Multifunctional unsaturated monomers

An idealized formulation guideline showing possible interactions of single vinyl unsaturated monomers, multifunctional vinyl cross-linking molecules and polymeric materials is represented by Pig. 5. 

Radiation sensitizers are multifunctional vinyl monomers (MFA) that are highly reactive towards free radicals. Since all common MFAs contain terminal unsaturation, it can be expected that addition/polymerization is the principal mechanism by which they react in the polymer compound. These additives are used mainly to accelerate the radiation-induced crosslinking in the polymers. The addition of MFA to the polymer formulations suppresses the chain scission reactions and allows more crosslinking to occur. 

Synthetic polymers used to form fibers are often classified on the basis of their mechanism of polymerization--step growth (condensation) or chain growth (addition) polymerization. Step growth polymerization involves multifunctional monomers which undergo successive condensation with a second monomer or with itself to form a dimer, which in turn condenses with another dimer to form a tetramer, etc., usually with loss of a small molecule such as water. Chain growth involves the instantaneous growth of a long molecular chain from unsaturated monomer units, followed by initiation of a second chain, etc. The two methods are outl ined below schematically ... 

The UV coating usually is comprised of unsaturated monomers that polymerize with UV light (in the presence of a photoinitiator, which absorbs radiation and begins polymerization). Unsaturated polyesters copolymerized with vinyl monomers or multifunctional acrylates often are utilized. Most recently, however, vinyl ether monomers for cationic curing with epoxies have shown... 

As in most chainwise polymerizations, q -> 1, the first observation arising from Eq. (3.180) is the very low value of the gel conversion. In actual systems, intramolecular cyclization and microgel formation produce an increase in the gel conversion. But reported values of xgei for the free-radical polymerization of systems containing multifunctional monomers are usually below 0.10. This is the case for the crosslinking of unsaturated polyesters (Af) with styrene (A2 ). 

Here, we shall focus on ruthenium-catalyzed nucleophilic additions to alkynes. These additions have the potential to give a direct access to unsaturated functional molecules - the key intermediates for fine chemicals and also the monomers for polymer synthesis and molecular multifunctional materials. Ruthenium-catalyzed nucleophilic additions to alkynes are possible via three different basic activation pathways (Scheme 8.1). For some time, Lewis acid activation type (i), leading to Mar-kovnikov addition, was the main possible addition until the first anfi-Markovnikov catalytic addition was pointed out for the first time in 1986 [6, 7]. This regioselectiv-ity was then explained by the formation of a ruthenium vinylidene species with an electron-deficient Ru=C carbon site (ii). Although currently this methodology is the most often employed, nucleophilic additions involving ruthenium allenylidene species also take place (iii). These complexes allow multiple synthetic possibilities as their cumulenic backbone offers two electrophilic sites (hi). 

Chainwise Polymerizations A typical example of a thermoset produced by a chainwise polymerizahon is the case of the cure of unsaturated polyesters with styrene by a free-radical mechanism. Styrene is a bifiinctional monomer, A2, characterized by the presence of one C=C group that is transformed into a -C-C- bond in the polymerization reaction. The unsaturated polyester is a multifunctional monomer, Aj, characterized by the presence of if 12) C=C groups in its chemical structure. The molar fraction of C=C groups belonging to the multifunctional monomer is given by ... 

Poly(ethylene terephthalate) is a linear polyester obtained from the reaction of difunctional monomers. Branched or network polyesters are obtained if at least one of the reagents is tri- or multifunctional. This can be achieved either by the use of polyols such as glycerol in the case of saturated polyesters (glyptal) or by the use of unsaturated dicarboxyhc acids such as maleic anhydride in the ease of unsaturated polyester. In the preparation of glyptal, glycerol and phthalic anhydride reaet to form a viseous liquid initially, which on further reaction hardens as a result of network formation (Equation 2.31). 

Unsaturated low molecular weight (M between 1000 and 10000), often branched, used as macromonomers for synthesis of thermosets (polyester resins), or thermosetting materials by themselves (alkyd resins). They are prepared from several monomers, namely phthalic and maleic anhydrides, adipic acid, iso-phthalic acid, natural fatty acids or triglycerides, and a great variety of multifunctional alcohols. In a few special cases, they may be saturated and/or linear for use as macromonomers in the production of polyurethanes or other polymers. 

Miniemulsion polymerization enables to incorporate water-insoluble materials such as resins, organic pigments, polymers, etc into the polymer matrix. The additive seed allows to control the particle number and particle size during the production process. Furthermore, miniemulsion polymerizations and copolymerizations carried out with acrylic and methacrylic monomers in the presence of unsaturated alkyd resins lead to the production of stable hybrid latex particles containing grafted and crosslinked alkyd resin/acrylic products as coating polymer [114]. In the reaction, the multifunctional resin acts as a hydrophobe as well as the costabilizer of the miniemulsion. 

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