Controlled activity polymers. VI

Journal of Controlled Release, 7 (1988) 101-108 Elsevier Science Publishers B.V., Amsterdam - Printed

CONTROLLED

ACTIVITY

POLYMERS.

101

in The Netherlands

VI

ACRYLAMIDE COPOLYMERS WITH STRUCTOPENDENT NAPHTHYLACETIC AND INDOLEACETIC ACID ESTERS: SYNTHESIS AND CHARACTERIZATION Charles

L. McCormick

and KiSoo Kim

Department

of Polymer Science,

University

of Southern

Mississippi,

Hattiesburg,

MS 39406

ACID

(U.S.A.)

(Received June 20, 1987; accepted in revised form October 2 1, 1987)

Auxin-containing monomers, 2- (1 -naphthylacetyl) ethyl acrylate (NAEA), 2- (1 -naphthylacetyl) butyl acrylate (NABA), 2-(I-naphthylacetyl) ethyl methacrylate (NAEMA), and 2-(indole-3-acetyl) ethyl acrylate (IAEA) were prepared utilizing a 4-pyrrolidinopyridine catalyst and dicyclohexylcarbodiimide. Copolymers of auxin-containing monomers with acrylamide (AM) were obtained by solution polymerization. Intrinsic viscosities, number-average molecular weights, and residual monomer contents were measured; molecular weights were found to be in the range 5.5-18.0~ 104. Copolymer compositional data were used to calculate reactivity ratios; the values obtained indicated random copolymerization behavior. The copolymer microstructures werepredieted utilizing statistical methods. These model structures are utilized for the assessment of structure/hydrolysis relationships in a subsequent paper.

INTRODUCTION

Polymeric bioactive agents are of great utility in the controlled release field since they may be tailored for specific applications. Although a large number of polymeric bioactive materials have been described, few have been thoroughly characterized as to macromolecular structure, mechanism of activity, kinetics of release, and biological activity [ 1 ] . Previous work in our laboratory has concerned copolymerization of herbicide-containing monomers with hydrophilic comonomers [l-4]. In particular, work centered on structural tailoring of copolymers containing metribuzin, a highly active triazine compound. As a continuation of this research interest we now report the synthesis and characterization of auxin-containing monomers and auxin mono-

0168-3659/88/$03.50

0 1988 Elsevier

Science Publishers

mer/acrylamide copolymers for evaluation as controlled release materials. Auxins are one class of plant growth regulators which act at exceptionally low levels, making them ideal candidates for controlled release studies [5]. Synthesis and characterization of auxin monomers and copolymers based on naphthaleneacetic acid and indoleacetic acid are described in this report; release properties of these materials are presented in the accompanying paper (J. Controlled Release, 7 (1988) 109).

EXPERIMENTAL Materials

1-Naphthylacetic acetic acid (IAA)

B.V.

acid (NAA) and indole-3from Sigma Chemical Co.

102

were recrystallized from deionized water and vacuum dried at 50 oC to give melting points of 135-136°C and 168-17O”C, respectively. Acrylamide (AM), reagent grade from Aldrich Chemical Co., was recrystallized from acetone and vacuum dried at room temperature ( m.p. 83.5-84.5° C ) . 2Hydroxyethyl acrylate (HEA), 4-hydroxybutyl acrylate (NBA) , and 2-hydroxyethyl methacrylate (HEMA), obtained from Polysciences, Inc., were distilled under reduced pressure (HEA at 0.25 mmHg, b.p. 43’ C; HBA at 0.5 mmHg, b.p. 71 oC; HEMA at 0.75 mmHg, b.p. 67°C) with hydroquinone prior to use. Azobisisobutyronitrile ( AIBN) was recrystallized twice from methanol. All other chemicals were reagent grade and were used without further purification. Solvents for polymerization and characterization were purified by distillation. Synthesis of 2-(1 -naphthylacetyl) rylate (NAEA) 2- (1-Naphthylacetyl)

