Tautomers as monomers and initiators

Prog. Polym Sci., Vol. 21,557~591, 1996 copyright0 1996Elsevier Science Lid Printed in Great Britain. AU rights reserved. 0079-6700/96 $32.00

Pergaunon 0079-6700(95)00025-9

TAUTOMERS

AS MONOMERS

AND INITIATORS

SEIZO MASUDA and KEIJI MINAGAWA Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokwhima, Minamijosanjima, Tokushima 770, Japan

CONTENTS 1. General introduction 2. Keto-enol tautomers 2.1. Introduction 2.2. Preparation 2.2.1. Preparation of unsaturated B-diketones 2.2.2. Preparation of unsaturated B-ketoesters 2.3. Tautomerization 2.4. Polymerization 2.5. Tautomers as initiators 2.6. Adsorption and separation of metal ions by tautomeric polymers 3. Oxime-nitroso tautomers 3.1. Introduction 3.2. Polymerization of acrolein oxime 3.2.1. Thermal polymerization 3.2.2. l3utyllithium-catalyzed polymerization 3.2.3. Cationic polymerization 3.2.4. Radiation-induced polymerization 3.2.5. Crystal structure of bicyclic compounds from vinyl oxime 3.3. Preparation of polyoximes 3.4. Oximes as polymerization catalysts 4. Internal olefin type tautomers 4.1. Introduction 4.2. Monomer-isomerization polymerization 4.2.1. Polymerization 4.2.2. Isomerization and equilibrium 4.3. Polymerization catalysts 4.3.1. Transition metal compounds 4.3.2. Alkylaluminum 4.3.3. Ternary catalysts 4.4. Kinetics and mechanism 4.5. Copolymerization and applications 5. Copolymerization of acidic and basic vinyl monomers 5.1. Introduction 5.2. Spontaneous copolymerization of acrylic acid and 4-vinylpyridine 5.3. Macroscopic and microscopic acid dissociations of the copolymer References

557

558 559 559 560 560 560 562 565 567 570 571 571 571 571 572 573 573 574 576 578 578 578 579 579 580 581 581 581 581 582 583 585 585 585 587 589

558

SEIZO MASUDA and KEIJI MINAGAWA

1. GENERAL

INTRODUCTION

The term tautomer is employed for structural isomers in rapid equilibrium. Tautomerism

is classified as cationotropy and anionotropy when a cationic or anionic species migrates within a molecule. When tautomerization involves only the shift of a proton, it is called prototropy or proton tautomerization. H-X-Y=Z~H+X--Y=Z~X=Y-Z-H+

S,X=y-Z-H

where X, Y and Z represent atoms such as carbon, oxygen and nitrogen. Prototropy is divided into the following, depending on the atoms represented by X, Y and Z. 1. 2. 3. 4. 5.

Keto-enol H-C-C=05 C-C-O-H Nitroso-oxime H-C-N=05 C-N-O-H Lactarn-lactim H-N-C=O%N=C-O-H Ketimine-enamine H-C-C=N% C-C-N-H Internal olefin H-C-C=C% C=C-C-H

The mobile proton may affect some reactions of the tautomer with other chemical species because of its high reactivity. For example, many tautomers form various complexes with metal compounds, and the resulting complexes can participate in subsequent reactions as a reactant or a catalyst. Moreover, tautomerism and other related reactions depend greatly on the reaction conditions because the equilibrium between tautomers is sensitive to the conditions. This fact means that a variety of reactions can be controlled through shifting of the equilibrium. Such a property of tautomers is of interest from the viewpoint of polymer synthesis. The tautomers can be a monomer and/or a polymerization initiator which are expected to have a well-controlled reactivity. Since vinyl polymerizations are extensively used for producing polymers, it has been useful to study the polymerization behavior of vinyl monomers having a tautomeric structure. Among the above five examples of prototropic tautomerization, internal olefins do not give a polymer having the structure of the original monomer because tautomerization involves a migration in the position of the carbon-carbon double bond.’ This type of polymerization is called monomer isomerization polymerization.2’3 On the other hand, the equilibrium between ethylideneamine and vinylamine lies greatly toward the latter, and so it polymerizes to give poly(vinylamine). H---r

fCH 2-FH)T

F C2H5

+H-NH) A H3

T

+c

CH=NH q L H3

CH,=CH-, t H2

C2H5

-(CH ,-CH), t H2

Vinyl alcohol is also an unstable tautomer. Therefore, it is impossible to carry out a direct polymerization to poly(viny1 alcohol).

