Crosslinking by free radical

The maleimide functionality has also been utilized as a modifying group to obtain polymers crosslinkable by free radical chemistry. Thus, hydroxy-terminated poly(oxy-2,6-dimethyl-1,4-phenylene) (164) was treated with p-maleimidobenzoyl chloride (165) to obtain resin (166 Scheme 81) (72FRP2122181). Resin (166) melted at 155 °C and could be coated on to... 

There are two types those that crosslink by free-radical mechanisms and those that crosslink by ionic mechanisms. In the first the monomers are often mixtures of di- and tiiacrylate esters. They are combined with photosensitive prepolymers, but lack adequate photosensitivity to crosslink by themselves. Instead, they rely upon photoinitiators as the sources of free radicals for the cure. The diacrylate esters usually serve as diluents to lower the viscosity of the mixtures. These can be various glycol diacrylates, such as butanediol diacrylate o o... 

Table 1. Swelling of polyacrylamide hydrogels obtained by free-radical crosslinking polymerization...

To determine the crosslinking density from the equilibrium elastic modulus, Eq. (3.5) or some of its modifications are used. For example, this analysis has been performed for the PA Am-based hydrogels, both neutral [18] and polyelectrolyte [19,22,42,120,121]. For gels obtained by free-radical copolymerization, the network densities determined experimentally have been correlated with values calculated from the initial concentration of crosslinker. Figure 1 shows that the experimental molecular weight between crosslinks considerably exceeds the expected value in a wide range of monomer and crosslinker concentrations. These results as well as other data [19, 22, 42] point to various imperfections of the PAAm network structure. 

For example, the molecular weight of unsaturated polyesters is controlled to less than 5000 g/mol. The low molecular weight of the unsaturated polyester allows solvation in vinyl monomers such as styrene to produce a low-viscosity resin. Unsaturated polyesters are made with monomers containing carbon-carbon double bonds able to undergo free-radical crosslinking reactions with styrene and other vinyl monomers. Crosslinking the resin by free-radical polymerization produces the mechanical properties needed in various applications. 

Recent work by Atalla(H) supports the idea that lignin is at least a semi-ordered substance in wood with the plane of the aromatic ring parallel to the cell wall surface. Woody plants synthesize lignin from trans-coniferyl alcohol (pines), trans-sinapyl alcohol 2 (deciduous), and trans-4-coumaryl alcohol 3 by free radical crosslinking initiated by enzymatic dehydrogenation(l2). Structures of these alcohols are given in Figure 1. 

The linkage between two chains can also be ionic. Thus the copolymer between ethylene and methacrylic acid (MA) (up to 15% MA), made by free radical polymerisation, yields a polymer with pendant carboxyl groups. Neutralisation with zinc ions gives a crosslinked, thermo-reversible polymer (Surlyn ). The resulting polymer (ionomer) has limited properties, although it is the favoured material for the outer covering of golf balls. 

Microgel Formation in Solution by Free-Radical Crosslinking Copolymerization... 

Several applications of hyperbranched polymers as precursors for synthesis of crosslinked materials have been reported [91-97] but systematic studies of crosslinking kinetics, gelation, network formation and network properties are still missing. These studies include application of hyperbranched aliphatic polyesters as hydroxy group containing precursors in alkyd resins by which the hardness of alkyd films was improved [94], Several studies involved the modification of hyperbranched polyesters to introduce polymerizable unsaturated C=C double bonds (maleate or acrylic groups). A crosslinked network was formed by free-radical homopolymerization or copolymerization. 

Dimethacrylate monomers were polymerized by free radical chain reactions to yield crosslinked networks which have dental applications. These networks may resemble ones formed by stepwise polymerization reactions, in having a microstructure in which crosslinked particles are embedded in a much more lightly crosslinked matrix. Consistently, polydimethacrylates were found to have very low values of Tg by reference to changes in modulus of elasticity determined by dynamic mechanical analysis. 

Up to the present time, use has been made almost entirely of dimethacrylates which are polymerized by free radical mechanisms to yield crosslinked products (81. Polymerization is initiated by redox systems, such as benzoyl peroxlde/aromatic amine, and by... 

Crosslinked polyacrylamide gels were obtained by free radical... 

Polyethylene may be crosslinked by electron beams or by free-radical initiators. These polymers have elastic memory, i.e., stretched crosslinked Aims or tubing shrinks to the original dimensions when heated. 

Monomers, (IV), which were crosslinkable using free radical polymerization with azoisobutyronitrile or with ring-opening metathesis polymerization catalyst, C12Ru(=CHC6H5)[P(C6H11)3] were prepared by Liaw et al. (4) and used in thermosets. 

Gels are prepared by free-radical bulk copolymerization. The comonomers (n-AMA and DMA at a specified molar ratio) are injected along with the crosslinker (DVB 0.1% w/w) and the initiator [2,2 -azobis(isobutyronitrile) (0.5% w/w)] between two glass plates. The glass is previously silanized by immersion for two days in a solution of dichlorodimethylsilane (2% v/v) in toluene. The plates are separated by Teflon spacers of specified thickness, and the whole assembly is held together by metal damps. Polymerization is accomplished by incubating the assembly in the vertical position under argon at 60 °C for 18 h. 

At present all commercial polystyrene (with average molecular weights between 100,000 and 400,000) is manufactured by radical polymerization, which yields atactic polymers.476 Peroxides and azo compounds are commonly used initiators. The suspension process (usually as a batch process in water at 80-140°C) produces a product with relatively high residual monomer content.223 More important is the continuous solution process (usually in ethylbenzene solvent at 90-180°C), which yields high-purity product. Styrene can be copolymerized with numerous other monomers.477 One of these copolymers, the styrene-divinylbenzene copolymer produced by free-radical polymerization, has a crosslinked stucture and is used in... 

Frechet [3] prepared bioactive microgels by free radical polymerization of acrylamide with acid-labile crosslinkers such as bisacrylamide 4-methox-ybenzaldehyde acetal, (IX), and bistrifluoroacetamide 4-(3-azidopropylether) benzaldehyde acetal, (X). 

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