ethyl

Anal. Calcd for C17H1604: C, 71.84; H, 5.63. Found: C, 71.80; H, 5.94. IR (neat) : aliphatic C-H, 2940; ester C=O, 1730; CH,=C, 1617,1633; aromatic (naphthyl) C=C, 1510,1579 cm-‘. ‘H NMR spectrum (DMSO-4): 6 4.14 (s, 2H, -CH,-naphthyl), 4.30 (s, 4H, -OCH&H20-), 5.80-6.30 (m, 3H, CH,=CH-) , 7.20-8.10 ppm (m, 7H, naphthyl group hydrogens) . 13C NMR spectrum (neat) (see also Fig. 1): 6 39.5 (s, -CH,-naphthyl) , 63.2 (s, -OCH,CH,O-), 125.0-133.0 (m, naphthyl group carbons), 133.2 (s, CH,=CH-), 134.9 (s, CH,=CH-) ,166.4 (s, -0-CO-CH,-naphthyl), 171.9 ppm (s, CH2= CH-CO-O-) .

Syntheses of 4-(1 -naphthylacetyl) butyl acrylate - (NABA), 2-(1 -naphthylacetyl) ethyl methacrylate (NAEMA), and 2-(indole-3acetyl) ethyl acrylate (IAEA)

ac-

ethyl acrylate (NAEA) was prepared by esterification of lnaphthylacetic acid (NAA) with 2-hydroxyethyl acrylate (HEA) utilizing a low-temperature esterification procedure modified from those reported in the literature [ 6,7]. To a stirred solution of 95.2 mmol NAA in 400 ml anhydrous ethyl ether were added 8 mmol 4pyrrolidinopyridine ( PPY) and 100 mmol HEA. Dicyclohexylcarbodiimide (DCC, 100 mmol ) was added to the reaction mixture, which was then stirred for 24 h at room temperature. N,N-Dicyclohexylurea was filtered and the filtrate was washed with water (1 x 300 ml ) , 5% acetic acid solution (3 x 300 ml), 5% sodium bicarbonate solution (3x300 ml), and water ( 1 x 300 ml), and then dried over MgSO,. The solution was filtered on a short silica column to remove particulates and the solvent was removed by evaporation to give the ester. Upon drying under vacuum, a pale yellow liquid was obtained in a 95% yield.

The syntheses of NABA, NAEMA, and IAEA followed the same procedure used for the NAEA preparation (Scheme 1). Physical data are given below for each of the synthesized monomers.

4- (1 -Naphthylacetyl) butyl acrylate (NABA): Anal. Calcd for C9HZ004: C, 73.04; H, 6.41. Found: C, 73.09; H, 6.64. IR (neat): aliphatic C-H, 2940; ester C=O, 1730; CH,=C, 1640; aromatic (naphthyl) C=C, 1510, 1597 cm-‘. ‘H NMR spectrum (CDCl,): 6 1.63 (s, 4H, -OCH,CH,C&CH,O-), 3.97 (d, 6H, -CH, -naphthyl and -OCH&H2CH2CH20-), 5.706.50 (m, 3H, CH,=CH-), 7.0-8.0ppm (m, 7H, naphthyl group protons). 13C NMR spectrum (neat) : 6 26.1 (s, -OCH,CH,CH,CH,O-) 39.8 -CH,-naphthyl), 65.1 (s, -OCH,CH,(s, CH,CH,O-), 124.9-132.1 (m, naphthyl group carbons), 133.2 (s, CH,=CH-) ,134.g (s, CH,= CH-), 166.5 (s, -0-CO-CH,-naphthyl), 171.9 ppm (s, CH,=CH-CO-O-) .

