Polymers that crosslink dimerization

Currently, industry uses many polymers that crosslink mostly by 2n + 2n type dimerizations with formations of cyclobutane rings. This is like the crosslinking reaction of the cinnamate group. Such materials are used in photoresists, photolithography, and in many ultraviolet light... 

Polymers that Crosslink by Dimerization of Nitrenes and by Other Combinations of Free-Radicals to Form Covalent... 

The metathesis polymerisation of dicyclopentadiene, an inexpensive monomer (commercially available cyclopentadiene dimer produced by a Diels-Alder addition reaction containing ca 95 % endo and ca 5 % exo form), leads to a polymer that may be transformed into a technically useful elastomer [144-146, 179] and thermosetting resin [180,181]. The polymerisation has characteristics that make it readily adaptable to the reaction injection moulding ( rim ) process [182], The main feature of this process comes from the fact that the polymerisation is carried out directly in the mould of the desired final product. The active metathesis catalyst is formed when two separate reactants, a precatalyst (tungsten-based) component and an activator (aluminium-based) component, are combined. Monomer streams containing one respective component are mixed directly just before entering the mould, and the polymerisation into a partly crosslinked material takes place directly in this mould (Figure 6.5) [147,168,183-186],... 

The cause of noninterpenetration in Vollmert s experiments was left up in the air because of the possibility of thermodynamic (and even steric) incompatibility. Braun (1976) endeavoured to circumvent this ambiguity by carrying out crosslinking reactions on identical polymer molecules. This was based upon the observation that 1,1-diphenylethylene in the presence of sodium metal forms radical anions that rapidly dimerize in quantitative yield. This reaction was applied to poly(l-phenyl-l-(4-vinylphenyl) ethylene-costyrene), which can in principle undergo both intramolecular and intermolecular reactions. Intramolecular reactions cause coil contraction, which is manifest by a decrease in the intrinsic viscosity of the polymer solutions. Intermolecular reactions stiffen the polymer chains, their structure... 

For the decomposition of dicumyl peroxide in LDPE (2% w.) at 160 °C, the efficiency of peroxide in crosslinking was about 87% [103]. Such a hi efficiency may be ascribed to the contribution of physical entanglements to the overall number of chemical crosslinks and to the presence of unsaturated C=C bonds in a polymer. We may recall that during dimerization of low molecular alkanes initiated by rert.butyl... 

In attempts to reconstitute nucleosomes on SV40 circular DNA, Cap-lan et al. (37) observed that if the histones were ADP-rihosylated prior to reconstitution assembly of nucleosomes was inhibited by as much as 80%. If nucleosomes were first allowed to assemble, then modified by ADP-ribosylation, no eflPect on stability was observed. These data led the authors to speculate that ADP-ribosylation of histones prior to nucleosome assembly might be of physiological importance. Perella and Lea (168, 169) have shown that in rat liver nuclei, polyamines cause an increase in histone Hi ADP-ribosylation and histone HI dimer synthesis, which is accompanied by a decrease in core histone ADP-ribosylation. Data such as these have led to speculation that Hi dimer formation may function in the condensation or stabilization of chromatin fibers (35, 119). The data of Lorimer et al. (124) indicate that dimer synthesis is inversely related to the nuclear activity of the poly(ADP-ribose) glycohydrolase. Thus, these authors (124) inferred that the Hl-Hl-polymer complex formation is of a transient nature. As pointed out by Purnell et al. (171), if this crosslink were to function in the stabilization of chromatin, the modified histone Hi would have... 

Anthracene and its derivatives undergo a [4-F4] photocycloaddition to cyclooctane-containing dimers. This reaction can be reversed with mechanical forces, 3delding fluorescent anthracenes. Poly(vinyl alcohol) was crosslinked with anthracene dimers to give polymers that could report microcracks in a similar way as the tricinnamate-containing polymers described above (Figure 11.3). 

