Thermally Initiated cationic with free radicals

In many cases of start reactions, a monomer is added on to the initiator to form an active center. The active center may be an anion, cation, or free radical in addition polymerization, or, for example, an electron-deficient compound or an unoccupied ligand position in polyinsertion (see Chapter 19). Spontaneous thermal addition polymerizations of monomers which proceed in the absence of added catalyst or initiator are relatively rare. The free radical thermal polymerization of styrene (Chapter 20) and the charge transfer copolymerization of monomers of opposed polarities (Chapter 22) are examples of genuine spontaneous polymerizations. These genuine spontaneous polymerizations can often only be distinguished with difficulty from nongenuine spontaneous polymerizations which are started by unsuspected impurities remaining in these systems. 

Three events are involved with chain-growth polymerization catalytic initiation, propagation, and termination [3], Monomers with double bonds (—C=C—R1R2—) or sometimes triple bonds, and Rj and R2 additive groups, initiate propagation. The sites can be anionic or cationic active, free-radical. Free-radical catalysts allow the chain to grow when the double (or triple) bonds break. Types of free-radical polymerization are solution free-radical polymerization, emulsion free-radical polymerization, bulk free-radical polymerization, and free-radical copolymerization. Free-radical polymerization consists of initiation, termination, and chain transfer. Polymerization is initiated by the attack of free radicals that are formed by thermal or photochemical decomposition by initiators. When an organic peroxide or azo compound free-radical initiator is used, such as i-butyl peroxide, benzoyl peroxide, azo(bis)isobutylonitrile, or diazo- compounds, the monomer s double bonds break and form reactive free-radical sites with free electrons. Free radicals are also created by UV exposure, irradiation, or redox initiation in aqueous solution, which break the double bonds [3]. 

Vinyl ketones are an interesting class of monomers because various members of this group polymerize via a radical, anionic, and cationic mechanism. Methyl vinyl ketone (MVK) - also named 3-butene-2-one - is its best examined representative. The physical properties of poly(methyl vinyl ketone) (PMVK) depend on the polymerization conditions and the degree of polymerization. PMVK ranges from a viscous oil to a hard plastic or rubbery mass. Polymers obtained with free radical initiators are amorphous materials with low softening points (about 40 to 80 °C) and poor thermal and chemical stability [274,275]. The molecular weights are relatively low because of the lability of the protons in the oc-position to the carbonyl groups. 

The features and detail of the IPN kinetics were also studied in other works [274-276]. The kinetics of thermally initiated cationic epoxy polymerization and free radical acrylate photopolymerization were investigated in [277]. It was found that the preexistence of one polymer has a significant effect on the polymerization of the second monomer. The reaction kinetics and phase separations were studied for sequential IPNs in [278]. The kinetics of IPN formation was studied for IPNs based on PDMS-cellulose acetate butyrate [279]. All these and other works [280-282] confirm the general regularities of the reaction kinetics and its connection with phase separation in forming systems. 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

Polymerization of substituted acetylenes has been carried out by a wide range of catalysts and condi-tions. Polymerization conditions include a homogeneous and heterogeneous Ziegler—Natta catalyst, transition metal complexes (Pd. Pt. Ru. W. Mo. Ni. etc.), free radical initiators such as 2.2 -azobis(isobu-tyronitrile) (AIBN). benzoyl peroxide (BPO). and di-tert-butylperoxide (DTBP). thermal polymerization, y-irradiation. cationic initiation with BF3. and anionic initiation by butyllithium. triethylamine. and sodium amide. 

Some special reaction with a particularly designed route can be used to synthesize azo BCs. For instance, a series of poly(vinyl ether)-based azo LCBCs were synthesized by using living cationic polymerization and free-radical polymerization techniques (Serhatli and Serhatli, 1998). As shown in Scheme 12.8, 4.4 -azobis(4-cyano pentanol) (ACP) was used to couple quantitatively two well-defined polymers of LC living poly(vinyl ether), initiated by the methyl trifluor-omethane sulfonate/tetrahydrothiophene system. Then the ACP in the main chain was thermally decomposed to produce polymeric radical, which was used to initiate the polymerization of MMA or styrene to obtain PMMA-based or PS-based azo BCs (AB or ABA types). 

Styrene is one of the few substances that can be polymerized equally well free-radically (thermally and with initiators), cationically, anionically, and with complex catalysts. Free radical, cationic, and most anionic polymerizations yield atactic polymers, whereas certain polymerizations of the polyinsertion type yield isotactic polymers. Only the free radical polymerizations are of commercial interest. 

The C=C unsaturation in all these structures is 1,2-disubstituted, so initiation and propagation by free-radical and cationic mechanisms are sluggish. Hence, they elicit slow processes and poor yields of macromolecular structures though, ultimately, crosslinked materials are generated (as in the case of classical oxido-polymerisation discussed in Chapter 2). Preparation of blown oils by thermal treatment >300 C is also a poor approach because of substantial losses associated with thermal degradation. It follows that applications are not particularly attractive in practical terms except in the traditional realm of siccative paints and inks. 

Fan et al. [56] attached an initiator to MMT via cation-exchange suitable for free radical initiation, investigating monofunctional and bifunctional (48, 49). The initiator-clay was thermally active and the clay rendered adsorptive rather than organophihc as no compatible groups with the matrix had been added to the clay. A comparative study of the monofunctional and bifunctional-modified days used to polymerize styrene found that the monofunctional modified days led to better exfoHated nanocomposites [57, 58]. 

General methods of obtaining functionalized HRs with carboxylic groups include thermal grafting with initiators forming free radicals, or with cationic catalysts of the Friedel-Crafts type. Also included are cationic co-oligomerization of unsaturated hydrocarbons or of some Diels-Alder adducts (obtained from dienic hydrocarbons) with MA or other filodienes with carboxylic functions [155,1561. 

Details of the procedures used in the preparation of commercial formaldehyde copolymers have not been fully disclosed. The principal monomer is trioxan and the second monomer is a cyclic ether such as ethylene oxide, 1,3-dioxolane or an oxetane ethylene oxide appears to be the preferred comonomer and is used at a level of about 2%. Boron trifluoride (or its etherate) is apparently the most satisfactory initiator, although many cationic initiators are effective anionic and free radical initiators are not effective. The reaction is carried out in bulk. The rapid solidification of the polymer requires a reactor fitted with a powerful stirrer to reduce particle size and permit adequate temperature control. The copolymer is then heated at 100°C with aqueous ammonia in this step, chain-ends are depolymerized to the copolymer units to give a thermally-stable product. The polymer is filtered off and dried prior to stabilizer incorporation, extrusion and granulation. 

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