Synthesis of Aromatic Polyesters Bearing Pendant Reactive Groups by

Chapter 18

Synthesis of Aromatic Polyesters Bearing Pendant Reactive Groups by Phase-Transfer Catalysis

Downloaded by MONASH UNIV on August 28, 2013 | http://pubs.acs.org Publication Date: February 1, 1997 | doi: 10.1021/bk-1997-0659.ch018

Shigeo Nakamura and Chonghui Wang Department of Applied Chemistry, Faculty of Engineering, Kanagawa University, Kanagawa-ku, Yokohama 221, Japan

Aromatic polyesters having reactive groups in the side chain have been prepared chemoselectively by a water phase/organic phase interfacial polycondensation using phase transfer catalyst. Polyesters bearing pendant carboxyl groups are prepared utilizing a higher nucleophilicity of phenolate compared to carboxylate, whereas high molecular-weight polyesters having pendant chlorohydrin groups are synthesized by controlling the molar ratio of bisphenolate to isophthaloyl chloride and the phase ratio of the water phase to the organic phase.

Since Schnell has first reported the preparation of polycarbonate by interfacial poly condensation (7), a variety of aromatic condensation polymers have been prepared by a water phase/organic phase interfacial polycondensation using phase transfer catalyst. Many reactive polymers have been synthesized, and their applications have been investigated extensively. However, these investigations are almost restricted to the addition polymers, and those of condensation polymers are very limited. The synthesis of polyesters bearing pendant reactive groups is very difficult due to the high reactivity of ester groups in the main chain. The synthesis of these polyesters involves the preparation of monomers having an additional functional group, the

© 1997 American Chemical Society

In Phase-Transfer Catalysis; Halpern, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

PHASE-TRANSFER CATALYSIS


232

chemoselective reaction among the reactive groups, and the degradation reaction of the ester groups of resulting polyesters with the pendant groups. Recently, attention has been focused on chemoselective synthesis of aromatic polyesters and polyamides with reactive groups using condensation agents (2). Aromatic polyesters bearing reactive groups can be used for the preparation of various functional polymers, which are expected for applications as polymer catalysts, amphoteric polyelectrolytes, chelating agents for metal ions, drug delivery systems and biodegradable polymers. Aromatic polyesters bearing pendant reactive groups can be also used for composite materials with inorganic materials, metals and other organic polymers, and those having pendant bifunctional or multifunctional groups can be blended with other polymers to produce intermolecular complexes and interpenetrating polymer networks (IPNs) or semi-IPNs. Most aromatic polyesters have been merely blended with condensation polymers such as other polyesters, polyimides and polycarbonates (3,4). Therefore, the polyesters bearing pendant reactive groups are expected to find wide applications in polymer blends due to specific interactions such as hydrogen bonding, ion-ion interaction and electron transfer (5). A cooperative effect has been proposed in the reaction of partially ionized poly (methacrylic acid) with bromoacetic acid (6,7). The reaction is affected not only by the long-range Coulomb forces but also hydrogen bonding or hydrophobic bond (8). Therefore, polyesters bearing pendant reactive groups behave differently from addition polymers due to the polarizability and reactivity of the ester groups in the main chain. This article describes the chemoselective synthesis of aromatic polyesters bearing reactive groups such as carboxyl groups and chlorohydrin moieties in the side chains by the water phase/organic phase interfacial polycondensation using phase transfer catalysts. Polyesters Having Pendant Carboxyl Groups Chemoselective Reaction of Phenolate. When a sodium carboxylate is reacted with an acid chloride in the presence of tertiary amine, an anhydride can be obtained almost quantitatively at room temperature (9). When sodium phenolate is reacted with an acid chloride at room temperature, an ester of phenol is obtained in a high yield even without a tertiary amine. Generally, the nucleophilicity of phenolate to acid chloride is much higher than that of carboxylate.


18.