TAUTOMERS AS MONOMERS AND JNITIATORS

559

Vinyl compounds with tautomeric groups as substituents such as acryloylacetone, acrolein oxime and acrylamide are also polymerized through the vinyl polymerization mechanism. Here the double bond in the tautomeric substituent does not participate in the polymerization, but the migration of the double bond, or a mobile hydrogen, significantly affects the polymerization depending on the reaction conditions. This review is concerned with tautomers as monomers and as polymerization initiators.

2. KETO-ENOL

TAUTOMERS

2.1. Introduction A well-known example of keto-enol tautomerization involves a proton shift between acetaldehyde and vinyl alcohol. However, this equilibrium tends extremely to the aldehyde, and its enolic form is virtually non existent. CH3-CHO%CH2=CH-OH Poly(viny1 ,alcohol) is normally prepared by a two-step process involving the polymerization of vinyl acetate followed by hydrolysis. An attractive, but hitherto unobtainable goal would be the direct polymerization of vinyl alcohol. One possibility is slow tautomerization of vinyl alcohol to acetaldehyde. Capon et ~1.~~~have reported that the stability of the enolic component (i.e., vinyl alcohol) stems from the reduced water concentration in the system, as well as from a favorable deuterium kinetic isotope effect. O-D vinyl alcohol is generated through the hydrolysis of ketene methyl vinyl acetal.5 Initial attempts at radical homopolymerization of vinyl alcohol yielded unsatisfactory results. However, by taking advantage of a stabilizing electron donor-acceptor interaction, the radical copolymerization of O-D vinyl alcohol and maleic anhydride proved to be successful.* I3-Dicarbonyl compounds are in equilibrium between the keto and enol tautomers. The enol contents increase with an increase in the electrophilicity of substituents R and R’. For example, the enol fraction R-:-CH,-t-R’ 0 R-t-CH 7-R’ 0

OH

,X

0 R-y=cH -c$R’ OH

0

of acetylacetone (R = R’ = CH$ (0.95 and 0.82 in hexane and ethanol,9 respectively) is larger than that of ethyl acetoacetate (R = CH3, R’ = O&Hs) (0.62 and 0.30 in hexane and ethanol,r’ respectively). It is known that B-carbonyl compounds are liable to form complexes with metal ions. Teyssie and Smetsi’ have studied metal adsorption on poly(acryloylacetone). Attempts have been made to clarify the relationship between the tautomeric structure and polymerizability of vinyl compounds with diketone groups as substituents.12

560

SEIZO MASUDA and KEIJI MINAGAWA

2.2. Preparation 2.2.1. Preparation

of unsaturated j3-diketones

Methacryloylacetone (MAA) was prepared by the condensation of ethyl methacrylate and acetone in the presence of sodium alcolate.11213However, the yield was poor and substituting acrylate for methacrylate did not lead to the production of acryloylacetone (AA)* ,CH3 CH,=C \

+(CH,),CC

R’ONa

w CH,=C

/

CH3

’ COCH,COCH,

COOR

Ponticello and Furman14 synthesized unsaturated B-diketones by using cyclopentadiene as a blocking agent during the Claisen condensation of the corresponding vinyl ketone and ethyl acetate.

+ CH,=!-COCH,

-&

e

_I

CH,=C-

P ;-CH,-i-CH,

COCH ,COCH,

0

(R=H,CH,) 0

Methyl p-vinylbenzoate condensed with acetone in the presence of sodium hydride to give p-vinylbenzoylacetone (VISA) in poor yield. l5 Chapin et al. l6 also prepared VISA by Claisen condensation of p-vinylacetophenone and ethyl acetate in the presence of sodium hydride. Reaction of (meth)acryloyl chloride with sodium acetylacetone leads to (meth)acryloylacetone. 17*18

2.2.2. Preparation

of unsaturated jl-ketoesters

Ethyl acryloyl acetate (EAA) and ethyl methacryloylacetate (EMAA) were obtained by the treatment of 5-acetylnorbornene and 5-acetyl-5methyl-norbomene with diethyl carbonate, respectively, followed by thermal decomposition. l9

561

TAUTOMERS AS MONOMERS AND INITIATORS

R

(C2H5)2m3

R

COCH,

n,

COCH,COOC,H,

?

lCH,=&~cH,-~c,H,

(R=H,CH,)

