Block Copolymers For Tooth Enamel Protection

Abstract

Described herein are block copolymers having hydrophobic blocks and hydrophilic blocks which are effective in binding to the surface of hard tissue; compositions comprising the same, as well as methods of making and using the same are also described.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser. No. 61/724,736, filed on Nov. 9, 2012, and U.S. Provisional Application Ser. No. 61/780,199, filed on Mar. 13, 2013, which both applications are incorporated by reference as if fully rewritten herein.

BACKGROUND

There is a need for oral care products offering superior protection against acid dissolution of tooth enamel that surpasses traditional fluoride approaches as awareness of erosion and the impact of dietary habits increases among dental practitioners and their patients. Extrinsic and intrinsic acid are the two most important factors governing demineralization, in which the former is prevalent because of the strikingly increased consumption of soft drinks. A. Wiegand et al., “Review on fluoride-releasing restorative materials—fluoride release and uptake characteristics, antibacterial activity and influence on caries formation,”Dental Materials, 2007, 23(3): 343-62. An interesting experiment that used soft drinks to etch tooth enamel indicated that the loss rate of enamel in a soft drink was as high as 3 mm per year. R. H. Selwitz et al., “Dental caries,” The Lancet, 2007, 369(9555): 51-9. For example, Coca Cola could reduce the hardness of enamel by 63% of the original enamel hardness after only 100 seconds of erosion. S. Wongkhantee et al., “Effect of acidic food and drinks on surface hardness of enamel, dentine, and tooth-colored filling materials,” Journal of Dentistry, 2006, 34: 214-220. Dietary acids such as citric acid are particularly damaging to tooth enamel because these acids not only have an acid pH, but they also have a calcium chelating capacity which enhances enamel dissolution. Hence, it is important to have new protective agents that are readily applied, are biologically suitable, and can coat tooth enamel and protect enamel from erosion and attack by foods such as dietary acids.

Currently, fluoride compounds are widely used to prevent caries formation and have also been identified as minerals that protect against acid erosion if formulated under the right conditions. A. Wiegand et al., “Review on fluoride-releasing restorative materials—fluoride release and uptake characteristics, antibacterial activity and influence on caries formation,” Dental Materials, 2007, 23(3): 343-62. R. H. Selwitz et al., “Dental caries,” The Lancet, 2007, 369(9555): 51-9. C. Hjortsjo et al. “The Effects of Acidic Fluoride Solutions on Early Enamel Erosion in vivo”, Caries Research, 2008, 43: 126-131. But high loading of fluoride may induce dental fluorosis. WHO, “Fluorides and Oral Health: Report of A WHO Expert Committee On Oral Health Status and Fluoride Use,” WHO Technical Report Series 846 Geneva, Switzerland, World Health Organization, 1994. A. K. Mascarenhas, “Risk factors for dental fluorosis: A review of the recent literature,” Pediatric Dentistry 2000, 22(4): 269-277. Nonfluoride functional agents have also been highlighted to deliver antierosion benefits. Ganss et al. “Efficacy of the stannous ion and a biopolymer in toothpastes on enamel erosion/abrasion” J. of Dentistry, 2012, 40: 1036-1043. There are many publications that also highlight remineralization processes. Nano hydroxyapatite has been employed for remineralization of tooth enamel. L. Li et al., “Bio-Inspired Enamel Repair via Glu-Directed Assembly of Apatite Nanoparticles: an Approach to Biomaterials with Optimal Characteristics,” Advanced Materials, 2011, 23(40): 4695-4701. L. Li et al., “Repair of enamel by using hydroxyapatite nanoparticles as the building blocks,” Journal of Materials Chemistry, 2008, 18: 4079-4084. Y. Cai et al., “Role of hydroxyapatite nanoparticle size in bone cell proliferation,” Journal of Materials Chemistry, 2007, 17: 3780-3787. P. Tschoppe et al., “Enamel and dentine remineralization by nano-hydroxyapatite toothpastes,” Journal of Dentistry, 2011, 39(6): 430-7. But the efficiency of enhancing remineralization is highly dependent on the nanostructure of apatite and varies a lot from case to case.

