Cationic polymerization of styrenes under essentially neutral conditions
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ORIGINAL PAPER
Cationic polymerization of styrenes under essentially neutral conditions Dmitry M. Kuznetsov & Vasily V. Tumanov & William A. Smit
Received: 17 October 2012 / Accepted: 25 March 2013 # Springer Science+Business Media Dordrecht 2013
Abstract An acid-free protocol for the cationic polymerization of styrene and its derivatives is described based upon utilization of hexafluoroisopropanol (HFIP) both as the sole proton source and a polar medium. For the electron-enriched styrenes no other activator is required while the polymerization of unsubstituted styrene proceeds readily only in the presence of the catalytic amounts of β-pinene as a co-initiator. Polymers thus prepared reveal MWD characteristics comparable to those for the products of the conventional non-living cationic polymerization. Keywords Acid-free cationic polymerization . Polystyrene . Hexafluoroisopropanol
Introduction Cationic polymerization of alkenes constitutes one of the most important pathways which is widely employed for the multiscale production of quite a number of practically useful polymers [1]. By now a huge diversity of methods for the cationic polymerization of arylolefins (including living polymerization) have been elaborated involving the usage of various acidic promoters (for the leading references on the scope of the reaction and a detailed discussion of the reaction mechanism, see data in Ref. [1–4]). Recently we have found that β-pinene is quantitatively converted into poly(β-pinene) upon mixing at the ambient temperature with an excess of hexafluoroisopropanol Electronic supplementary material The online version of this article (doi:10.1007/s10965-013-0128-2) contains supplementary material, which is available to authorized users. D. M. Kuznetsov (*) : V. V. Tumanov : W. A. Smit N. D. Zelinsky Institute of Organic Chemistry, 47 Leninsky Prospekt, Moscow 119991, Russian Federation e-mail: [email protected]
(HFIP) in the absence of any other initiators [5, 6]. This finding seemed to be quite surprising in view of a rather low Brønsted acidity of HFIP (pKa =9.3, cf. for acetylacetone pKa =9.0). The observed reactivity of HFIP as a protontransfer agent was explained by a unique set of properties such as high ionizing power, low nucleophilicity, exceptional hydrogen bond donating capacity and specific ability to solvate anions. These properties are well documented in the literature (for the general reviews see Ref. [7–9]). An obvious novelty of the disclosed phenomenon prompted us to undertake a study of its scope and generality and here we report our data obtained for the proton-induced and at the same time acid-free cationic polymerization of several arylolefins: styrene (St), p-methylstyrene (p-MeSt), p-methoxystyrene (p-MeOSt) and p-(N,N-dimethylamino)styrene (p-Me2NSt) (preliminary results of these studies were presented [10, 11]).
Results and discussion Polymerization promoted solely by HFIP Our initial attempts to perform the styrene polymerization in neat HFIP did not appear encouraging: at ambient t
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