0

0

Other preparative methods have been reported by Pichat et aE.” and Ceccherelli et al. 21922Crotonyl chloride and methacryloyl chloride are condensed at low temperature with an excess of the lithio derivative (prepared by metalation of trimethylsilylethoxycarbonyl acetate using butyllithium) and the non-isolated intermediate is hydrolyzed with spontaneous decarboxylation, giving rise, respectively, to ethyl crotonyl and methacryloylacetate. 20This procedure failed with acryloyl acetate. Y

C=c,

R

H’

R’ ‘(2 H’

,COCI + Li-CH 2

, COOSi (CH,) 3 > ‘COOC,H,

R’ ,COCH,COOC,H, * ‘cqR2

,COCHCOOSi (CH 3) 3 c ‘R2

H’

kOC,H,

a:R’=CH,, b:R’=H,

R2=H R2 =CH,

Rhodium(I1) acetate catalyzes the conversion of cr-diazo-&hydroxy esters into the corresponding D-ketoester (for example, EAA from acrolein). CH,== CH-CHO

+ N,yOOC,H,

-wCH,=CH-YH-fCOOC,H,

Li ----w

Ho

N2

CH,=CH-rCH,-f-OC,H, 0

0

The acid-catalyzed cyclic reaction of diketene and acetone forms diketene-acetone adduct (2,2,6-trimethyl-4H-1,3-dioxin-4-one).239” One of the postulated mechanisms involves the reaction of enolized acetone with ketene via isopropenyl acetoacetate.25y26 However, isopropenyl acetoacetate has never been isolated. 27 Hyatt2* synthesized vinyl acetoacetate (VAA) by using cyclohexadiene as a blocking agent during the reaction of vinyl acetate and diketene.

562

SEIZO MASUDA and KEIJI MINAGAWA

P

A,

CH,= C-OCOCH,COCH,

2.3. Tautomerization

It is known that in theiH-NMR spectra of B-diketone and l3-ketoester compounds the peaks of the ketonic methylene and enolic methine protons lying between the carbonyl groups appear near 3.5 and 5 ppm, respectively. Though AA has three theoretically possible forms (one ketonic and two enolic forms), itslH-NMR spectrum revealed the existence of only one of the enolic forms. CH,= CH-C-CHq-CH, 0

! (1) CH,=CH--=CH-f--H, 0‘H

1 0

\ CH,=CHfi 0

-CH%-CH, 0 H’

(2)

(3)

The chemical structure of the enolic AA was determined by use of a selective decoupling technique. The peak at 199.55 ppm in a non-decoupled13C-NMR spectrum of AA which is assigned to the carbonyl carbon appears as a quintet with a coupling constant of 5.2 Hz. The coupling of the carbonyl carbon is removed by irradiating the methyl proton at its resonance frequency so that the carbonyl carbon appears as a doublet due to spin-spin splitting by the methine proton on the adjacent carbon. This clearly shows that the carbonyl group is adjacent to the methyl group. That is, the chemical structure of AA may be most accurately represented by Formula (2).2g In the structure of CH2=CH-CO-CH~R, the stronger the substituent R pulls the electron, the more predominantly the enolic component forms. As described above, AA is only in the enolic form. On the other hand, EAA, EMAA and VAA coexist as ketonic and enolic tautomers because of the poor electron-donating ester group. MAA and VBA, S-diketone compounds, also exist in tautomeric mixtures though the ketonic content is not very high. Table 1 lists the ketonic fraction of unsaturated tautomers.30-33 The ketonic fraction

563 Table 1. The ketonic fraction of unsaturated tautomers No. 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17 18 19 20 21 23 24 25

Solvent

Ketonic fractiona EAAb EMfLAb

Hexane Cyclohexane Triethylamine Carbon tetrachloride l-Butanol 1:ZDimethoxyethane l-Propanol Ethyl acetate Dioxane Tetrahydrofitran Methanol Benzene A-c&one Acetonitrile Anisole Chloroform Dlichloromethane l.,ZDichloroethane Pyridine DGmethylformamide Dlimethyl sulfoxide Formamide

0.179

0.240 0.490 0.446 0.638 0.336

VAAb

VBAb

0.370

0

0.612

0.017

0.370 0.374 0.389 0.733 0.748 0.071 0.043

0.884 0.224 0.479 0.759 0.478 0.873 0.421 0.470 0.485 0.606 0.770 0.904

0.860 0.492 0.798 0.796

0.760 0.894 0.938

0.032 0.095 0.194

0.817 0.931 0.899

0.075 0.143 0.195

0.068 0.046 0.822 0.856 0.891

a The NMR measurements of EAA and EMAA were made in hydrogenated solvents. b EAA: ethyl acryloylacetate, EMAA ethyl methacryloylacetate, VAA: vinyl acetoacetate, VBA p-vinylbenzoylacetone

increases with increasing polarity of the solvent. Such a tendency is widely noted in the keto-enol tautomerization for ethyl acetoacetate. As a description of the solvent effects on chemical processes and spectrometric properties, linear multiparametric equations have been suggested by many authors.34-40 Kamlet and Taft proposed a “Linear Solvation Energy Relationship (LSER)“,41 and pointed out a better correlation between the solvatochromic parameters and other empirical solvent scales.42