Casein phosphopeptides-armohous calcium phosphate (CPP-ACP) complexes are known to bind to tooth enamel and provide a way for remineralization of the enamel. Srinivasan et al. “Comparison of the remineralization potential of CPP-ACP and CPP-ACP with 900 ppm fluoride on eroded human enamel: An in situ study”, Archives of Oral Biology, 2010, 57: 541-544. E. C. Reynolds. “Remineralization of enamel subsurface lesions by casein phosphopeptide-stabilized calcium phosphate solutions,” Journal of Dental Research, 1997, 76: 1587-1595. M. Panich et al. “The effect of casein phosphopeptide-amorphous calcium phosphate and a cola soft drink on in vitro enamel hardness,” Journal of American Dental Association, 2009, 140; 455-460.” However, CPP and other dairy products may have potential health risks such as allergic reactions, ranging from minor swelling of the mouth to serious anaphylaxis, which can be potentially life threatening. G. H. Docena et al., “Identification of casein as the major allergenic and antigenic protein of cow's milk,” Allergy, 1996, 51(6): 412-416. B. Schouten et al., “Acute allergic skin reactions and intestinal contractility changes in mice orally sensitized against casein or whey,” International Archives of Allergy and Immunology, 2008, 147(2): 125-134. In view of the latter problems, alternate materials are needed which will not only provide effective protection of tooth enamel, but also are non-toxic, biologically suitable and provide a readily usable synthesis to provide materials which may be effectively used for enamel protection.

SUMMARY

Block amphiphilic copolymers having hydrophobic blocks and hydrophilic phosphonated or phosphorylated or carboxylated blocks have been developed where the copolymers are effective to bind to hard tissue which includes hydroxyapatite (HA) and enamel. These copolymers bind to and protect the hard tissue from acid erosion. The hydrophilic phosphonated or phosphorylated or carboxylated blocks are effective to bind to the hard tissue and the hydrophobic blocks are effective to protect the hard tissue from loss of calcium by at least 5 percent after exposure of the hydroxyapatite to the polymers for 0.1-10 minutes and subsequent exposure of the polymer coated hydroxyapatite to a 0.3-1% citric acid solution, such as for 15 minutes at 37° C. as compared to hydroxyapatite which is not bound to the block copolymers. It should be noted that other temperatures and time periods may also be used to illustrate the effect of the composition. In some embodiments, hydrophilic phosphonated or phosphorylated or carboxylated block copolymers are effective to protect the hard tissue from loss of calcium by at least 10 percent. In some embodiments, hydrophilic phosphonated or phosphorylated or carboxylated block copolymers are effective to protect the hard tissue from loss of calcium by at least 15 percent. In some embodiments, hydrophilic phosphonated or phosphorylated or carboxylated block copolymers are effective to protect the hard tissue from loss of calcium by at least 20 percent. In some embodiments, hydrophilic phosphonated or phosphorylated or carboxylated block copolymers are effective to protect the hard tissue from loss of calcium by at least 25 percent. In some embodiments, hydrophilic phosphonated or phosphorylated or carboxylated block copolymers are effective to protect the hard tissue from loss of calcium by about 30 percent. In some embodiments, hydrophilic phosphonated or phosphorylated or carboxylated block copolymers are effective to protect the hard tissue from loss of calcium by at least 30 percent.

In one form, the block copolymers have a molecular weight (Mn) in a range of from about 1,000 to 1,000,000. According to one form, the block copolymers have a molecular weight in a range of 1,000 to 10,000. The hydrophilic blocks may include blocks with pending functional groups such as phosphonic, phosphoryl, carboxyl, sulfonic, amino, hydroxyl groups, or other hydrophilic groups. In an important aspect, the phosphonated or phosphorylated or carboxylated blocks have a molecular weight in a range of from about 200 to about 1,000,000. According to one form, the hydrophobic blocks have a molecular weight in a range of from about 200 to about 1,000,000. The phosphonated or phosphorylated or carboxylated blocks generally comprise from about 10 to about 90 weight percent of the copolymers and the hydrophobic blocks comprise from about 10 to about 90 weight percent of the block copolymers. In any event, the block copolymers are dispersible in an aqueous media and effect protection of tooth enamel from acid erosion. The polymers may be polymers having two blocks (bi-block copolymers), three blocks (tri-block polymers) where there are two blocks which may be hydrophobic and one hydrophilic block or two hydrophilic blocks and one hydrophobic block and multi-armed blocks. Arms extend from a common core and the arms may have one or more blocks.

In one aspect, the polymers have molecular weights of from about 1,000 to about 1,000,000 and hydrophobic and hydrophilic blocks have molecular weights of from about 1,000 to about 1,000,000 which provide a good solubility in water in a range of from about 0.001 to about 100 g/l at 25° C.

