Accelerated Curing Speed of Ethyl α-Cyanoacrylate by Polymer with Catecholamine Groups

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* E-mail: [email protected] or [email protected] Received May 24, 2012; accepted July 5, 2012; published online XXXX, 2012. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc. 201200507 or from the author. Chin. J. Chem. 2012, XX, 1—6 © 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1

DOI: 10.1002/cjoc.201200507

Accelerated Curing Speed of Ethyl α-Cyanoacrylate by Polymer with Catecholamine Groups

Zhang, Fenga(张峰) Liu, Siwei*,a(刘四委) Zhang, Yia(张艺) Xu, Jiaruia(许家瑞) Wei, Yen*,b(危岩)

a PCFM Lab, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, Guangdong 510275, China

b Department of Chemistry, Tsinghua University, Beijing 100084, China

Four kinds of poly(ethylene glycol) (PEG) derivatives with the similar backbone and different side groups have been synthesized successfully. When both catecholamine and double bond are tethered to polymer backbone, i.e., the PEG backbone, simultaneously, the polymer can accelerate the curing speed of ethyl α-cyanoacrylate (commer-cially available as 502) greatly under the same conditions (the curing time of such system is no more than 5 s). Probably this is due to the autoxidation of catecholamines. Through the redox-cycling, catecholamines can produce, collect free radicals, and thus initiate the free radical polymerization. Due to the fast-curing of such material when mixed with α-cyanoacrylate, we could design and develop a new bicomponent super bioglue used in the dentistry or other bioenvironment requiring super fast settlement for further surgical operations.

Keywords catecholamines, dopamines, radical reactions, bioadhesion

Introduction It is well known that, anionic polymerization is a

kind of repetitive conjugate addition reaction, i.e., the “Michael reaction” in organic chemistry.[1] Generally speaking, the electron withdrawing groups (EWGs, such as ester, cyano) or groups with double bonds (phenyl, vinyl) are required to be linked with double bond di-rectly, so as to stabilize the propagating species by resonance. For initiation to be successful, the free en-ergy of the initiation step must be favourable. If the propagating anion is not very strongly stabilized, a powerful nucleophile is required as initiator. On the other hand, a rather weak nucleophile will be successful as initiator. Therefore, α-cyanoacrylates can be initiated easily even by water, a weak nucleophile, because the two EWGs linked on it are so effective in stabilizing anions. Since the two strong EWGs are attached to the same carbon atom of the double bond in α-cyano-acrylates, the electron cloud density is reduced greatly at the β carbon atom, leading to susceptibility to nucleo-philic attack. Moreover, the β carbon atom is sensitive to the nucleophile because of the absence of substitu-tional group(s). The curing mechanism of α-cyano-acrylates is widely accepted as anionic bulk polymeriza-tion.

Free radical polymerization is a method of polym-erization by which polymers formed by the successive

addition of free radical building blocks. Free radicals could be generated via a number of different mecha-nisms usually involving separate initiator molecules.[2] Following its generation, the initiating free radical adds monomer units, thereby growing the polymer chains.

The compounds would be prone to anionic polym-erization if strong EWG(s) is (are) linked to double bonds, especially when multiple EWGs are connected to double bond in an asymmetric manner. If the EWG tethered to the double bonds is weak, the vinyl com-pounds would be prone to free radical polymerization. Of course, it is well known that the vinyl compounds can be cationic polymerization if the electron donating groups (EDGs) attached to the double bond. We would like to say that the polymerizations of olefins with dif-ferent molecular structure have different polymerization characteristics.

The compounds containing phenolic hydroxyl and/or amino-group(s) are efficient free radical inhibitors. For example, hydroquinone is often used as stabilizing agent for vinyl compounds. Even in the typical anionic po-lymerization system, such as α-cyanoacrylates, hydro-quinone is often added as a stabilizer. Maybe this is the reason why a lot of reactions were carried out in sealed glass tube filled with inert gas for the vinyl compounds polymerization containing phenolic hydroxyl and/or amino-groups.[3]

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Recently we have synthesized a series of poly(ethyl-ene glycol) (PEG) derivatives. These polymers have the similar main chain, i.e., like PEG backbone. According to the different functional side groups joined to the back-bone, the polymers can be divided into four types: (I) both hydroxyl group and double bond are tethered to the main chain; (II) only catechol group is linked to the main chain; (III) both catechol group and double bond are connected to the main chain; (IV) only double bond as side group is attached to the main chain (Figure 1).

