Ceric ion initiation of methyl methacrylate using polytetrahydrofuran diol and polycaprolactone diol

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Ceric ion initiation of methyl methacrylate

using polytetrahydrofuran diol and polycaprolactone

diol

HuÈlya Arslan

a

, Baki Hazer

a, b,

*

a_{Zonguldak Karaelmas University, Department of Chemistry, 67100, Zonguldak, Turkey}

b_{TUBITAK-Marmara Research Center, Food Science and Technologies Research Institute, Gebze, 41470, Kocaeli, Turkey}

Received 28 April 1998; accepted 27 August 1998

Abstract

Polymerization of methyl methacrylate (MMA) initiated by ceric ammonium nitrate in combination with polytetrahydrofuran diol (PTHF-diol) and polycaprolactone diol (PCL-diol) has been investigated in aqueous nitric acid. PMMA-b-PTHF and PMMA-b-PCL block copolymers were obtained. Block copolymer yield was increased when using tetrabutyl ammonium hydrogen sulphate as a surfactant. The eects of nitric acid and ceric ions on the block copolymer yields were investigated. Block copolymers were characterized using GPC,1_{H-NMR, DSC, TGA}

1. Introduction

Ceric salts show high reactivity in aqueous media and have been used either alone or in combination with reducing agents as initiators of vinyl polymerization [1±12]. Alcohols, aldehydes, ketones, acids, amines, thiols and thiourea were reported to be suitable reducing agents [3]. Some biodegradable polymers containing hydroxyl groups such as poly (vinyl alcohol) [4, 7], cellulose [5], chitin [6], starch [8]

and polyethylene glycol [9±12] as reducing agents have also been reported as initiating vinyl polymerization to obtain graft or block copolymers. The ceric ion functions via a single-electron transfer with the for-mation of free radicals from the reducing agents. The oxidation of alcohols by Ce(IV) is believed to proceed by disproportionation of coordination complexes. According to the complex mechanism , unimolecular disproportionation of complex C yields cerous ion, a proton and a free radical on the alcohol substrate [4]:

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Block and graft copolymers can also be obtained by the polymerization of vinyl monomers with macro-initiators [13±17]. In this manner, macro-macro-initiators based on biodegradable units (ca. PTHF diol with MW 1000 and PCL diol with MW 1250) have been used to prepare block copolymers [16, 17].

The present work refers to the polymerization of methyl methacrylate (MMA) initiated by ceric ion/ PTHF-diol and ceric ion/PCL-diol in order to obtain PMMA-b-PTHF and PMMA-b-PCL block copolymers. 2. Experimental

2.1. Materials

Poly-THF-diol (with Mw of 1000 g/mol) was

sup-plied by BASF and used without puri®cation. PCL-diol (with Mw of 1250 g/mol) was supplied by

Polysciences, and used without puri®cation. It was reported that hydroxyl end-functionalization was achieved by reacting of PCL with equimolar amount of 1,4-butanediol. Its hydroxyl functionality was 2.0. MMA was supplied by Fluka AG and freed from in-hibitor by vacuum distillation over calcium hydride.

Ceric ammonium nitrate, Ce(NH4)2(NO3)6, (CAN),

was supplied by Fluka AG. and used as received. All other chemicals were reagent grade and used as received.

2.2. Polymerization procedures

Polymerization was carried out in a nitrogen atmos-phere at 408C. The general experimental procedure was as follows. An appropriate amount of polymer was put in a pyrex tube, then the CAN solution (in-itiator) and MMA (monomer) were added. Approximately 10 mg of tetrabutyl ammonium hydro-gen sulphate,TBAHS, was also added in order to increase the solubility of the diols. Nitrogen was intro-duced for 1 min through a needle into the homogenous mixture to expel the air. The tube was covered with a

stopper and kept at 408C for a given time with stirring during the polymerization. The product was precipi-tated in methanol, collected by ®ltration and dried to constant weight.

2.3. Characterization

In order to separate block copolymers from unreacted homopolymers, the fractional precipitation method was carried out by measuring g, the volume ratio of nonsolvent (methanol) to the solution of copo-lymers in chloroform, to separate pure block copoly-mers from the corresponding homopolycopoly-mers [13]. For this purpose, approximately 0.5 g of polymer was dis-solved in 10 mL of CHCl3; and methanol was

gradu-ally added to this solution until the polymer precipitated. Then the g value of the polymer was cal-culated by taking the volume ratio of methanol used in the CHCl3solution.

Gel permeation chromatography, GPC, was used to determine molecular weights and distributions with a Waters instrument (410 Dierential Refractometer) in THF. The elution rate was 1 ml minÿ1 _{. Waters}

Styragel columns HR1 and HT6E were used and mol-ecular weights were calibrated with polystyrene stan-dards (TOSOH Corp.).

1_{H NMR spectra of products were recorded in a}

Bruker- AC 200 L, 200 MHz NMR spectrometer, using CDCl3as solvent. Dierential scanning

calorime-try, DSC, and thermogravimetric analysis, TGA, measurements were carried out under nitrogen by using a Dupont DSC- 9100 and Dupont TGA-951 , re-spectively, with a TA-9900 data processing system. 3. Results and discussion

PMMA-b-PTHF and PMMA-b-PCL block copoly-mers were synthesized via redox initiation at 408C in the presence of TBAHS to increase solubility of the monomer and the diols. Polymerization reactions can be represented as follows:

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After these reactions, b-PTHF and PMMA-b-PCL block copolymers could be ABA type of block copolymers [14].