In another aspect, compositions which are effective for use in connection with dental hygiene, such as toothpaste, mouthwash, strips, and gel containing trays which include the block copolymers described herein, are effective for reconstituting protection of tooth enamel from acid erosion as described herein. Regular applications of the compositions, which include the block copolymers, are effective for providing a protective layer on tooth enamel at a first time of application, and thereafter. Regular use of the compositions, as by brushing teeth or use of mouthwash, gels, or strips provide a way of regularly applying the copolymers for protection against acid erosion of tooth enamel.

In an important aspect the phosphonated or phosphorylated block copolymers have the general formula:

In another aspect, the carboxylated copolymers have the general formula:

Where in the above formulas I and II A is selected from the group consisting of (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for substituent A p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0 or 1;

R₁ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₁ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₂ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₂ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₃ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₃ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₄ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₄ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₅ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₅ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₆ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₆ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₇ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₇ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₈ is selected from the group consisting of an alkali metal, an ammonium, protonated alkyl amine, H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₈ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₉ is selected from the group consisting of an alkali metal, an ammonium, protonated alkyl amine, H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₉ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₁₀ is hydrogen, alkali metal, or ammonium;

m and n are each independently in a range from about 5 to about 3000.

The block copolymers can be synthesized from reversible addition fragmentation chain transfer radical polymerization (RAFT), atomic transfer radical polymerization (ATRP) which often use a catalyst such as a transition metal catalyst and which can effect multi-armed blocks, other chain transfer polymerization, free radical polymerization, ionic polymerization or direct coupling from homopolymers.

Initiators include, but are not limited to, benzoyl peroxide, dicumyl peroxide, t-butyl peroxybenzoate, 2,2-azobisisobutyronitrile (AIBN) and other materials that can generate radicals in direct or indirect approaches. The initiators for ATRP can be 2-bromoisobutyryl bromide or others with similar structure.

The general chemical formula for the chain transfer agent (CTA) for RAFT polymerizations is shown below:

where Z and R can be the same or different substitutes. Typical chain transfer agents include, but are not limited to, cumyldithiobenzoate, 2-cyano-2-yl-dithiobenzoate and 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid with their structure shown as below.

In an important aspect the copolymers are the reaction product of hydrophobic monomers such as acrylates (alkyl (meth)acrylate, alkyl acrylate), styrene, olefins (ethylene, propylene, butylenes, butadiene), vinyl monomers (vinyl acetate, vinyl ether), fluoro monomers (perfluorocarbon, tetrafluoroethylene), acrylonitrile, which will provide the hydrophobic block after polymerization and other hydrophilic monomers to provide the hydrophilic block. The hydrophilic monomers contain polymerizable groups and active phosphate acid, phosphonic acid and related esters, as well as other phosphorous containing monomers, such as alkyl (meth)acryloyloxyethyl phosphate, bis(2-methacryloxyethyl) phosphate, vinyl phosphonic acid and other monomers.

In another aspect, the block copolymers comprise from about 0.001 to about 50 weight percent of a dental hygienic composition such as an ingredient which forms the basis of toothpaste or gel which also includes abrasive particulates such as aluminum hydroxide, calcium carbonate, dicalcium phosphate, and silicas; flavorants, humectants, antibacterial agents, and remineralizers such as fluoride, hydroxyapatite and phosphates such as calcium phosphate. The block copolymers also may be included in aqueous compositions which form the basis of mouthwash which also include fluoride, alcohol, chlorhexidine gluconate, cetylpyridinium chloride, hexetidine, buffers such as benzoic acid, methyl salicylate, benzalkonium chloride, methylparaben, hydrogen peroxide, domiphen bromide and fluoride, enzymes, and calcium. Mouthwash can also include other antibacterials such as, e.g., phenol, thymol, eugenol, eucalyptol or menthol as well as sweeteners such as sorbitol, sucralose, sodium saccharin, and xylitol. In this aspect the copolymers are dispersible in an aqueous media and the block copolymers form from about 0.001 to about 20 weight percent of the aqueous composition which forms the mouthwash.

In yet another aspect, the phosphonated or phosphorylated block copolymers are formed in a two-step reversible addition-fragmentation transfer (RAFT) polymerization or a one pot RAFT polymerization reaction. Illustrative of the two step RAFT reaction is shown below.

Monomer-1 (hydrophobic monomer)+Chain Transfer Agent (CTA)+Free Radical Initiator→Poly(monomer-1)−Chain Transfer RAFT agent

then

Poly(monomer-1)−Chain Transfer RAFT agent+Monomer-2 (phosphorous monomer)+Free Radical Initiator→

In another aspect, the carboxylated block copolymers are also formed in a two-step RAFT polymerization or a one pot RAFT polymerization reaction. Illustrative of the two step RAFT reaction is shown below.

