(ethylene glycol)s (PEGs) were tested as macromolecular chaintransfer agents (macroCTA) in the reversible addition-fragmentation transfer-mediated polymerization of vinyl acetate (VAc) and N-vinylpyrrolidone. The macroCTA leaving group played a determining role in the preparation of the block copolymers. PEG-b-PVAc and PEG-b-PVP diblock copolymers were obtained when the macroCTA had a propionyl ester leaving group, whereas under the same experimental conditions the macroCTA with a phenylacetyl ester leaving group inhibited the polymerization. In situ 1 H NMR spectroscopy polymerizations were performed with low molecular weight xanthate analogues to investigate the cause of inhibition. Block copolymers were prepared with the macroCTA which did not inhibit the polymerization and were characterized via size exclusion chromatography, high-performance liquid chromatography, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The ability to produce narrowly distributed (PDI < 1.4) block copolymers end capped with a xanthate moiety with little to no homopolymer contaminant is presented. IntroductionPoly(ethylene glycol) (PEG) is widely used in the pharmaceutical and biomedical fields. It is a nonionic polymer, soluble in water and most common organic solvents. The incorporation of a PEG segment in a macromolecule modulates its solution properties. Many synthetic pathways are available for the preparation of block copolymers comprising a PEG block. Each block can be prepared separately and connected by postpolymerization coupling of functional end groups. 1 Commercially available PEGs prepared via anionic polymerization can be found with one or two hydroxyl end functionalities, which enable almost unlimited chemical modification 2 and the preparation of di-, tri-, or multiblock copolymers. The main prerequisite for this approach is that the second polymeric block must be quantitatively end functionalized. Thus, this method has been used mostly to prepare biodegradable block copolymers of PEG with a second block obtained via polycondensation or ring opening polymerization. 3 Poly(N-vinylpyrrolidone) (PVP) and poly(vinyl acetate) (PVAc) are typical examples of widely used industrial polymers that can only be prepared via free-radical polymerization. Conventional free-radical polymerization does not normally provide end functionality because of transfer and termination reactions. By taking advantage of transfer reactions, however, Ranucci et al. synthesized a variety of low molecular weight PVPs bearing chain-end functionality. 4 Another synthetic approach consists of growing a second block from an endfunctional PEG precursor. By selecting a macromolecular precursor bearing a functional group capable of controlling the polymerization of the second comonomer, it is possible to not only obtain block copolymers but also control the molecular weight distribution of the blocks. The recent development of

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The monomer N‐vinylpyrrolidone (NVP) undergoes side reactions in the presence of R group functional xanthates and impurities. The fate of the monomer NVP and a selection of six O‐ethyl xanthates during xanthate‐mediated polymerization were studied via NMR spectroscopy. A high number of by‐products were identified. Significant side reactions affecting NVP include the formation of an unsaturated dimer and hydration products in bulk or in solution in C6D6. In addition, the xanthate adjacent to a NVP unit was found to undergo elimination at moderate temperature (60–70 °C), resulting in unsaturated species and the formation of new xanthate species. The presence of the chlorinated compound α‐chlorophenyl acetic acid, ethyl ester, a precursor in the synthesis of the xanthate S‐(2‐ethyl phenylacetate) O‐ethyl xanthate, resulted in a dramatic increase in the rate of side reactions such as unsaturated dimer formation and a high ratio of unsaturated chain ends. The conditions for the occurrence of such side reactions are discussed in this article, with relevance to increasing the control over the polymerization kinetics, endgroup functionality, and control over the molar mass distribution. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6575–6593, 2008

Aldehyde omega-endfunctional poly(N-vinylpyrrolidone) was synthesised via quantitative conversion of a xanthate endfunctional precursor obtained via RAFT-mediated polymerisation.

The selection of a suitable mediating agent in Reversible Addition‐Fragmentation Chain Transfer (RAFT) mediated polymerization is crucial to the degree of control that can be achieved. An overview of work from the Stellenbosch group is presented in which the use of NMR spectroscopy as a tool for evaluating RAFT‐agents is highlighted. The occurrence of selective initialization, i.e. the selective conversion of a RAFT‐agent into its single monomer adduct is discussed for various classes of monomers, as well as for copolymerization. One of the general rules for living polymerization is that chains should start growing early in the polymerization reaction. Selective initialization is claimed to be the extreme case where all chains have begun growing after the conversion of only one monomer equivalent per RAFT‐agent.

The trithiocarbonate 2-(benzylsulfanylthiocarbonylsulfanyl) propanoic acid is formed as minor by-product in the synthesis of the dithioester 2-((2-phenylthioacetyl) sulfanyl) propanoic acid via the Grignard route. The mechanism for this side reaction is not clear. The isolated trithiocarbonate may act as unsymmetrical but bifunctional RAFT agent in the aqueous polymerization of NN-dimethyl acrylamide. Therefore, it is important to separate it completely from the dithioester before engaging the latter in controlled free radical polymerization to guarantee a maximum control

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