Chain crosslinking polymerization

Photopolymerization and photocrosslinking processes have been in use for many years in the electronics industry, for example in the making of printed circuit boards and in the fixation of color dots in TV tubes. More recent applications of light-induced chain crosslinking polymerization processes are the replication of optical discs (1,2) of aspherical lenses (3,4) and the in-line coating of optical fibers (5,6). 

With the chain crosslinking polymerization of undiluted tetrafunctional monomers gelation starts almost immediately (Fig. 1) and at present such systems cannot be described kinetically. Since the cause of the autoacceleration is a physical one (sup-... 

The last class of dielectric coatings to be mentioned here is formed by the UV cationic epoxy systems. In most cases photoinitiators as invented by Crivello are used to initiate the chain crosslinking polymerization of di-epoxides or mixtures thereof with monoepoxides. 

Simulation of Chain Crosslinking Polymerization with a Ptfcolation Model... 

Reaction (5.W) is interesting inasmuch as either the AA or BB monomer must be present to produce crosslinking. Polymerization of AB with only Aj- (or Bf) introduces a single branch point, but no more, since all chain ends are mis-oriented for further incorporation of branch points. Including the AA or BB molecule reverses this. The bb unit which accomplishes this in reaction (5.W) is underscored. 

Once cured, PDMS networks are essentially made of dimethylsiloxane polymeric chains crosslinked with organic linkages. The general and inherent molecular properties of the PDMS polymers are therefore conferred to the silicone network. Low surface energy and flexibility of siloxane segments are two inherent properties very useful in adhesion technology. 

We can create crosslinks during chain growth polymerization by copolymerizing dienes with vinyl monomers. When the two vinyl functions of the diene are incorporated into separate chains, a crosslink is formed. This process is shown in Fig. 2.18. When we use a low concentration of dienes, we produce a long chain branched polymer, while high concentrations of dienes create a highly crosslinked polymer network... 

Chain-growth polymerizations are diffusion controlled in bulk polymerizations. This is expected to occur rapidly, even prior to network development in step-growth mechanisms. Traditionally, rate constants are expressed in terms of viscosity. In dilute solutions, viscosity is proportional to molecular weight to a power that lies between 0.6 and 0.8 (22). Melt viscosity is more complex (23) Below a critical value for the number of atoms per chain, viscosity correlates to the 1.75 power. Above this critical value, the power is nearly 3 4 for a number of thermoplastics at low shear rates. In thermosets, as the extent of conversion reaches gellation, the viscosity asymptotically increases. However, if network formation is restricted to tightly crosslinked, localized regions, viscosity may not be appreciably affected. In the current study, an exponential function of degree of polymerization was selected as a first estimate of the rate dependency on viscosity. 

Figure 5.7 Calculated dependence of the weight-average degree of polymerization of molecules, (P)w, and hard clusters, (Pc)w, on conversion in a stoichiometric Adh) + B2(h) + B2(s) system (h - hard, s - soft). The system corresponds to a mixture of short and hard chains crosslinked with a tetrafunctional crosslinking agent...

Dusek, K., Network formation by chain crosslinking (co)polymerization, in Haward, R. N. (ed.), Development in Polymerization, Vol. 3, Applied Science Publ., Barking, 1982, pp. 143-206. 

Perhaps the most viable short-term use for dendritic macromolecules lies in their use as novel catalytic systems since it offers the possibility to combine the activity of small molecule catalysts with the isolation benefits of crosslinked polymeric systems. These potential advantages are intimately connected with the ability to control the number and nature of the surface functional groups. Unlike linear or crosslinked polymers where catalytic sites may be buried within the random coil structure, all the catalytic sites can be precisely located at the chain ends, or periphery, of the dendrimer. This maximizes the activity of each individual catalytic site and leads to activities approaching small molecule systems. However the well defined and monodisperse size of dendrimers permits their easy separation by ultrafiltration and leads to the recovery of catalyst-free products. The first examples of such dendrimer catalysts have recently been reported... 

Porous materials used for chromatography result from a chemically induced phase separation using chain-wise polymerization of vinyl-containing monomers crosslinked with a portion of divinyl functional monomers. Frechet has improved this technique for the preparation of porous PS beads [48]. In this approach the inner phase consists of a mixture containing the reactive styrene and divinylbenzene monomers as well as an unreactive polymeric porogen. After polymerization, the soluble polymeric porogen is removed, leaving behind ma-croporous beads with pore sizes of around 100-500 nm. 