Monomer-1 (hydrophobic monomer)+CTA+Free Radical Initiator→Poly(monomer-1)−Chain Transfer RAFT agent

then

Poly(monomer-1)−Chain Transfer RAFT agent+Monomer-2 (carboxylated monomer)+Free Radical Initiator→

where A is selected from the group consisting of (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for substituent A p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0 or 1;

R₁ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₁ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₂ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₂ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₃ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₃ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₄ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₄ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₅ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₅ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₆ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₆ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₇ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₇ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₈ is selected from the group consisting of an alkali metal, an ammonium, protonated alkyl amine, H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₈ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₉ is selected from the group consisting of an alkali metal, an ammonium, protonated alkyl amine, H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₉ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₁₀ is hydrogen, alkali metal, or ammonium; and

m and n are each independently in a range from about 5 to about 3000.

The block copolymers can be synthesized from reversible addition fragmentation chain transfer radical polymerization (RAFT), atomic transfer radical polymerization (ATRP), other chain transfer polymerization, free radical polymerization, ionic polymerization or direct coupling from homopolymers.

Initiators include, but are not limited to, benzoyl peroxide, dicumyl peroxide, t-butyl peroxybenzoate, 2,2-azobisisobutyronitrile (AIBN) and other materials that can generate radicals in direct or indirect approaches. The initiators for ATRP can be 2-bromoisobutyryl bromide or others with similar structure. The general chemical formula for the chain transfer agent for RAFT is shown below:

where Z and R can be the same or different substitutes.

In the “one pot” method, the reaction for phosphonated or phosphorylated block copolymer proceeds as follows as part of a single step with the phosphorous acid being added to the reaction mixture having the hydrophobic block:

Monomer-1 (hydrophobic monomer)+Chain Transfer Agent+Free Radical Initiator→Poly(monomer-1)−Chain Transfer RAFT agent+Monomer-2 (phosphorous monomer)→

or, in another aspect, the reaction for carboxylated block copolymer proceeds as follows as part of a single step with the carboxylated monomer being added to the reaction mixture having the hydrophobic block:

Monomer-1 (hydrophobic monomer)+Chain Transfer Agent+Free Radical Initiator→Poly(monomer-1)−Chain Transfer RAFT agent+Monomer-2 (phosphorous monomer)→

where A is selected from the group consisting of (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for substituent A p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0 or 1;

R₁ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₁ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₂ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₂ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₃ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₃ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₄ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₄ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₅ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₅ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₆ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₆ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₇ is selected from the group consisting of H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₇ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₈ is selected from the group consisting of an alkali metal, an ammonium, protonated alkyl amine, H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₈ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₉ is selected from the group consisting of an alkali metal, an ammonium, protonated alkyl amine, H_a, (CH₂)_p, (CH₂CH₂O)_q, (phenyl), (C(═O)—O)_y, or any combination thereof, where for R₉ p, q=0, 1, 2, 3 . . . 20; x=0, 1, y=0, 1, a=0, 1;

R₁₀ can be hydrogen, an alkali metal, or an ammonium; and

m and n are each independently in a range from about 5 to about 3000.

The block copolymers can be synthesized from reversible addition fragmentation chain transfer radical polymerization (RAFT), atomic transfer radical polymerization (ATRP), other chain transfer polymerization, free radical polymerization, ionic polymerization or direct coupling from homopolymers.

Initiators include, but are not limited to, benzoyl peroxide, dicumyl peroxide, t-butyl peroxybenzoate, 2,2-azobisisobutyronitrile (AIBN) and other materials that can generate radicals in direct or indirect approaches. The initiators for ATRP can be 2-bromoisobutyryl bromide or others with similar structure. The general chemical formula for the chain transfer agent for RAFT is shown below:

where Z and R can be the same or different substitutes.

In the third aspect, the amphiphilic copolymers are prepared by using a free radical polymerization without RAFT chain transfer agent or by using an atom transfer radical polymerization (ATRP) either from ‘one-pot’ polymerization or ‘two step’ polymerization as that for RAFT.

The water solubility and/or dispersibility of the block copolymers may be controlled by the molecular weight of the hydrophilic portion of the copolymer and the ratios of the two blocks. This is done for example by having a feeding ratio and polymerization time of the phosphorous monomer such that a dispersibility or solubility of the block copolymer is from about 0.001 g/L to 100 g/L in water at 25° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 indicates the controlled synthesis of hydrophobic block via RAFT method by using different monomer/chain transfer RAFT agent/initiator ratios. MMA stands for methyl methacrylate, CTA stands for chain transfer agent, AIBN stands for azoisobutyronitrile, and MOEP stands for methacryloyloxyethyl phosphate.