OO

OH

O

O

O

O

HN

O

OH

OH

OO

O

O

O

HN

O

OH

OH

O

O

I

III IV

m n

n

m n

n

II

Figure 1 The synthesized four kinds of poly(ethylene glycol) derivatives with the similar backbone and different side groups.

Previous reports have shown that the catecholamine is an important source of free radical production.[4] In the central nervous system, the generated free radicals could be inhibitable by ascorbic acid, dithiothreitol, glutathione, superoxide dismutase, and tetrahydrobiop-terin.[5] Catecholamines can autoxidize to form quinones in the following order: 6-hydroxy-dopamine, dopamine, norepinephrine and epinephrine.[6] The superoxide radi-cals in the striata of mice treated with methamphetamine are increased. Probably this is due to the release of do-pamine in the brain after methamphetamine administra-tion and subsequent dopamine oxidation with its termi-nals.[7] And further redox-cycling of dopaquinone formed during dopamine catabolism would enhance the concentration of oxygen-based free radicals with dopa-mine terminals.[8] The electron spin resonances investi-gation of 3,4-dihydroxyphenylalanine melanin revealed that the free radicals could be trapped by the stabilized semiquinonoid form of the polymeric pigment.[9] Finally, the trapped free radicals are consumed. Probably this is why the polymer containing dopamine moieties is an efficient autoxidation, which could be related with its interesting ability of scavenging reactive free radi-

cals.[10] A very interesting experimental result was found:

when the four kinds of the polymers were simply blended with 502 (super glue, with the main composi-tion as ethyl α-cyanoacrylate, ＞87 wt%) without inert gas protection respectively, only type III polymer accel-erated the curing speed of the mixture system while the other three mixtures had no obvious changes in curing process. Under the same condition, the curing speed of type III polymer with 502 is at least two times faster than that of 502 alone.

Experimental Materials

Triisobutylaluminum (1.0 mol/L in toluene, Aldrich), 2,3-epoxypropan-1-ol (glycidol, 96%, Aldrich) and ethyl vinyl ether (99% Aldrich) were used as received. Allyl glycidyl ether (AGE) purchased from Aldrich was dried over calcium hydroxide, distilled under vacuum, and treated with triisobutylaluminum for 15 min in a glass flask equipped with PTFE stopcocks to remove traces of impurities. It was finally distilled again under vacuum and stored under vacuum at room temperature in sealed glass tubes prior to use. Toluene was purified by treating with polystyryllithium followed by distilla-tion under vacuum and then stored in sealed glass tubes before use. Tetraoctylammonium bromide (98%, Al-drich) was dissolved into dried toluene and stored in glass tubes on a vacuum line fitted with PTFE stopcocks. Ethoxyethyl glycidyl ether (EEGE) was synthesized from glycidol and ethyl vinyl ether following the previ-ous reported procedures.[11] Upon drying over calcium hydroxide and distillation, the product was obtained in a yield of 90%. Dopamine hydrochloride, succinicanhy-dride, 1,3-dicyclohexylcarbodiimide (DCC), N-hydroxy-succinimide (NHS) and tris(hydroxymethyl) amino-methane hydrochloride (Tris-HCl) were purchased from Aldrich and used as received. The molecular weight was measured by Waters Alliance GPC2000.

Synthesis of the type I polymer The synthesis method was accomplished following a

modified literature procedure.[12] As a typical procedure, toluene (10.0 mL), EEGE (1.0 mL, 6.6 mmol), AGE (0.32 mL, 2.6 mmol), tetraoctylammonium bromide (0.035 g, 0.06 mmol) and triisobutylaluminum (0.26 mL, in 1.1 mol•L－1 toluene solution) were charged into a glass reactor equipped with a magnetic stirrer under argon. The reaction was carried out at 0 ℃ for 12 h. Then ethanol was added to terminate the reaction. The wax-like product was obtained in nearly quantitative yield after removal of the solvents at 50 ℃ under vac-uum. Then the obtained product (1.70 g, 0.077 mmol) was dissolved in 150 mL ethanol containing 3% (V/V) hydrochloric acid (0.054 mol•L－1). After stirring for 4 h at room temperature, the solution was neutralized by adding 55 mL of 1 mol•L－1 sodium carbonate aqueous


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solution. The reaction mixture was filtered, vacuum dis-tilled and dried at 50 ℃ for 12 h under vacuum to af-ford the polymer as a slightly yellow viscous liquid (1.24 g, 99.8% yield) (the type I polymer). Mn is 1.8×104 g/mol, with the polydispersity of 2.1. 1H NMR (300 MHz, CD3OD/TMS, δ) for type I polymer: 3.10 (2H×m, CHCH2O and 2H×n, CHCH2O), 3.67 (1H×m, CH2CHO and 1H × n, CH2CHO), 3.72 (2H × m, CH2OH), 3.82 (2H × m, CH2CHO and 2H × n, CH2CHO), 4.00 (2H×n, OCH2CH＝), 5.20 (2H×n, CH＝CH2), 5.88 (1H×n, CH＝CH2). FT-IR ν: 3500, 3010, 2864, 1600, 1330, 1285, 985, 868, 820 cm－1.

