Crosslinked polymers insolubility

In polymer science and technology, linear, branched and crosslinked structures are usually distinguished. For crosslinked polymers, insolubility and lack of fusibility are considered as characteristic properties. However, insoluble polymers are not necessarily covalently crosslinked because insolubility and infusibility may be also caused by extremely high molecular masses, strong inter-molecular interaction via secondary valency forces or by the lack of suitable solvents. For a long time, insolubility was the major obstacle for characterization of crosslinked polymers because it excluded analytical methods applicable to linear and branched macromolecules. In particular, the most important structural characteristic of crosslinked polymers, the crosslink density, could mostly be determined by indirect metho ds only [ 1 ], or was expressed relatively by the fraction of crosslinking monomers used in the synthesis. 

Under acidic conditions, furfuryl alcohol polymerizes to black polymers, which eventually become crosslinked and insoluble in the reaction medium. The reaction can be very violent and extreme care must be taken when furfuryl alcohol is mixed with any strong Lewis acid or Brn nstad acid. Copolymer resins are formed with phenoHc compounds, formaldehyde and/or other aldehydes. In dilute aqueous acid, the predominant reaction is a ring opening hydrolysis to form levulinic acid [123-76-2] (52). In acidic alcohoHc media, levulinic esters are formed. The mechanism for this unusual reaction in which the hydroxymethyl group of furfuryl alcohol is converted to the terminal methyl group of levulinic acid has recendy been elucidated (53). 

Another class of silicon-containing polymers that have great potential to be extremely useful precursor materials are poly(chlorocarbosilanes).14f 46 Poly (chlorocarbosilanes) are not useful without modification because of the rapid hydrolysis of Si—Cl bonds, forming HC1 and an insoluble crosslinked polymer network. However, nucleophilic substitution of these Si—Cl bonds with various reagents produces materials widi a broad range of properties that are determined by the nature of the nucleophile used.47 Poly(chlorocarbosilanes) can be easily synthesized by ADMET (Fig. 8.18) without any detrimental side reactions, since the Si—Cl bond is inert to both catalysts 12 and 14. Early studies produced a polymer with Mn = 3000.14f... 

Irradiation of a hexane solution of the polymer 1 with a 15-W low-pressure mercury lamp under a nitrogen atmosphere, produced the insoluble crosslinked polymer whose IR spectrum shows a weak absorption at 2150 cm- attributed to the stretching vibration of an Si-H bond. 

We use gel content analysis to determine the weight fraction of a crosslinked polymer that is bound into an insoluble network. We immerse a stainless steel mesh basket containing a known weight of the crosslinked polymer in a suitable solvent (which may be heated to facilitate dissolution). If necessary, we can slice or grind the sample to increase its surface area. After 24 hours or more, we remove the basket from the solvent and dry it to constant weight. We calculate the gel content from Eq. 5.6. 

Because of their insolubility, the restricted access of chemical reagents and the influence of the neighborhood on the mobility of chain segments and functional groups of crosslinked polymers, the determination of residual reactive or functional groups in crosslinked polymers is much more difficult than in linear or branched polymers. This is especially true for densely crosslinked polymers prepared from tetrafunctional monomers, such as DVB. 

For a long time crosslinking reaction steps in the polymerization of unsaturated monomers have been considered to lead inevitably to insoluble polymer materials, even with small amounts of the crosslinking component. Moreover, small crosslinked polymer particles were a nuisance in the production and characterization of polymers as unpredictable products of side reactions. 

Crosslinked polymer networks formed from multifunctional acrylates are completely insoluble. Consequently, solid-state nuclear magnetic resonance (NMR) spectroscopy becomes an attractive method to determine the degree of crosslinking of such polymers (1-4). Solid-state NMR spectroscopy has been used to study the homopolymerization kinetics of various diacrylates and to distinguish between constrained and unconstrained, or unreacted double bonds in polymers (5,6). Solid-state NMR techniques can also be used to determine the domain sizes of different polymer phases and to determine the presence of microgels within a poly multiacrylate sample (7). The results of solid-state NMR experiments have also been correlated to dynamic mechanical analysis measurements of the glass transition (1,8,9) of various polydiacrylates. 

Under free radical conditions, we found that the bulk polymerization of 4-allyloxystyrene gave an insoluble crosslinked polymer with AIBN. Similar results were previously reported for the polymerization with benzoyl peroxide (8). The cationic polymerization of /7-alkoxystyrene monomers have been shown to proceed at rates that are comparable to vinyl ethers (12,13). As expected from these studies, we found that alkenyloxystyrene monomers also have a high degree of cationic reac-... 