FIGS. 2a through 2f illustrate Fourier transform Infrared (FTIR) spectra of enamel treated with polymers at different pHs and concentrations. FIGS. 2a, 2c and 2e relate to aqueous compositions where the polymers are solubilized at 1 g/L. FIGS. 2b, 2d and 2f relate to aqueous compositions where the polymers are solubilized at 0.2 g/L. The pH of the treatment solution is 3.1 in FIG. 2a, 3.7 in FIG. 2b, 4.2 in FIGS. 2c and 2d and 7.0 in FIGS. 2e and 2f. The insert in FIG. 2a is a FTIR spectrum of a phosphate copolymer.

FIGS. 3-1 through 3-3 indicate the UV spectra of a phosphate block copolymer before and after binding with HA powder at different conditions. FIG. 3-4 through FIG. 3-6 illustrate the UV spectra of carboxylic block copolymers before and after binding with HA at different conditions. FIG. 3-7 compares the binding efficiencies of phosphorylated polymer and carboxylated polymer at different concentrations and pHs.

FIG. 4 illustrates the effects of the block copolymers described herein on calcium erosion by acid environments.

FIG. 5 indicates the anti-erosion efficiency of the phosphorylated or carboxylated diblock copolymers and the enhanced performance in presence of fluoride.

FIG. 6-1 is the enamel surface morphology after exposing to acid. FIG. 6-2 is the enamel surface morphology which was treated by phosphate block copolymer and then exposed to acid erosion.

DETAILED DESCRIPTION

Examples and Tests

1. Controlled synthesis of hydrophobic blocks
2. Block copolymer synthesis
3. Polymer/enamel binding
4. Quantitative analysis of polymer/HA binding
5. Anti erosion test by phosphate block copolymer
6. Anti erosion test by phosphate block copolymer in presence of fluoride
7. SEM observation on the surface morphology of enamel

1. Controlled Synthesis of Hydrophobic Blocks

Typically, 10 mmol MMA, 0.25 mmol chain transfer RAFT agent (e.g. the chain transfer agent 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid) and 0.1 mmol AIBN were dissolved in 10 ml 1,4-dioxane. After purging with Argon for 1 h, the system was heated to 70° C. for a period of time. Gel permeation chromatography (GPC) was used to monitor the average macromolecular weight (Mn) of hydrophobic block. For example, Mn of polymethyl methacrylate (PMMA) can be well controlled using different monomer/CTA/initiator ratios as shown in FIG. 1.

2. Block Copolymer Synthesis

The synthesis of PMMA-b-PMOEP is shown in Scheme 1. Once the targeted Mn of PMMA segment was achieved, certain amounts of methacryloyloxyethyl phosphate (MOEP) in 1,4-dioxane was then injected into the system with syringe and the reaction was further allowed to continue for different reaction times. The composition of PMMA-b-PMOEP could be adjusted by using different feeding ratios and different polymerization times as shown in Table 1.

TABLE 1
Composition of PMMA-b-PMOEP using different
polymerization time and feeding ratios
	RAFT -chain			Number of	Number of
MMA	transfer	AIBN	MOEP	MMA in	MOEP in
(mol)	agent(mol)	(mol)	(mol)	copolymer	copolymer
100	2.5	1	50	17	9
100	2.5	1	50	19	14
100	2.5	1	80	17	14
100	2.5	1	80	20	35

The synthesis of PMMA-b-PAA by RAFT polymerization is shown in Scheme 2. Once the targeted Mn of PMMA segment was achieved, certain amounts of acrylic acid (AA) in 1,4-dioxane was then injected into the system with syringe and the reaction was further allowed to continue for different reaction times. The composition of PMMA-b-PAA could be adjusted by using different feeding ratios and different polymerization times as shown in Table 2. PAA stands for poly acrylate acid.

