Chapter 11
The Heterogeneous Cationic Polymerization of Aromatic Alkenes by Aluminum Triflate
Downloaded by UNIV OF OKLAHOMA on April 15, 2013 | http://pubs.acs.org Publication Date: May 1, 1997 | doi: 10.1021/bk-1997-0665.ch011
Alessandro Gandini and Yao-Hong Yang Matériaux Polymères, Ecole Française de Papeterie et des Industries Graphiques, Boite Postal 65, 38402 Saint Martin d'Hères, France
Aluminium triflate is a very convenient initiator for the polymerization of styrenic monomers. The systems investigated in this work call upon the use of that strong Lewis acid suspended in non polar solvents. The behaviour of 4-methylstyrene and indene as representative monomers for the C9 petroleum cut was explored both in heterogeneous homopolymerization and copolymerization conditions. Initiation was found to take place only through cocatalysis and monomer transfer was an important chain-braking event at room temperature. The low DPs thus obtained were deemed interesting in the context of the preparation of resinous materials. After the study of these batch reactions, a continuous mode was tested and the results showed that the catalyst operates adequately, but needs frequent regenerations.
The use of heterogenous means to initiate cationic polymerization has attracted sporadic attention since the late 1940s, with various laboratories becoming involved from time to time with the study of a specific solid catalyst. However, as underlined in a previous survey (7), up to the end of the 1970s, these contributions often lacked the necessary scientific approach to justify reliable mechanistic conclusions. It was at that time that a thorough investigation of the activity of numerous metal perchlorates and inflates in relation to both typical electrophilic reactions and cationic polymerizations was carried out in our laboratory in order to assess both the relative reactivity of the salts used, the initiating mechanisms involved and the kinetic features of the most promising systems (1-3). In this study, the metal salts were employed in different media, of which some were entirely heterogeneous (7), others involved the presence of both dissolved and solid species (3) and finally, some allowed the complete dissolution of the catalyst (2,3). Aluminium triflate (AT) was found to be one of the most interesting initiators, capable of operating under the different conditions mentioned, albeit through different mechanisms. Among the conventional electrophilic reactions, alkylations, acylations,
© 1997 American Chemical Society
In Cationic Polymerization; Faust, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
113
114
CATIONIC POLYMERIZATION
carbinol dehydrations, alkene hydrations and esterifications gave very satisfactory results in terms of yields and selectivity (3). Cationic polymerization systems were of course also thoroughly scrutinized using this initiator (1-3). In solvents of low polarity (ε~2), A T was entirely insoluble and acted therefore at the liquid/solid interphase essentially through cocatalysis. In solvents of medium polarity (ε-10), direct initiation was shown to take place even for the predominantly insoluble portion of the salt. In highly polar media, viz. solvents like nitromethane or some bulk ndonor monomers, AT was soluble up to fairly high concentrations and its mode of initiation was found to occur following its selfionization:
Downloaded by UNIV OF OKLAHOMA on April 15, 2013 | http://pubs.acs.org Publication Date: May 1, 1997 | doi: 10.1021/bk-1997-0665.ch011
2 Al(OTf)
3
^
^
(OTf) Al® + 2
(OTf) Al© 4
and the subsequent addition of the aluminium cation to the alkenyl unsaturation or the heteroatom of the monomer. AT can be readily prepared by different synthetic routes, of which two were selected in that work (1-3), namely: (i) the reaction of aqueous triflic acid with an excess of aluminium powder, or (ii) the reaction of triethyl aluminium with a dry solution of triflic acid in methylene chloride. It was found that, provided the salt prepared by the former method was correctly dried, both preparations led to the same species in terms of qualitative and quantitative activity. A large variety of monomers were found to respond to activation by AT. These included aliphatic and aromatic alkenes, vinyl ethers and other η,π-donor structures, as well as numerous heterocycles (1-3) and their relative reactivity led to the conclusion that this novel initiator ought to be considered as a particularly strong Lewis acid. The present study deals with a more specific approach in which we called upon AT as a potentially useful heterogeneous catalyst for the polymerization of aromatic alkenes in a more practical context. It was in fact thought that we could use this initiator to convert what is usually known as the "C9 cut" into a resinous product. This fraction, arising from petroleum cracking, contains a mixture of aromatic structures which include polymerizable species like substituted styrenes and indene together with non-polymerizable components like xylenes. It is used industrially as a source of resins for paint, printing inks, etc. after the various alkenes have been oligomerized cationically by conventional initiators. The replacement of the latter acidic catalysist by AT seemed an interesting proposition because of the possibility of using fixed-bed heterogeneous catalysis, i.e. a continuous polymerization system for the C9 cut. In order to acquire a sound understanding of this new system, it was decided to conduct a systematic study of the polymerization of two monomers representative of the C9 cut, namely 4-methylstyrene ( M S ) and indene (IN), as well as of their copolymerization in toluene, which again would simulate adequately the mixture of aromatic solvents in that cut. Experimental Aluminium triflate was synthesized by the simpler route, namely the reaction of aqueous triflic acid with a 20% excess of pure aluminium fine powder at 50°C for twenty hours. After filtration to separate the remaining unreacted metal, the salt was
In Cationic Polymerization; Faust, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
Downloaded by UNIV OF OKLAHOMA on April 15, 2013 | http://pubs.acs.org Publication Date: May 1, 1997 | doi: 10.1021/bk-1997-0665.ch011
11.
GANDINI & Y A N G
Polymerization of Aromatic Alkenes by Al Triflate
isolated by vacuum drying at 150°C to constant weight. Various samples of A T were prepared following this straightforward technique and gave reproducible results, provided the same average crystal size was employed. A T was kept at room temperature in a dry atmosphere since it is quite hygroscopic. Whenever needed, it was submitted to cascade sieving under dry nitrogen in order to separate different granulometric fractions. A T is a remarkably stable salt in terms of its resistance or its indifference to photolysis and thermal degradation. For the sake of comparison, two standard cationic initiators were also employed, namely boron fluoride etherate and titanium tetrachloride. Both were purchased as one molar solutions in methylene chloride and used as received. The two monomers used in this work, M S and IN, were comercially available and were purified by fractional distillation. Each middle portion was collected over calcium hydride and left for at least 48 hours before use, in order to ensure adequate drying. The purity of these samples was checked by G L C (>99.5%) and their residual moisture content by Karl-Fischer titration (
