Mechanism (i) the induction corresponds to the time required to produce sufficient radicals to convert all of the Co(II) species into Co(III)–R, which controls the polymerization via a slow RDC mechanism. (ii) The continued influx of radicals from V-70 changes the process to a faster DT-based polymerization in which both ln([M]0/[M]) vs. time and Mn vs. conversion increase linearly. (iii) After the influx of new radicals from V-70 has ended, the polymer ization reaction reverts to the slower RDC pathway .
reversible deactivation by coupling (RDC) mechanism F. di Lena, K. Matyjaszewski / Progress in Polymer Science 35 (2010) 959–1021
Catalytic chain transfer (CCT) , when its rate is sufficiently high, the molecular weight distribution of the resulting oligomers can be described by a Shultz–Flory statistics (Mw/Mn ∼2). Efficient CCT has been reported with certain Mo, Fe, and especially Co complexes.
Degenerative transfer (DT) process F. di Lena, K. Matyjaszewski / Progress in Polymer Science 35 (2010) 959–1021
Since the Co–C bond cleavage of Organo cobalt(III) complexes can be achieved under mild conditions, organocobalt complexes have been used as sources of carboncentered radicals for organic synthesis and in polymer chemistry.
A. Debuigne et al. / Progress in Polymer Science 34 (2009) 211–239
(TMP)Co-CH2C-(CH3)3 1 Addition of (TMP)CoII ·reduces the rate of polymerization, and the poly merization process can be effectively stopped and restarted repetitively by cycling the reaction temperature between 5 ℃ and 60 ℃. A linear increase in Mn with MA conversion along with relatively small PMA polydispersities (Mw/Mn = 1.1-1.2) indicates that a preponderance of the polymer chains are growing in a manner characteristic of a living polymerization process.
Bradford B. Wayland / J. Am. Chem. SOC. 1994,116, 7943-7944
In 2004, the Wayland research group showed that, in the presence of a radical source such as AIBN or V-70, the polymerization of MA could be mediated by catalytic amounts of (TMP)Co. At 60◦C and in benzene, after a short induction period, PMA of molecular weight up to 120,000 and polydispersity as low as 1.06 were produced. Low polydispersity poly(MA-b-BA) copolymers were also efficiently synthesized.
F. di Lena, K. Matyjaszewski / Progress in Polymer Science 35 (2010) 959–1021
Transition metal complexes that have been reported to polymerize olefins via a RDC mechanism include Ti, Mo, Os, Co, and Rh compounds.
(TMP)Co-MA 2
Reaction of 2 wiБайду номын сангаасh MA at 60 ℃ in benzene results in formation of PMA with relatively small polydispersity (1.1-1.3) and linear increase in Mn (1 X 104 to 1.7 X 105) with MA conversion (5-80%, DP = 125-2000). In spite of the processes that can limit polymer growth , observation of linear increases in Mn with conversion, formation of block copolymers, and relatively small polydisper- sities clearly demonstrate that 1 and 2 initiate an effective living radical polymerization of acrylates.
F. di Lena, K. Matyjaszewski / Progress in Polymer Science 35 (2010) 959–1021
Organometallic mediated radical polymerization, is based on the fast and reversible homolytic cleavage of a metal-carbon bond in the metal complex. Depending on the type of monomer used, the specific metal/ligand combination and the initiation strategy, OMRP can occur through either RDC or DT or both mechanisms. Chain-breaking reactions such as CCT may also competitively take place.
Free Radical Polymerization
Controlled Radical Polymerization
requirements
(i) The rate of initiation is faster than that of propagation, so that all chains form and grow simultaneously. (ii) The concentration of active radicals is low in order to slow down termination reactions. (iii) The concentration of propagating chains is high so only a small fraction of them are terminated. (iv) The polymerization system remains sufficiently homogeneous, so that the active centers are readily available.
F. di Lena, K. Matyjaszewski / Progress in Polymer Science 35 (2010) 959–1021
The accuracy of theoretical calculations in terms of molecular structure, energetics and physical properties has made enormous progress in the last couple of decades. The application of computational tools, especially Density Functional Theory (DFT) methods, in view of their relatively good performance at comparatively low computational cost, is becoming routine in a variety of different areas of chemistry and controlled radical polymerization is no exception.