TABLE 2
Composition of PMMA-b-PAA using different feeding ratios
Molar ratios for monomers,
RAFT agent and initiatora
MMA	AA	RAFT agent	AIBN	Structure of block copolymer b
100	200	2.5	1	P(MMA)20-b-P(AA)19
100	500	2.5	1	P(MMA)18-b-P(AA)29
100	1000	2.5	1	P(MMA)17-b-P(AA)35

3. Polymer/Enamel Binding

The structure of block copolymer used in this test is P(MMA)₁₉-b-P(MOEP)₁₄. Before polymer treatment, the surface of bovine enamel was pre-conditioned by immersing the enamel in 1% citric acid solution (pH=3.8) for 5 min. Polymer solution with different concentrations (0.2 and 1.0 g/L) and different pHs (3.1, 4.2 and 7.0) were used to treat the bovine enamel surface for 5 min at 50 rpm. Then the treated surface was washed with phosphate buffer solution (pH=7.0) and acid solution (pH=3.8) for three cycles (5 min/cycle). The treated and etched enamel was characterized by FTIR spectroscopy after air dry. The FTIR spectra are shown in FIG. 2. The peaks at 1452, 1407, and 869 cm⁻¹ could be assigned to the existence of carbonated hydroxyapatite on the surface. The peak at 1730 cm⁻¹ could be ascribed to the characteristic absorption peak of C=0 in block copolymers. Both the effects of polymer concentration and pH on the binding were evaluated. When increasing the polymer concentration from 0.2 to 1.0 g/L, the relative intensity of peak at 1730 cm⁻¹ was increased, indicating higher polymer concentration could facilitate the binding efficiency. This could be ascribed to the strong interaction between phosphate groups in the block copolymer and the active site on enamel. Also, from the FIGS. 2a, 2c and 2e, when the pH is increased from 3.1 to 7.0 and the polymer concentration is kept constant as 1.0 g/L, less polymer could be adsorbed onto the enamel surface. The first and second dissociation constants, pK_al and pK_a2 for phosphoric acid are 2.12 and 7.21, respectively. The phosphate groups of the copolymer are believed to exist in the form of R—HPO₄⁻ and R—PO₄²⁻, where R stands for the polymer side chains attached to the backbone. The former moiety (R—HPO₄⁻) will be dominant over the latter one at the pH range (3.1-7.0) in this test. The phosphate block copolymer with negative charge could bind with the calcium domains on HA surface via electrostatic interaction. A lower pH value of the polymer solutions appears to facilitate the binding.

4. Quantitative Analysis of Polymer/Hydroxyapatite (HA) Binding

The structures of block copolymers used in this test are P(MMA)₁₉-b-P(MOEP)₉ and P(MMA)₁₇-b-P(AA)₃₅. Polymer solutions of 5 ml with different concentrations and different pH values were mixed with 100 mg HA powder for 2 h at room temperature. After centrifuging for 10 min at 10000 rpm, the solution was used tested by UV-vis spectroscopy. The absorbance of thiocarbonyl group (C═S) before and after binding were utilized to calculate the adsorbed polymer onto HA powder. The calibration curve was performed by using polymer solution with known concentrations. The UV spectra of phosphorylated or carboxylated block copolymer before and after binding are shown in FIG. 3-1 to FIG. 3-6. The calculated adsorbed polymer bound to HA is shown in FIG. 3-7. It can be seen that when the polymer concentration is gradually increased from 0.06 to 1.0 g/L, more and more polymer could be adsorbed onto the HA surface.

5. Anti Erosion Test of Phosphate Block Copolymer

The structure of block copolymer used in this test is P(MMA)₁₇-b-P(MOEP)₁₂ and P(MMA)₁₈-b-PAA₂₉. Atomic absorption (AA) spectrometry is one of the most reliable and sensitive methods on evaluating the dental erosion by monitoring the mineral loss. The typical testing procedure used was as follows. First, sintered hydroxyapatite (HA) discs were immersed in 1% citric acid (pH=2.5) for 15 min at room temperature, then soaked in water and sonicated for 30 min. HA discs were fixed on a 6 well plate by using KERR compounds. Note that only the top surface of HA was exposed to the solutions. After air drying, the fixed HA discs were challenged by 1% citric acid (pH=3.8) for 15 min at 37° C. with a shaking speed of 50 rpm. The solution was collected and the calcium concentration was designated as [Ca]_ref. The HA discs were washed with phosphate buffer solution (PBS, pH=7.0) and then treated with polymer solution (1 g/L) or PBS (as blank) for 2 min. After another washing with PBS, the HA was again challenged with citric acid for another 15 min. The solution was collected and the calcium concentration was measured by AA spectrometry [Ca]_treat. Because of the heterogeneity among HA samples, the relative calcium level (Ca level), calculated as the following equation (51), was utilized as an index to assess the protecting efficiency against acid erosion.

