Crosslinked polymers, formation

Assuming that classical chemical kinetics are valid and that the crosslinking reaction rate is proportional to the concentrations of polymer radicals and pendant double bonds, it was shown theoretically that the crosslinked polymer formation in emulsion polymerization differs significantly from that in corresponding bulk systems [270,316]. To simplify the discussion, it is assumed here that the comonomer composition in the polymer particles is the same as the overall composition in the reactor, and that the weight fraction of polymer in the polymer particle is constant as long as the monomer droplets exist. These conditions may be considered a reasonable approximation to many systems, as shown both theoretically [316] and experimentally [271, 317]. First, consider Flory s simplifying assumptions for vinyl/divinyl copolymerization [318] that (1) the reactivities of all types of double bonds are equal, (2) all double bonds... 

Redistribution reactions, crosslinked polymer formation 350, 366, 371, 373, 381 Redistribution reaetions, radieal generation-reeombination 350, 389-392 Redistribution reaetions, transamidation 153, 181, 351-353, 370 Redistribution reaetions, transesterifieation 348, 351, 354-356, 372, 387,1003 Redox 1014,1370,1371... 

Tobita, H. Takekuma, K., Analytical Calculus and Monte Carlo Simulation of Crosslinked Polymer Formation in the Copolymerization of Tetraethoxysilane and Poly(dimethylsiloxane). Macromol. Theory Simul. 2000, 9,181-187. 

In suspension polymerization, the crosslinked polymer formation from mono- and poly-unsaturated monomers takes place by free-radical crosslinking of the chains (Elliott and Bowman 2002). At the beginning of (co)polymerization, only a few linear polyfunctional macromolecules (that remain dissolved in monomers) are formed. 

U rea-formaldehyde N,N -Bis(hydroxymethyl)urea Crosslinking, polymer formation... 

Literature articles, which report the formation and evaluation of difunctional cyanoacrylate monomers, have been published. The preparation of the difunctional monomers required an alternative synthetic method than the standard Knoevenagel reaction for the monofunctional monomers, because the crosslinked polymer thermally decomposes before it can revert back to the free monomer. The earliest report for the preparation of a difunctional cyanoacrylate monomer involved a reverse Diels-Alder reaction of a dicyanoacrylate precursor [16,17]. Later reports described a transesterification with a dicyanoacrylic acid [18] or their formation from the oxidation of a diphenylselenide precursor, seen in Eq. 3 for the dicyanoacrylate ester of butanediol, 7 [6]. 

In distinction to other esters of acrylic acids containing double bonds in the alcohol radical and, therefore exhibiting a tendency to cyclopolymerization43 and formation of crosslinked polymers, 10 reacts with AN in DMF solution41 or in benzene/DMF42 only with the vinyl group of the acid part due to deactivation of the double bond in the 3-chloro-2-butenyl group by the chlorine atom. The copolymer of structure 11 is formed. 

To explain the formation of non-crosslinked polymers from the diallyl quaternary ammonium system, Butler and Angelo proposed a chain growth mechanism which involved a series of intra- and inter-molecular propagation steps (15). This type of polymerization was subsequently shown to occur in a wide variety of symmetrical diene systems which cyclize to form five or six-membered ring structures. This mode of propagation of a non-conjugated diene with subsequent ring formation was later called cyclopolymerization. 

Care ill choosing the particular polymer to be employed in a frac-turing treatment is needed because some polymers can interfere with the function of the crosslinker. Some of these polymers are also stable to high temperature steam and have been successfully used to treat high temperature steam injection wells. Recent developments in organic polymer formation damage control polymers are discussed in chapter 10 of this book. 

Photopolymerizations were initiated with either ultraviolet or visible blue light of varying intensity (1-150 mW/cm2). In general, the high concentration of double bonds in the system and the multifunctional nature of the monomer (two double bonds per monomer molecules) led to the formation of a highly crosslinked polymer system in a period of a few seconds, depending on the initiation rate. 

The first type, termed sequential IPN s, involves the preparation of a crosslinked polymer I, a subsequent swelling of monomer II components and polymerization of the monomer II in situ. The second type of synthesis yields materials known as simultaneous interpenetrating networks (SIN s), involves the mixing of all components in an early stage, followed by the formation of both networks via independent reactions proceeding in the same container (10,11). One network can be formed by a chain growth mechanism and the other by a step growth mechanism. 

Scheme 8.11 Thermodynamic formation of crosslinked polymer 54 via radical crossover reaction of alkoxyamines in copolymers 52 and 53 [42],...

It is possible that microbubble shell may be shattered during the interaction with an ultrasound pulse. Indeed, drastic variation of microbubble size, up to several-fold in less than a microsecond, has been reported [33], with linear speeds of the wall motion of microbubble approaching hundreds of meters per second in certain conditions. At these rates, it is easy to shatter the materials that would otherwise flow under slow deformation conditions. In some cases (e.g., lipid monolayer shells, which are held together solely by the hydrophobic interaction of the adjacent molecules), after such shattering the re-formation of the shell maybe possible in other cases - e.g., with a solid crosslinked polymer or a denatured protein shells - the detached iceberg-like pieces of the microbubble shell coat would probably not re-form and anneal, and the acoustic response of microbubbles to the subsequent ultrasound pulses would be different [34]. 

The discussions until this point have been concerned with the polymerization of bifunctional monomers to form linear polymers. When one or more monomers with more than two functional groups per molecule are present the resulting polymer will be branched instead of linear. With certain monomers crosslinking will also take place with the formation of network structures in which a branch or branches from one polymer molecule become attached to other molecules. The structures of linear, branched, and crosslinked polymers are compared in Fig. 1-2. 

Radical chain polymerizations are characterized by the presence of an autoacceleration in the polymerization rate as the reaction proceeds [North, 1974], One would normally expect a reaction rate to fall with time (i.e., the extent of conversion), since the monomer and initiator concentrations decrease with time. However, the exact opposite behavior is observed in many polymerizations—the reaction rate increases with conversion. A typical example is shown in Fig. 3-15 for the polymerization of methyl methacrylate in benzene solution [Schulz and Haborth, 1948]. The plot for the 10% methyl methacrylate solution shows the behavior that would generally be expected. The plot for neat (pure) monomer shows a dramatic autoacceleration in the polymerization rate. Such behavior is referred to as the gel effect. (The term gel as used here is different from its usage in Sec. 2-10 it does not refer to the formation of a crosslinked polymer.) The terms Trommsdorff effect and Norrish-Smith effect are also used in recognition of the early workers in the field. Similar behavior has been observed for a variety of monomers, including styrene, vinyl acetate, and methyl methacrylate [Balke and Hamielec, 1973 Cardenas and O Driscoll, 1976, 1977 Small, 1975 Turner, 1977 Yamamoto and Sugimoto, 1979]. It turns out that the gel effect is the normal ... 

Molecules suitable for the formation of macromolecules must be at least bifunctional with respect to the desired polymerization they are termed monomers. Linear macromolecules result from the coupling of bifunctional molecules with each other or with other bifunctional molecules in contrast, branched or crosslinked polymers are formed when tri- or poly-functional compounds are involved. 

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