Here x0 is the value of the given property x in the standard solvent. u* (polarity), 6 @oh&ability), (Y(hydrogen bond donor acidity) and l3 (hydrogen bond acceptor basicity) are referred to as the solvatochromic parameters, and s, d, a, and b as the solvatochromic coefficients. In order to elucidate factors governing the solvent effect, the Kamlet-Taft equation was applied to the tautomeric constants (Kr = [keto]/[enol]). As shown in Fig. 1, regression analysis of the tautomeric constants of EAA with the solvatochromic parameters gives a good linear correlation. ln Kr = 0.711 - 1.199(?r* - 0.2996) - 0.709~~+ 0.1920

R (Relative coefficient) = 0.936

564

SEIZO MASUDA and KELJI MlNAGAWA

-1.0

-0.5

0

0.5

Xo+s(n*+dS)+aa+bp Fig. 1. Regression

analysis of the tautomeric constants of EAA with solvatochromic Same symbols as in Table 1.

parameters.

A comparison of the solvatochromic coefficients reveals that the polarity and the hydrogen bond donor acidity of the solvent are major factors in the Kr variation. In addition, both factors shift the equilibrium to the ketonic side. The tautomeric constants are provided in Fig. 2 as a function of concentration. The tautomeric equilibria exhibit a remarkable dependence on concentration. Dilution with carbon tetrachloride, less polar than the solute, favors the shift of the equilibrium to the intramolecular chelate tautomer, whereas the opposite effect occurs in acetonitrile solutions. These concentration effects probably result from differences between the polarities of the solvent and/or solute-solvent interactions. Similar concentration effects on tautomerit equilibria have been reported by Roussel et aZ.43 A relationship between the tautomeric constant and the reciprocal temperature allows the determination of the enthalpy change and entropy change. Table 2 summarizes the thermodynamic parameters for these tautomerizations. A number of investigations have been presented on the phototautomerization of l3dicarbonyl compounds. Petkov et al. examined the influence of W-irradiation on the keto to enol ratio of poly(ethy1 acryloylacetate) (PEAA) and poly(acryloylacetone) (PLL~)~~in solution. PEAA and PAA have both ketonic and enolic pendant groups which are in tautomeric equilibria. The major process of photoinduced tautomerization is ketonization which is rapid. Two mechanisms for photoketonization were proposed. The first mechanism involves an internal molecular process in each B-carbonyl unit. The second involves intermolecular proton transfer. From comparing the photosensitivities of monomer and polymer, it was concluded that the polymer structure is an important parameter in the process of photoinduced ketonization.

565

TAUTOMERSAS MONOMERSAND INITIATORS

0.5

1.0

mole fraction of EAA Fig. 2. Influence of dilution on the equilibrium constants of E&Y in carbon tetracbloride (0) and acetonitrile (0).

2.4. Polymerization Polymerization of B-diketones and l3-ketoesters was kinetically investigated in polar and nonpolar solvents using 2,2’-azobisisobutyronitrile (AIBN) as the initiator. The polymerization is signiCcantly affected by the solvent. The rate of polymerization is higher in a nonpolar solvent than in a polar solvent. The kinetic results deviate from the conventional model of radical polvmerization. [AB3N]“.8[h]1.47 for AA in benzene29 [AIBN]0.48[AA]1.67 for AA in acetonitrile29 [AIBNJ0.45[EAA]1.39 for EAA in benzene3’ [AIBN]0.48[EAA]‘.35 for EAfI in acetonitrile3’ [AIBN]0.45[EMAA]1.54 for EMAA in benzene46 [AIBN1°~45[EMAA]‘~85 for EMAA in DMS046 Table Z!.Thermodynamic parameters for tautomerization of EAA, EMAA, VAA and VBA Tautomer

EMAA VAA VBA

Solvent

AH (kJ/mol)

AS (J/mol*K)

Reference

cc4

0.08

4.23 - 8.67 4.61 - 0.12 0.29 - 4.16 5.74

- 10.0 22.6 - 25.9 33.8 - 3.6 23.4 - 47.3 7.4

Ref. 41

CH$N CCb CH&N CC14 CH&N

cc4 CHXN

Ref. 39 Ref. 42 Ref. 43

566

and KIWI MINAGAWA

SEIZO MASUDA

Table 3. Monomer reactivity ratios for the copolymerizations of tautomer (MI) with styrene (Mr) No.

