WO1996010994A1

WO1996010994A1 - Process for the preparation of colon specific reduction sensitive polymers

Info

Publication number: WO1996010994A1
Application number: PCT/IB1995/000834
Authority: WO
Inventors: Etienne Schacht; Ian Wilding
Original assignee: Danbiosyst Uk Ltd.
Priority date: 1994-10-05
Filing date: 1995-10-05
Publication date: 1996-04-18
Also published as: JPH10507219A; AU7443694A; AU690573B2; CA2201966A1

Abstract

Specific reduction sensitive polymers selected from the group consisting of (i) azo-containing polymers, (ii) disulfide-containing polymers, and (iii) azo- and disulfide-containing polymers, are prepared by a process comprising the steps of copolymerizing an azo- and/or disulfide-containing α,φ-difunctional reagent by polycondensation or polyaddition with a α,φ-difunctional comonomer.

Description

PROCESS FOR THE PREPARATION OF COLON SPECIFIC REDUCTION SENSITIVE POLYMERS

The present invention relates to a process for the preparation of specific reduction sensitive polymers selected from the group consisting of (i) azo-containing polymers, (ii) disulfide-containing polymers, and (iii) azo- and disulfide-containing polymers, the process comprising the steps of copolymerizing an azo- and/or disulfide-containing α,ω-difunctional reagent by polycondensation or polyaddition with a suitable α,ω-difunctional comonomer.

The process according to the invention has for purpose the preparation of azo- and/or disulfide-containing polymers which can be completely degraded or decomposed into monomeric units by bacteria in the reductive medium of the human colon intestine after a short incubation period. Said polymers are useful for manufacturing tablets and capsules intented for a site specific drug delivery in the lower part of the gastrointestinal tract.

From W091/11175, a process is already known for preparing colon specific reduction sensitive polymers selected from the group consisting of

(i) azo-containing polymers;

(ii) disulfide-containing polymers, and

(iii) azo- and disulfide-containing polymers.

Said process comprises the step of copolymerizing an azo- and/or disulfide-containing α,ω-difunctional comonomer according to the general reaction scheme illustrated below : n Y-R₁-R-XX-R-R₂Y + n HX-R₃-XH →

Y-R₁-R-XX-R-R₂-[-Z-X-R₃-X-Z-R₁-R-XX-R-R₂-]_n-1-Z-X-R₃-XH with

-XH = -NH₂, -OH

-Y = -COOH, CO-Hal, COOAlkyl, -N=C=O,

, -SO₂Hal

-XX- = -N=N-, -S-S- and

R = alkyl, aryl

R₁, R₂ = alkyl, aryl, alkylaryl groups optionally

substituted

R₃ = alkylidene, arylidene, alkylarylidene

optionally substituted

polyether, polyester

hal = halogen radical, e.g. Cl, Br

with

-Y + H-X- → -Z-X- and

Z = C=O, CH₂-CH-OH, SO₂ and whereby

X and Y are interchangeable in the above formulas. Said process allows the preparation of a multitude of azo- and/or disulfide-containing polymers through a variation in the α,ω-difunctional reagents. The final physicochemical and physical properties (hydrophilicity, permeability, thermal properties, rheoligical properties) can be widely varied.

The molecular weight of the resulting polymers is advantageously determined by gel permeation chromatography. It can be altered by adjusting the degree and type monomers used in the reaction mixture.

The molecular weight of the polymers prepared by the process of the invention is adjusted by modifying the molar ratios of the functional groups reacting with one another. For high molecular weight polymers equivalent amounts of reactive functional groups should be used.

It is well known to those skilled in the art that molecular weights for polymers must be above about 1000 to avoid adsorption in the gastrointestinal tract and that polymers for use in the gastrointestinal tract can have average molecular weights up to 10,000,000. (See U.S. Patent No. 4,298,595, Col. 25, lines 14-33).

The polymers are especially appropriated for preparing specific drug delivery systems. The latter can be matrix type systems or reservoir type systems. In the former case the azo- and/or disulfide-containing polymer is a major part of the drug containing compartment. In the latter part the azo- and/or disulfide-containing polymer is used to encapsulate a drug loaded core.