Synthesis of the type III polymer The synthesis of type III polymer was achieved by

using a procedure modified from previous reported pa-pers.[13] The type I polymer (3.90 g, where the amount of hydroxide radical was estimated to be ca. 29.7 mmol based on the 1H NMR integration of the EEGE to AGE ratio) and butanedioic anhydride (7.43 g, 74.3 mmol) were dissolved in 150 mL toluene in a round-bottom flask under argon. After the mixture was heated to re-flux for 6 h, the solvents were removed by rotary evaporation. The residue was dissolved in 80 mL me-thylene dichloride, precipitated in 500 mL precooled ether and filtered. Such a dissolution and precipitation process was repeated at least 5 times. The obtained pre-cipitate was dried at room temperature under vacuum for 12 h and dissolved in methylene dichloride (80 mL). To this solution, EDC (7.6 g, 40 mmol) and NHS (4.6 g, 40 mmol) were added under argon atmosphere. After reaction for 24 h at room temperature with stirring, the solvents were removed by vacuum distillation. The residue was dissolved in 50 mL absolute ethanol, pre-cipitated from precooled ether and filtered at least 5 times. The precipitate was dried under vacuum at room temperature for 12 h and then dissolved in 300 mL ab-solute ethanol. Dopamine hydrochloride (7.6 g, 40 mmol) and N-methylmorpholine (7.6 mL, 69 mmol) were added to the solution under argon atmosphere. Af-ter reaction for 2.5 h at room temperature with constant magnetic stirring, the reaction mixture was concentrated to about 20 mL by vacuum distillation. The crude white product was washed with precooled ether several times. After dialysis for 2 d (pH＝3—4, MWCO＝3000 Da) in ultrapure water at room temperature and lyophilization, 4.5 g of the final product, the type III polymer, was ob-tained as a slightly brown viscous liquid in 47% yield after dried at 50 ℃ under vacuum. 1H NMR (300 MHz, CD3OD/TMS, δ) for type III polymer: 2.48 (2H×m, CH2COO), 2.63 (2H×m, CH2CONH), 2.80 (2H×m, CH2C6H3), 3.02 (2H×m, CH2NH), 3.10 (2H×m, CHCH2O and 2H × n, CHCH2O), 3.67 (1H × m, CH2CHO and 1H × n, CH2CHO), 3.82 (2H × m, CH2CHO and 2H × n, CH2CHO), 4.00 (2H × n, OCH2CH＝), 5.20 (2H×n, CH＝CH2), 5.88 (1H×n, CH＝CH2), 6.50—6.74 (3H×m, C6H3(OH)2). FT-IR ν: 3495, 2935, 2874, 2776, 1706, 1652, 1525, 1459, 1369,

1287, 1233, 1114, 1080, 991, 868, 816 cm－1.

Synthesis of the type II polymer The procedures are similar to method used in syn-

thesis for type I and III polymer. The yield is about 49%. Mn is 3.2×104 g/mol, with the polydispersity of 1.7. 1H NMR (300 MHz, CD3OD/TMS, δ) for type II polymer: 2.50 (2H×m, CH2COO), 2.61 (2H×m, CH2CONH), 2.82 (2H×m, CH2C6H3), 3.03 (2H×m, CH2NH), 3.08 (2H×m, CHCH2O), 3.66 (1H×m, CH2CHO), 3.80 (2H×m, CH2CHO), 6.50—6.74 (3H×m, C6H3(OH)2). FT-IR ν: 3485, 2960, 1710, 1654, 1530, 1460, 1370, 1290, 1240, 1080, 991, 870 cm－1.

Synthesis of the type IV polymer The procedures are similar to method used in syn-

thesis for type I polymer. The yield is about 97%. Mn is 3.7×104 g/mol, with the polydispersity of 2.5. 1H NMR (300 MHz, CD3OD/TMS, δ) for type IV polymer: 3.12 (2H×n, CHCH2O), 3.62 (1H×n, CH2CHO), 3.80 (2H×n, CH2CHO), 4.00 (2H×n, OCH2CH＝), 5.23 (2H×n, CH＝CH2), 5.86 (1H×n, CH＝CH2). FT-IR ν: 2985, 2880, 1585, 1340, 1290, 980, 860, 824 cm－1.