3 4 8 9 10 11 12 13 14 15 16 21 22 23 24

Solvent Trimethylamine Carbon tetrachloride Toluene Ethanol Ethyl acetate Dioxane Tetrahydrofuran Methanol Benzene Acetone Acetonitrile Pyridine Hexamethyl phosphoramide Dimethylformamide Dimetyl sulfoxide

AA-St rt

r2

I1

r2

2.68 1.22 1.52 1.91 2.07 1.91 1.73 1.38 2.46 2.69

0.16 0.031 0.066 0.171 0.173 0.179 0.070 0.049 0.121 0.240

0.64 1.64

0.12 0.19

1.48 1.23

2.93 3.19 4.11

0.014 0.044 0.028

EAA-St

EMAA-St rl

r2

0.67

0.0062

0.62

0.27

0.27 0.17

0.56

0.17

0.66 0.89

0.15 0.15

0.57

0.22

0.40 0.66 0.47 0.39 0.42

0.11 0.10 0.18 0.16 0.062

0.76 0.66

0.18 0.18

0.43 0.46

0.20 0.10

For acrylic acid esters, the literature shows a deviation in the order with respect to monomer concentration. 47 AA exists in only the enolic form, while EAA and EMAA coexist as two tautomers. Therefore, deviation from the conventional model of radical polymerization is thought to be due to the B-dicarbonyl group rather than to tautomerism. Tables 3 and 4 show copolymerization data for the systems of unsaturated keto-enol tautomer with styrene. Monomer reactivity ratios rl are dependent on the solvent, while r2 are essentially independent except for the system of AA and styrene. Figure 3 shows an example of the relationship between the monomer reactivity ratio and solvatochromic parameters. The regression equation of monomer reactivity ratio rl for the EAA(Mr)styrene(M2) system is expressed as follows. ln(l/rr)=

-1.515+3.217(x*-0.1396)+0.999a-1.5470 R = 0.971

Table 4. Monomer reactivity ratios for the copolymerizations of tautomer (MI) with MMA (M2) No. 8 9 10 12 13 14 15 16 23 24

Solvent Toluene Ethanol Ethyl acetate Tetrahydrofuran Methanol Benzene Acetone Acetonitrile Dimethylformamide Dimethyl sulfoxide

EAA-MMA rt

r2

rl

f-2

3.48 2.01 2.98 1.66 1.11 1.73 1.75 0.65 1.60 1.14

0.60 0.41 0.56 0.431 0.29 0.56 0.62 0.22 0.74 0.60

2.57

0.60

0.73 .12 0.93 1.68 1.11 1.01 0.72 0.64

0.37 0.50 0.42 0.48 0.60 0.61 0.65 0.60

EMAA-MMA

TAUTOMERS AS MONOMERS AND INITIATORS

567

kV 0.5 v5

O-

-0.5 -

-1.0-

I

-1.0

I

-0.5

I

0

0.5

-1.515+3.217(n* - 0.1396)+0.999a+ 1.5478

Fig. 3. Regression analysis of rl for the system of EAA (MI) and styrene (Mz) with solvatochromic parameters. Same symbols as in Table 1.

Comparison of the solvatochromic coefficients reveals that the relative reactivity depends on polarity, hydrogen bond donor acidity, and hydrogen bond acceptor basicity, with the first factor being especially important. An increase in the polarity and hydrogen bond donating power of the solvent results in reduced rl values, whereas an increase in hydrogen bond accepting power of the solvent should produce an opposite effect. 2.5. Tautomers as initiators The thermal stability of acetylacetone with various mono-, di- and tri-valent metals was determined at 191”C.48 No general correlations were observed between the thermal stability and properties of the acetylacetone, nor with the properties of the parent-metal ions. Amett and Mendelsohn49 found that the autoxidative decomposition of Fe(II1) acetylacetona.te at 100°C under oxygen is inhibited by benzoyl peroxide and azobisisobutyronitrile (AIBN), both of which usually serve as good autoxidation initiators, and conversely that the reaction is not inhibited by usually good antioxidants such as phenol, 2,4,6-tri-tert-butylphenol and hydroquinone. These results suggest that rather stable radicals are formed by the thermal decomposition of the chelate and that these radicals are not intercepted by inhibitors but rather by the radicals produced from the decomposition of the initiators. The chelates which undergo facile autoxidation can initiate the polymerization of styrene. The proposed mechanism for destructive autoxidation includes the formation of an acetylacetone radical by a homolytic dissociation of the metal-xygen bond.”

