A photocurable filling restoration material called a composite resin is generally widely used for restoration of a tooth damaged by caries from an aesthetic viewpoint. Such composite resin (dental curable composition) is a material containing a polymerizable monomer (monomer), a filling material (filler), and a photopolymerization initiator as main components and having further added thereto a pigment, an additive, and the like as necessary. In this case, a radically polymerizable monomer having radical polymerizability is generally used as the polymerizable monomer. Therefore, polymerization shrinkage inevitably occurs upon curing of the dental curable composition, that is, a polymerization reaction of the radically polymerizable monomer.
Hence, when the dental curable composition is filled into a cavity of a tooth that requires restoration and then subjected to polymerization curing, a gap is liable to be formed between the cured dental curable composition and an inner wall of the cavity. When such gap is formed, plaque is liable to be accumulated in the gap, and hence secondary caries is liable to develop. Further, the cured dental curable composition is liable to be detached out of the cavity. Accordingly, it is preferred that the polymerization shrinkage of the composite resin upon curing be as small as possible.
As a method of suppressing the polymerization shrinkage of the dental curable composition upon curing, there are given: a method involving using a radically polymerizable monomer having a high molecular weight; and a method involving using a cationically polymerizable monomer having a smaller polymerization shrinkage than that of the radically polymerizable monomer and subjecting the polymerizable monomer to polymerization curing through cationic ring-opening polymerization. Further, in addition to the foregoing, there are also proposals concerning methods involving using dendritic polymers having dendritically branched molecular chains, such as a hyperbranched polymer and dendrimer (see Patent Literatures 1 to 3).
The dendritic polymers are broadly divided into, for example, a hyperbranched polymer and dendrimer. A conventional polymer generally has a linear shape, while such dendritic polymer has actively introduced branches. Therefore, as compared to the conventional polymer having a linear shape, the dendritic polymer has, for example, the following features: 1) having a unique molecular structure; 2) having a size on a nanometer order; 3) being capable of forming a surface retaining a large number of functional groups; 4) being capable of being reduced in viscosity as compared to the linear polymer; 5) exhibiting a fine particle-like behavior because of less entanglement of molecules; and 6) having voids in the molecule. In this case, the hyperbranched polymer has an advantage in terms of its ease of synthesis as compared to the dendrimer, and thus is particularly advantageous in industrial production. In general, the dendrimer is synthesized by repeating protection and deprotection, while the hyperbranched polymer can be synthesized in one stage and can be synthesized simply.
For example, in Patent Literature 1, there is a proposal of a technology involving using a (meth)acryloyl terminated hyperbranched polyester having at least one ethylenically unsaturated moiety as a polymerizable resin for forming a dental material for the purpose of, for example, reducing a polymerization shrinkage of the dental material. Further, in Patent Literature 2, there is a proposal of a technology involving using a dendritic polymer as a polymerizable composition that may be utilized as a dental material and is low in shrinkage. For example, in Patent Literature 2, there is a disclosure of an example using a hyperbranched polyesteramide as a hyperbranched polymer. In addition, in Patent Literature 3, there is a proposal of a technology involving using, as a dental material, a dendritic compound in which a core, first shell, and second shell for forming a molecule are bonded via a polyurethane group and the second shell is modified by a reaction with a (meth)acrylate. According to the patent literature, undesired polymerization shrinkage is advantageously affected by this technology.
Further, in addition to Patent Literatures 1 to 3, there is a proposal of a technology involving using, as a crystal component for forming a dental material, one having a dendritic, hyperbranched, or star-shaped structure (Patent Literature 4). According to the patent literature, the crystal component to be used for the dental material preferably has a hydroxyl group at the end.