Owing to the property that azo- and/or disulfide polymers are stable in the fluids of the mouth, the stomach and the upper intestine the polymers according to the invention can pass the mouth, stomach and upper intestine without being destructed. In the lower part of the gastrointestinal tract, these reduction sensitive polymers are too slowly cleaved and fragmented by the reductive medium, all or not enzym mediated, so that enclosed active agent is not completely released at the site of degradation.

Several segmented disulfide-containing polyamides and hydrophilic groups suitable for reductive cleavage were prepared by polycondensation of 3,3'-dithiobissuccinimidylpropionate and a,ω-amino-terminated poly(tetramethyleneoxide) or tetraethyleneglycol diamine (JEFFAMINE EDR-192). Polymer films were incubated in a 15 ml sample taken from a SHIME reactor, t.m. Simultating Human Intestinal Microbial Ecosystem, which is simulating the microbial contents of human colon.

Also, segmented polyurethanes comprising disulfide and hydrophilic groups intended for the preparation of capsules which specific dry delivery were synthetized by reaction of 4,4'-diphenylmethane diisocyanate (MDI), an α,ω-hydroxy-terminated prepolymer and a disulfidecontaining chain extender. As hydroxy-terminated prepolymers were used poly (tetramethyleneoxide), poly(propyleneoxide), polycaprolactone, poly(ethyleneoxide), poly(ethyleneoxide/polypropyleneoxide).

In a same manner, segmented polyurethanes comprising azo-aromatic and hydrophilic groups were synthesized by reaction of m-xylylene diisocyanate with a mixture of m,m'-dihydroxyazobenzene, poly (ethylene glycol) and 1,2-propanediol. A hydrophilic drug (FOY-305) was enclosed in a capsule made of these polymers or coated with these polymers. The capsules or coated pellets were incubated in a SHIME-reactor reproducing a culture of human intestinal flora. Samples of reduction sensitive polymers are taken over time at measured intervals. The degradation in molecular weight is observed by performing gel permeation chromatography on the samples. Although the drug was released from these pellets, the molecular weight did not decrease substantially. Hence there was no degradation of the main chain azo groups to amine groups.

A series of azo-containing polyamides were prepared by polycondensation of an azo-containing diacid chloride with an oligomeric α,ω-diamine. The polymers were incubated in an in-vitro reductive medium. A plurality of tests have indicated that numerous azo-containing polymers become stickly and colourless during incubation in a SHIME-reactor simulating a culture of human intestinal flora. Changes in structure were monitored, by ¹³C, Raman, UV and GPC analysis. The observed discolouring of these polymers is not necessarily due to degradation, but can be due to partial reduction of the azo to the hydrazo form.

The thermodynamic conditions for the reductive cleavage of the azo polymers were determined by voltametrie measurements.

It was shown that for the hydrophilic azo-containing polyamide, incubation in the reductive medium led to degradation of the azo bond with formation of amines.

The molecular weight of the azo-polymers does not decrease substantively. The partial reduction of azoto hydrazo-group occurs slowly and there is no degradation of the main claim azo groups to amine groups.

Duration incubation, the polymers become stickly and colourless. During reoxidation, the original colours reappears and the molecular weight raises to the initial value.

These experiments indicate that for this type of polyamide no degradation of the azo-bond to aminogroups occurs. Changes in colour and mechanical properties are due to a conversion of the azo- to the hydrazogroups. These finding are in good agreement with those of Kimura et al. in EP-A-0 398 472. They have shown in an article published in Polymer 33(1992)5294-5299 that by incubation of azo-containing polyurethanes in a culture of human intestinal flora, no molecular weight decrease occured. As from these results, they concluded that there was no degradation of azo groups to amines, only to hydrazo groups.

The present invention aims to improve the thermodynamic conditions for the reductive cleavage of azo or disulfide containing polymers by incubation of said polymers in a similar reductive medium.

To achieve this improvement, the present invention proposes a process of the type described in the preambule of claim 1. This process is characterized in that HX-R₃-XH is a hydrophilic comonomer

with

-XH = -NH₂, -OH

-Y = -COOH, CO-Hal, COOAlkyl, , -SO₂Hal

-XX- = -N=N-, -S-S- and

R = alkyl, aryl

R₁, R₂ = alkyl, aryl, alkylaryl groups optionally substituted by methoxy-groups

R₃ = alkylidene, arylidene, alkylarylidene all or not substituted

polyether, polyester

hal = halogen radical, e.g. Cl, Br

with

-Y + H-X- → -Z-X- and

Z = C=O, CH₂-CH-OH, SO₂

and whereby

X and Y are interchangeable in the above formulas.

