Free-radical and Ionic Polymerization

An initiator, decomposed under the influence of heat or UV-irradiation, generates two free radicals 1 containing highly reactive unpaired electrons. Radical 1 reacts with methyl methacrylate 2 to produce the intermediate free 

Editor s note For more information about epoxy adhesives and their chemistry and formulation, the reader may refer to the chapter Epoxy Adhesives , which will be published in Volume 5 of this handbook. 

almost 50% of the semiconductors encapsulated in plastic packages are made for surface-mount assembly that subjects the devices to a considerable thermal shock during the soldering process. Within a few seconds, the internal package temperature rises to 215-260°C and the moisture absorbed by the plastic encapsulant and the organic adhesive evaporates explosively. This sometimes results in package cracks that start at the interface between the chip and the die pad or in delamination within the die attachment layer. To investigate the relationship between the chemical structure of epoxies and this so-called popcorn elfect , a series of polyfunctional resins has been evaluated [5]. They include new experimental epoxy novolacs whose chemical formulae have been previously displayed [4]. 

Because of their excellent adhesive properties on most hard materials and relatively low production cost, poly(glycidyl ethers) have been used for more than 30 years to formulate high-volume consumer-orientated adhesives as well as high-performance structural adhesives. When not loaded with large amounts of inorganic fillers, these resins have dynamic viscosities that are convenient for most applications. However, the addition of metal or oxide fillers dramatically increases the viscosity to a level higher than 10 Pa s. To lower the viscosity of the adhesive 

The chemical formulae of dicyandiamide 37 and of two ureas 3-(4-chloro-phenyl)-l,l-dimethylurea 38 and 3-(3,4-dichlorophenyl)-l,l-dimethylurea 39, which are accelerators frequently used in combination with 37, are drawn in 

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A factor in addition to the RTD and temperature distribution that affects the molecular weight distribution (MWD) is the nature of the chemical reaciion. If the period during which the molecule is growing is short compared with the residence time in the reactor, the MWD in a batch reactor is broader than in a CSTR. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are very short hved. In cases where the growth period is the same as the residence time in the reactor, the MWD is narrower in batch than in CSTR. Polymerizations that have no termination step—for instance, polycondensations—are of this type. This topic is treated by Denbigh (J. Applied Chem., 1, 227 [1951]). 

An important difference between free radical and ionic polymerization is that a counter ion only appears in the latter case. For example, the intermediate formed from the initiation of propene with BF3-H2O could be represented as... 

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

Funt and Williams reported that the copolymer compositions of methyl methacrylate and acrylonitrile varied at the same electrode depending upon the salts used in the saturated dimethyl formamide solution (Table 5) (19). They believed in the simultaneous occurrence of free radical and ionic polymerization in the system. Yield of polymers also differed with a variety of salts in the polymerization of methyl methacrylate in dimethyl sulfoxide (<5). Nevertheless no pattern of correlation has been given on the aspects relating to the supporting electrolytes. 

Greater differences between the optical activity of monomers and of polymers have been recently observed by Schulz and Hartmann 133) when investigating the free-radical and ionic polymerization of a number of variously substituted N-vinyl compounds. The above authors also observed in one case a large dependence of the optical activity of the polymers on the type of solvent used. 

Radiation-induced polymerization, which generally occurs in liquid or solid phase, is essentially conventional chain growth polymerization of a monomer, which is initiated by the initiators formed by the irradiation of the monomer i.e., ion radicals. An ion radical (cation radical or anion radical) initiates polymerization by free radical and ionic polymerization of the respective ion. In principle, therefore, radiation polymerization could proceed via free radical polymerization, anionic polymerization, and cationic polymerization of the monomer that created the initiator. However, which polymerization dominates in an actual polymerization depends on the reactivity of double bond and the concentration of impurity because ionic polymerization, particularly cationic polymerization, is extremely sensitive to the trace amount of water and other impurities. 

The formation of polymer, which is by far the most important product, has been attributed to both free-radical and ionic-polymerization reactions. 

Chain Homopolymerization Mechanism and Kinetics Free radical and ionic polymerizations proceed through this type of mechanism, such as styrene polymerization. Here one monomer molecule is added to the chain in each step. The general reaction steps and corresponding rates can be written as follows ... 

Items 2 and 3 arise from the fact that both the "counterion" and the medium itself can markedly affect the nature of the growing chain end. Thus, the growing chain end may assume various forms that depend on the extent of electrical charge separation and range all the way from a polarized covalent (sigma) bond to a completely dissociated state of free ions. This characteristic presents the greatest distinction between the mechanisms of free-radical and ionic polymerization. 

A great many reactions in physics and chemistry proceed via chain mechanisms. This large family of mechanisms includes free radical and ionic polymerization, Fischer Tropsch synthesis, gas phase pyrolysis of hydrocarbons, and catalytic cracking. Nuclear reactions, of both the power generating and the explosive kind, are also chain processes. Notice that chemical chain reactions can be catalytic or non-catalytic, homogeneous or heterogeneous. One is almost tempted to say that chain reactions are the preferred route of conversion in nature. 

Both free radical and ionic polymerizations are restricted to certain types of monomers. Many olefinic and acrylic monomers are readily polymerizable by a free radical mechanism, whereas other compounds such as oxiranes (epoxides)... 

With the exclusion of the actual start step, radiation-initiated polymerizations proceed like regular initiated free radical and ionic polymerizations. The only reason for treating radiation-initiated polymerizations in a separate chapter is because in many cases the growth mechanism cannot be predicted. Radiation polymerizations, on the other hand, frequently do not proceed according to an ionic or a free radical mechanism. 

There is another important difference between free-radical and ionic polymerization. In free-radical polymerization, there are system-immanent, unavoidable termination reactions, the bimolecular disproportionation and combination between two radicals. Because these termination reactions are very fast (k = 10 -10 s ) compared to propagation kp = 10 -10 s ) and radical formation... 

Another difference between free-radical and ionic polymerization that must also be kept in mind is economic rather than scientific. Most of the monomers can be polymerized by radical or ionic mechanisms (see Table 7.1), but in most cases the requirement for purity of monomers and solvents in ionic polymerization is much higher, and initiators are more expensive, so ionic polymerization is chosen only if the monomers do not polymerize by a radical pathway, or if ionic polymerization offers other advantages, such as access to molecular stmctures like block copolymers or specially designed molecular weight distributions. 

Polymerizations are generally exothermal reactions with specific energies up to 3600 kJ kg , corresponding to an adiabatic temperature rise of up to 1800 K. Some typical reaction enthalpies are presented in Table 11.2, together with the specific heat of reaction and adiabatic temperature rise obtained for mass polymerization. Most free-radical and ionic polymerizations have negative standard enthalpies and standard entropies thus at higher temperatures these reactions must be considered reversible [Eq. (15)]. 

The initiation of free radical and ionic polymerization will be described in Section 5.2, while chemical transformations in neat linear synthetic polymers and biopolymers, and in polymers both in the solid state and in solution, will be treated in Sections 5.3, 5.4, and 5.5, respectively. The importance of high-energy radiation-induced processes for technical applications will be discussed in Section 5.6. 

There are further important distinctions between free-radical and ionic polymerizations. For example, many ionic polymerizations proceed at very much higher rates than is usual for free-radical polymerization, largely because the concentration of actively propagating chains is very much... 

The oxidation of various dyes of iodonium salts and the use of these systems for both free-radical and ionic polymerization has been reported by... 

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