Reactions homopolymerization

When the curing process is based on the reaction between the reactive epoxy molecules only, the reaction is homopolymerization (see Fig. 2.12). The cured structure is essentially made up only of the original epoxy molecules linked together through their own reactive sites. The reactive compound that initiates the homopolymerization reactions is generally known as a catalyst. (Note Sometimes the term hardener is used interchangeably for catalyst, although this is not the convention followed in this book.) 

The catalyst does not make up part of the final epoxy network structure or have a significant effect on the final properties of the cured resin. Thus, the final cured properties of the epoxy system are primarily due to the nature of the epoxy resin alone. Homopolymerization normally provides better heat and environmental resistance than polyaddition reactions. However, it also provides a more rigidly cured system, so that toughening agents or flexibilizers must often be used. In adhesive systems, homopolymerization reactions are generally utilized for heat cured, one-component formulations. 

The catalytic curing agents commonly used include tertiary amines, Lewis acids and bases, and dicyandiamide. Since their function is truly catalytic, the catalyst is added at relatively low concentrations (0 to 5% by weight) to the epoxy formulation. Homopolymerization generally requires both the presence of catalysts and elevated temperatures for the reaction to proceed. Like the polyaddition reaction, the homopolymerization reaction is accelerated by hydroxyl groups or tertiary amines. 

As epoxy resins can vary enormously as suggested above, so can the means for curing these resins. Curing reactions specific to specific types of curing agents or catalysts are described in Chap. 5. 

---

In this chapter we deal exclusively with homopolymers. The important case of copolymers formed by the chain mechanism is taken up in the next chapter. The case of copolymerization offers an excellent framework for the comparison of chemical reactivities between different monomer molecules. Accordingly, we defer this topic until Chap. 7, although it is also pertinent to the differences in the homopolymerization reactions of different monomers. 

The lower yield for olefin than for S02 was explained by scavenging of the formed free olefin on the cationic sites of the irradiated polymer in a homopolymerization reaction, thus reducing G(olefin). Adding cation scavengers it was found that the overall product yield was reduced with concurrent reduction of the S02/olefm ratio towards unity73. Thus it can be concluded that the homopolymerization of the olefin is occurring by a cationic mechanism. 

In an attempt to generate convenient routes to organofunctional phosphazenes which could undergo homopolymerization reactions, we explored the reactions of lithium enolates, particularly that of acetaldehyde, with halophosphazenes which leads to the series of organofunctional monomers N3P3ci6 n(0CH=CH2)n15 16... 

M.M. Reis, M. Uliana, C. Sayer, P.H.H. Araujo and R. Giudici, Monitoring emulsion homopolymerization reactions using ET-Raman spectroscopy, Braz. J. Chem. Eng., 22, 61-74 (2005). 

Recently, FTIR spectroscopy studies have been reported which support the above observations. Moacanin et al 3) concluded that two reactions dominate the TC3fDA/DDS cure epoxy-primary amine addition is the principal reaction occurring during the early stage of cure followed by the epoxy-hydroxyl addition reaction. Indeed they find that the rate of epoxy-hydroxyl addition is at least an order of magnitude slower than for the epoxy-primary amine reaction at 177 C. Furthermore, Morgan et al (4) report that the epoxysecondary amine addition and epoxy-epoxy homopolymerization reactions also occur at 177°C but at rates that are approximately 10 and 200 times slower, respectively, than the epoxy-primary amine react ion. 

The homopolymerization reaction proceeds through an intramolecular transfer reaction between macroradicals and monomers ... 

Other possible reactions, such as homopolymerization (epoxide+epoxide) and epox-ide+hydroxyl group (in the latter stages of cure), can be neglected when the ratio of epoxide to amine is stoichiometric and in the absence of catalyst or accelerator [194], For TGDDM/DDS resins, the homopolymerization reaction may be neglected at cure temperature below 180°C [84], At temperatures between 177°C and 300°C, dehydration and/or network oxidation occur, which results in formation of ether cross-linkings with loss of water. Decomposition of the epoxy-OH cure reaction can also take place, which results in propenal... 

Once the vinylbenzylanion (II) forms, it may add to another divinylbenzene molecule resulting in DVB "homopolymerization" (Reaction 2). Likewise, the vinylbenzylanion (II) may attach a pendant vinyl group of another polymer chain (Reaction 3) to give the alkylbenzylanion (III). 