According to a particularity of the invention, the process comprises the copolymerization of dithio bis-(succinimidyl propionate) with a hydrophilic α,ω-difunctional comonomer.

The reduction sensitive polymers are prepared according to the methods described hereunder.

Description of the starting materials

Jeffamine ED-600 (Mn=600) (7) is a commercial product purchased from Texaco Chemical Company (Austin, USA). The polymer is dried by azeotropic removal of water from toluene solution.

The amine content is determined by titration :

Jeffamine ED-600 : 2.96 meq/g. Azobenzene-4,4'-diacid chloride (5) was prepared from the corresponding azobenzene-4,4'-dicarboxylic- acid (4) as described in the literature [23]. All other reagents were obtained from Janssen Chimica (Beerse, Belgium).

The chloroform was washed with sulfuric acid and water and subsequently dried over calciumhydride.

Trietylamine was purified by reacting with tosylchloride and ninhydrin, respectively.

It was finally dried over calciumhydride.

Methods

IR spectra were recorded on a Beckmann IR 4230 apparatus. UV spectra were measured on a Kontron Uvikon 810 double beam spectrophotometer.

Η-NMR spectra were recorded using a 360 MHz

Bruker WH-360 apparatus.

¹³C-NMR spectra were obtained at 100 MHz on a Varian Unity-400 spectrometer using a 5 mm broadband probe.

The molecular weight of the polymers was determined by gel permeation chromatography. A PL gel mixed D (5 μm) column was used with chloroform as eluent. For calibration, polystyrene standards were used.

FT-Raman spectra were recorded using a Bruker IFS 66FT IR spectrometer equipped with a FRA 106 FT Raman module.

Voltametric measurements were carried out on a EG&G Princeton Applied Research Potentistat/Galvanostat Model 273 using a Pd electrode as working electrode and a saturated calomel electrode (SCE) as reference electrode.

Viscosity was measured using a Ubbelohde viscosimeter (Haake).

The preparation of a polyamide starting from Jeffamine ED-600 and azobenzene-4,4'-diacid chloride is achieved according to a method described by M. BALASUBRAMANIAN et al., in Makromol. Chem. 180 (1979) 2517-2519. The 10 g (14.80 mmol) of Jeffamine ED-600, dissolved in 100 ml dry, alcohol free chloroform, was placed in a three necked flask and stirred under nitrogen atmosphere. To this solution 7 ml (50 mmol) triethylamine was added. The mixture was stirred for 15 min at -10°C. Then 4.4 g (13.03 mmol) azobenzene-4, 4'-diacid chloride, dissolved in 100 ml, dry ethanol free chloroform, was added at once at the reaction mixture. This mixture was stirred for 15 minutes at -10°C and then allowed to reach room temperature. Stirring was continued for 24 hours. The reaction mixture was extracted with 0.1 M HCl (3 × 150 ml), water, 0.1 M NaOH (3 × 150 ml). The chloro- form layer was dried over MgSO₄, filtered and evaporated to dryness. The resulting polymer was obtained as en orange coloured waxy solid.

¹H NMR spectroscopy (CDCl₃; 360 MHz) : δ = 1,1 ppm (d, CH₃ γ of PPG ether); δ = 1,35 ppm (d, CH₃ β of PPG ether); δ = 3,65 ppm (m, CH₂ a of PEG ether); δ = 3,8 ppm (m, CH β of PPG ether); δ = 4,4 ppm (broadened, N-H amide); δ = 7,95 ppm (m, aromatic protons).

IR-spectroscopy (film on KBr discs) : υ = 3300 cm^-1 N-H stretching vibration, v = 1650 cm^-1 C=O stretching vibration, υ = 1540 cm^-1 N-H bending vibration, v = 1600 cm^-1 C-H aromatic stretching vibration, υ = 1100 cm^-1 C-O stretching vibration.

Casting of films

A concentrated solution (ca. 30%) of the azo polyamide in chloroform was casted on a siliconized glass plate. After evaporation of the solvent in a hot air ventillated oven, the film was removed from the glass plate. Preparation of in-vitro reductive medium

A solution of sodium phosphate buffer (0.25 M, pH 6.5) was boiled and further cooled under an atmosphere of Argon. While stirring, sodium sulphide and cystein are added to give concentrations of 50 mM for each.

