Highly Crosslinked Polybenzoxazines from Monobenzoxazines: The Effect of Meta-Substitution in the Phenol Ring

Abstract

It is possible to control the crosslink density of polymers derived from monobenzoxazines by switching the type of substituents in the phenolic ring and their relative position with respect to the phenol group. We prepared several substituted monobenzoxazines in the para and meta positions of the phenolic ring and studied how these substituents affected the polymerization temperature of monomers and the thermal stability of the final polymers and, more extensively, how they affected the crosslink network of the final polymers. Gel content and dynamic mechanical analysis confirm that ortho- and para-orienting substituents in the meta position generate highly crosslinked materials compared to para ones. This fact can lead to the design of materials with highly crosslinked networks based on monobenzoxazines, simpler and more versatile monomers than the commercial bisbenzoxazines currently in use.

Review on the Accelerated and Low-Temperature Polymerization of Benzoxazine Resins: Addition Polymerizable Sustainable Polymers

“…Furan ring of furfuryl amine undergoes electrophilic aromatic substitution reactions more readily than a benzene ring of aniline [ 130 ]. The presence of fluorine at the meta -position to phenol ring facilitated both polymerization and crosslinking reactions, which is accounted to its smaller size and electron withdrawing nature [ 390 ]. The existence of phenolic-OH [ 391 , 392 , 393 , 394 , 395 ], phenyl thioether and hydroxyl [ 104 , 282 , 396 , 397 ] moieties lowers the polymerization temperature.…”

Enhancing the material performance of chloroprene rubber (CR) by strategic incorporation of zirconia

“…The halogenated polymers like CR usually show flame resistance as they provide hydrogen halides which can serve as very good radical scavengers for H and OH radicals. 36 We have theoretically studied the flame retardancy of the synthesised composites by calculating the char yield and LOI (limiting oxygen index) values [43][44][45] (Table 3). The char yield of the filled composites increases from 32% (unfilled compound) to 37-45%.…”

Implementation of meta-Positioning in Tetrafunctional Benzoxazines: Synthesis, Properties, and Differences in the Polymerized Structure

“…Moreover, their extraordinarily rich molecular design flexibility allows further improvement in properties as the desired properties can be easily introduced into both monomeric and polymeric structures by simply using suitable starting phenols/amines; this makes them an attractive candidate for various kinds of applications. − One of the main problems researchers are still trying to resolve associated with these resins is their high curing temperature (180–250 °C). Many progresses have been made in the past, lowering polymerization temperature by using catalysts/initiators − or molecularly designed monomers with special functional groups or backbone structure. − Adding catalysts/initiators such as metal/phosphorous halides, imidazoles, p -toluene sulfonates, amines, phenols, etc., was successful in lowering the polymerization temperature. Apart from the shelf life/miscibility issues related to addition of these catalysts into monomers, it affects the actual performance properties of the resulting polybenzoxazines due to the structural changes in the final polymer caused by these promoters.…”

“…Therefore, it is important to focus on designing molecules that can lower the ROP temperature while balancing/not sacrificing the properties of the polybenzoxazines they inherit in the first place . The attractive and obvious approach is by utilizing their versatile and rich molecular design flexibility, and plethora of starting materials (petroleum/renewable-based phenols/amines ,− or even replacing formaldehyde with other aromatic/aliphatic aldehydes), and various synthetic approaches/designing tools ,, available at researcher’s disposal; new benzoxazine molecules that can provide a catalytic effect on ROP are on the rise.…”

“…The other factor that influences the ROP of benzoxazines is reportedly electronic effects, either resulting from bridging groups (−SO 2 – ( T p = 215 °C) < −CO– ( T p = 239 °C) < single bond ( T p = 257 °C) < −O– ( T p = 260 °C) < −CH 2 – (T p : 264 °C) < −C­(CH 3 ) 2 – ( T p = 267 °C)) between two benzoxazine moieties or the substituents present on the backbone due to their electron-withdrawing/donating character. − Introducing electron-donating/withdrawing substituents on the para -position to the phenolic part of a monofunctional benzoxazine (phenol-aniline-based, T p = 272 °C) showed drastic differences in curing temperature, and a decreasing trend was reported with electron-withdrawing groups (NO 2 ( T p = 211 °C) < CN ( T p = 227 °C) < CHO ( T p = 231 °C) < Br ( T p = 243 °C) < CF 3 ( T p = 251 °C) < Cl ( T p = 260 °C) < OCF 3 ( T p = 272 °C) < F ( T p = 275 °C)), whereas almost negligible differences were reported with electron-donating groups (OCH 3 , OCH 2 CH 3 , OCH 2 CH 2 CH 3 , SMe, Ph, Me, t Bu, etc., mostly T p = 260–274 °C). , However, when the electron-donating group such as methoxy was substituted at the meta -position instead of the para -position to the phenolic part of oxazine ring, it lowered the curing temperature in the order of m 7-OCH 3 ( T p = 231 °C) < m 5-OCH 3 ( T p = 255 °C) < p -OCH 3 ( T p = 259 °C). On the other hand, when it was substituted with an electron-withdrawing group such as NO 2 at the meta -position, the curing temperature showed the opposite trend of p -NO 2 ( T p = 211 °C) < m 7-NO 2 ( T p = 236 °C) < m 5-NO 2 ( T p = 248 °C). , Introducing these electron-withdrawing/donating substituents on the para -position to the amine part of this benzoxazine monomer had less/no impact on lowering the curing temperature (mostly T p = 260–279 °C), with the exception of strong electron-donating dimethylamino (NMe 2 , T p = 233 °C) and diethylamino (NEt 2 , T p = 250 °C) groups . Other benzoxazine monomers reported with low curing temperature as a consequence of electronic effects were resorcinol-based benzoxazine ( T p = 179 °C) and meta -phenylenediamine-based benzoxazine ( T p = 193 °C) due to the location of two electron-donating aryl ethers/tertiary amines of oxazine ring at the meta -position of the aromatic ring. , In addition, isomeric bisphenol F-aniline-based benzoxazines were reported to show the reactivity rate on ring-opening polymerization (at 160 °C) in the order of 2,2′ > 2,4′ > 4,4′ substitution .…”