103

RCOOH

+

DCC

+

f

R-C-O-C-R

fl

,

PPY 1

R-!-f&g

+ R-F-6

l 9

/R *

R’= - (CH,),OC$

L

R’OH

+

CH, n-

2,

R*z

H, CH,

Scheme 1. PPY-catalyzed

4

DCC method for the esterification

2-(I-Naphthylacetyl) ethyl methacrylate (NAEMA): Anal. Calcd for C1sHi804: C, 72.48; H, 6.04. Found: C, 73.44; H, 6.38. IR (neat): aliphatic C-H, 2940; ester C=O, 1730; CH,=C, 1636; aromatic (naphthyl) C=C, 1510, 1597 cm -‘. ‘H NMR spectrum (CDCI,): 6 1.80 (s, 3H, CH,=CCH,-), 4.04 (s, ZH, -CH2- naphthyl), 7.0-8.0 ppm (m, 7H, naphthyl group protons). 13CNMR spectrum ( neat) : 6 18.9 (s, CH,=CCH,-) , 39.5 (s, -CO-CH,-naphthyl), 63.2 (s, -OCH&H20-), 124.8-133.5 (m, naphthy1 group carbons), 134.9 (s, CH,=CCH,-), 136.9 (s, CH,=CCH,-), 167.4 (s, -0-CO-CH,naphthyl), 171.8 ppm (s, CH,=CCH,-CO-O-).

2-(Indole-3-acetyl)

ethyl acrylate (IAEA):

Anal. Calcd for CisH,,N04: C, 65.93; H, 5.49; N, 5.12. Found: C, 65.65; H, 5.93; N, 5.51. IR (neat) : N-H stretching, 3400; ester C=O, 1725; CH,=C, 1620, 1633; aromatic (indolyl) C=C, 1510,158O cm-‘. ‘H NMR spectrum (CDC&) : 6 3.70 (s, ZH, -CH,-indolyl), 4.38 (s, 4H, -OCH,CH,O-), 5.57-6.23 (m, CH,=CH-), 6.947.55 ppm (m, 6H, indolyl group protons). 13C NMR spectrum (neat): 6 32.0 (s, CH,-indolyl), 63.4 (s, -OCH2CH20-), 108.0- 128.8 (m, indolyl group carbons), 132.4 (s, CH,=CH-) ,

reaction.

137.5 (s, CH,=CH-CO-O-), 167.0 (s, -O-COCH,-indolyl), 173.3 ppm (s, CH,=CH-CO-O-).

Poly(auxin

monomer-co-acrylamides)

The copolymerizations of auxin monomers, NAEA, NABA, NAEMA, and IAEA, with acrylamide (AM) were conducted at 65°C in dimethylformamide (DMF) solution using AIBN as the initiator. The initiator concentrations were 0.25-1.00 mol%. The concentration of comonomers in solution was 0.8 mol/l. Table 1 lists reaction parameters for the copolymerizations of auxin monomers with AM. Series of the copolymers of 2- (1-naphthylacetyl) ethyl acrylate ( NAEA) with acrylamide (AM) were prepared under the reaction conditions shown in Table 2 to obtain copolymers with varying auxin contents and different molecular weights. AIBN and potassium persulfate were used as the initiators and DMF and dimethylsulfoxide (DMSO) were used as polymerization solvents. A typical reaction procedure for the preparation of NAEA (22.5) -AM is described below. In 50 ml of freshly distilled DMF, 2.27 g (8 mmol) NAEA and 2.27 (32 mmol) AM were dissolved and placed in a lOO-ml three-neck

104

TABLE 1 Reaction parameters

for the copolymerizations

of NAEA, NABA, NAEMA, and IAEA with AM at 65’ C in DMF

Copolymer sample

Feed molar ratio ( Auxin monomer: acrylamide)

AIBN (mol% )

Reaction time (h)

Conversion (%)

Auxin monomer in copolymer (mol%)

NAEA( 22.5-AM) NABA (18.0) -AM NAEMA (20.5) -AM IAEA(18.4)-AM

NAEA:AM = 28 NABA:AM = 2:8 NAEMA:AM = 2:8 1AEA:AM = 2:8

0.25 1.00 1.00 1.00

4 6 6 5

63.0 83.4 83.2 37.1

22.5 18.0 20.5 18.4

TABL,E 2 Reaction parameters for the copolymerization differing auxin content and molecular weight Copolymer sample

Monomer ratio (M,:M,)

