Network addition polymerization

The addition polymerization of diisocyanates with macroglycols to produce urethane polymers was pioneered in 1937 (1). The rapid formation of high molecular weight urethane polymers from Hquid monomers, which occurs even at ambient temperature, is a unique feature of the polyaddition process, yielding products that range from cross-linked networks to linear fibers and elastomers. The enormous versatility of the polyaddition process allowed the manufacture of a myriad of products for a wide variety of appHcations. 

Figure lc. Scheme of a glasslike structure, modified by an additional, polymeric network (interpenetrating). 

Figure 1. The synthesis of sequential IPN above and simultaneous interpenetrating networks, SIN, below. For the synthesis of SIN, two different reactions operate simultaneously such as condensation polymerization and addition polymerization. Reproduced with permission from Ref. 23. Copyright 1981, Plenum Publishing.

With decreasing UV dose T(tan 6niax) also decreases. When free monomer is still present additional polymerization causes a stepwise increase of E and T(tan max) during a thermal scan. In the presence of sufficient monomer the tan 6 peak splits up into two peaks one at a constant and the other at a dose-dependent position, representing monomer and network, respectively. 

We synthesized [13] IPNs composed of polyethylene oxide) (PEO) (polymer A) and poly(N-acryloylpyrrolidine) (PAPy) (polymer B). The IPN was synthesized by simultaneous crosslinked polymerization of APy and PEO. The overall reaction scheme for IPN synthesis by radical polymerization for APy (polymer A) and addition polymerization for PEO (polymer B) is shown in Fig. 3. This pair shows simple coacervation behavior in water. The IPN is constructed from PEO and PAPy networks as shown in Fig. 4. Chemically independent networks of polymer A and polymer B are interlocked and macroscopic phase separation in water swollen states is avoided. 

The nature of the reactive centre defines the chemistry of the polymerization, the rate and conditions under which high polymer may form, and particular features of the polymer architecture (such as the tacticity see Section 1.1.2). The nature of the reactive centre and the monomer may also control the side reactions (such as branching) and defect groups that may be introduced, which may affect the subsequent performance of the polymer. In the following, we will consider the most common types of addition polymerization since this may define the properties of the polymer that then control the chemorheology. Certain of these reactions are more important than others in reactive processing, and the particular examples of reactions that occur in forming networks as well as modification of polymers will be considered in more detail. 

The formation of networks by addition polymerization of multifunctional monomers as minor components included with the monofunctional vinyl or acrylic monomer is industrially important in applications as diverse as dental composites and UV-cured metal coatings. The chemorheology of these systems is therefore of industrial importance and the differences between these and the step-growth networks such as amine-cured epoxy resins (Section 1.2.2) need to be understood. One of the major differences recognized has been that addition polymerization results in the formation of microgel at very low extents of conversion (<10%) compared with stepwise polymerization of epoxy resins, for which the gel point occurs at a high extent of conversion (e.g. 60%) that is consistent with the... 

Figure 1.27. A schematic diagram of a network crosslinked by addition polymerization showing regions of high crosslink density (mierogel particles, arrowed) in regions of resin of lower crosslink density. Chains from the latter region that conneet gel partieles are also shown. Adapted from Pascualt et al. (2002).

A form of addition polymerization is that of copolymerization in which two or more different monomers are linked together, either at random or alternating, to form one single copolymer chain or network ... 

Polymers are frequently classified in terms of bonding in one dimension versus bonding in two or three dimensions. Bonding in one dimension results in linear polymers with single-strand chains. Bonding in two or three dimensions results in cross-linked polymers having infinite sheets or three-dimensional networks. Linear polymers are produced by addition polymerization if the reactant has only one double bond or by condensation polymerization if the reactant or reactants each have two reactive sites. Such polymers are usually soluble in suitable solvents. Since they also tend to soften when heated, they are called thermoplastic polymers. Cross-linked polymers may be produced by addition polymerization if the reactant has more than one double bond, or by condensation polymerization if the reactant or reactants each have more than two reactive sites. Such network polymers are usually insoluble and Infusible and are called thermosetting polymers. 

As in conventional glass-ionomers, the acidic component in resin-modified glass-ionomers is either poly(acrylic acid) or acrylic/maleic acid copolymer. In many brands, this polymer is simply blended with the monomer HEMA in aqueous solution. However, in certain brands, the polymeric acid is modified with side chains that allow it to participate in the addition polymerization process and thereby form a copolymer network with the HEMA. 

Descriptions of networks in terms of cross-link index and density provide no information on the internal architecture, that is, the homogeneity, of the network. Depending on how they are produced, most networks are more or less inhomogeneous, that is, the local density has a distribution. With multifunctional polycondensation, the gel point is most often reached at relatively high yields, and the network formed is quite homogeneous. In addition polymerization, the gel point occurs already at relatively low yields. The polymerization continues around the spatially fixed network structured centers, and, so, densely cross-linked centers are produced within a less densely cross-linked matrix. 

Polymeric network topology heavily reflects on drug diffusion as it concurs in determining the drug diffusion coefficient as later on discussed. Additionally, polymeric network can also be responsible for a non-Fickian diffusion. Indeed, in the presence of a very complex topology. 

Allyl esters, unsaturated polyesters, as well as some of what are known as vinyl or acrylic esters are cured by free radical addition polymerization. In the case of allyl esters, the monomers, themselves, are cross-linked. On the other hand, unsaturated polyesters are copolymerized with monomers such as styrene or methyl methacrylate. Since the unsaturated polyesters have many main-chain double bonds and the structure of a cross-linked network is fixed after quite low conversions, only a few double bonds actually react. These unconverted double bonds can then react later with atmospheric agents, and so produce poor weathering properties of the crosslinked networks. In addition, the polymerization produces many free chain ends that contribute nothing or even disadvantageously to the mechanical properties. The newly developed vinyl or acrylic esters avoid both of these problems in that the monomers capable of cross-linking only have unsaturated double bonds at the molecular ends (see also Section 26.4. S). 

While step polymerization methods lead to more or less statistical networks and good agreement with theory, addition polymerization and vulcanization nonuniformities lead to networks that may swell as much as 20% less than theoretically predicted (115,116). 

Copolymerization of styrene with small amounts of bifunctional monomers such as divinylbenzene is used for the synthesis of networks. The polymerization technique of choice is bead polymerization. Polymer porosity can be controlled by the addition of polystyrene, which can be extracted after polymerization has been completed. Sulfonation of such networks yields cation-exchange resins anion-exchange resins can be synthesized... 

In the creation of network structures by chemical bonding (covalent bonding), there is a method of (1) crosslinking at the same time as polymerization or (2) crosslinking by chemical reaction after linear polymer chains have been synthesized. The latter method can be further divided into the addition polymerization in the presence of divinyl conqjounds (radical polymerization, anionic polymerization, ionic polymerization, etc.) or the formation of crosslinked structures by polycondensation of multifunctional compoimds. In the addition reaction, free radical polymerization is generally utilized. In this free radical polymerization method, initiators are usually used, but light, radiation, and plasmas can also be used. 

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