RSD 816 - Esthetic Dentistry I Dental Polymers I. Polymers wrap) is shown below (Fig. 3). The monomer molecule is ethylene gas. A polymer molecule is a long molecule which is made by chaining together smaller units called mers. The starting molecule from which the polymer is formed is called a mono- mer. The structure of the monomer and mer may be the same or they may be different. Fig. 2. An assembly of polymer molecules is called a polymer. Each dash represents a mer. Solid polymers often contains 10s of thousands of molecules are tangled up like a ball of monomer mer string. Real polymers are tangled in three molecule dimensions. mer Fig. 1. A polymer molecule. The monomer H H H H H H H H H H molecule supplies fundamental unit that when C C C C C C C C C C repeated produces a long polymer molecules. H H H H H H H H H H The number of molecules can number in the polyethylene 10s or 100s of thousands. In addition poly- ethylene monomer polymer merization, the mer has the same molecular weight as the monomer. In other types of Fig. 3. A single ethylene molecule and a seg- polymers, the mer often has a smaller molecu- ment of a single polyethylene polymer mole- lar weight than the monomer. cule. II. Acrylic Polymers A polymer molecule (Fig. 1) usually consists of thousands of mers. The degree of poly- The methyl methacrylate molecule (MMA) merization is the number of mers in a poly- is similar to the ethylene molecule. Its mono- mer molecule. A polymer is an assembly of mer and a segment of a polymethyl methacry- many millions (typically) of polymer molecules. late (PMMA) molecule are shown in Fig 4. Random polymers can be envisioned as a Note that the ethylene and MMA monomers three-dimensional tangled ball of string. If are unsaturated hydrocarbons; that is, both each mer is drawn as a short dash, a polymer atoms contain double bonded carbon atoms. assembly would appear as in Fig. 2. These double electron bonds can be broken open, thereby freeing electrons, which can The monomer molecule and a segment of a form bonds with carbon atoms in two other polymer molecule of polyethylene (plastic monomer molecules. Consequently, monomer 1 Dental Polymers RSD 816 - Esthetic Dentistry I molecules can react, building a chain of mers H H H CH H CH connected by a carbon "backbone". 3 3 C C C C C C H C O H C O H C O H C H CH3 C H3 C H3 CH3 CH3 3 O C C O O H C O C O C O C O C O C O H H R O O O O O O acrylic methacrylic methacrylate acid acid ester C H3 CH3 C H3 C H3 CH3 CH3 N Fig. 5. Acrylic acid, methacrylate acid, and methyl methacrylate mer monomer polymethyl methacrylate meth- acrylate ester. Note that each of these (PMMA) can be used as monomers from which "acrylic" polymers can be formed. If the "R" in the Fig. 4. A methyl methacrylate monomer mole- methacrylate ester is CH3, the molecule is cule and a segment of a polymethyl methacry- called methyl methacrylate. If the "R" is late molecule. Note that in the format of the C2H5, the molecule is called ethyl meth- acry- right-hand molecule does not show the carbon late atoms along the polymer's "backbone" and the The base material for methacrylate two hydrogen atoms that are bonded to every monomer, polyacrylic acid, is the active ingre- second carbon atom. dient in the liquids of zinc polyacrylate and PMMA is an example of an acrylic poly- glass-ionomer cements. In addition, polymers mer. Acrylic polymers are polymers that are derived from PMMA are used in: formed from acrylic acid, methacrylic acid or resin composites their esters. As Fig. 5 shows, both acrylic acid resin cements for etched metal & porce- and methacrylic acid contain a carboxyl (- lain COOH ) group. Acids that contain this group adhesive resin cements are carboxylic acids . In the methacrylate pit and fissure sealants ester, the -COOR side chain is an ester group. When R is a methyl group (-CH3), the ester is components of dentin bonding systems called methyl methacrylate (abbreviated resin modified glass-ionomer cements MMA) and its polymer is called polymethyl core build-up materials methacrylate (abbreviated PMMA). If R is porcelain repair materials an ethyl group (-C2H5), the monomer is called orthodontic cements ethyl methacrylate and the polymer is called Because of its importance in dentistry, polyethyl methacrylate. PMMA will be the focus on most of the discus- Acrylic Polymers in Dentistry sion of polymer properties which follows. PMMA is the most important dental poly- Addition Polymerization mer. Unmodified PMMA is used in: Polymerization is the formation of long denture base materials polymer molecules from an assembly of mon- hard and soft denture reliners omer molecules. PMMA undergoes