Pure block copolymers were isolated by fractional precipitation of chloroform solution with methanol from the crude polymer mixture. g-Values of pure block copolymers were determined as the ratio of volume of chloroform solution to volume of non-sol-vent (methanol). Polymerization conditions and results are tabulated in Table 1. Polymer yields increased with increasing -diol concentration in the initial feed. g-Values of pure block copolymers were 2.5±8.0, depend-ing on PTHF or PCL content, while those of homo-PMMA were 0.5±0.8 and polydiols>20 (soluble in methanol). The higher the PTHF or PCL content in block copolymer, the higher the g-value. Interestingly, pure block copolymer fraction of PTHF had higher g than PCL. This may arise from the higher water solu-bility of PTHF over that of PCL.

Acid concentration in¯uenced block copolymer yield. When we compare runs 8 and 9, we see the

smal-ler acid concentration causes higher PMMA-b-PTHF block copolymer yield (polymer yield of 9 twice that of 8). Similarly, the same eect on the PMMA-b-PCL block copolymers can be seen when comparing runs 23 and 24. In the case of runs 8 and 11, the eect of cer-ium concentration was investigated. Higher cercer-ium concentration causes higher block copolymer yield (polymer yield of 24 three times that of 23).

Molecular weights and polymer contents of pure block copolymers are listed in Table 2. Molecular weights of pure block copolymers ranged from 9.6 104 _{to 52 10}4 _{and their GPC chromatograms}

were unimodal.

NMR spectra of the products showed the character-istic peaks of PTHF, PCL and PMMA in block copo-lymers as reported in the literature. dppm: PMMA: 0.9±

1.05, d; 1.8±2.0, d; 3.6,s; PTHF: 1.6,m; 3.4,m, PCL: 1.4,m; 1.6,m; 2.3, t; 4.1,t. PTHF and PCL inclusions calculated from the NMR spectra were between 0.5 and 10% (mol) (see Table 2).

Table 1

Results and conditions of the polymerization of MMA by CeIV_{/-diol redox system at 408C}

Polymerization initial feed concentration Block copolymer fractionation, % Run no -diol, g [MMA],

mol/l (ml) [HNOmol/l3], Ce

+4_,

mol/l (ml) Total polymeryield, g g2.5±3.8 g3.1±4.1 g3.3±5.0 g5.0±8.0 g>20 3 PTHF 1.02 3.7 (2) 3 0.05 (3) 0.324 Ð 72 Ð Ð 28 7 PTHF 1.54 3.7 (2) 3 0.05 (3) 0.423 Ð 58 Ð Ð 42 8 PTHF 3.04 3.7 (4) 3 0.05 (6) 0.737 Ð 79 Ð Ð 31 9 PTHF 3.14 3.7 (4) 0.5 0.05 (6) 1.814 Ð 77 Ð 77 23 10 PTHF 1.53 3.7 (2) 3 0.1 (3) 0.196 Ð Ð 61 Ð 39 11 PTHF 3.00 5.3 (4) 3 0.05 (3) 0.539 Ð 65 Ð Ð 35 21 PCL 1.05 3.7 (2) 3 0.05 (3) 0.07 23 Ð Ð Ð 77 22 PCL 3.12 3.1 (2) 3 0.05 (4) 0.98 23 Ð Ð Ð 77 23 PCL 5.05 1.6 (2) 3 0.05 (10) 1.21 26 Ð Ð Ð 74 24 PCL 5.00 1.6 (2) 0.5 0.05 (10) 3.40 Ð Ð 12 57 31 Table 2

Molecular weights and polymer contents of the block copolymers

GPC Block copolymer analysis

Run no Mw104 Mn104 P.D PTHF, % PCL, % 3 35.4 15.6 2.27 2 Ð 7 48.8 17.9 2.72 8 Ð 8 33.1 15.7 2.11 10 Ð 9 46.7 20.5 2.27 7 Ð 11 23.2 10.0 2.3 7 Ð 21 9.6 6.1 1.58 Ð 0.5 22 52.0 18.8 2.76 Ð 3 23 45.6 19.3 2.37 Ð 5 24 28.1 13.5 2.07 Ð 10

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Interestingly, NMR spectra of the samples obtained using the highest initial concentration of PTHF (run 11) and PCL (run 24) contained characteristic vinyl peaks at 5.3 and 5.7 ppm. We tentatively say that cer-ium ion oxidizes the diols to some vinyl groups together with the radical formation.

Thermal characterization of PMMA-b-PTHF and PMMA-b-PCL block copolymers were achieved by recording TGA and DSC curves. TGA curves exhib-ited decomposition temperatures (Td) at 2308C for

PTHF blocks, 2408C for PCL blocks and 3708C for PMMA blocks. Soft PTHF and PCL segments shifted

the decomposition temperature, Td , of PMMA blocks

to 3708C from 4308C. In order to understand the ther-mal behaviour of the copolymers, DSC curves of copo-lymers were recorded (see Fig. 1). In each curve, glass transition (Tg) of PMMA blocks at around 1108C is

observed. PTHF units have large melting transitions at 308C (runs 3 and 7) and 158C (run 8). The small melt-ing transitions at 67, 65 and 458C may arise from some miscible PTHF±PMMA random copolymer domains formed in the block copolymer structure. In run 23 in Fig. 1, Tgof PMMA units at 1028C and Tm

of PCL units at 458C were observed. The results of

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both TGA and DSC studies can be taken as proof of the presence of blocks of the corresponding homopoly-mers in the backbone.

In conclusion, to insert the biodegradable units such as PTHF and PCL into the block copolymer matrix is very important for the synthetic polymer in order to gain biodegradability. Ceric initiation system with these kinds of polimeric diols initiates vinyl polymeriz-ation to obtain such block copolymers. In this system, acid concentration, water solubility of polymeric diol and monomer used in¯uence copolymer yield. Therefore, to increase water solubility of the reactants, surface active agents can be used.

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