The nature of the cure reactions in these epoxies can be confirmed by monitoring the epoxide consumption via near infra-red spectroscopy for a series of epoxide-amine mixtures containing a range of amine contents. A plot of % epoxide consumption vs. amine concentration for DGEBA-T403 epoxies is illustrated in Fig. 2. This plot confirms that the DGEBA-T403 epoxy system forms exclusively from epoxide-amine addition reactions, because (i) 100% epoxide consumption is attained at the stoichiometric amine concentration associated with exclusive epoxide-amine addition cure reactions and (ii) extrapolation of this plot to zero amine content indicates there is no epoxide consumption i.e. there are no epoxide homopolymerization reactions. 

The homopolymerization reactions of impure TGDDM (MY720) in the presence and absence of a BF3 NH2C2H5 catalyst and, also, pure TGDDM were monitored by FTIR as a function of cure temperature from 177 to 300 °C. The intensities of the epoxide, hydroxyl, ether and carbonyl bands at 906, 3500, 1120 and 1720 cm-1 respectively were determined from spectral differences and are plotted as a function in cure conditions in Figs. 10,11,12 and 13 respectively. The 906,1120 and 1720 cm-1 band intensities were normalized to the 805 cm-1 band and the 3500 cm-1 to the 1615 cm 1 band. The 805 and 1615 cm-1 bands are associated with the phenyl group which is assumed to chemically unmodified during the homopolymerization reactions. 

Special Case where K2 9 0, q = 0, and q-> — 0. In this case the two homopolymerization reactions are reversible the alternative steps are irreversible. Here Equation 35 is valid (10, 29) ... 

Homopolymerization reactions were also carried out with the use of methyl methacrylate monomer. 

The percentage conversion of monomer to polymer for homopolymerization reactions and the grafting yields for the grafting ones were calculated according to the following equations ... 

Both polyaddition and homopolymerization reactions can result in increased molecular weight and crosslinking. Both types of reaction occur without the formation of by-products. The curing reactions are exothermic, and the rates of reaction increase with temperature. 

Figure 2 shows a series of cloud-point curves determined for the system ethylene-2-ethylhexyl acrylate-poly(ethylene-co-2-ethylhexyl acrylate). Each cloud-point curve corresponds to one stationary copolymerization condition in CSTR1. The compositions and concentrations referring to the five monomer-polymer mixtures, including one ethylene homopolymerization reaction (Experiment 1), are listed in Tab. 1. FA is the concentration of the acrylate units within the copolymer (in mole-%),/P and/A denote the concentrations of polymer and of acrylate monomer in the monomer-polymer mixture, respectively. As can be seen from Fig. 1 and from Tab. 1, increasing acrylate content in the copolymer lowers the cloud-point pressure. 

Exact definition of the cure chemistry is difficult since formulations of this type may also contain auxiliary catalysts (Lewis Acids) which enhance the homopolymerization reaction. 

G. Oshanin and M. Moreau, Influence of transport limitations on the kinetics of homopolymerization reactions, J. Chem. Phys., 107 (1995) 2977-2985. 

The synthesis of homo- (Ti-Ti) and heterobinuclear (Ti-Zr) complexes linked by 1,2-G2H4 linker groups as shown in Scheme 316 has been reported. The molecular structures of the dimethylamido derivatives have been determined by X-ray diffraction methods. In the presence of binuclear borate activators, the methyl complexes produce long-chain branched polyethylene and polystyrene in homopolymerization reactions and ethylene-styrene co-polymers. The polymerization behavior differs from that obtained with the mononuclear compound (3-ethylindenylSiMe2-NBiOTiMea (Scheme 3 1 7).762"764... 

Table 1-Representatlve Homopolymerization Reactions Of Fungicidal Monomers...

Major part of the ABA homopolymerization reaction leads to the formation of oligomers (dimers). 

An homologous series of epoxy resins with various EEWs was synthesized by standard advancement techniques and subsequently esterified with 15 wt% 1300X13. Isothermal aging kinetics were followed at 175C. Aliquots of resin were withdrawn hourly, rapidly cooled to room temperature, and titrated for EEW. No precautions were taken to exclude air during the isothermal aging. Table Xll summarizes the formulations, reactive moiety equivalent weights, and kinetic calculations for the epoxide-alcohol addition reaction and the epoxide homopolymerization reaction. 

The kinetics of copolymerization provides a partial explanation for the copolymerization behavior of styrenes with dienes. One useful aspect of living anionic copolymerizations is that stable carbanionic chain ends can be generated and the rates of their crossover reactions with other monomers measured independently of the copolymerization reaction. Two of the four rate constants involved in copolymerization correspond at least superficially to the two homopolymerization reactions of butadiene and styrene, for example, and k, respectively. The other... 

The major homopolymerizations reactions of isocyanates are shown in Scheme 1. 

---