This article deals with recent research on construction and control of network of conjugated polymers, mainly poly(acetylene)s and poly (phenyleneethynylene)s utilizing intra/intermolecular interaction and/or crosslinking. Bi- and multi-functional substituted acetylene monomers give poly(acetylene)-based network polymers exhibiting stimuli-response, chiral recognition and catalysis. Poly(acetylene)s substituted with benzoxazine moieties are thermally highly stable due to the rigid poly(acetylene) backbone. Poly(phenyleneethynylene)s give stable network polymers by crosslinking at the side chains. The higher order structures of platinum-containing poly(phenyleneethynylene)s are controllable by network formation with bisphosphine ligands.

As mesogenic groups containing aromatic rings have rigid structure, introduction of such mesogenic groups into the networked polymer chains effectively suppresses the molecular mobility resulting in improved thermal property. In addition, when a suitable flexible chains coexist, self-organization of the mesogenic groups in the network chains occurs and liquid crystalline phases are formed. The liquid crystal orientation can be fixed into the networked polymer chains mainly by optimizing the curing reaction condition (temperature / external field) and by selecting curing agents. The obtained polymer materials by this method exhibited high toughness, high heat resistance and high thermal conductivity and their properties depended on the degree of molecular chain orientation. Moreover, the influence on network chain orientation caused by loading inorganic filler is introduced.

Monosaccharides and their analogues have plural hydroxyl groups applicable to their chemical modifications leading to the conversion into compounds bearing fused ring systems. Such fused ring systems are under restriction in their conformational changes and thus useful as core structures to design rigid monomers. This review paper deals with 1) several sugar-derived rigid monomers such as isohexides that are rigid diols derived from monosaccharides, 2) another series of rigid diols bearing fused ring systems that can be derived from acyclic sugars through their acetalization, and 3) myo-inositol that is naturally occurring cyclic hexaol available from rice bran. The current status of their applications to networked polymers and their perspectives are discussed.

Network polymers show excellent properties such as a high mechanical strength and solvent resistance. On the other hand, the removal of network polymers is difficult due to insolubility. In order to solve the issue, de-cross-linkable network polymers have attracted much attention.In this review, the design of network polymers having degradable units is introduced with focusing on the main-chain degradable polyperoxides-based network polymers and curable hyperbranched polymers having degradable linkages densely. Curable adhesives are one of the applications of network polymers, and de-crosslinkable network polymers can be used as dismantlable adhesives, which demonstrate both sufficient bonding strength in use and easy debonding on demand. Increase in bonding strength by the network formation and debonding by degreasing a mechanical strength and solubilization in response to de-cross-linking are reported.

Photo (Light) curing materials consisting of various monomers, pre-polymers, and additives such as photo-cationic initiators, are used for coatings, adhesives, moldings, and photolithography. The chemical reactions such as polymerization/cross-linking of these materials, may occur quickly by photo-irradiation. For that reason, these materials and technologies are very useful and essential in various fields of industries such as electricity, mobility, semiconductors, etc. For the consideration of the global environment, photo curing technologies are developed as efficient productive technologies with high productivity, low energy, requiring little space, and so on. As features of these technologies, solvent-free reaction can be possible, and low heat resistant materials can be also applicable. In this review, a summary of photo-initiators, application of networked polymers formed by photo-cationic polymerization with a central focus on sulfonium salts, are explained.

Radical ring-opening polymerization (RROP), which is similar to cationic and/or anionic ring-opening polymerization, affords polymer structures containing the functional moieties originated from the monomer structures in the main-chain. RROP is one of the most significant strategies for not only an inducement of functional moieties in the polymer main-chain but also synthesis of various functional polymers, whereas such polymers cannot be synthesized by typical radical polymerization from vinyl monomers. In addition, the networked polymers are synthesized from multifunctional monomers such as variety of vinyl monomers, monomers with a radical ring-opening property, and thiol derivatives for radical addition reaction based on RROP. In this review, we describe the typical molecular design and synthesis of monomers undergoing RROP, and also demonstrate the synthesis and properties of polymers and networked polymers based on radical ring-opening system from recent our works.