NAKAMURA & WANG

Synthesis of Aromatic Polyesters

233


The higher nucleophilicity of phenolate than that of carboxylate can be utilized in the interfacial polycondensation and polyesters having pendant carboxyl groups can be obtained chemoselectively from aromatic diols having a carboxyl group, leaving the carboxylic group unreacted (10). Phase Transfer Catalyst. A low-molecular quaternary ammonium salt, tert-n~buty\phosphonium bromide (TBPB) and a polyester having quaternary ammonium salt groups in the side chain and amino acid moieties in the main chain (poly-cat) were used as phase transfer catalysts (PTC). The polymer catalyst, poly-cat was prepared as reported previously (11) by an organic phase/organic phase interfacial polycondensation of isophthaloyl chloride (IPC) with triethyl-2,3-propanediolammonium chloride and Ν,Ν-άι- (2-hydroxyethyl)-3-aminopropionic acid. The molar ratio of the quaternary ammonium salt groups to the amino acid groups in the poly-cat was 56/44. Preparation of Polyesters. Polyester bearing pendant carboxyl groups (PEA) was prepared from IPC and 4,4-bis(p-hydroxyphenyl)valeric acid (diphenolic acid) (DPA) according to Equation 1 of Scheme 1 (10). A solution polycondensation of IPC with DP A was carried out in DMAc using TEA as an acid acceptor to compare with the phase transfer-catalyzed polycondensation. Although the yield is high (76%), the molecular weight of the product is very low (r\JC: 0.09dL/g). Thus, the polycondensation in an organic solvent without phase transfer catalyst is insuitable for the synthesis of high molecular-weight polyester PEA because the resulting oligomer precipitates at the initial stage of reaction. 0.02 Mole of IPC in 80 mL of organic solvent was added to 0.02 mole of DP A in 180 mL of aqueous 0.06 Ν NaOH solution in the presence of 0.2 g of PTC. The mixture was stirred for 3 h at 30°C and then the pH of the solution was adjusted to about 3 by 2 Ν hydrochloric acid to precipitate polyester (PEA). Poly(ester-amide-thioester) (PEATA) and copolyester (ΡΕΒΑ) were obtained by substituting 0.01 mole DP A with 0.01 mole of 2-aminothiophenol (ATP) or 0.01 mole of bisphenol A (BP A), respectively (Equations 2 and 3 of Scheme 1 ). Constituent ratios of the resulting copolyesters were 59/41 of DP A/ATP for PEATA and 48/52 of DP A/BP A for ΡΕΒΑ. In the IR spectra of polyester PEA, copolyesters PEATA and ΡΕΒΑ, the band due to anhydride groups is not observed at 1800-1850 cm*. The carboxylic acid content of the resulting polymers was determined by titration and agrees well with the calculated values. Therefore, linear polyesters having pendant carboxyl groups are obtained by interfacial polycondensation. Molecular weight and yield of the resulting polymer are remarkably affected by the phase transfer catalyst, base and reaction medium as seen in Table 1. 1



234


PEA

Table 1. Effects of P T C and Solvent on the Reduced Viscosity of Polyesters Bearing Pendant Carboxyl Groups Yield

^sp/c

%

dL/g

Run no

PTC

Reaction media

PEA-1

TBAB

H 0/toluene

43

2

TBAB

H 0/CCI

36

—)

3

TBAB

H 0/CHCI

74

0.30

4

TBAB

H 0/CH CI

2

84

0.41

H 0/CH CI

2

17

0.19

2

2

4

2

2

3

2

5

TBAB+TEA

6

Poly-cat

H 0/CCI

7

Poly-cat

H 0/CHCI

8

Poly-cat

H 0/CH CI

PEATA

Poly-cat

H 0/CH CI

ΡΕΒΑ

poly-cat

H 0/CH CI

2

2

2

2

2

2

2

62

4

2

2

2

2

—> 2

76

0.62

2

87

0.83

2

87

1.65

2

87

1.15

3

1

Measured at 0.5g/dL in tetrachloroethane-phenol (2:3 by weight) containing 4 . 7 6 % (by volume) of H S 0 at 30°C. 2

The value of η

8 ρ

/

4

0

i s

v e r

Y

, o w

-


NAKAMURA & WANG



18.


235

236


Effect of Phase Transfer Catalyst and Reaction Medium. When TBAB is used as a phase transfer catalyst in a H 0/CCI system, the yield of PEA is only 36% and the molecular weight is low because the complex of TBAB with phenolate is difficult to diffuse from the water phase to the CC1 phase. Both the yield and molecular weight of PEA are increased to 62% when poly-cat is used as a phase transfer catalyst. In H 0/CHCI and H 0/CH CI systems, the yield becomes higher when either TBAB or poly-cat is used. However, the molecular weight is remarkably increased when poly-cat is used. The phase transfer process of poly-cat may be different from that of TBAB because poly-cat contains amino acid moieties as nucleophilic catalyst. 2