568

SEIZO MASUDA and KFUI MINAGAWA

Kastning et ~1.~~examined the efficiency of some metal chelates as initiators for the polymerization of styrene and ethylene and for the copolymerization of styrene with vinyl acetate or ethylene. The initiation efficiency depended both on the central atom of the metal chelate and on the type of organic ligand. The metal chelates from groups VII and VIII of the periodic table were shown to be the most effective. Bamford and Lind52 were the first to deal with the kinetics of vinyl polymerizations by metal acetylacetonates. They found that the initial rate is proportional to square root of the metal chelate concentration and directly to the monomer concentration in MMA polymerization initiated by manganese(II1) acetylacetonate. They concluded that the polymerization proceeds through a radical mechanism. The results of supplementary examinations by Otsu et al.53 were different from those obtained by Bamford: the order with respect to monomer concentration was observed to be higher than unity. This observation suggests the participation of monomer in the initiation step of the polymerization. That is, the initiation involves the formation of a complex of monomer and metal chelate with the formation of a MMA radical by the reduction of the metal chelate caused by the coordination process. The copolymerization of vinyl monomers in the presence of metal chelates was also investigated.53-55 The copolymerization parameters obtained are close to the values for conventional copolymerization, which supports a radical mechanism and no participation of a metal chelate in the propagation step. The copper chelate of ethyl acetoacetate can also initiate MMA polymerization and its copolymerization with styrene.56 In homopolymerization, the rate equation deviates from the conventional model of radical polymerization as well as in the case of copper(II)57 and manganese(III)53 chelates of diketonate. The composition curve for the system of styrene and MMA is, however, typical for a radical copolymerization process. EAA, an unsaturated keto-ester compound, forms a complex with various metal ions. If the metal chelate of EAA is subjected to redox reactions and to radical formation, it can be radically polymerized without any catalysts. That is, EAA behaves as both a catalyst and a monomer. The polymerization of EAA in methanol was carried out at 50°C in the presence of cobalt(I1) acetate. As shown in Figs 4 and 5, log-log plots of the polymerization rate vs concentrations of both cobalt(B) acetate and EAA give good linear correlations. The rate equation is expressed as

R, =k[Cobalt acetate]“.51[EAA]3.12 The rate of polymerization is approximately proportional to the square root of the concentration of cobalt(B) nitrate. This suggests the participation of cobalt(I1) ion in the

TAUTOMERSAS MONOMERSAND INITIATORS

569

-4.3 -

I

-4.9 -2.5

-1.5

-2.0

Log [CO(OAC)~] (molI I)

Fig. 4. Relationship between polymerization rate and concentration of cobalt@) acetate for the polymerization of EAA. [EAA] = 2.07 mol/l.

initiation step and of two propagating radicals in the termination step. On the other hand, the higher o:rder with respect to monomer concentration suggests the participation of monomer in both the initiation and propagation steps. The elementary reaction of the polymerization of EAA in the presence of fIobalt(II) nitrate may be expressed as follows. Complex formation: 3EAA + Co(OAc), S Co-EAA3 + 2AcOH

-5’

’

0.2

I

I

0.4

0.6

Log [EAA] (mol /I)

Fig. 5. Relationship between polymerization rate and monomer concentration for the polymerization of EL4A. [CO(OAC)~]= 2.07 x lo-’ mol/l.

570

SEIZO MASUDA and KEIJI MINAGAWA

Initiation:

CoEAAs + EAA g Co-E& Co-E&

$ Co.EAA3 + EAA.

