Norbornenes crosslinking

Cyclopentene is readily available as a byproduct in the ethylene production. Norbornene 2-ethylhexyl carboxylate is obtained by the Diels-Alder reaction of 2-ethylhexyl acrylate with cyclopenta-diene (5). Norbornene isobornyl carboxylate, norbornene phen-oxyethyl carboxylate, and other related monomers are synthesized according to the same route. Polymers obtained from these esters exhibit excellent properties in terms of controlling the crosslinking density, the associated product modulus, and the glass transition temperature (Tg), thus allowing tailoring the properties of elastomers, plastics and composites. Other suitable monomers are summarized in Table 1.1 and sketched in Figure 1.2. 

Monomers are shown in Table 2.1 and in Figure 2.1. The most important monomer is norbornene. Norbornene is in made from di-cyclopentadiene (DCPD) and ethene by a Diels-Alder reaction. 5-Tri-ethoxysilyl-2-norbomene is used for crosslinkable compositions (2). 

Only two spectroscopic studies on sulfur vulcanisation of EPDM by Fujimoto and coworkers are available [73-74], Using attenuated total reflectance (ATR) IR spectroscopy they showed that during sulfur/TMTD/MBT/ZnO/stearic acid vulcanization, the C=C bands at 3035, 966 and 870 cm 1 of the residual unsaturations of the EPDM third monomers, DCPD, 1,4-hexadiene (HD) and 5-methylidene-2-norbornene (MNB), respectively, decreased in intensity as a function of time at 140 and 150 °C. The relative decrease in intensity was shown to correlate with the increase in crosslink density. In Sections 6.2.2.2 and 6.2.2.3 it will be shown that this decrease of intensity should not be interpreted as a loss of unsaturation during sulfur vulcanisation of EPDM. 

Norsorex A process for making a thermosetting polymer from norbornene by ring-opening metathesis (ROMP). The product contains double bonds and can be crosslinked it can be made... 

Liaw [4] synthesized norbornene monomers containing crosslinkable side chains (VII), which underwent a ROMP reaction with a ruthenium-based... 

The pre-polymer has a very low DP (x < 3). The norbornene-ended polyimide (norbornene-2,3-dicarboxylic imide also known as nadic imide) is further polymerized by heating it at about 300 C. The thermosetting process of norbornene-ended polyimide can be investigated by Py-GC/MS. For example, the crosslinking of norbornene-ended polyimide PMR-II, was investigated using Py-GC/MS [20] (the pre-polymer PAR-11 structure is shown below) ... 

The addition copolymerization of norbornene-type monomers with a-olefins [21] forms the basis of EPDM (ethylene propylene diene monomer) technology. Incorporation of smaU amounts of DCPD or ethylidene norbornene (ENB) in olefinic vinyl addition polymers provides latent crosslink sites in EPDM elastomers. It is weU known in the hterature that incorporation of higher amounts of rigid, bulky multicychc olefins results in materials with higher TgS [22]. In fact, more recent work has concentrated on increasing the Tg of norbornene-type monomer/a-olefin copolymers [23]. The use of late transition metal catalysts to prepare such copolymers is reviewed in Section 4.3. 

Recent studies in our laboratories have been concerned with the physical properties of sulfonated ionomers such as sulfonate ethylene/propylene/ethylidene norbornene terpolymers (4, or lightly sulfonated polystyrene (S-PS) (11). These ionomers exhibit pronounced ion pair association (at sulfonate levels > 15 milli-equivalents/100 g polymer) to a degree that they appear crosslinked covalently. These interactions can be dissipated by the addition of a polar additive, thereby showing that such associations are indeed physical and do not arise due to covalent crosslinking. 

At elevated temperatures the terminal norbornene rings presumably undergo addition polymerization to promote crosslinking with minimal formation of volatile by-products. 

The mechanism of the crosslinking reaction has been postulated as (a) dissociation of the terminal cyclopentadiene-N-arylma1-eimide Diels-Alder adduct to the monomeric precursors, which immediately react to form an adduct which initiates the homopolymerization of the undissociated terminal norbornene rings to form a saturated polymer (j>). 

The crosslinked products resulting from the high temperature "pyrolytic polymerization" (jj) curing of the norbornene end-capped... 

Elastomers or rubbers are flexible materials that are mainly used in tires, hoses, and seals as adhesives or as impact modifiers of thermoplastics. They exhibit high resistance to impact, even at low temperatures at which materials increase their rigidity. Eor some of the applications (e.g., tires or hoses), these materials have to be slightly crosslinked once they are formed into the desired shape in order to impart them dimensional stability, since otherwise they tend to slowly flow. Elastomers are polymers that are used above their glass-transition temperature (Tg). Some examples of common elastomers are polybutadiene, which is used as an impact modifier of rigid plastics SBR (copolymer of styrene and butadiene), mainly used in tires EPDM (copolymer of ethylene, propylene, and a diene monomer, usually norbornene) NBR (copolymer of acrylonitrile and butadiene) and so on. 

A series of [2.2.1]bicycloheptenyl (norbornene) functional prepol)nners have been prepared via the cycloaddition reaction of cyclopentadiene monomer with corresponding acrylics. When these materials are formulated with an appropriate multifuntional thiol crosslinker and photoinitiator and irradiated, a rapid, exothermic, crosslinking reaction takes place. When the acrylic precursors are organic resins, the derived polymers behave like toughened plastics. The choice of a norbornene functional polydimethylsiloxane precursors gives elastomeric products. 