NAEA NAEA NAEA NAEA NAEA NAEA NAEA NAEA NAEA

1:9 2:8 3:7 5:5 7:3 3:7 3:7 3:7 3:7

(9.7) -AM (22.5) -AM (30.9) -AM (48.3) -AM (70.0) -AM (30) -AM-A (30) -AM-B (30) -AM-C (30) -AM-D

feed

of 2- (I-naphthylacetyl)

Solvent

Initiator (mol%)

DMF DMF DMSO DMF DMSO DMF DMSO DMSO DMSO

AIBN AIBN K,S,O, AIBN K,S,Os AIBN K&O, K&O, DMSO

flask equipped with a nitrogen inlet tube, a rubber septum, and a condenser connected to a water trap. This mixture was purged with oxygen-free nitrogen for 15 min. Initiator dissolved in DMF was injected through a rubber septum. After a designated reaction time with continuous stirring, the resulting polymer was precipitated into 1000 ml anhydrous diethyl ether which contained a small amount of hydroquinone. The polymer was then dried under vacuum at room temperature. Conversion was determined gravimetrically. The polymer was further purified by reprecipitation from DMF solution using ethyl ether. Characterization

methods

Elemental analyses for carbon, hydrogen, and nitrogen were performed by M-H-W Labora-

ethyl acrylate with acrylamide

and cont.

(0.25 ) (0.25) (0.1) (0.5) (0.5 1 (0.25) (0.5) (0.1) (0.5)

for copolymers

of

Reaction temp. (“C)

Reaction time (h)

Conversion (%)

65 65 40 65 40 65 40 40 40

6 4 24 4 2 4 24 24 24

75.3 63.0 63.0 51.7 36.8 57.6 85.1 63.0 38.6

tories of Phoenix, AZ. The error in determination for each element was reported to be t 0.2%. Viscosity measurements of copolymers were performed in DMSO at 30” C using a Ubbelohde dilution capillary viscometer. The intrinsic viscosities were obtained by extrapolating the reduced viscosities and the inherent viscosities to zero concentration. Membrane osmometry measurements were performed with a Knauer Membrane Osmometer in N,N-dimethylacetamide (DMAC) at 465°C utilizing a GOOW-type membrane from Arro Laboratories, Inc. IR spectra were recorded with a PerkinElmer 283B grating infrared spectrophotometer. UV spectra were obtained with a PerkinElmer 320 spectrophotometer. ‘H NMR data were recorded using a Varian EM-360 spec-

105 TABLE 3 Intrinsic

viscosity,

molecular weight, and residual monomer

content

Copolymer sample

Auxin monomer in polymer (mol%)

[VI

NAEA(22.5)-AM NABA (18.0) -AM NAEMA (20.5) -AM IAEA(18.4)-AM NAEA (9.7) -AM NAEA(22.5)-AM NAEA (30.9) -AM NAEA (48.3 ) -AM NAEA (70.0) -AM NAEA (30) -AM-A NAEA (30) -AM-B NAEA (30) -AM-C NAEA (30) -AM-D

22.5 18.0 20.5 18.4 9.7 22.5 30.9 48.3 70.0 30.3 30.2 30.9 34.1

0.29 0.19 0.20 0.14 0.17 0.29 0.44 0.18 0.37 0.20 0.37 0.44 0.58

of auxin-containing

copolymers DP”

M”

Residual NAEAb content (mol%)

x 10-4

5.1 4.6 4.9 3.4

370

5.1 9.0

370 670

5.5 7.0 9.0 18.0

400 520 670 1300


“DP = degree of polymerization. bMol% of incorporated NAEA.

/ t? B f&=;H-y-$H,;H,-O;-SH,

\

-

8

\ , 6

3

I

Fig. 1. ‘“C NMR spectrum

I 150

I 100

I

of NAEA monomer

(neat).

I

I 50

I

I 0

106

tnt

DMSO

h2.7

150

200

Fig. 2. “‘C NMR spectrum

I

I

I

wm

of NAEA-acryl~ide

copolymer,

I 0

I

50

100

in DMSO.