Propagation: EAA. + EAA & EAA. kt Termination: 2EAA. 5 P or 2P where Co*EAA3 and COW& represent cobalt ions coordinated with three and four molecules of EAA, respectively. These elementary reactions lead to a rate equation where the rate is proportional to the square root of cobalt acetate concentration and to the third power of EAA concentration. The systems of PEAA and copper(I1) nitrate and PAA and cobalt(I1) nitrate can initiate MMA polymerization. 58In these cases, the monomer participates in the initiation step of the polymerization. However, no polymerization occurs by the use of metal acetate in place of nitrate. 2.6. Adsorption and separation of metal ions by tuutomeric polymers Compounds containing a l3-diketone group can be used as chelating agents to extract metal ions from aqueous and non-aqueous solutions.59’60Hoeschele et al.60 prepared PAA by anionic polymerization of methyl vinyl ketone and simultaneous condensation with acetic anhydride, and examined its chelating properties for metal ions. When compared to its monomeric analogue, the chelating ability of the polymer is similar for copper(I1) ions and about three orders of magnitude larger for uranyl(II) ions. Tomida et aL61 also reported dissociation and overall binding constants of the polymer obtained by radical polymerization of AA. The resulting values are different from those reported by Hoeschele. This is probably due to the solvent used. The adsorption equilibria of PAA with metal ions conform to the Langmuir isotherm. The maximum adsorption on PAA is in the order Hg(I1) > > Cu(I1) > Ni(I1) > Co(I1) > Cd(II), showing high selectivity for mercury(I1) ion. The magnitude of the stability constants is also in agreement with this order. The adsorption depends significantly on the pH of the solution. The separation of copper(I1) ion from aqueous solutions containing Cu(I1) and Ni(I1) ions was successfully achieved by a stepwise decrease in the pH of the eluent. The adsorption of copper(I1) and mercury(I1) ions on PA4 was kinetically investigated by taking account of external mass transfer. Model equations were proposed and the diffusion coefficients were determined by matching the calculated uptake curves with the experimental results.62 One metal ion was also separated from a mixture of light rare-earth metal salts.63 Even with pairs of neighboring elements, a fairly good separation was achieved. Better resolution was attained with an increase in the duration of the stepwise decrease in pH. Poly(p-vinylbenzoylacetone) and poly(methacryloylacetone-co-divinylbenzene) were prepared and their adsorptivities for nine metal ions (mainly divalent ions) were investigated by batch operations.59 These polymers were highly selective for Fe(II1) in the acidic region, but had no significant affinity for other metal ions. Marmor and Kidene64 prepared a poly(&diketone) by the oxidation of poly(viny1 alcohol) with chromic acid. This polymer was effective in removing metal ions from solution. The metal ions adsorbed to the polymer may be completely recovered with dilute aqueous acid, and the polymer can be reused.

TAUTOMERSASMONOMER!3ANDINlTLATORS 3. OXIME-NITROSO

571

TAUTOMERS

3.1. Introduction

Oximes are obtained by the reaction of an aldehyde or a ketone with hydroxylamine. An oxime has two different types of isomerism: geometrical isomerism and tautomerism. R

‘C=N’

Syn-anti

ai

w

oxime-nitroso

R’ ‘C=N__OHe w

#

R’

p)C=N_ R ‘CH=N-0 w

There are orrly a few reports concerning oxime-nitroso tautomerism. The tautomerism of 5,8-diethyl-7-hydroxy-6-dodecanone oxime6’ and ar-hydroxyimino-2-quinolylacetonitrile66 was spectrometrically studied in various solvents. The NMR spectral data supported the conclusion that the nitroso forms of these compounds were unstable. The nitro-aci-nitro tautomerism for nitroparaflins was also reviewed. 67 Brokenshire reported on the silver oxide oxidation of a number of oximes. The reaction involves the intermediate formation of iminoxy radicals which are in equilibrium with “unstable” dimers. In practice, acrolein oxime (AOX) inhibits or retards the polymerization of vinyl monomers such as styrene, methyl methacrylate, and acrylonitrile.69 The great inhibiting or retarding ability of AOX is derived from the formation of inactive AOX radicals by chain transfer reactions. On the other hand, some oximes and the binary system of an oxime and a metal salt can often initiate vinyl polymerization.70-73 AOX, a vinyl compound having an oxime group as a substituent, can be polymerized by heating without any catalyst to give an oligomer with an average molecular weight of around 1000. The mechanism of the thermal polymerization seems not to be radical but ionic in nature. AOX can retard or inhibit the radical polymerization of vinyl compounds because of a remarkable chain transfer effect of the oxime group. On the other hand, there are unsaturated oxime compounds such as diacetone acrylamide oxime which can be polymerized radically. The rate of polymerization of diacetone acrylamide oxime with azobisisobutyronitrile was much higher than with peroxides.74 However, in the case of perester initiating systems, a cobalt salt promoted the polymerization rate markedly. It is well-known that the oxime group has a powerful affinity for metal ions. There are many reports concerning metal adsorption and separation by polymers with pendant oxime groups. 3.2.