Chain-growth polymerization. A 1,2-polybutadlene polymer is crosslinked with t-butylstyrene, utilizing a free radical initiator. Reaction rates include... 

The free-radical crosslinking polymerization can be regarded as a special example of specific diffusion control, in which the tendency to microgel formation and decrease of apparent reactivity of Internal double bonds depends on the size of the mlcrogel which in turn depends on the molecular weight of the primary chain. Polymerization of diallyl monomers exhibits much less of these features (W) because the degree of polymerization of their primary chains is extremely low due to degradative chain transfer. 

The work that follows pertains primarily to actin networks. Many proteins within a cell are known to associate with actin. Among these are molecules which can initiate or terminate polymerization, intercalate with and cut chains, crosslink or bundle filaments, or induce network contraction (i.e., myosin) (A,11,12). The central concern of this paper is an exploration of the way that such molecular species interact to form complex networks. Ultimately we wish to elucidate the biophysical linkages between molecular properties and cellular function (like locomotion and shape differentiation) in which cytoskeletal structures are essential attributes. Here, however, we examine the iri vitro formation of cytoplasmic gels, with an emphasis on delineating quantitative assays for network constituents. Specific attention is given to gel volume assays, determinations of gelation times, and elasticity measurements. 

In ideal random crosslinking polymerization or crosslinking of existing chains, the reactivity of a group is not influenced by the state of other groups all free functionalities, whether attached or unattached to the tree, are assumed to be of the same reactivity. For example, the molecular weight distribution in a branched polymer does not depend on the ratio of rate constants for formation and scission of bonds, but only on the extent of reaction. Combinatorial statistics can be applied in this case, but use of the p.g.f. simplifies the mathematics considerably. 

In crosslinking polymerization, the effect of unequal reactivity and dilution may be combined, because long "primary chains possibly very rich in divinyl units with unreacted double bonds are predominantly present in the beginning of the copolymerization in a very dilute solution of the monomem and possibly other diluents. In addition, inhomogeneous crosslinking can induce local gel effects (acceleration of polymerization at the gel point) and thus contribute to the overall inhomogeneity of the system. 

Systematic studies of macrosyneresis in crosslinking dissolved polymer chains are lacking the available experimental data indicate, that eventual phase separation is a frequently occurring phenomenon 149, 11,2, 81). In crosslinking polymerization the effect of increasing cross-linking density is well established and always favors phase separation irrespective of the solvent power of the diluent (157). 

As a preliminary step in the manufacture of unsaturated polyester thermoset plastic one uses low molecular weight linear polyester (Mr 10,000) obtained by a polycondensation of polyglycols with saturated and unsaturated dicarboxylic acids. The precondensate can then be dissolved and stored in the stabilized comonomer, e.g. styrene, with which it will be crosslinked later. The crosslinking polymerization reaction between the polyester chains and the styrene bridges is initiated with the help of organic peroxides which are added dispersed in plasticizers. The reaction begins at 60-90 °C and then proceeds exothermally. In addition to this a cold hardening reaction can also be carried out. For this reaction cold accelerators are necessary, e.g. tertiary amines or cobalt naphthenate. 

In chain-growth polymerizations, epoxy reactants containing more than one epoxy ring per molecule can form tightly crosslinked, three-dimensional networks, as each epoxy group acts as a difunctional reactant. 

Based on the principle of the equal and simultaneous solvation of the polymer and the bound peptide chains in different solvents, Sheppard and coworkers developed a number of polyacrylamide-type supports for solid phase peptide synthesis 50 55). In this case, the crosslinked polymeric support, in addition to possessing the good mechanical characteristics like polystyrene, is much more structurally related to the peptide than in the case of polystyrene. The polar polyacrylamide support in this case is prepared by the emulsion copolymerization of a mixture of dimethylacrylamide (7), ethylenebisacrylamide (2) and acryloylsarcosine methylester (5), initiated by ammo-nium persulphatesl). 

The acetates, unlike the acetates of transition metals like Cr , Fe ", and Ru , do not adopt oxo-centred structures with M3O cores. Instead, [Ln(OAc)3.1.5(H20)] (Ln = La-Pr) have structures with acetate-bridged chains crosslinked by further acetate bridges [Ln(OAc)3. (H2O)] (Ln = Ce-Pr) have one-dimensional polymeric structures with acetate bridges and [Ln(OAc)3.4H20)] (Ln = Sm-Lu) are acetate-bridged dimers. 

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