Network, polymeric

Network polymerization, in silicate and phosphate glasses, 12 574 Neuberg, Carl, 11 8 Neupogen, 3 817 Neural networks, 7 507... 

Several other synthetic techniques have also been described. Redistribution polymerization was outlined in COMC II (1995) (chapter Organopolysilanes, p 99) and proceeds by phosphonium salt-catalyzed redistribution of chlorodisilanes.133 Disproportionation polymerization, which is a similar process, has been described for the formation of polymers by ethoxide-catalyzed disproportionation of alkoxydisilanes via silyl anion intermediates.134 These procedures give rise to network polymeric products of rather low molecular weight (see below, Section 3.11.7.1). 

Most of the isolated borate anions shown above may polymerize in a variety of ways to form extended chains, sheets, and networks. Polymerization occurs by either sharing an exocyclic oxygen atom or by sharing an intracyclic tetrahedral boron atom. These are exemplified by the structures of the important industrial minerals colemanite [29], Ca[6304(0H)3] H2O... 

It is possible to classify polymers by their structure as linear, branched, cross-linked, and network polymers. In some polymers, called homopolymers, merely one monomer (a) is used for the formation of the chains, while in others two or more diverse monomers (a,p,y,...) can be combined to get different structures forming copolymers of linear, branched, cross-linked, and network polymeric molecular structures. Besides, on the basis of their properties, polymers are categorized as thermoplastics, elastomers, and thermosets. Thermoplastics are the majority of the polymers in use. They are linear or branched polymers characterized by the fact that they soften or melt, reversibly, when heated. Elastomers are cross-linked polymers that are highly elastic, that is, they can be lengthened or compressed to a considerable extent reversibly. Finally, thermosets are network polymers that are normally rigid and when heated do not soften or melt reversibly. 

A variation of these types of chiral stationary phases was reported by Anderson et al. [42], who synthesized a series of network polymeric stationary phase based on para-substituted AA -dialkyl-L-tartaramide dibenzoates. 

S. Anderson, S. Allenmark, P. Moller, B. Persson, and D. Sanchez, Chromatographic separation of enantiomers on V,V -diallyl-L-tartardiamide-based network—Polymeric chiral stationary phases, J. Chromatogr. 741 (1996), 23. 

Network-polymeric CSPs based on cross-linked tartaric acid diamides. Recently, a new class of network-polymeric-based CSPs has been proposed by Allenmark et al. [I94j. CSPs of this class are based on A,A -diallyl-(/ .R)-tartaric acid diamide (DATD) as chiral monomers. The chiral monomers are polymerized and cross-linked with multifunctional hydrosilanes, yielding a network polymer, which... 

Fig. 9.11. Reaciion scheme for the synthesis of network-polymeric CSPs and representative chromatograms, (a) Derivatization of A, A -diallyl-(R.R)-tartaric acid diamide (DATD) to give the bifunctional monomers used as chiral SO units, (b) Cross-linking and immobilization by hydrosilylation with multifunctional hydrosilane (alternatively, cross-linking and immobilization can be performed first with DATD followed by O-derivatization). (c) Enantioseparation of 2-(octylsulphinyl)benzoic acid. The chromatograms illustrate the column performance under non-overloadcd (left) and overloaded conditions (right). CSP network polymer from /V. -diallyl-i/il.Rl-tartaric acid diamide fc/.s-. i.S-dimethylbenzoatc bound to. ) pm 1.50 A Kromasil. Mobile phase hexane-THF (80 20 v/v) with 0.0.55 - of TFA (reprinted with permission from Ref. [194]).

As shown earlier in Section 3.2.2, scanning temperamre DSC provides a mpid method for measuring the total heat of reaction for network polymerization, and there are many such applications to provide baseline data for polymer chemorheoiogy. For example, in the... 

A very important time in the creation of a network polymeric composition is the choice of oligomer, because its chemical composition and structure determine characteristics of the created material. This is true also for RubCon, the liquid phase of which consists from rubbers with various microstructures of polymeric chains. Liquid rubbers in projected compositions are capable, if acted on by special sulfur-accelerating systems, to be vulcanized with formation of space-linked net polymers, the space net of which mainly determines the positive properties of the hard base of the RubCon composite. 

Andersson, S., Allerrmark, S., Moeller, P., Persson, B., and Sanchez, D. (1996) Chromatographic separation of enantiomers on N,f/-diallyl-L-tartardiamide-based network-polymeric... 

However, as In the stepwise network polymerization. It would be valuable to determine relationships between the molecular structure and rheological properties. The Flory-Stockmayer theory for the structural buildup In a network forming radical chain growth pol3nnerlzatlon (9,10) predicts a conversion for gelation which Is much less than that found experimentally (11). Analysis of experimental results has determined the cause of this deviation to be the formation of Intramolecular crosslinks, l.e. cycllza-tlon (12). 

Mesophases can be locked into a polymer network by making use of polymerizable LCs [59]. These molecules contain moieties such as acryloyl, diacety-lenic, and diene. Self-organization and in situ photopolymerization under UV irradiation will provide ordered nanostmctured polymers maintaining the stable LC order over a wide temperature range. A number of thermotropic liquid crystalline phases, including the nematic and smectic mesophases, have been successfully applied to synthesize polymer networks. Polymerization of reactive lyotropic liquid crystals also have been employed for preparation of nanoporous polymeric materials [58, 60]. For the constmction of nanoporous membranes, lyotropics hexagonal or columnar, lamellar or smectic, and bicontinuous cubic phases have been used, polymerized, and utilized demonstrated in a variety of applications (Fig. 2.11). 

For a loosely cross-linked network, polymerized above the glass transition temperature, R should approach the flexible limit, i.e., R, because the distance from the last attachment to the network is significantly larger and the overall mobility of the system is increased by polymerization above the Tg, As the polymerization temperature is lowered below Tg, the distance back to the last network attachment point (or entanglement) becomes less important, and the mobility of the radical chain end is reduced to the point where it is virtually immobile on the time scale of propagation. In this case, the rigid limit should be applicable, and R should approach R m, just as it would for a highly cross-linked network. 

Interpenetrating Networks Polymeric networks that pass through voids within each other to become topologically entangled such that they are inseparable without breaking bonds. 

Some of the factors that control the morphology of IPNs are now reasonably clear they include chemical compatibility of the polymers, interfacial tension, crosslink densities of the networks, polymerization method, and the IPN composition. While these factors may be interrelated, they can often be varied independently. Their effects are summarized here. 

Solutions of silicic acid thicken slowly and finally form a gel. Since the gel appears outwardly like organic gels, it was generally thought that Si(OH)4 polymerized into siloxane chains (i.e., chains with Si—O—Si bonds) that branched and crosslinked like many organic polymers. However, in his book. Her (10) clearly states that silicic acid polymerizes into discrete particles that in turn aggregate into chains and networks. Polymerization occurs in three stages ... 

It must be noted that this apparent miscibility is only mechanical in nature, a product of the high cross-link level of a sequential IPN. Sfechez and co-workers (33) note that the key factor controlling the apparent miscibility in sequential IPNs is the cross-linking density of the network polymerized first. 

Networks Polymerization of sugar-based multifunctional allylm ... 

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