TABLE 4 Structural

data for the auxin-confining

Copolymer

copolymers Blockiness

(theoretically

(mol% )

calculated by Q and e values) Alternation (mol% ) MI-&.

Mean seq. length FM,

ih

MI-M,

MY-&

0.98

81.38

17.65

1.12

9.33

4.20

64.08

31.72

1.27

5.04

48.29 22.13

43.12 49.55

1.40

3.24

NAEA (48.3) -AM

8.59 28.33

NAEA (70.0) -AM NAEMA (20.5) -AM

51,07 6.55

8.27 41.53

40.66 51.91

2.10 3.58 1.25

1.93 1.40 2.61

NAEA

(9.7) -AM

NAEA(22.5)-AM NAEA (30.9) -AM

M, = NAEA or NAEMA and M, =

AM.

107

trometer and 13C NMR spectra were obtained at 22.5 MHz on a JEOL F-90 Q spectrometer using lo-mm tubes. Solid-state 13C NMR spectra were obtained at 50.32 MHz on a Bruker MSL 200 NMR spectrometer utilizing the CP/ MAS technique. The amount of residual monomer contained in the purified polymers was determined using a dialysis/LC method and DMSO.

RESULTS AND DISCUSSION Copolymer

macrostructure

and properties

Homopolymers of auxin-containing monomers would not be expected to give adequate release rates due to their highly hydrophobic Copolymers with acrylamide were nature. therefore prepared in order to enhance polymer hydrophilicity and release rates. It has been shown that hydrolysis rates of the pendent bioactive moiety can be modified by changing the nature of the susceptible bond and/or its distance from the macromolecular backbone. Chemical structure, molecular weight, molecular weight distribution, hydrophilicity, and physical dimensions may be altered, although it is often difficult to discern the effect of each parameter separately on release rates. In order for an appropriate comparison of the effects of molecular structure on release behavior, auxin monomers were synthesized with differing spacer lengths, bulkiness near the vinyl double bond, and degrees of hydrophobic character. These were copolymerized with acrylamide (AM ) ; the reaction parameters are shown in Table 1. Auxin monomer content in the polymers was measured by UV spectroscopy. Since naphthyl and indolyl moieties are strong UV chromophores (A,,, = 283.5 nm, E,,, = 6700 for NAA and A,,,= 284 nm, E,,,= 6500 for IAA) , these systems are easily analyzed both for copolymer composition and released auxin concentration. The copolymers for this study show an auxin monomer content of 20 2 2 mol% (Ta-

ble 1). Intrinsic viscosities and molecular weights were obtained. The latter ranged from 3 x lo4 to 5 x 104, which correspond to degrees of polymerization from 250 to 400. Residual monomer content was below 0.1 mol% as measured by RPLC (Table 3). Table 2 shows the reaction parameters for preparing two series of the copolymers of differing auxin content and molecular weight. The influences of both have been previously noted [3,8] but the effects on release were not assessed quantitatively. The auxin monomer content in the polymer was varied from 9.7 mol% to 70.0 mol% by changing the monomer feed ratios in coplymerizations. Table 3 lists polymer properties of the copolymers. NAEA ( 30 ) AM-A has the lowest molecular weight, 5.5 x 104, and NAEA (30) -AM-D the highest molecular weight, 1.8 x 105, corresponding to degrees of polymerization of 400 and 1300, respectively. The structure of the NAEA-AM copolymer was also verified by 13C NMR. Assignments are given in Fig. 2; a 13C spectrum of NAEA monomer is shown for comparison in Fig. 1. Copolymer

microstructure

The influence of copolymer microstructure on the hydrolysis properties of a polymer system is important in designing materials for controlled release technology. The reactivity ratios, rland r,,of NAEA ( M, ) and AM ( M, ) were determined experimentally by copolymerization studies. The Fineman-Ross [ 91 and the Kelen-Tudos methods [lo] were employed, and yielded values of rl= 1.10 and r2= 0.93.The NAEA-AM comonomer combination (rl and r2 both near 1 and r,r,= 0.94) possesses a random tendency in copolymerization. Statistical microstructure of NAEMA (M, )-AM (M,) was calculated from the theoretical reactivity ratios calculated by the Q-e scheme [ 11,121. Q and e values of NABA and IAEA were assumed to be same as that of NAEA (&=0.78 and e= 0.95) due to their similar molecular structures. Q and

