Cationic polymerization procedure

The ionic liquid process has a number of advantages over traditional cationic polymerization processes such as the Cosden process, which employs a liquid-phase aluminium(III) chloride catalyst to polymerize butene feedstocks [30]. The separation and removal of the product from the ionic liquid phase as the reaction proceeds allows the polymer to be obtained simply and in a highly pure state. Indeed, the polymer contains so little of the ionic liquid that an aqueous wash step can be dispensed with. This separation also means that further reaction (e.g., isomerization) of the polymer s unsaturated ot-terminus is minimized. In addition to the ease of isolation of the desired product, the ionic liquid is not destroyed by any aqueous washing procedure and so can be reused in subsequent polymerization reactions, resulting in a reduction of operating costs. The ionic liquid technology does not require massive capital investment and is reported to be easily retrofitted to existing Cosden process plants. 

The most important approximations of the HMO method are, the consideration of only the it-electrons, the neglect of the interaction between them and the neglect of all atom-atom-interactions, if the atoms are not neighbouring. Contrary to the other methods, the molecular geometry does not play a role in the HMO method because only the topologic connection of the atoms in the molecule is considered. In spite of these drastic approximations, the HMO method proved its worth as the first semiempirical procedure with a wide field of applications as well as in the theoretical interpretation of many chemical phenomenons. The method was broadly used in the field of cationic polymerizations in the 60-70 years too (see 2)). 

Cationic polymerization was considered for many years to be the less appropriate polymerization method for the synthesis of polymers with controlled molecular weights and narrow molecular weight distributions. This behavior was attributed to the inherent instability of the carbocations, which are susceptible to chain transfer, isomerization, and termination reactions [48— 52], The most frequent procedure is the elimination of the cation s /1-proton, which is acidic due to the vicinal positive charge. However, during the last twenty years novel initiation systems have been developed to promote the living cationic polymerization of a wide variety of monomers. 

The first attempts at ROP have been mainly based on anionic and cationic processes [4,5]. In most cases, polyesters of low molecular weight were recovered and no control on the polymerization course was reported due to the occurrence of side intra- and intermolecular transesterification reactions responsible for a mixture of linear and cyclic molecules. In addition, aliphatic polyesters have been prepared by free radical, active hydrogen, zwitterionic, and coordination polymerization as summarized in Table 2. The mechanistic considerations of the above-mentioned processes are outside the scope of this work and have been extensively discussed in a recent review by some of us [2 ]. In addition, the enzyme-catalyzed ROP of (di)lactones in organic media has recently been reported however, even though this new polymerization procedure appears very promising, no real control of the polyesters chains, or rather oligomers, has been observed so far [6]. 

A typical example of the experimental conditions for the cationic polymerization of various olefinic and diolefinic monomers is illustrated in the Procedure section. 

The use of a precision digital density meter as supplied by Mettler Instruments (Anton Paar, Ag.) appeared attractive. Few references on using density measurements to follow polymerization or other reactions appear in the literature. Poehlein and Dougherty (2) mentioned, without elaboration, the occasional use of y-ray density meters to measure conversion for control purposes in continuous emulsion polymerization. Braun and Disselhoff (3) utilized an instrument by Anton Paar, Ag. but only in a very limited fashion. More recently Rentsch and Schultz(4) also utilized an instrument by Anton Paar, Ag. for the continuous density measurement of the cationic polymerization of 1,3,6,9-tetraoxacycloundecane. Ray(5) has used a newer model Paar digital density meter to monitor emulsion polymerization in a continuous stirred tank reactor train. Trathnigg(6, 7) quite recently considered the solution polymerization of styrene in tetrahydrofuran and discusses the effect of mixing on the reliability of the conversion data calculated. Two other references by Russian authors(8,9) are known citing kinetic measurements by the density method but their procedures do not fulfill the above stated requirements. 

The behavior of cationic intermediates produced in styrene and a-methyl-styrene in bulk remained a mystery for a long time. The problem was settled by Silverman et al. in 1983 by pulse radiolysis in the nanosecond time-domain [32]. On pulse radiolysis of deaerated bulk styrene, a weak, short-lived absorption due to the bonded dimer cation was observed at 450 nm, in addition to the intense radical band at 310 nm and very short-lived anion band at 400 nm (Fig. 4). (The lifetime of the anion was a few nanoseconds. The shorter lifetime of the radical anion compared with that observed previously may be due to the different purification procedures adopted in this experiment, where no special precautions were taken to remove water). The bonded dimer cation reacted with a neutral monomer with a rate constant of 106 mol-1 dm3s-1. This is in reasonable agreement with the propagation rate constant of radiation-induced cationic polymerization. 

N-Benzyl and iV-alkoxy pyridinium salts are suitable thermal and photochemical initiators for cationic polymerization, respectively. Attractive features of these salts are the concept of latency, easy synthetic procedures, their chemical stability and ease of handling owing to their low hygroscopicity. Besides their use as initiators, the applications of these salts in polymer synthesis are of interest. As shown in this article, a wide range of block and graft copolymer built from monomers with different chemical natures are accessible through their latency. 

This chapter presents an updated overview of the current status of the controlled polymer syntheses via the modem generation of cationic polymerizations that are mostly living or controlled what and how one can design and eventually synthesize novel polymers with well-defined structures and functionalities. Thus, the following sections are devoted to each of these classes of polymers (Fig. 2), with emphasis on the general methodologies and specific examples. The last section (Section VII) briefly covers the experimental procedures in living cationic polymerization and related polymer synthesis. 

The preparation of block copolymers by combination of thermally radical and photoinduced cationic polymerization processes has also been reported [151], Indeed, styrene/cyclohexene oxide (CHO) copolymers have been synthesized by using a bifunctional azobenzoin initiator such as ABME, previously described, through a two-step procedure. In the first step, thermal Iree radical polymerization of styrene in the presence of the above azobenzoin initiator gives poly(styrene) prepolymers with benzoin photoactive end groups, as reported in Scheme 38. These prepolymers, upon photolysis and subsequent oxidation to the corresponding carbocations in the presence of l-ethoxy-2-methylpyridinium hexafluoro phosphate (EMP+PFg ), finally give block copolymers by cationic polymerization of cyclohexene oxide (Scheme 45). 

Many different approaches have been used to synthesize star-block copolymers including anionic, cationic, radical, and condensation polymerization techniques, and even combinations of them [9]. The majority of the molecules produced thus far were prepared by anionic polymerization procedures. The dominant way of preparing star-block copolymers by anionic polymerization is the coupling of preformed diblock or triblock living copolymer chains with a suitable compound to produce the central linking point. In this way divinylbenzene (DVB) was first used in order for a central core to be created [ 10]. This was achieved by adding a predetermined amount of the divinyl compound to a solution of living diblock chains (Scheme 1). 

Only a few examples of macromonomers prepared cationically have appeared in the literature despite the development of many systems susceptible to living cationic polymerization. Similarly, with all the living procedures the polymerizable end group can be introduced by either functional termination or functional initiation. 

Initiation of Cationic Polymerization by Free-Radical Initiators. A new procedure for the initiation of cationic polymerization was developed by Ledwith (13. 23. 71. 72). This procedure consists of... 

Living cationic polymerizations of propylene oxide and of epichlorohydrin have been reported. The procedure is carried out in an alcohol with a strong acid catalyst such as fluoroboric acid. Interestingly enough, the procedure is not applicable to the polymerization of ethylene oxide [56]. 

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