Double-End Crosslinkable

In Fig. 19, uncrosslinked refers to the situation where only one end of the chromophore is attached to the polymer backbone (prescursor polymer) while crosslinked refers to the situation where both ends of the chromophore are attached to the polymer backbone (final hardened polymer) [138]. The double-end crosslinking results in improvement in the dynamic thermal stability by more... 

Figure 1. Scheme for synthesis of a double-end crosslinkable (DEC) chromophore monomer. 

Replication of the viral DNA occurs only after a second stage uncoating and this step requires protein synthesis (Joklik, 1964) because the addition of cycloheximide at this stage of replication strongly inhibits DNA replication (Kates and McAuslan, 1961b). The double-stranded, crosslinked genome is replicated from each end semiconservatively (Esteban and Holowczak, 1977 Esteban et al.,... 

In order to create an ideal, stand-alone, and spherically shaped NLO material that does not need to involve the complexity of incorporating an NLO chromophore in a separate polymeric matrix, multiple NLO chromophore building blocks can be placed into a dendrimer in order to construct a precise molecular architecture with predetermined chemical composition. Crosslinkable NLO dendrimer 39 is one such system exhibiting a very large optical nonlinearity and excellent thermal stability. NLO dendrimers were constmcted using a double-end funrtionalized, three-dimensional thio-phene-stilbene-based NLO chromophore as the central core and crosslinkable trifluorovinylether-containing dendrons as the exterior moieties. Spatial isolation from the dendrimer shell decreases chromophore-chromophore elertrostatic... 

BMI was also used as a crosslinking agent for poly(iminoethylene). The Michael addition takes place with the nucleophilic nitrogen of the imino group and the double bonds of the electrophilic BMI. The Michael addition of BMI is now adopted as a crosslinking reaction for polymers with amino end groups [2]. 

Networks obtained by anionic end-linking processes are not necessarily free of defects 106). There are always some dangling chains — which do not contribute to the elasticity of the network — and the formation of loops and of double connections cannot be excluded either. The probability of occurrence, of such defects decreases as the concentration of the reaction medium increases. Conversely, when the concentration is very high the network may contain entrapped entanglements which act as additional crosslinks. It remains that, upon reaction, the linear precursor chains (which are characterized independently) become elastically effective network chains, even though their number may be slightly lower than expected because of the defects. 

Alkenes with two reactive carbon-carbon double bonds per molecule like 1,5-hexadiene or diallyl ether are used in the synthesis of silicone compounds which can be later crosslinked by hydrosilylation. A sufficiently high excess of double bonds helps to prevent the dienes from taking part in silane addition across both olefmic ends, but trouble comes from double bond isomerization (Eq. 2). 

NMR resonances attributable to Y-links have been reported in polyethylene after irradiation to low doses. These crosslinks are suggested to be formed by reaction of a chain radical with a C=C double bond at the end of another molecule. 

DMTA of partially polymerized samples of TEGDA reveals an increase of Young s modulus due to thermal aftercure near 120°C. Parallel DSC-extraction experiments show that this aftercure requires the presence of free monomer. Near the end of the polymerization the free monomer is exhausted and only crosslinks are formed. T(tan then increases markedly with double bond conversion. 

Using labeled initiator and radiometric analysis, the number of initiator fragments from the number of end-groups, R, can be evaluated and the number of crosslinking points can be calculated. The number of unreacted double bonds can be found by the bromometric method. On the basis of such analysis, we can find the percentage of units in the multimonomer which had reacted according to template mechanism. The results of the calculations for two examined systems are presented in Tables 5.3 and 5.4... 

They should consist of elastically effective chains only. An elastically effective chain should connect two different crosslinks, and two such crosslinks should be tied by only one elastic chain. This means that the gel should contain no defects such as pendant chains (one end of which only is connected with a crosslink), loops (chains linked at both ends to the same crosslink), or double connections. Physical crosslinks (permanent entanglements) should be prohibited, too. 

Crosslinked LC elastomers (Figure 19d) are very promising for piezoelectric and ferroelectric applications, and also as non-linear optic materials. The first synthetic step to such materials is the preparation of usual side chain or combined LC copolymers doped with a small part of side chains containing a polymerizable >C=C< double bond at the end (Figure 23 shows a particular example of a crosslinkable LC polymer64). The copolymer can be further photocrosslinked, giving an elastic polymer film which reveals... 

Many side reactions can occur because when BMIs are heated above 200°C, double bonds can also polymerize by a chain polymerization mechanism with the formation of a crosslinked polymer. A typical formulation contains a molar excess of BMI with respect to a diamine. Linear maleimide end-capped aspartimides are formed, which can then be cross-linked by further heating with or without an initiator ( a catalyst ). 

---