The redox potential of this medium, determined with a Pt redox-electrode, was -430 mV (+ 10 mV) and remained constant for at least 4 days. The reductivity of this medium was assayed by incubation of sulfalazine. The formed 5-ASA was determined qualitatively and quantitatively by HPLC analysis.

Incubation of films in chemical reductive medium

An azo polyamide ca. 0,1 g (tickness + 100 μm) was incubated in a solution of 30 ml reductive medium for periods from 3 hours varying to 2 days. At regular time intervals, films were removed from the medium, washed with water, dried in vacuo and analysed by GPC. FT-Raman and ¹³C-NMR measurements.

Incubation of films in simulating human intestinal medium (SHIME)

An azo polyamide ca. 0.05 g (thickness ± 100 μm) was incubated in 15 ml batch medium of a SHIME-reactor for periods from 12 hours varying to 3 days. The SHIME-reactor consists of 5 vessels which represent the ileum (2 vessels) and the colon (3 vessels). The redox potential -250 mV (+ 50 mV) is daily measured with a Pt redox-electrode. The incubation flasks were kept at 37°C. At regular time intervals, films were removed from the medium, washed with water, dried in vacuo and analysed by GPC, Raman and ^l3C-NMR measurements.

Results and Discussion

I) Synthesis of co-polyamides containing disulfide

The disulfide-containing polymers were prepared by polycondensation of 3,3'-dithiobissuccinimidyl propionate and α,ω-amino terminated prepolymer.

3,3'-dithiobissuccinimidyl propionate (DDSP) was synthetized according to the following reaction schema : HOOC- (CH₂) ₂-S-S- (CH₂) ₂-COOH

To a solution of 5 gm (23.778 mmol) 3,3'-dithiobissuccinimidyl propionate (DDSP) in 125 ml dry dioxane in 250 ml two neck flask under nitrogen atmosphere was added a solution of 6.25 gm (54.3 mmol) N-hydroxysuccinimide (NHS). The reaction mixture was cooled to 0°C in an ice-acetone bath followed by addition of 11.25 gm (54.52 mmol) dicyclohexylcarbonate (DCC). The reaction mixture was stirred for 1 hour at 0°C and the stirring was continued overnight at room temperature. The precipitated dicyclohexylurea (DCU) was filtered off, washed with dry dioxane, the filtrate was concentrated to one/third and was left overnight in refrigerator.

The precipitated traces of DCU was filtered off and the filtrate was evaporated on rotary evaporator. The white crude product was recrystallized by refluxing it in a mixture (150 ml) of dry acetone and diethyl ether (1:1) for 1-2 hrs. The DCU impurities was found to be more soluble in the refluxing mixture and the pure DDSP was less soluble, the insoluble product was filtered and dried in vacuum. The last step was repeated 2-3 times till no DCU was evidented in the NMR of the product.

B) Copolymerization of DDSP with Jeffamine EDR-192 and

PTHF-diamine 750

B-l) 1:1 PTHF-750 : EDR-192

2.

Procedure

To a cooled solution (0°C) of 2.51 gm

(24.75 meq) EDR-192 in 15 ml dry chloroform, 10.55 gm

(24.75 meq) PTHF-750 in 15 ml dry CHCl₃ and 13.69 ml triethylamine was added a solution of DDSP (49.5 meq) in

65 ml CHCl₃. The reaction mixture was stirred at 0°C for

1 hour and the stirring was continued for 36 hrs at room temperature.

The chloroform layer was extracted with HCl 0.1 M (3x), H₂O (3x), NaOH 0.1 M (3x) and finally with H₂O

(3x). The CHCl₃ layer was dried over MgSO₄ overnight and then was filtered and the filtrate was evaporated in rotary evaporator. The product was further dried on vacuum.

Results

1) Good film forming

2) GPC (NMP) : Weight average molecular weight : 21791

Dispersity : 2.3

3) Colonic degradation : see Table 1. II) Degradation of disulfide-containing co-polvamides in isolated colonic medium

Polymer films were incubated in a 15 ml sample taken from a SHIME reactor. The SHIME reactor (Simulating Human Intestinal Microbial Ecosystem) is simulating the microbial contents of human colon. The flasks were degassed by flushing it with nitrogen and then was incubated at 37°C. At regular times samples were taken and analyzed by GPC (eluent : NMP). Undissolved polymer samples were washed, dried under vacuum and the analyzed by GPC. The soluble polymers were removed by freeze-drying the medium and the residue was dissolved in NMP, filtered to avoid inorganic salts and the analyzed by GPC.