4

4


2

3

2

2

2

Effect of Organic Base. When TBAB is used together with TEA as a complex catalyst, the yield is very low. Aromatic acid chloride is considerably less reactive than aliphatic acid chlorides. Therefore, when Schotten-Baumann reaction is used for preparation of polyester, aqueous sodium hydroxide or organic base must be added to the reaction system. However, the use of organic base triethylamine (TEA) is not effective in water phase/organic phase interfacial polycondensation as seen in Table 1. Catalytic Mechanism. As shown in the catalytic mechanism of TEA (Equations 4, 5 and 6 of Scheme 2), the partial positive charge δ on the carbonyl carbon of the inter mediate I is increased and the nucleophilic attack of carboxylate and water occurs on the intermediate I. In the IR spectra of reaction products, the characteristic absorption bands of an hydride groups are observed at 1770 and 1830 cm . The poly-cat is a complex catalyst and contains both quaternary ammonium groups and amino acid groups. The quaternary ammonium groups acts as a phase transfer catalyst and the amino acid moiety as a nucleophilic catalyst. +

1

Polymer Catalyst. Previously, we have reported the synthesis of copolyesters having tertiary amine groups in the main chain from terephthaloyl chloride (TPC), TV-ethyl diethanolamine (EDA) and bisphenol A (BPA) by an interfacial polycondensation (72). The resulting copolymer contains higher EDA residues when Na C0 is used as an acid acceptor. If the self-nucleophilic catalysis of the tertiary amine groups in the main chain of the polymer occurs in the interfacial polycondensation, the content of EDA residues in the resulting copolymers is higher because aliphatic diols do not react with TPC when Na2C0 is used as an acid acceptor. As the same reason, the intermediate of nucleophilic catalysisfrompoly-cat and IPC is formed during the reaction. Polymer effect is exerted by the intermediate of nucleophilic catalysis resulting from the amino acid moiety of the poly-cat and IPC; that is, the nucleophilic attack of carboxylate and water on the carbonyl carbon of the 2

3

3


NAKAMURA & WANG



18.


238



propagating chain end is disturbed by the hydrophobic nature of polymer chain segments in the vicinity of the intermediate. It has been reported that high molecular-weight aromatic polyesters can be ob tained in the presence of surfactants or quaternary ammonium salts (75). When a quaternary ammonium salt is added, it acts as a phase transfer catalyst and accelerates the transportation of reactants from the aqueous phase to the organic phase. Phase Transfer of Reactants. However, when the poly-cat is used as a complex catalyst, two types of transportation occur. The complex produced by bisphenolate with the quaternary ammonium salt group transfers from the aqueous phase to the organic phase and the intermediate from the reaction of IPC with the amino acid moiety of polycat transfersfromthe organic phase to the aqueous phase. The phase transfer mechanism is represented in Equations 7 and 8 of Scheme 3. The resulting polyester is soluble in basic aqueous solution, and the intermediate transfersfromthe organic phase to the aqueous phase. The mechanism of the interfacial polycondensation differs from that of polyarylate formation because the rate of phase transfer of polymer chain segments from the aqueous phase to the organic phase is slower for polymers with phenolate end groups than that of the low-molecular bisphenolates. The relation between the reduced viscosity and the reaction time is given in Figure 1 for the polycondensation of IPC with DPA at 20°C using poly-cat and TBAB as chain transfer catalysts keeping the molar ratio of IPC to DPA to unity. As expected, the rate of propagation of polyester PEA is faster for poly-cat than that for TBAB as a catalyst. The poly-cat contributes to the increase in the molecular weight due to the nucleophilic catalytic action of the amino acid moiety in the main chain and the phase transfer of the resulting intermediate from the organic phase to the aqueous phase. Effect of the molar Ratio of Reactants. The molecular weight of polyarylate is significantly affected by the molar ratio of acid chloride to BPA (14), because the molar ratio of acid chloride to BPA affects the mechanism of the interfacial polycondensation. Usually, monomer or dimer reacts with the propagating polymer in the interfacial polycondensation. At the initial stage of reaction, the solubility of oligomer in the reaction media is governed by the end groups of the oligomer itself. In the synthesis of polyarylate, therefore, very high molecular weight can be attained when the molar ratio of BP A to acid chloride is larger than unity (75), because the propagating oligomer or polymer having phenolate end groups is insoluble in both the organic phase and the aqueous phase. However, the molecular weight of PEA is remarkably affected by the molar ratio of reactants (Table 2). When the molar ratio of DPA to IPC is 1.15 and 1.20, the reaction tends to occur in



h2

Ο II c—ci +

2

CH COO N a

?

+

2

2

2

2

Poly-cat

2

CH I CH COOH

2

~OCH CH NCH CH 0

Poly-cat

2

5 3

~

c f N (C H )

2

+

2

~ - OCHCH 0 I CH 2

+

-1 /

Ο

Partially soluble in water

C

Partially soluble in organic solvents

CH COO Ν

Synthesis of Aromatic Polyesters Bearing Pendant Reactive Groups by

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