addition tray resins polymerization. To polymerize methyl meth- acrylate, the double bonds joining carbon denture teeth atoms must be broken. Highly reactive mole- bite guard resins cules called free radicals can break open mouth protector materials 2 Dental Polymers RSD 816 - Esthetic Dentistry I these bonds. An unpaired electron within the After each reaction, the polymer chain con- free radical accounts for its high reactivity. tinues to act as a free radical. The chain prop- Such groups of atoms are often denoted A " , agates rapidly. Note that the radical A• is where the dot indicates the unpaired electron. incorporated in the growing chain. The free Fig. 6 shows the action of a free radical on radical A• is called an initiator. methyl methacrylate. The most common initiator for dental PMMA and its derivatives is benzoyl peroxide. H CH H CH3 3 When heated or reacted with appropriate acti- A + C C A C C vating chemicals, benzoyl peroxide decomposes forming free radical initiators. H C O H C O free O O O O radical CH3 CH3 C O O C Fig. 6. The initiation step of addition poly- merization. O O This step is called . Note that initiation C O C O the product of the reaction between methyl methacrylate and the free radical is itself a free radical. This new free radical can the Fig. 8. An example of an initiator for addition action of a free radical on methyl methacry- polymerization - benzoyl peroxide. This mole- late. react with other monomer molecules. cule will form free radicals if heated or if Each time this occurs a new free radical is reacted with certain activating chemicals. formed. This process is called chain propa- The process used in forming PMMA is gation or propagation. After three such called addition polymerization. The charac- reactions, the chain appears as shown in Fig. teristics of addition polymerization include: 7. 1. The monomers contain double bonds. CH CH CH CH 3 3 3 3 2. There is no composition change during A polymerization. C O C O C O C O O O O O unpaired The monomer and mer are the same. electron CH3 CH3 CH3 CH3 3. No by-products are formed. Properties during polymerization. Fig. 7. The propagation step of addition poly- Methyl methacrylate (MMA) is a clear water- merization. Shown here is the reaction after like liquid with a boiling point just over that of the addition of four mers. Note that the grow- water (100.8oC). During the polymerization of ing chain is still a free radical and can contin- MMA, thousands of polymer molecules are ue to react with monomer molecules. As long growing simultaneously. As the molecules as monomer molecules are available, this mol- within an ensemble of molecules grow, the liq- ecule will grow. PMMA molecules can contain uid thickens, forming a syrup. Further poly- millions of mers. merization, produces a paste, then a dough, then a rubber, and finally a rigid polymer. Two factors contribute to the increasing viscos- 3 Dental Polymers RSD 816 - Esthetic Dentistry I ity and strength of the ensemble of polymer inhibitor is added to the monomer. The usual molecules as their average length increases: inhibitor is an hydroquinone (less than 0.006%). The hydroquinone reacts with free First, as length increases, pairs of mole- radicals, thereby, slowing spontaneous poly- cules are increasing likely to have stretches of merization. molecule that are close to one another. At sites where molecules are sufficiently close, Polymerization inhibitors. Eugenol and Van der Waals forces and, sometimes, hydro- oxygen retard polymerization by reacting with gen bonding will bond the molecules to one free radicals. This is why zinc oxide-eugenol another. The more segments of polymer cements should not be used adjacent to setting chains that are close enough to produce such resin composites (whose polymer matrix is bonding, the stronger the polymer ensemble. made from an acrylic- derived monomer); the eugenol will slow the polymerization of the The second reason for the increase in matrix of the composite. strength is that with increasing average length the molecules become increasingly When acrylic polymers are polymerized in entangled. To stretch a solid polymer, one will air, one can sometimes notice a soft, sticky have to untangle some of the polymer mole- layer at the surface. The oxygen in the air will cules. Unentangling will be easier if one can compete with monomer for free radicals. As a break some of the - C - C - bonds that form result, the average length of the polymer mole- each molecule's backbone. But since these are cules at air-inhibited surfaces will be less than covalent bonds, a strong primary bond, break- for polymer deeper within the polymer.