Step polymerization crosslinking

Chemical Crosslinking. Only linear polymers are produced from bifunctional monomers. The reaction system must include a polyfunctional monomer, i.e., a monomer containing 3 or more functional groups per molecule, in order to produce a crosslinked polymer. However, the polyfunctional reactant and/or reaction conditions must be chosen such that crosslinking does not occur during polymerization but is delayed until the fabrication step. This objective is met differently depending on whether the synthesis involves a chain or step polymerization. In the typical... 

Many thermosets are inhomogeneous. This may be the result of the polymerization mechanism (chemically induced inhomogeneities, such as those produced by the free-radical crosslinking polymerization of multifunctional monomers or by the step polymerization of three different monomers), or of the decrease of solubility of reaction products (thermodynamic driving force) or of both factors acting simultaneously, such as the case of several UP formulations. Inhomogeneities formed in the course of polymerization are fixed by the crosslinking reactions. 

Homogeneous ideal networks, also called closed networks, result from a single-step polymerization mechanism of a stoichiometric mixture of monomers, reacted to full conversion. Many amine-crosslinked epoxies of Tg < 200°C and polyurethanes obtained using a single isocyanate monomer and a single polyol belong to this family. 

Viewing a single polymerizing droplet in a suspension polymerization, we see it is surrounded by an aqueous phase and consits of the monovinyl compound, the divinyl compound, and the inert solvent. Because of the large amount of crosslinking agent and the dilution, we have an increased number of intramolecular crosslinking steps. 

Each of these reactions and their reverse reactions may, in fact, be involved in major reaction pathways in coal conversion. Reaction 10, for instance, is expected to be a key step in H-transfer and aromatization, reaction 11 can lead to crosslinking and polymerization while reaction -11 breaks up complex molecules, and reaction 12 should provide a steady source of free radicals in many pyrolytic systems even in the absence of weak covalent bonds (vide infra). Furthermore, these free radicals contain weak C-H bonds that may rupture to yield H atoms, which can in turn lead to breaking of C-C or C-0 bonds through aromatic displacement reactions (reaction 5). 

High modulus C-fibers from special pitch 1st step polymerization > 300 C to mcsophase pitch 2nd step melt spinning 3rd step oxidative crosslinking 4th step inert carbonization at 1500 to 3000"C... 

Simultaneous luterpenetrating Network, SIN. The monomers and/or prepolymers plus crosslinkers and activators of both components are mixed, followed by simultaneous polymerization via non-interfering reactions, see Figure 6.1B. Typical syntheses involve chain and step polymerization kinetics. While both polymerizations proceed simultaneously, the rates of the reactions are rarely identical. 

A host of crosslinkers have been employed, see Table 6.2. The mixed functionality crosslinkers often serve as grafting sites between networks of quite different nature, perhaps one network chain polymerized, and the other step polymerized. Then, each type of functionality reacts with each type of monomer during polymerization. 

An alternative method to the step polymerization of a mixture of phenols with methanal to form a copolymerized network is the free radical polymerization of substituted aromatics containing polymerizable, double bonds to produce products with improved properties. In this way, a highly crosslinked network is obtained by copolymerization of allyl Xylok with 4,4 -bismaleimidodiphenyl methane in presence of diisopropylbenzyl peroxide and imidazole (Scheme 25). 

Eceiza et al investigated a series of thermoplastic PUs synthesized in bulk by two-step polymerization, obtained with the isocyanate-chain extender couple MDI-BG and various SS macrodiols. The chemical structure, the SS molecular weight and the HS content were varied. The effects on the thermal and mechanical properties were investigated [77]. The changes in the macrodiols in the case of PUs based on MDl-BG, resulted in a modulus curve that showed a plateau indicating the existence of physical crosslinks because of the increase in the size and inter-connectivity... 

From the standpoint of elementary reaction steps, free-radical crosslinking (co)polymerization starts with initiation, proceeds by propagation (in which the monomers are added to the active ends of the growing chains accompanied by chain transfer), and ends with termination reactions (when the active chain ends are deactivated). 

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