Showing papers by "Ezio Rizzardo published in 2000"

Living free radical polymerization with reversible addition : fragmentation chain transfer (the life of RAFT)

Abstract: Free radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization) is discussed with a view to answering the following questions: (a) How living is RAFT polymerization? (b) What controls the activity of thiocarbonylthio compounds in RAFT polymeriza- tion? (c) How do rates of polymerization differ from those of conventional radical polymerization? (d) Can RAFT agents be used in emulsion polymerization? Retardation, observed when high concentra- tions of certain RAFT agents are used and in the early stages of emulsion polymerization, and how to overcome it by appropriate choice of reaction conditions, are considered in detail. Examples of the use of thiocarbonylthio RAFT agents in emulsion and miniemulsion polymerization are provided. # 2000 Society of Chemical Industry

Living Polymers by the Use of Trithiocarbonates as Reversible Addition−Fragmentation Chain Transfer (RAFT) Agents: ABA Triblock Copolymers by Radical Polymerization in Two Steps

Abstract: (calcd) is basedon the assumption that all chains contain one trithio-carbonate moiety, it is important to realize that a smallnumber of dead chains are produced by radical-radicaltermination.Polymers prepared with symmetrical trithiocarbon-ates should have the active functionality located in thecenter, e.g.,

Molecular weight characterization of poly(N-isopropylacrylamide) prepared by living free-radical polymerization

Abstract: A series of relatively monodisperse samples of poly(N-isopropylacrylamide) were synthesized by reversible addition−fragmentation chain transfer (RAFT) over the molecular weight range 2 × 103−3 × 105. For molecular weights below 4 × 104, conditions were found so that polydispersity remained below 1.4 up to high conversion (72%). Molecular weight distributions of polymer obtained using GPC in THF and MALDI were in accord for the low molecular weight range (typically 105). Particular care is required in GPC sample preparation: it is necessary to ensure that trace amounts of water are initially present when drying a polymer sample prior to dissolution in THF, to avoid irreversible chain aggregation. The log/log plot of intrinsic viscosity against molecular weight for polyNIPAM was found to be linear for molecular weights <105, after which the hydrodynamic volume seems to be independent of molecular weight. The Mark−Houwink parameters obtained from the lower molecular weight data are K = 10-(4.24±0.42) dL g-1...

Living polymerization: Rationale for uniform terminology

Abstract: Polymer chemistry textbooks (e.g., B. Vollmert, Polymer Chemistry, Springer-Verlag: New York, 1973, p 37; G. Odian, Principles of Polymerization, 3rd ed., Wiley: New York, 1991, p 8; H. G. Elias, An Introduction to Polymer Science, VCH: Weinheim, 1997, p 51) classify polymerization reactions as chain, step, and living according to the dependence of their degree of polymerization ( ) or molecular weight ((M) over bar) on conversion. This article discusses the rationale for uniform terminology in living polymerization. (C) 2000 John Wiley & Sons, Inc.

Free-Radical Ring-Opening Polymerization of Cyclic Allylic Sulfides. 2. Effect of Substituents on Seven- and Eight-Membered Ring Low Shrink Monomers

Abstract: The effect of substituents on the free-radical ring-opening polymerization behavior and polymerization volume shrinkage of the cyclic allylic sulfides 6-methylene-1,4-dithiacycloheptane (2a) and 3-methylene-1,5-dithiacyclooctane (2b) has been investigated. The monomers were 2-(hydroxymethyl)-6-methylene-1,4-dithiacycloheptane (3a), 6-methylene-1,4-dithiacycloheptan-2-ylmethyl acetate (3b), 6-methylene-1,4-dithiacycloheptan-2-ylmethyl methacrylate (3c), 7-methylene-1,5-dithiacyclooctan-3-ol (4a), 7-methylene-1,5-dithiacyclooctan-3-yl acetate (4b), 7-methylene-1,5-dithiacyclooctan-3-yl benzoate (4c), and the bicyclic monomer bis(6-methylene-1,4-dithiacycloheptan-2-ylmethyl) diglycolate (5). The monomers were polymerized in bulk with thermal (AIBN, VAZO88) and photochemical initiators (Darocur 1173) with selected solution polymerizations also performed. The presence of a substituent on the monomers gave clear amorphous polymers unlike the crystalline polymers obtained from the unsubstituted parent monomers (...

Chain transfer in the sulfur-centered free radical ring-opening polymerization of 3-methylene-6-methyl-1,5-dithiacyclooctane

Abstract: 3-Methylene-6-methyl-1,5-dithiacyclooctane (MDTO, 1) was polymerized in both the presence and absence of a number of chain transfer agents, viz. 1-butanethiol (BuSH), dibutyl disulfide (BuSSBu), and dibutyl sulfide (BuSBu). Mark−Houwink−Sakurada (MHS) constants for poly(MDTO) were found to be K = 23 × 10-5 dL g-1 and α = 0.67. Using these parameters, chain transfer constants were obtained at 60 °C and are reported as 0.13 (BuSH), 0.19 (BuSSBu), and 0.0025 (BuSBu). These values are compared to the corresponding chain transfer constants obtained for styrene (STY) and methyl methacrylate (MMA). Subsequently, the chain transfer activity was measured over a range of temperatures (40−80 °C), and Arrhenius parameters were determined. Significant differences in chain transfer activity between carbon- and sulfur-centered radicals were observed. In addition, the extent of transfer to monomer was determined across the range of temperatures 40−100 °C, yielding a value for CM of 55 × 10-4 at 60 °C. The likely extent o...