Union Carbide has developed a process whereby poly-p-xylylenes can be produced which are completely linear non-crosslinked polymers. This process involves the conversion of p-xylene to a white dimeric solid known as di-p-xylylene and then pyrolysing this dimer at about 650°C in a vacuum of 0 1-0 5 Torr. The essential feature of this process is that the dimer dissociates into a monomer, p-xylylene, which immediately polymerises on contact with a surface forming a coating. These polymer films are called parylenes by Union Carbide, who have a series of patents covering the chemistry of the materials and the apparatus for their vapour deposition. 

Therefore, for highly flexible polymers, metal complex dimerization occurs either in a single chain (linear polymer) or in the chain site between the crosslinked knots. It was reported that a ratio of mononuclear and binuclear complexes of Cu with polymeric acids depends on the degree of neutralization of the latter low charge density provides binuclear complexes and monomer formation as the charge density increases. 

In contrast, observation of the c.d. with the addition of Ca(OH)2, as a function of d.p., demonstrated that terminal and central units react differently towards Ca ". This is illustrated in Fig. 27 for the dimer and the polymer. Again, the intensity of the c.d. band decreases as the polymer binds calcium and begins to gel. Results for both salt forms are attributed to a helix having a two-fold screw-symmetry, in analogy with calcium pectates. The gelling would then involve a multi-chain association, with crosslinking by the calcium ions to form an egg box structure. ... 

Philippova and Starodubtzev have also extensively studied the complex-ation behavior of polyacids and PEG, especially, the system of crosslinked of poly(methacrylic acid) and linear poly(ethylene glycol) (Philippova and Starodubtzev, 1995 Philippova et al., 1994). They observed that decreasing the molecular weight of PEG from 6000 to 1500 resulted in its slower diffusion into the swollen network of PMAA, and a drastic decrease in both the stability and equilibrium composition of the intermacromolecular complex. Analysis of dried polymer networks of PMAA with absorbed PEG chains by FT-IR spectroscopy revealed the presence of two types of hydrogen bonded structures (1) dimers of methacrylic acid at absorption frequency of 1700 cm-1 and (2) interpolymer complexes of PMAA and PEG at 1733 cm-1. In addition, they also suggested as a result of their studies, that the hydrogen bonded dimer of PMAA forms preferentially to the intermacromolecular complex between the PMAA network and PEG chains. 

A scheme for bicyclic dimer formation from HMCTSN under plasma conditions has been proposed in our previous paper (J.) According to this scheme, formation of new Si-N bonds with tertiary nitrogen between trisilazane rings leads to crosslinking of the polymer, and involves the production of hydrocarbons such as methane and ethane. Indeed, gas chromatographic analysis of the gaseous residue after plasma polymerization has shown that it consists mainly of three hydrocarbons methane, ethane and ethylene in the 5 33 4 ratio. 

The polymer is prepared by crosslinking p-CyD in DMSO with toluene 2,4-diisocyanate (TDI) in the presence of cholesterol as the template. In order to obtain homogeneous samples, the amount of TDI is kept small. Matrix-assisted laser desorption/ionization time-of-flight mass spectra (MALDI-TOFMS) are presented in Fig. 5.5. In the presence of the template (a), both the dimers of P-CyD (mass number (M) = 2000-3500) and its trimers (M = 4000-4500) are efficiently formed. In its absence (b), however, virtually all the products are monomeric P-CyDs (M = 1000-2000). The template enormously accelerates the bridging between two P-CyD molecules. Each of the signals in the spectra corresponds to different amount of substitution by TDI. These analyses clearly show that dimeric P-CyDs (the binding sites for choles-... 

The reactivity of functionalised crosslinked polymers may significantly differ from that of analogous soluble reagents. Polymer-bound benzyne, generated by Pb(OAc)4-oxidation of resin-bound 1-aminobenzotriazole 3, for example was converted into aryl acetates 4. This reaction was not observed if an analogous 1-aminobenzotriazole derivative 6 was oxidised in solution, where the formation of dimers (7 and 8) was predominant (Scheme 1.6.3). 

The temperature dependence of the electrical resistance values of the polymer in the range of 30-125°C is such that it can be used as a thermal sensor." When solid carbazole crystals are immobilized on an electrode surface, oxidative dimerization and polymerization can also be achieved in solid state." By means of electrochemical nanolithography, conducting nanopatterns due to the selective oxidative crosslinking of PVK can be produced. 

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