108 e values of NAEMA were assumed to be similar tothoseofHEMA (&=0.93ande=0.40) [13]. The calculation of the statistical distribution of monomer sequences, Mi-M1, M2-M2, and M,-M, was performed utilizing the method of Igarashi [ 141. Monomer sequence lengths, p1 and ,u2,were also calculated from the reactivity ratios of the comonomer pairs [ 151. The calculated structural data for the NAEA-AM copolymers with a series of comonomer ratios and NAEMA-AM are listed in Table 4. The mean sequence length of acrylamide, ,uAM,varies from 9.33 at a 9.7/90.3 mole ratio of NAEA/AM in the copolymer to 1.40 at a 70.0/30.0 mole ratio. For those copolymer compositions, values of ,uNA~Awere 1.12 and 3.58, respectively. The NAEMA-AM copolymerization shows a tendency to form longer hydrophobic blocks as compared to the NAEA-AM copolymer of approximately the same composition (Table 4). Hydrophilicity of these polymers depends on microstructural features. Longer sequences of hydrophilic copolymer should enhance release performance. Correlations of microstructural features with release performance are presented in the accompanying paper of this series.

behavior of the auxin monomer with acrylamide. This point is particularly important, since the relative degree of hydrophilicity of these copolymers is governed by copolymer microstructure. The molecular weight, copolymer composition, and microstructural data of this report are employed to assess structure hydrolysis (release) property relationships in the accompanying paper of this series. REFERENCES 1

2 3

CONCLUSIONS

Auxin-containing copolymers of naphthaleneacetic acid and indoleacetic acid have been prepared to assess the effects of polymer structural characteristics on the rate of auxin release. These structural characteristics include variation of monomer structure, copolymer composition, and molecular weight. Copolymer microstructure was shown to be controllable via choice of the appropriate copolymer composition, a result of the random copolymerization

9 10 11 12

13 14 15

C.L. McCormick, K.W. Anderson and B. Hutchinson, J. Macromol. Sci., Rev. Macromol. Chem. Phys., C22(1) (1982) 57-87. K.W. Anderson, Ph.D. Thesis, University of Southern Mississippi, Hattiesburg, MS, 1984. C.L. McCormick, Macromolecules as Drugs and Carriers for Biologically Active Material, Ann. N.Y. Acad. Sci., 446 (1985) 76. C.L. McCormick, Z.B. Zhang and K.W. Anderson, J. Controlled Release, 4 (1986) 97-109. C.G. Gebelein and C.E. Carraher, Bioactive Polymer Systems, Plenum Press, New York, NY, 1985, p. 69. A. Hassner and V. Alexanian, Tetrahedron Lett., 46 (1978) 4475. B. Neises and W. Steiglich, Angew. Chem., Int. Ed. Engl., 7 (1978) 17. F.W. Harris, J.W. Thomson and S. Amdur, in: Proceedings of 6th International Controlled Release Pesticide Symposium, Controlled Release Society, Lincolnshire, IL, 1979, p. 22. M. Fineman and S. Ross, J. Polym. Sci., 5 (1950) 259. T. Kelen and F. Tudos, J. Macromol. Sci., A9 (1975) 1. T. Alfrey, Jr. and C.C. Price, J. Polym. Sci., 2 (1947) 101. T. Alfrey, Jr. and L.J. Young, The Q-e scheme, in: G.E. Ham (Ed.), Copolymerization, Wiley-Interscience, New York, NY, 1964, Chap. II. R.B. Yokum and E.B. Nyquist, Functional Monomers, Vol. 1, Marcel Dekker, 1973, p. 308. S. Igarashi, J. Polym. Sci., Polym. Lett. Ed., 1 (1963) 359. C.W. Pyun, J. Polym. Sci.,AZ (1970) 1111.