The curing experiments Firstly, we want to say here that, we do have done

the experiments by using the four kinds of polymers with pure ethyl α-cyanoacrylate. Such mixtures have the same trends in curing speed with that of the polymers with 502. Therefore, we choose 502 instead of the pure ethyl α-cyanoacrylate to carry out our experiments (by using the pure ethyl α-cyanoacrylate with these synthe-sized polymers, the curing speed of all the mixtures is faster than that using 502). The curing experiments were conducted as follows: under room temperature without inert gas protection, 502 (0.8 g) was dropped onto two glass slides simultaneously. On each of the glass slide, the synthesized polymer (0.4 g) was added to the 502 and stirred immediately. The curing time of the system was defined as the time from adding polymer into 502 to the mixture could not be stirred. The curing time ob-tained by such method is 3 s and 10 s on average for the mixture of the type III polymer with 502 and other mixtures, including 502 alone, respectively.

Results and Discussion As it is well known, the phenolic compounds such as

hydroquinone and 4-tert-butyl-1,2-dihydroxybenzene are very effective inhibitors, often used as stabilizer in active vinyl compounds to eliminate the trace of free radicals and block the polymerization. According to this under-standing, we believe that the type II and III polymers should block or slow down the polymerization speed (curing speed) of the systems mentioned above, at least do not accelerate the curing speed (if the bulk polymerization is carried out in free radical polymerization manner).

To our surprise, we found that the type III polymer was not only without inhibition or slowed down the cur-



ing speed when it was mixed with 502, but accelerated the curing speed instead. This is because that, for one thing, if the bulk reaction is carried out in free radical polymerization, type II and III polymers should extend the curing time of the mixtures; for another, if carried out in anionic polymerization, all of the four kinds of poly-mers should not significantly affect the curing time. The curing speed had been accelerated at least more than two times faster than that of 502 alone. However, through the control experiments we found that the type I, II and IV polymers had nothing to do with the curing speed as they blended with 502 respectively.

The four polymers have the similar backbone: like PEG main chain. The only difference is the functional side groups tethered to the backbone. The side groups can be divided into three types: hydroxyl group, double bond and catechol group. Double bond is connected to the PEG backbone through an ether linkage. But the ether group is not directly linked to double bond. There is a methylene between the double bond and ether group. So the inductive effect of the oxygen atom on the dou-ble bond is not very strong. Because of the weak elec-tron withdrawing ability of the ether group, olefins as type I, III and IV polymers are prone to be initiated as free radical polymerization, rather than the anion or cation polymerization. In our experiments, the polymer could accelerate the curing speed significantly while blended with 502 only if both catechol group and dou-ble bond are tethered to the main PEG chain simultane-ously, as type III polymer. However, the polymers with other combinations, as type I with both hydroxyl group and double bond, type II with catechol group and type IV with double bond only, do not speed up the curing speed while they blended with 502 respectively.

According to the previous reports, we can deduce that dopamine (catecholamine) has three main features related with free radicals: (1) catecholamine can pro-duce free radicals through the process of autoxidation; (2) catecholamine, due to its stabilized semiquinonoid form, can be considered as a trap to enrich free radicals. However, the trapped free radicals, with certain stability, will not be consumed immediately; (3) catecholamine has the ability of free radical scavenging. The free radi-cals will be eliminated by catecholamine and/or other reagents or other methods finally.

According to this argument, we may explain why only type III polymer could accelerate the curing speed with 502, while the other three kinds of polymers do not. For the type III polymer, it can generate free radicals during the redox cycling due to the changes of the do-pamine side groups, i.e., the autoxidation process. Dur-ing such oxidation process, the stabilized intermediated semiquinonoid could act as a trap to enrich free radicals. It should be noted that there are lots of double bonds tethered to the PEG backbone. Because of pedesis of the side groups/polymer, the double bonds may have the opportunity to be very close to the enriched free radical centers. So, the free radical polymerization of the poly-