$\begin{array}{cc}\mathrm{Ca}\mathrm{level}=\frac{{\left[\mathrm{Ca}\right]}_{\mathrm{treat}}}{{\left[\mathrm{Ca}\right]}_{\mathrm{ref}}}*100\%& \mathrm{Equation}S1\end{array}$

The different polymer treatments on HA surface could influence the calcium level as shown in FIG. 4. The calcium level after phosphorylated polymer treatments with different polymer treating times was decreased from 91% for blank (non-polymer treated) to 50%, 48%, 34%, 17% for 0.5, 1, 2, or 5 minutes polymer treatment, respectively. The calcium level after carboxylated polymer treatments with different polymer treating time was decreased from 91% for blank (non-polymer treated) to 56%, 60%, 64%, 31% for 0.5, 1, 2, or 5 minutes polymer treatment, respectively. The possible reason is that the adsorbed polymer onto enamel/HA could form a protective layer and prevent the mineral from release.

6. Anti Erosion Test of Phosphate Block Copolymer in Presence of Fluoride

Since the fluoride ion is widely used in oral care to protect enamel against acid attack, phosphorylated or carboxylated block copolymers can greatly enhance the efficiency of this traditional treatment. Another anti erosion test based on phosphorylated or carboxylated block copolymer and fluoride was performed using the pH stat instrument. In this experiment, the HAP discs were immersed in 15 ml 0.3% citric acid solution (pH 3.8) for 15 minutes before and after 2-minute treatment. The treatments were polymer aqueous solution, or NaF aqueous solution or the mixture of polymer and NaF aqueous solution. The amount of the 10 mM HCl added over time to keep pH 3.8 was recorded. The % reduction (anti-erosion efficiency) is calculated as

$\left(1-\frac{{\left(\mathrm{acid}\mathrm{addition}/\mathrm{time}\right)}_{\mathrm{after}}}{{\left(\mathrm{acid}\mathrm{addition}/\mathrm{time}\right)}_{\mathrm{before}}}\right)*100.$

The corresponding results are shown in FIG. 5. It is clearly shown that the anti-erosion efficiency of the mixture of NaF and polymer is increased by 15-30% compared with the other treatments.

7. Surface Morphology

The protective layer that is formed on the enamel surface could prevent the mineral loss as indicated by previous data. This layer could also protect the surface morphology of enamel surface by obstructing the diffusion of external acid. Without any treatment, enamel could be easily etched by acid as shown in FIG. 6-1. When the surface was treated by phosphate block copolymer first, the surface morphology before and after acid erosion, the tooth surface was largely preserved as shown in FIGS. 6-2.

Claims

1. A block copolymer having at least one hydrophobic block and at least one hydrophilic block which are effective to bind to hydroxyapatite.

2. The block copolymer of claim 1, wherein the block copolymer is effective to protect the hydroxyapatite from loss of calcium by at least about 10 percent after exposure of the hydroxyapatite to the copolymer and subsequent exposure of the copolymer coated hydroxyapatite to citric acid.

3. The block copolymer of claim 1, wherein the hydrophilic block is selected from the group consisting of phosphonated block, phosphorylated block, carboxylated block, sulfate substituted block, amino substituted block, hydroxyl substitued block and mixtures thereof.

4. The block copolymer of claim 2, wherein the hydrophilic block is selected from the group consisting of phosphonated block, phosphorylated block, carboxylated block, sulfate substituted block, amino substituted block, hydroxyl substitued block and mixtures thereof.

5. The block copolymer of claim 4, wherein the block copolymer has a molecular weight in the range of from about 1,000 to about 1,000,000, individual hydrophilic blocks having a molecular weight in the range of from about 200 to about 1,000,000, and individual hydrophobic blocks having a molecular weight in the range of from about 200 to about 1,000,000.

6. The block copolymer of claim 4, wherein the hydrophilic blocks comprise from about 10 to about 90 weight percent of the block copolymer and the hydrophobic blocks comprise from about 10 to about 90 weight percent of the block copolymer.

7. The block copolymer of claim 4, wherein the block has a molecular weight and the polymers have a total molecular weight effective to provide a solubility in water of from about 0.001 to about 100 g/l.

8. The block copolymer of claim 4, wherein the block copolymer is selected from the group consisting of a diblock copolymer, a triblock copolymer and a multi-armed copolymer.