The tensile strength of the polymer samples was measured using a Hounsfield type H10KM tensile machine. The samples was tested at 25°C at a crosshead speed (tensile rate) of 20 mm/min using 100 N cell.

Film preparation

Polymer solutions containing 10% w/w of the polymer in chloroform were cast on a Teflon plate. The plate was covered with a watch glass to slow down solvent evaporation, solidified by atmospheric air drying. The films were carefully removed from the plate and dried to constant weight in vacuum at 30°C. The thickness of the film was measured using micrometer Forster-isometer S 2.320 (Belgium). The film was cutted for tensile strength measurements using hand press cutting knife from Berg & Schmid model HK500.

1.3 Disulfide-containing polyurethanes

The disulfide-containing polyurethanes are pre pared by polycondensation of 4,4'-diphenylmethane diisocyanate (MDI), an α,ω-hydroxy-terminated prepolymer and a disulfide-containing chain extender, according to the following reaction scheme :

As hydroxy-terminated prepolymers were used : poly(tetramethyleneoxide), poly(propyleneoxide), poly(caprolactone), poly(ethyleneoxide), poly(ethyleneoxide/propyleneoxide).

Synthesis of azo-containing polyamides

The azo-containing polymers were synthesized by polycondensation of azobenzene-4,4'-diacid chloride and oligomeric α,ω-diamines in chloroform as solvent and triethylamine as proton acceptor. The azobenzene-4,4'-diacid chloride was prepare from the corresponding azobenzene-4,4'-dicarboxylic acid by reaction with thionylchloride in toluene as solvent. The oligomeric diamines selected was hydrophylic (Jeffamine ED-600) (7) which resulted in water soluble (9) polymer :

Characterisation of the polymers was done by ¹H-NMR and IR spectroscopy. Table 1 shows the molecular weight of both azo polymers as determined by GPC using polystyrene as standards.

Reduction of azo-containing polyamides in chemical reductive medium

The degradation of the azo polymers was investigated in an in-vitro reductive medium. During incubation, the films become sticky and colourless. Upon drying of the incubated polymer at 50°C in air, the samples showed a rapid colour recovery from colourless to orange.

Table 3 shows molecular weight as determined by GPC using polystyrene as standards and the results of viscosity measurements for said hydrophylic azo polymer (9) before and after incubation in a chemical reductive medium.

GPC analysis of samples taken at different incubation times (table 3) show a remarkable decrease in molecular weight. After 3 hours incubation, the molecular weight is apparently reduced of a factor 10 faible original value and remains so for the further 48 hours of incubation (see table 3). However, during reoxidation, the original colour appears and the molecular weight as determined by GPC raises to the initial value. The films dissolved completely in the reductive medium after 6 hours of incubation.

Spectroscopic evidence for the formation of the hydrazo bond during reduction of the azo bond

Evidence for the formation of hydrazo groups during reduction of the hydrophobic azo polymer (8) was gained by ¹³C-NMR analysis.

Table 4 gives different chemical shifts for the oxidized and reduced form of the hydrophobic azo polymer. The aromatic carbon resonances were assigned based on additivity rules [25] using a para X-Ph-Y structure with Y = -N=N-Ph and X = -CON(CH₃)₂ for the oxidized polymer and with Y = -NH-NH₂ and X = -CON(CH₃)₂ for the reduced one. The calculated values are in good agreement with those determined experimentally for the oxidized and the reduced form.

In conclusion it can be stated that both polymer forms, having a few characteristic carbon resonance lines (e.g. C3 and C5), can easily be discriminated from each other by ¹³C-NMR.

The ¹³C-NMR spectra of the reoxidized polymers, after incubation in the chemical reductive medium and in the SHIME reactor, are clearly mixtures of the reduced and oxidized forms. The polymer which was incubated in the chemical reductive medium, reoxidized for three days in air atmosphere, still contains mainly (about 90%) of the reduced form. The polymer which was incubated in the SHIME-reactor, reoxidized for about two weeks in air atmosphere, contains mainly (about 55%) of the oxidized form.

