Polymerization monomer reactant

Structural modifications were envisioned early to overcome these limitations. A first improvement was outlined by preparing copolymers, which were soluble in the state of full imidation, mainly poly(ester-imide)s and poly(amide-imide)s [2,4, 5]. As an alternative to these conventional copolymers, addition polyimides were developed in the 1970s as a new class of thermosetting materials. Thus, bismaleimides, bisnadimides, and end-capped thermocurable polyimides were successfully developed and marketed [6,7]. These resins were the precursors of the modern PMR (polymeric monomer reactants) formulations [8]. 

We now turn to two of the problems we have sidestepped until now. In this section we consider the polymerization of reactants in which a stoichiometric imbalance exists in the numbers of reactive groups A and B. In the next section we shall consider the effect of monomers with a functionality greater than 2. 

Bis-maleimide resins composed of BMI and diamines have been reported in the early 1960s in the patent literature. Since that time, a number of patents have appeared describing improvements in their properties and uses [3]. Although many bis-maleimide resins are commercially developed, relatively few reports of their use as adhesives are to be found in scientific journals [4-10]. Improvements of maleimide resins are mirrored in the improvements of thermosetting polyimides. For example, the method of in situ polymerization of monomer reactants (PMR method) was developed [6]. 

The monomer reactant of this polymerization reaction contains a double bond. The product polymer has no double bond, so an addition reaction must have occurred. Thus, this reaction is an addition polymerization reaction. Since the monomer s name is tetrafluoroethene, the product s name is polytetrafluoroethene. 

The random copolymer of propylene and ethylene (EP) lacks the good symmetry of it PP and is a flexible elastomer. Since this copolymer is used as an elastomer, it is customary to add a small amount of a diene, such as ethylidene norbomene, to the monomer reactants before polymerization to allow subsequent cross-linking or curing. 

Solution polymerization. Solution polymerization involves polymerization of a monomer in a solvent in which both the monomer (reactant) and polymer (product) are soluble. Monomers are polymerized in a solution that can be homogeneous or heterogeneous. Many free radical polymerizations are conducted in solution. Ionic polymerizations are almost exclusively solution processes along with many Ziegler-Natta polymerizations. Important water-soluble polymers that can be prepared in aqueous solution include poly(acrylic acid), polyacrylamide, poly(vinyl alcohol), and poly(iV-vinylpyrrolidinone). Poly(methyl methacrylate), polystyrene, polybutadiene, poly(vinyl chloride), and poly(vinylidene fluoride) can be polymerized in organic solvents. 

The basis of modern organic resin production incorporates the same principles described for their predecessors but depends upon an entirely different polymerization mechanism first applied by D Alelio in 1944, called addition or vinyl polymerization. The mechanism is one of free radical induced polymerization between reactants monomers) carrying ethenyl (or vinyl) double bonds (—GH=CH2). One of the reactants must contain at least two ethenyl double bonds to effect crosslinking. Again, an understanding of the resin synthesis is afforded by showing separately in Scheme 2.2 what are in fact simultaneously occurring complex polymerization reactions between all reactant permutations. 

NASA Lewis Research Center further developed the norbornene project, culminating in the PMR concept, where a class of addition type poiyimides was formed by the in situ Polymerization of Monomer Reactants (PMR) [42]. 

Initiator i- ni-she- at [LL initiatus, pp of initiare, fr. L, to induct, fr. initium] (ca. 1573) vt. An agent that causes a chemical reaction to commence and that enters into the reaction to become part of the resultant compound. Initiators differ from catalysts in that catalysts do not combine chemically with the reactants. Initiators are used in many polymerization reactions, especially in emulsion polymerizations. Initiators most commonly used in polymerizing monomers and resins having ethenic unsaturation (-C=C-) are the organic peroxides. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. 

Again the steady state with its general approximations is assumed in which the concentrations of the reactants, such as the monomer, free radicals, and transfer agent, do not vary with time. Hence, in equation 83 the number of polymerized monomer units can be substituted with the rate of polymerization and the numbers of end groups by the rate of their formation. 

Vapor deposition is a highly desirable method for modifying surfaces with fluo-ropolymers and has been extensively reviewed [1-6], In chemical vapor deposition (CVD), one or more reactants are metered into a vacuum chamber. Inside the CVD reactor, monomer vapors undergo polymerization and thin film formation in a single step. In the case of vinyl polymerization, monomer vapors are inttoduced directly into the CVD chambers as reactants. Alternatively, less stable monomers, such as difluorocarbene (CFj ), can be generated via in situ reaction (Table 7.1). 

Figure 9.18 Chemical sequences involved in the polymerization of monomer reactants (PMR) process. The average molecular weight of intermediate oligomer 43 is controlled by the relative amounts of monoester 41, diester 42, and diamine 34. Chain extension of bisnadimide 43 occurs at high temperature by the reverse Diels-Alder polymerization process.

The reaction conditions can be varied so that only one of those monomers is formed. 1-Hydroxy-methylurea and l,3-bis(hydroxymethyl)urea condense in the presence of an acid catalyst to produce urea formaldehyde resins. A wide variety of resins can be obtained by careful selection of the pH, reaction temperature, reactant ratio, amino monomer, and degree of polymerization. If the reaction is carried far enough, an infusible polymer network is produced. 

As with polyesters, the amidation reaction of acid chlorides may be carried out in solution because of the enhanced reactivity of acid chlorides compared with carboxylic acids. A technique known as interfacial polymerization has been employed for the formation of polyamides and other step-growth polymers, including polyesters, polyurethanes, and polycarbonates. In this method the polymerization is carried out at the interface between two immiscible solutions, one of which contains one of the dissolved reactants, while the second monomer is dissolved in the other. Figure 5.7 shows a polyamide film forming at the interface between an aqueous solution of a diamine layered on a solution of a diacid chloride in an organic solvent. In this form interfacial polymerization is part of the standard repertoire of chemical demonstrations. It is sometimes called the nylon rope trick because of the filament of nylon produced by withdrawing the collapsed film. 

For a fixed extent of reaction, the presence of multifunctional monomers in an equimolar mixture of reactive groups increases the degree of polymerization. Conversely, for the same mixture a lesser extent of reaction is needed to reach a specified with multifunctional reactants than without them. Remember that this entire approach is developed for the case of stoichiometric balance. If the numbers of functional groups are unequal, this effect works in opposition to the multifunctional groups. 

Uses. Furfuryl alcohol is widely used as a monomer in manufacturing furfuryl alcohol resins, and as a reactive solvent in a variety of synthetic resins and appHcations. Resins derived from furfuryl alcohol are the most important appHcation for furfuryl alcohol in both utihty and volume. The final cross-linked products display outstanding chemical, thermal, and mechanical properties. They are also heat-stable and remarkably resistant to acids, alkaUes, and solvents. Many commercial resins of various compositions and properties have been prepared by polymerization of furfuryl alcohol and other co-reactants such as furfural, formaldehyde, glyoxal, resorcinol, phenoHc compounds and urea. In 1992, domestic furfuryl alcohol consumption was estimated at 47 million pounds (38). 

Since acrylic polymerizations liberate considerable heat, violent or mnaway reactions are avoided by gradual addition of the reactants to the kettie. Usually the monomers are added by a gravity feed from weighing or measuring tanks situated close to the kettie. The rate of monomer addition is adjusted to permit removal of heat with full flow of water in the condenser and a partial flow in the cooling jacket. Flow in the jacket can be increased to control the polymerization in cases of erroneous feed rates or other unexpected circumstances. A supply of inhibitor is kept on hand to stop the polymerization if the cooling becomes inadequate. 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

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