9. The block copolymer of claim 4, wherein the block copolymer has a molecular weight in a range of 1,000 to 1,000,000.

10. The block copolymer of claim 6, wherein the block copolymer has a molecular weight in a range of 1,000 to 10,000.

11. The block copolymer of claim 8, wherein the block copolymer has a molecular weight in a range of 1,000 to 10,000.

12. The block copolymer of claim 2, wherein the block copolymer has the structure of formula I or formula II:

where A is selected from the group consisting of (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for substituent A p, q=0, 1, 2, 3... 20; x=0, 1, y=0 or 1;

R1 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R1 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R2 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R2 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R3 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R3 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R4 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R4 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R5 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R5 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R6 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R6 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R7 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R7 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R8 is selected from the group consisting of an alkali metal, an ammonium, protonated alkyl amine, Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R8 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R9 is selected from the group consisting of an alkali metal, an ammonium, protonated alkyl amine, Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R9 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R10 is hydrogen, alkali metal or ammonium; and

m and n are each independently in a range from about 5 to about 3000.

13. The block copolymer of claim 12, wherein the block copolymer has a molecular weight in the range of from about 1,000 to about 1,000,000, individual hydrophilic blocks having a molecular weight in the range of from about 200 to about 1,000,000, and individual hydrophobic blocks having a molecular weight in the range of from about 200 to about 1,000,000.

14. The block copolymer of claim 12, wherein the hydrophilic blocks comprise from about 10 to about 90 weight percent of the block copolymer and the hydrophobic blocks comprise from about 10 to about 90 weight percent of the block copolymer.

15. An oral hygienic composition comprising: an orally acceptable carrier; and a block copolymer having at least one hydrophobic block and at least one hydrophilic block which are effective to bind to hydroxyapatite and the block copolymer is effective to protect the hydroxyapatite from loss of calcium by at least about 10 percent after exposure of the hydroxyapatite to the copolymer and subsequent exposure of the copolymer coated hydroxyapatite to citric acid.

16. The oral hygienic composition of claim 15, wherein the block copolymer has the structure of formula I or formula II:

where A is selected from the group consisting of (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for substituent A p, q=0, 1, 2, 3... 20; x=0, 1, y=0 or 1;

R1 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R1 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R2 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R2 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R3 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R3 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R4 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R4 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R5 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R5 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R6 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R6 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R7 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R7 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R8 is selected from the group consisting of an alkali metal, an ammonium, protonated alkyl amine, Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R8 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R9 is selected from the group consisting of an alkali metal, an ammonium, protonated alkyl amine, Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R9 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R10 is hydrogen, alkali metal or ammonium; and

m and n are each independently in a range from about 5 to about 3000.

17. A method for protecting tooth enamel from acid erosion, the method comprising:

applying a block copolymer to tooth enamel, the block copolymer having at least one hydrophobic block and at least one hydrophilic block which are effective to bind to hydroxyapatite.

18. The method of claim 17, wherein the block copolymer is effective to protect the hydroxyapatite from loss of calcium by at least about 10 percent after exposure of the hydroxyapatite to the copolymer and subsequent exposure of the copolymer coated hydroxyapatite to citric acid.

19. The method of claim 18, wherein the block copolymer has a molecular weight in a range of from about 1,000 to have 1,000,000.

20. The method of claim 18, wherein the block copolymer has a molecular weight in the range of from about 1,000 to about 1,000,000, individual hydrophilic blocks having a molecular weight in the range of from about 200 to about 1,000,000, and individual hydrophobic blocks having a molecular weight in the range of from about 200 to about 1,000,000.

21. The method of claim 20, wherein the hydrophilic blocks comprise from about 10 to about 90 weight percent of the block copolymer and the hydrophobic blocks comprise from about 10 to about 90 weight percent of the block copolymer.

22. The method of claim 18, wherein the block copolymer has the structure of formula I or formula II:

where A is selected from the group consisting of (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for substituent A p, q=0, 1, 2, 3... 20; x=0, 1, y=0 or 1;

R1 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R1 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R2 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R2 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R3 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R3 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R4 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R4 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R5 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R5 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R6 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R6 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R7 is selected from the group consisting of Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R7 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R8 is selected from the group consisting of an alkali metal, an ammonium, protonated alkyl amine, Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R8 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R9 is selected from the group consisting of an alkali metal, an ammonium, protonated alkyl amine, Ha, (CH2)p, (CH2CH2O)q, (phenyl), (C(═O)—O)y, or any combination thereof, where for R9 p, q=0, 1, 2, 3... 20; x=0, 1, y=0, 1, a=0, 1;

R10 is hydrogen, alkali metal or ammonium; and

m and n are each independently in a range from about 5 to about 3000.

23. The block copolymer of claim 2, wherein the hydrophobic block comprises a monomer selected from the group consisting of alkyl acrylate, styrene, olefin, a vinyl monomer, a fluoro monomer, acrylonitrile, and a combination of two or more thereof.