Benzoxazine monomers, ring-opening

The polybenzoxazines (PBZs) provide a new class of phenolic resins that were first described by Ishida in 1998 (Ref. 15). Synthesis of the resins involves three components a phenol, a primary amine and formaldehyde. The first stage involves the formation of a multifunctional benzoxazine monomer Figure 23.30 a)). The monomer can then be ring-opened at elevated temperatures (160-220 C) to yield a polymeric stmcture (Figure 23.30(b)). 

Photoinitiated Cationic Polymerization of Unusual Monomers Recently, novel monomers and macromonomers (Chart 11.12), namely, benzoxazines, monothiocarbonates, thiophene, and epoxy and vinyl ether functional polymers were reported to undergo photoinitiated cationic polymerization [2,118], Among them, benzoxazine monomers deserves a special attention as they yield thermosets with several excellent characteristics including heat resistance good flame retardance stable dielectric constants low water absorption and perfect dimensional stability. This type of thermosets is also obtained by thermally activated ring-opening polymerization at elevated temperatures without catalysts [19]. 

The 3,4-dihydro-3-substituted-l,3-benzoxazine monomers cure by a ting-opening mechanism that avoids the release of volatiles. The heterocyclic ring reacts preferentially with the ortho positions of free phenolic compounds to form methylene-amine-methylene bridge structures (equation 44). 

Schreiber, for the preparation of phenolic resins with improved thermal, mechanical, and dielectric characteristics. However, no systematic study was reported in Schreiber s patents. Thus, it was of interest to examine in more detail several points such as 1) the synthesis of an homologeous series of benzoxazines, 2) their reaction with phenols, which may be considered as the initiation step of the polymerization by a ring opening mechanism, 3) the pol3nnerization kinetics of benzoxazines, initiated either thermally or with phenols, and 4) the application possibilities of these monomers and polymers. 

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