Polymerization and Crosslinking Processes

The manufacturing procedure of silicone rubber consists of various processes starting with the production of dime-thyldichlorosilane from powdered silicon and ending in the rubber formation. Polymerization and crosshnking are two processes of paramount importance in order to understand the material structure. Both are considered to increase the chains size however, they are two different processes sequentially occurring. 

Polymerization is a process where a big molecule is formed by appropriately connecting a number of smaller molecules (monomers). In the case of PDMS the polymerization process starts from the hydrolysis of dimethyldichlo-rosilane (Reaction 1.1), followed by condensation Reactions 

The molecular weight of the obtained macromolecules is determined by the polymerization conditions and can be controlled by various methods, such as the addition of active monomers (for example, examethyldisilane). The polymer finally manufactured is a mixture of linear and cyclic macromolecules, which are further used to produce larger molecules by anionic or cationic ring opening polymerization of the cyclic oligomers or by polycondensation of the silanol end-block linear ohgomers. 

Depending on the molecular weight, the material produced is in gas or liquid form (low intermolecular forces). In order to acquire a sohd material, the development of connections between the polymers molecules that will stabihze the material structure are required. Crosslinking or vulcanization is the process followed in this direction. Two of the vulcanization mechanisms that are applied in the case of sih-cone rubber (outdoor insulation) are described below. 

There are two mechanisms for the network formation characterized as RTV [45, 46, 54]. The first is a condensation reaction of silanol groups to form siloxanes with the participation of water and drastic groups, catalyzed by acid or base, as shown in Reaction 1.6. The process is triggered by the atmosphere humidity and the coimections are developed with oxygen atoms also by the byproducts formed, usually alchohol. 

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Simple polyesters of the type described by Eq. 2-170 are too limited to be of commercial interest. Almost all crosslinked polyesters are either unsaturated polyesters or alkyd polyesters. These offer a greater ability to vary the final product properties to suit a targeted market. Also, they offer greater process control since different chemical reactions are involved in the polymerization and crosslinking reactions. A typical unsaturated polyester... 

The polymerization and crosslinking of phenol-formaldehyde is a highly useful industrial process. However, the reactions that take place are quite difficult to handle in a quantitative manner for a number of reasons. The assumption of equal reactivity of all functional groups in a monomer, independent of the other functional groups in the molecule and of whether the others are reacted, is dubious in this polymerization. Consider, for example, the routes by which trimethylolphenol (XXIb) can be produced in this system ... 

Polymerization and crosslinking by reactions of this type can explain the formation of the explosive coke . The compn of the gases evolved was found to vary with the extent of reaction and with the temp (Ref 64). The complicated nature of the process is also shown by the fact that the evolved gases contain not only water, corresponding to the formation of the products cited above, but also CO, N2)N0,N20, and even acetylene (Ref 64)... 

Photoinitiated polymerization and crosslinking of monomers, oligomers, and polymers constitute the basis of important commercial processes with broad applicability, including photoimaging and UV curing of coatings and inks. 

It has been demonstrated that molecular orientation can be achieved starting with a low molecular weight species which is oriented in an elongational flow and subsequently cured under UV-irradiation. The orientation of the monomer is frozen-in by the ultra-fast process of polymerization and crosslinking. Both extrusion and stretching can be carried out at relatively low temperatures and pressures. Polymer filaments produced in this way are definitely anisotropic as is evidenced by their birefringence and by a strong increase of the tensile modulus and a decrease of the thermal expansion coefficient in the axial direction. 

The mixture is homogeneous at first, but the polymer eventually precipitates because of limited solubility in the medium. Although the catalysis itself may be considered to be (at least in the beginning) homogeneous, many complications arise—e.g., in the termination process. Precipitative polymerization and crosslinking are examples of this type. 

The MI method utilizes initiator-functionalized linear chains, which initiate the polymerization and crosslinking of a difunctional monomer (e.g., divinylbenzene). The active chain ends also attack the neighboring linear chains ends, and a core with crosslinked microgel is formed. In the meanwhile, certain numbers of linear chains are attached to the core. However, it is always difficult to obtain star polymers with narrow distribution of arm numbers. Quite often, many linear polymers are not attached to the core, which leads to problems in the course of the purification and for finally applications. By using multifunctional coupling agents, it is possible to get stars with uniform arm numbers. But the purification process is always unavoidable and difficult. 

Crosslinkable oxadiazole monomers with pending acrylic units have been described. The s)mthesis is shown in Figure 10.2. The molecules can be oriented and this orientation can be frozen by a polymerization or crosslinking process, yielding a material with anisotropic properties. 

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