Curing ring opening

Fig. 9. Initiation of epoxy cure. Irradiation of a triaryl sulfonium salt produces a radical cation that reacts with an organic substrate RH to produce a cation capable of releasing a proton. The proton initiates ring-opening polymerization. X = BF , PFg, AsFg, and SgFg. ...

Biernath et al. concluded that phenolic novolac and epoxidized cresol novolac cure reactions using triphenylphosphine as the catalyst had a short initiation period wherein the concentration of phenolate ion increased, followed by a (steady-state) propagation regime where the number of reactive phenolate species was constant.85 The epoxy ring opening was reportedly first order in the steady-state regime. 

For commercial application, diepoxides such as those derived from bisphenol A are employed, and they are cured via ring-opening crosslinking reactions, into which the epoxy group enters readily. Bisphenol A is so-called because it is formed from two moles of phenol and acetone (Reaction 1.7). 

The ion thus produced may itself react with another epoxy-group in a process which forms the first crosslink (Reaction 4.16). This reaction may occur at both ends of the molecule of the diglycidyl ether, so that a crosslinked stmcture can easily be built up from these substances. Reaction becomes complicated by the fact that the epoxy-group may also react with the hydroxy-groups that form as the epoxy ring opens up during cure. Thus the finished resin may contain a complicated array of structures within the three dimensional network. 

None of the above described ring opening polymerization methods has, as yet, proved useful for the formation of polysilazane preceramic polymers. However, Si-N bond cleavage and reformation, as it occurs in reaction (13), is probably responsible in part for the curing or thermoset step in transition metal catalyzed dehydrocoupling polymerization of hydridosilazanes (31), as described below. 

The synthesis and proposed cure mechanisms of this resin are described in reference 2. While the cure mechanism of the BCB terminated resin is not yet known, it is speculated that it reacts via one of two different routes. Initially the strained four member ring of the benzocyclobutene undergoes a thermally Induced ring opening. The opened rings then react with one another by a linear type addition to form a network type of structure or by cycloaddition to form linear polymer chains. An Illustration of the proposed polymerization mechanism of benzocyclobutene (BCB) terminated resins is shown below. 

The cure mechanism of the BCB/BMI blend is believed to differ from that of the BCB oli omer. Following the thermally induced ring opening of the strained four member ring of the benzocyclobutene it can then react with the dienophile introduced to the system by the BMI resin via a Diels-Alder reaction An illustration of that reaction is shown below. 

Curing of epoxy resins can also be achieved by ring-opening polymerization of epoxide groups using either Lewis acids (including those generated photochemically) or Lewis bases (Sec. 7-2). 

Ring-Opening Polymerization of a Cyclic Oligosiloxane to a Linear, High-Moiecuiar-Weight Polysiloxane with Hydroxy End Groups Curing of the Polymer... 

The possible reactions related to vacuum curing termination in the EAP resist are 1 exposure-induced ring-opened epoxy radical with an azide, 2 exposure-induced ring-opened epoxy ion with an azide, 3 exposure-induced... 

These results indicate that vacuum curing occurs through a radical reaction mechanism and is terminated by reaction of the ring-opened epoxy group with the azide group (not nitrene) under exposure. There is a possibility that polymerization initiated by an exposure-induced radical cation may occur. Furthermore, it is thought that reaction products from both the azide and epoxide serve as dissolution inhibitors, because the sensitivity of EAP is almost the same as that of EP, as shown in Figures 1 and 2. 

For a review of ring opening of cyclopropyl anions and related reactions, see Boche Top. Cure. Chem. 1988, 146, 1-56. 

The most common method of preparation of PMTFPS is through the base-catalyzed ring-opening polymerization of the corresponding cyclic trimer.61-62 Details are in Section 2.11.1. A specific cure site for peroxide curing is developed by incorporating 0.2 mol% of methylvinyl siloxane.63 Typically, fluorosilicone elastomers are copolymers of 90 mol% of trifluoropropylsiloxy and 10 mol% of dimethylsiloxy monomers, but the fluorosilicone content in commercial products ranges from 40 to 90 mol%.63... 

Since the epoxy resin cures primarily by a ring-opening mechanism, it exhibits a smaller degree of cure shrinkage than other thermosetting resins. In these reaction processes, the epoxy group may react in one of two different ways anionically and cationically. Both are of importance in epoxy resin chemistry. In the anionic mechanism, the epoxy group may be opened in various fashions to produce an anion, as shown in Fig. 2.10. 

Figure 1 shows kinetic curves of the DGER-mPhDA (P = 1) cure reaction at different T.ure. From these curves, the existence of the conversion at which the reaction stops due to diffusion control, adlf, can be seen. The measurement of the total reaction heat Q at different Tcure and its comparison with the estimated value based on the specific heat of epoxy ring opening give the values of adif for any T 16). 

Hydrogen bonds are one of the most representative structural features of epoxy polymers. Each step of the cure reaction involving epoxy ring opening forms one OH group, whose concentration in a cured polymer is high. 

Another curing mechanism is operative in the curing reaction initiated by a polymerization catalyst for epoxy ring opening, where anionic catalysts such as tertiary amines 1S-l9) and imidazols 20 21> and cationic catalysts such as amine complexes of... 

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