mer (as a macromonomer) would be initiated. Although α-cyanoacrylates are very prone to proceed with anionic polymerization, they do have the ability to be polymer-ized by free radical polymerization.[14] Therefore, we believe that the accelerated curing speed of mixture system by the type III polymer with 502 is due to the free radical polymerization between α-cyanoacrylates and macromonomers (the type III polymer) (Figure 2). Because of the high molecular weight and multiple double bonds of the type III polymer (Mn is 2.2×104 g/mol, with the polydispersity of 1.8 and the molar ratio of double bonds is about 29%), crosslinked product would be generated, thus leading to accelerated curing speed for this system. By comparing the 1H NMR spec-tra of the mixed system curing products with the 502-curing product stand alone, we find there are some small peaks ranging from δ 3.4 to 3.9 (the curing prod-ucts were extracted by ethanol before measured by 1H NMR. The course of dissolution was very slow, longer than 24 h, Figure 3). We believe that these peaks come from the methylenes in the backbone of the type III polymer (PEG backbone), indicating that some of the polymers are tethered to poly(ethyl α-cyanoacrylate) through chemical bonds. However, the curing process of such mixtures is dominated by anionic polymerization. Only the curing speed is accelerated through little amount free radical polymerization, the bulk polymeri-zation is anionic.

For the type II polymer, under the same condition as type III, it can not accelerate the curing speed with 502, although it has the same functional side group as type III polymer, catecholamine. The catecholamine groups may produce free radicals through autoxidation and even enrich free radicals around it. However, there are no double bonds prone to be initiated as free radical polymerization. Therefore, we do not find any acceler-ated curing speed while this polymer blended with 502. The 1H NMR spectrum of the curing product (type II polymer blended with 502) is identical to that of 502-curing product.

It is relatively easy to understand why the other two polymers, type I and IV, do not accelerate the curing speed with 502 respectively. There are no functional side groups as catecholamines in these kinds of poly-mers. Therefore, there are no free radicals generating sources, and no areas/points to trap free radicals. Thus, there are no enriched free radical areas/points. Although there are many double bonds prone to free radical po-lymerization, these double bonds are difficult to be ini-tiated for free radical polymerization due to the absence of enough free radicals (there are some hydroquinone in 502. So, in the induction period, some free radicals would be consumed by inhibitors and/or retarding agents). We did not find any differences in 1H NMR spectra of the curing products either (type I and IV blended with 502 respectively vs. 502-curing product).

In fact, we also found that the type III polymer could turn from semi-liquid to solid if it was preserved for a


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Figure 2 Redox cycling of type III polymer. Dopamine can form quinone, and can also undergo redox cycling with generation of free radi-cals (A). Moreover, the formed stabilized semiquinonoid can enrich free radicals. Thus the free radical polymerization would be initiated (B).

long time under nitrogen in refrigerator (the preservation condition is not extremely strict), indicating there was some degree of crosslinking (this was verified by its solubility and 1H NMR spectra). Furthermore, we did some small additional experiments to validate our speculations. When a catecholamine, 4-(2-aminoethyl) benzene-1,2-diol hydrochloride, was added into 502, there was no accelerated curing process happened. It looked like that the hydrochloride (white powder) dis-solved in 502. However, if we put both the hydrochlo-ride and methyl methacrylate together into 502, the cur-ing speed of the mixture is slightly accelerated. But the curing speed is much slower than that of the blended system of type III polymer with 502. Maybe this is something related to the molecular weight of vinyl monomers: for type III polymer with 502, once the free radical polymerization initiated, the molecular weight

will be sharply increased, maybe forming a crosslinked polymer and resulting in fast curing speed.

Conclusions In summary, we found that a series of polymers, with

poly(ethylene glycol) as backbone, double bond and catecholamine as side groups, can accelerate the curing speed of ethyl α-cyanoacrylates greatly. The control experiments show that the catecholamine and double bond play a very important role for the accelerated cur-ing speed, and both of them are indispensable. The backbone, in our view, should have no decisive influ-ence on such accelerated curing process. The cate-cholamine may produce and collect free radicals. Per-haps the enriched free radicals initiate the free radical polymerization of the double bonds in the synthesized



Figure 3 The curing product 1H NMR of 502 with type III polymer and 502 alone in DMSO-d6 respectively. The curing products were extracted by ethanol for 24 h before using.

polymer with α-cyanoacrylates. Although only a mini-mal amount of substances was initiated by free radical reaction, such polymerization process could lead to dramatic changes in molecular weight of the cured prod-ucts, resulting in fast-curing. Due to the fast-curing of such materials when mixed, we could design a bicom-ponent super bioglue used in dentistry or other bioenvi-ronment requiring super fast settlement for further sur-gical operations.

Acknowledgement This work was supported by the National Natural

Science Foundation of China (Nos. 50903096, 21134004), the Department of Science Technology of Guangdong Province (No. 2008B090500196), and the Fundamental Research Funds for the Central Universi-ties.

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Accelerated Curing Speed of Ethyl α-Cyanoacrylate by Polymer with Catecholamine Groups

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