Mechanistic studies on ceric-amine initiated aqueous polymerization of methylmethacrylate by endgroup analysis

Summary Endgroup analysis of polymethylmethacrylate obtained by initiation with Ceric ion-amine initiator systems in acid aqueous media has been carried out using Palit's dye partition technique. The amines used are diethylamine, triethylamine, mono-ethanolamine, diethanol amine and triethanol amine and also ethylene diamine. All the amines incorporated amine endgroups in the polymer samples to the extent of approximately one endgroup per chain, except ethylene diamine which incorporated approximately two amine endgroups per macromolecule. Besides amine endgroups, the three ethanol amines incorporated hydroxyl endgroups to different extents depending on the hydroxyl content of the particular compound. In the light of endgroup results initiation through amine, ethylene diamine or ethanol amine radicals have been suggested. Probable termination mechanism has also been discussed.With 3 tables     

Mechanistic studies on ceric-thiourea-initiated polymerization of methyl methacrylate in acid-aqueous media from end group analysis

The mechanism of the redox polymerization of methyl methacrylate in acid-aqueous medium was studied for ceric-thiourea initiator system by analysis of polymer end groups using the dye partition method. Polymerization at room temperature and at pH 2.30 with this initiator system incorporates mostly amino end groups in the polymers to the extent of 1.18-1.31 and a small amount of hydroxyl end group not exceeding 0.08 per polymer molecule. Under the same pH condition, a higher polymerization temperature increases the amine end group content to 2.17-2.30 and hydroxyl end group content to 0.14-0.18 per polymer molecule. A lower polymerization temperature (5°C), or a lower pH (ca. 1) of the reaction medium, produces polymers with exclusively amine end groups, hydroxyl end groups being totally absent. At pHs greater than 3, amine end group incorporation decreases significantly, with simultaneous increase in hydroxyl incorporation. On the basis of end group results the initiating radical has been identified and the mechanism of initiation has been suggested. The possible mode of termination has also been discussed.     

Mode of initiation in ceric salt initiated aqueous polymerization of methylmethacrylate

Summary The basic reaction for the production of initiating radicals in Ceric salt initiated polymerization of methylmethacrylate is strongly dependent on the acidity of the medium and independent of the nature of the anion associated' with the ceric ion. In moderately acid medium the primary reaction is the formation of hydroxyl radicals by ceric ion oxidation of water. When ceric sulfate is used as the initiator these hydroxyl radicals initiate the polymerization and appear as hydroxyl endgroups in the polymers. On the other hand, when ammonium ions are present, as in case of ceric ammonium sulfate initiator or ceric sulfate initiator in the presence of ammonium sulfate, some of the hydroxyl radicals generated in the primary reaction, react with the ammonium ions producing ammonium radicals and both hydroxyl and ammonium radicals act as the initiator giving rise to both hydroxyl and amine endgroups in the polymers. In strongly acid medium the basic initiating reaction changes into one in which the monomer transfers an electron to the ceric ion producing monomer radicals which initiate the polymerization and polymers with no endgroups are formed.

---

Zusammenfassung Die Grundreaktion zur Erzeugung von Startradikalen aus Cersalz-initiierter Polymerisation von Methylmethacrylat ist von der Aciditäat des Mediums stark abhängig und unabhängig von der Natur des Anions, das mit den Cerionen assoziiert ist. Im mäßig sauren Medium besteht die primäre Reaktion in der Bildung von Hydroxylradikalen durch das Cerion auf Basis der Oxidation von Wasser. Wenn Cersulfat als Initiator verwendet wird, starten die Hydroxylradikale die Polymerisation und erscheinen als Kettenendgruppen. Andererseits, bei Gegenwart von Ammonium, wie im Falle des Cerammoniumsulfats oder Cersulfats in Gegenwart von Ammoniumsulfat, reagieren einige der Hydroxylradikale in primärer Reaktion mit den Ammoniumionen und erzeugen Ammoniumradikale, und darn geben beide-Hydroxyl-und Ammoniumradikale - als Initiatoren Anlaß für Hydroxyl-und Aminendgruppen der Hydroxylketten. In stark saurem Medium verändert sich die Startreaktion in eine andere, in der Monomere ein Elektron an das Cerion transferieren und hierdurch Monomerradikale erzeugen, die die Polymerisation starten und Polymere ohne Endgruppen bilden.

---

---

With 5 tables     

Studies on the kinetics of ceric–thiourea-initiated aqueous polymerization of methyl methacrylate. I. Mechanistic features in moderately acid solution

The kinetics of the aqueous polymerization of methyl methacrylate by a ceric-thiourea initiator system in moderately acid solution (pH 2.15) was studied. The rate of polymerization was proportional to 0.41 power of ceric concentration, 0.32 power of thiourea concentration, and 1.18 power of monomer concentration. The degree of polymerization was smaller than expected from the rate of polymerization. Initiation efficiency was less than one. There was no evidence of any ceric ion termination in the concentration range of 2.50 × 10^?4–2.00 × 10^?3M studied. The results are explained in terms of partial primary radical termination; the principal mode of termination, however, was bimolecular.     

Measurement of the ground state 2n pickup probability for 28Si+68Zn and its role in sub-barrier fusion enhancement

The ground state and excited state transfer yields for the 2-neutron pickup channel in the ²⁸Si+⁶⁸Zn system have been measured explicitly. The recoil mass separator at the nuclear Science Centre, New Delhi was used for the measurement. A NaI(T1) detector was used for detecting the deexcitation γ’s from the transfer products. The kinematic coincidence technique was employed for the transfer measurement. Simplified coupled channels calculations show that out of all transfer channels the major contribution to the sub-barrier enhancement comes from the ground state 2 neutron pickup channel with a ground state Q-value of+1.83 MeV.     

Comparison of thermomagnetic history effects in weakly pinned single crystals of R 3Rh4Sn13 (R=Yb, Ca)

A comparative study of the thermomagnetic memory effects of J _c in two weakly pinned low T _c superconductors, Ca₃Rh₄Sn₁₃ (CaRhSn) and Yb₃Rh₄Sn₁₃ (YbRhSn), is presented. In both the systems, the peak effect (PE) phenomenon appears as an order-disorder transformation through stepwise amorphization of the flux line lattice (FLL). However, in CaRhSn. we can witness another disorder-driven transition (Bragg glass (BG) to a vortex glass (VG) in a distinct manner as in a single crystal of high T _c YBa₂Cu₃O_7-δ for H‖c.     

Das Thiol/Dimethylsulfoxid-System als Initiator fiir die Polymerisation in benzolischem Medium. 2. Mitt. Kinetische Studien

Ohne Zusammenfassung     

Recent Advances On Direct Formylation Reactions

Aldehydes serve as the key functional group in organic synthesis and are valuable intermediates. The various advanced methods of direct formylation reactions have been reviewed in this article. Overcoming the drawbacks of the traditional methods of formylation, newer methods involving homo and heterogenous catalysts, one pot reactions, solvent free techniques are elaborated, which can be performed under mild conditions and using inexpensive resources.