MODEL STUDIES Early in this study it appeared that [2.2.1]bicyclic olefin resins added conventional crosslinking thiols in a rapid, exothermic, manner. These results appear to contradict earlier reports that internal olefins and cyclic olefins such as cyclohexene and cydopentene react only slowly with thiols. In reality, [2.2.1]bicydic olefins represent a separate dass of reactive olefins. These results are also consistent with reports (16-19) that bicyclic olefins such as norbomadiene are quite reactive to the addition of monofunctional thiols and thiyl radicals. In order to quantify the relative reactivity of norbornene resins with other "standard" ene components, a model study of the addition reaction was undertaken. A "typical" thiol (ethyl mercaptoacetate) was examined in a series of competitive reactions in which there was a defidency of olefin (Figure 4). Olefin substrates that were compared were norbornene, styrene, butyl vinyl ether, [2.2.2]bicydooctene and phenyl allyl ether. The results of that study are listed below in Table I. 

CROSSLINKED NETWORK MODEL CALCULATIONS Our research has shown that tetrathiol crosslinked norbomene resins form a densely crosslinked, three dimensional network. Recently there has been considerable interest in crosslinked networks from a theoretical and practical point of view (29-31). As part of our study we attempted to analyze the polymer network using the Miller-Macosko formalism as applied by Bauer (32). For the purposes cf this analysis we assumed that the curable formulation was an A2 (ene) B4 (thiol) type system. We also assumed, based on HPLC analysis of EBPA DN and acrylate precursor batches that the norbomene resins was a mixture of oligomers consisting of difunctional olefin (85%) and monofimctional olefin (15%). The thiol crosslinker was assumed to be essentially tetrafunctional. Furthermore, we made the not unreasonable assumption that there would be no thiol-thiol or norbornene-norbornene reactions. In one case, a chain extending... 

HGURE 6 GEL POINT CALCULATION FOR A CROSSLINKED THIOL-NORBORNENE RESIN SYSTEM... 

NORBORNENE TERMINATED SILICONE FLUIDS Our efforts to expand this technology have also focused on the preparation a,o>-norbornenyl functional silicone fluids as precursors to photocured elastomers. The results of this work indicate that indeed, thiol-norbomene photoinitiated crosslinking will produce high elongation elastomers with physical properties comparable to those obtained from the photopx)lymerization of silicone acrylates (. These properties are listed below in Table V. The synthetic methodology that we... 

When a butyl rubber, an ethylidene norbornene (ENB)-type ethylene propylene diene monomer (EPDM), or a mixture of desired proportions of a butyl rubber and an ENB-type EPDM is thermally crosslinked using an... 

Poly (ethylene-co-propylene). See EPM rubber Poly (ethylene-co-propylene-co-5-methylene-2-norbornene). See Ethylene-propylene-ethylidene norbornene Poly (ethylene-co-tetrafluoroethylene). See Ethylene/tetrafluoroethylene copolymer Poly (ethylene-co-vinyl acetate). See Ethylene/VA copolymer Polyethylene, crosslinked Synonyms XLPE XPE... 

In 2010, Buchmeiser [56] developed a similar system that capitalized on the thermally reversible carboxylation [11] of NHCs (Scheme 31.13, inset). By employing the NHC-CO2 adduct (which essentially is a protected NHC), the reaction conditions did not have to be stringently air- and moisture-free to prevent NHC decomposition. Synthesis of the norbornene-functionalized monomer 37 allowed the molybdenum-catalyzed ROMP with l,4,4a,5,8,8a-hexahydro-l,4,5,8-exo-ewdo-dimethanonaphthalene (a ditopic norbornene) to produce crossHnked polymer 38 with pendant CO2-masked NHCs (Scheme 31.13). Upon heating in the presence of Rh, Ir, or Pd species, the NHC-metal-functionalized polymers 39 were formed and found to contain >20mol% metal, as determined with inductively coupled plasma optical emission spectrometry (ICP-OES). The C02-masked NHC material was found to catalyze the carboxylation of carbonyl compounds and the trimerization of isocyanates upon thermal deprotection (i.e., decarboxylation). Moreover, the NHC-metal-crosslinked materials were found to catalyze Heck reactions, transfer hydrogenations, and also the polymerization of phenylacetylene (M = 8.4 kDa, PDI = 2.45, as determined with GPC in DMF against PS standards). This modular system provides an array of options for catalysis from simple modifications of polymer-supported, C02-masked NHCs. 

The fiber reinforcement was prepregged with a solution of the monomers in alcohol, followed by removal of the solvent. The prepared prepreg was then heated at 150-200°C involving an in situ condensation reaction to form the norbornene-endcapped imide prepolymer. Crosslinking was subsequently achieved by applying pressure and heating at 250-300° C. 

If DCPD is copolymerized with other NBE-type monomers like 2-norbornene-5-methylester (NBE-ME), using 4 as catalyst, the Tg of the so obtained copolymers decrease linearly with the NBE-ME content from 145°C (pure DCPD) to 57°C (pure NBE-ME) and the swelling in toluene increases from 100 to >900%, pointing to a sharp decrease in the crosslink density. This and similar experiments with other comonomers suggest a random incorporation of the comonomer into the poly(DCPD) network. 

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