The axial immobilization of chiral [Mn((S,S)-salen )j (where salen = (Si,Sj-NiN -bis(3,5-diR -salicylidene)-l,2-diRbethane-diamine R = Bu, R -R = -(CH2)4- R = Bu, R = Ph R = Pn, R R = -(CH2)4-) complexes was achieved by reaction of [Mn(salen )Cl] with sulfonic acid- or phenol-substituted crosslinked and insoluble polystyrene resins [45]. The resulting polymer-immobilized [Mn(salen )j complexes were active and enantioselective for the asymmetric epoxidation of... 

All the enzymes used in the work described above are quite stable at room temperature and can be used in a free form. They can also be used in an immobilized form to improve the stability and to facilitate the recovery. Many immobilization techniques are available today (25). The recent procedure developed by Whitesides et al using water-insoluble, cross-linked poly(aerylamide-acryloxysuccinimide) appears to be very useful and applicable to many enzymes (37). We have found that the non-crosslinked polymer can be used directly for immobilization in the absence of the diamine cross-linking reagent. Reaction of an enzyme with the reactive polymer produces an immobilized enzyme which is soluble in aqueous solutions but insoluble in organic solvents. Many enzymes have been immobilized by this way and the stability of each enzyme is enhanced by a factor of greater than 100. Horse liver alcohol dehydrogenase and FDP aldolase, for example, have been successfully immobilized and showed a marked increase in stability. 

Figure 6.19 Ester-functionalized thiourea 59, insoluble crosslinked polymer-bound thioureas (60 61) and soluble PEG-bound thiourea 62 screened in the asymmetric Michael addition of diethyl malonate to trarrs-P-nitrostyrene at rt in toluene (59, 60, and 61) and dichloromethane (62).

Various a-methylenemacrolides were enzymatically polymerized to polyesters having polymerizable methacrylic methylene groups in the main chain (Fig. 3, left). The free-radical polymerization of these materials produced crosslinked polymer gels [10, 12]. A different chemoenzymatic approach to crosslinked polymers was recently introduced by van der Meulen et al. for novel biomedical materials [11]. Unsaturated macrolactones like globalide and ambrettolide were polymerized by enzymatic ROP. The clear advantage of the enzymatic process is that polymerizations of macrolactones occur very fast as compared to the chemically catalyzed reactions [13]. Thermal crosslinking of the unsaturated polymers in the melt yielded insoluble and fully amorphous materials (Fig. 3, right). 

Thus, to improve upon the physical and mechanical properties of the polymer material, one must not only consider the materials used, but also the conditions under which the polymer was formed. These reaction conditions, along with the type of monomer system chosen, will completely control the conversion of functional groups in the system. More importantly, the conversion will ultimately determine the mechanical, physical, and wear properties of the material. Since most dental materials are crosslinked polymers, characterizing the polymerization reaction becomes even more important since the physical nature of a crosslinked polymer is fixed upon completion of the polymerization. For example, not only is the microstructure (i.e. the degree of crosslinking) largely unalterable after polymerization, but the system is insoluble and fixed macroscopically. Clearly, to produce crosslinked networks with the desired material properties, one must ascertain the appropriate reaction conditions and the effects of the reaction conditions on the network structure. 

These discussions will embrace homogeneous solutions of polymer-metal complexes. Of course one of the important advantages offered by the use of a polymer ligand, especially a crosslinked polymer ligand, in catalysis is the insolubilization of the attached complexes the insolubility of the polymer catalyst makes it very easy to separate from the other components of the reaction mixture. Several polymer-metal complexes have been used for this purpose, although such applications are not covered in this article. The aim here is (1) to characterize polymer-metal complexes and their behavior in such simple but important elementary reactions as complex formation, ligand substitution, and electron transfer, and (2) to describe their catalytic activity. 

The importance of crosslinked polymers, since the discovery of cured phenolic formaldehyde resins and vulcanized rubber, has significantly grown. Simultaneously, the understanding of the mechanism of network formation, the chemical structure of crosslinked systems and the motional properties at the molecular level, which are responsible for the macroscopic physical and mechanical properties, did not accompany the rapid growth of their commercial production. The insolubility of polymer networks made impossible the structural analysis by NMR techniques, although some studies had been made on the swollen crosslinked polymers. 

The stoichiometry used to synthesize the first polymer can in principle lead to a completely crosslinked and insoluble polymer however, when xylenes were used as solvent, a 60% yield of soluble precursors were obtained. When octane was used as solvent, the soluble fraction was only 40-50%. The second polymer type, which in principle should give only linear materials, also gave 50-60% yields of soluble precursor. 

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