FT-Raman spectroscopy is an attractive tool to study the reduction of the azo groups. The aromatic ring vibrations in both spectra appear at 1613 cm^-1 and 1607 cm^-1. The typical Raman shift at 1460 cm^-1 can be assigned to the N=N functional group and is present in the oxidized form. This Raman signal can be used to differentiate between the oxidized and reduced compounds. FT-Raman can thus be applied as a complementary nondestructive technique for this purpose. For the reduced polymer the N=N stretching band at 1460 cm^-1 is missing and replaced by a broad band at 1298 cm^-1. This band is due to the symmetrical NH-NH stretching. The reoxidized polymers contain a shift at 1460 cm^-1, characteristic for the N=N group. However, the higher relative intensity of the Raman line at 1460 cm^-1 of the in SHIME- reactor incubated polymer can be correlated with a higher amount of the oxidized polymeric form. These results are totally in agreement with the ¹³C-NMR data.

Voltametric experiments

Literature data [28-29) show that the electro-chemical reduction of an azo bond proceeds via a two- electron step mechanism towards the hydrazo group. Reduction surpassing the hydrazo-derivatives generally requires more energy than the azo-hydrazo reaction. If the difference in energy is great enough, the reduction may be represented by two waves (figure 5).

In some cases, however, azo compounds are known to be reduced to the amines, by a four-electron step reaction (figure 6) .

Voltametric measurements have been carried out in water, in which tetrabutylammoniumbromide was used as electrolyte. By using water as a solvent, only the water soluble, hydrophylic polymer (9), could be investigated.

From table 5 we can notice that the redox potential of

(9) is -230 mV vs. SCE (+ 6 mV vs. SHE). This explains why the polymer is reduced in the chemical reductive medium.

Groups such as hydroxy, amino, methoxy, which increase the electron density on the azo linkage, show a shift towards higher (less negative) redox potentials and proceed by a four-electron step reduction towards amines [30,31). Voltametric measurement of the water soluble azobenzene-4,4'-dicarboxylic acid (4) showed a lower half wave potential than the methoxy substituted analogue (10).

There is a difference of 140 mV in redox potential between both products. This implies that the reduction of the methoxy substituted diacid is more favourable.

When the hydrophylic azo polymer is incubated in a similar reductive medium, degradation proceeds with reduction of azo groups to amines. The solubility of the azo polymers in the reductive medium is the determining factor. Voltametric measurements have shown that all water-soluble azo compounds have a redox potential, more positive than the reductive medium. These results indicate that the thermodynamic conditions for the reductive cleavage of azo bonds are fulfilled.

Claims

C L A I M S

1. - A process for the preparation of colon specific reduction sensitive polymers selected from the group consisting of

(i) azo-containing polymers;

(ii) disulfide-containing polymers, and

(iii) azo- and disulfide-containing polymers, comprising the step of copolymerizing an azo- and/or disulfide-containing α,ω-difunctional reagent by polycondensation or polyaddition with a suitable α,ω-difunctional comonomer according to the general reaction scheme illustrated below : n Y-R₁-R-XX-R-R₂Y + n HX-R₃-XH →

Y-R₁-R-XX-R-R₂-[-Z-X-R₃-X-Z-R₁-R-XX-R-R₂-]_n-1-Z-X-R₃-XH characterized in that

HX-R₃-XH is a hydrophilic comonomer

with

-XH = -NH₂, -OH

-Y = -COOH, CO-Hal, COOAlkyl, , -SO₂Hal

-XX- = -N=N-, -S-S- and

R = alkyl, aryl

R₃ = alkylidene, arylidene, alkylarylidene all or not substituted

polyether, polyester

hal = halogen radical, e.g. Cl, Br with

-Y + H-X- → -Z-X- and

Z = C=O, CH₂-CH-OH, SO₂

and whereby

X and Y are interchangeable in the above formulas.

2.- The process according to claim 1, characterized in that it comprises the copolymerization of dithio bis (succinimidyl propionate) with a hydrophylic α,ω-difunctional comonomer.

3.- The process according to claim 1 or 2, characterized in that it comprises the copolymerization of dithio bis (succinimidyl propionate) with a hydrophylic α,ω-diamine functional comonomer.