Polymerization of acrolein oxime

3.2.1. Thermal polymerization AOX can ble polymerized just by heating. The thermal polymerization of AOX is not accelerated by the addition of radical initiators and is not inhibited by radical scavengers.

572

SEIZO MASUDA and KEIJI MINAGAWA

This suggests that the polymerization proceeds via a non-radical mechanism.75 Both cationic and anionic catalysts accelerate the polymerization of AOX. Monomer reactivity ratios for the thermal copolymerization of AOX with styrene, methyl methacrylate, and acrylonitrile agree with those for the butyllithium-catalyzed copolymerizations. Therefore, the thermal polymerization of AOX was concluded to proceed through an anionic mechanism, in which the initiating species is the anion, CH2 =CH-CH=N-O- , arising from the dissociation of the molecular aggregates of AOX. The aggregation number varies depending on solvents. Accordingly, the kinetic order with respect to monomer concentration is also dependent on the solvent.76 When AOX is added to a growing chain, there are five possible paths: (a) l,Zaddition, (b) 3,4-addition, (c) l,Caddition, and (d, e) hydrogen transfer reactions. -CH,-

Hz H=N-Cl-l

(a)

-YHT CH,=CH

(b CH *= CH

\

OH

Y

AH=N-OH e) -CH,-?H CH-N(OH)

\

11(4

P

-cH2-fH CH- NH-O-

- CH,-

7H2 CH =N-O-

On the basis of functional group analysis and spectral data, the distribution of structural units for an AOX polymer with an average molecular weight of 2100 was determined as CHrCH-CH=N-0

f CH,yH

m

CHz-CH2-CH-N-O mCHZCH=CH-y

CH=N-OH TCH,-CH=CH-NH-O

12.6

F OH

H

AOX is an equilibrium mixture of two isomers ([syn form]/[anti form] = 18/83).77 The syn form is a viscous liquid and the anti form white needle-like crystals with a melting point of 33-35°C. When syn form rich and anti form rich AOXs are polymerized thermally, the syn-anti ratio of the content remaining unpolymerized approaches that in the equilibrium state with time during the polymerization, as shown in Fig. 6. 3.2.2. Butyllithium-catalyzed

polymerization

A characteristic time-course was observed for the butyllithium (BuLi)-catalyzed polymerization. 78A rapid reaction initially occurs in bulk polymerization. The use of dichloromethane gives a linear relationship between time and conversion, whereas the polymerization in tetrahydrofuran was violent during its initial stage and afterward proceeded at a constant rate. The initial rate was proportional to the square root of BuLi concentration for the bulk polymerization, while for the solution polymerization, it was proportional to the fist power of the concentrations of both BuLi and AOX.

TAUTOMERS AS MONOMERS AND INITIATORS

0

5

10

15

573

20

cs 40

Time (h) Fig. 6. Syn-anti isomerization

of AOX.

A, 45°C; 0,6O”C;

l,7O”C.

It was concluded from these findings that: (1) the polymerization involves no termination process and a stable polymer is produced by a chain transfer reaction with monomer, and (2) the polymerization proceeds through an initiation step by the reaction of BuLi with AOX, each of which has different aggregation factors depending on the kind of solvent used. An equimolar reaction of AOX and BuLi was studied at 0°C in various solvents.79 The structure of the reaction products was determined by 13C-NMR and GC-MS. Lithium acrolein oxime was the main product. When the reaction mixture was treated with dilute hydrochloric acid, about 80% of the AOX was recovered along with many other products, such as heptanal oxime, 2-methylhexanal oxime, butyl vinyl ketone oxime, and butyl hexyl ketone oxime. The initial processes of AOX polymerization by BuLi was discussed on the basis of the reaction products. 3.2.3. Cationic polymerization The polymerization of AOX initiated by hydrochloric acid was studied in 1,2-dichloroethane over the temperature range from 0 to 40°C. The reaction is generally incomplete, giving limited yields of polymer, depending on the initial catalyst concentration, but not on that of the monomer.sO Individual values for the rate constants were estimated on the basis of the nonstationary chain reaction proposed by Pepper.81 The molecular weight of the polymer, ,which is determined by chain transfer rather than termination, is very low (
SE120 MASUDAand KEIJI MINAGAWA

574

5 . oc” 8 -I

-6.

-7 .

3

4

5 1000/T

Fig. 7. Arrhenius plot for the polymerization of AOX in methyl tetrahydrofuran. [AOX] = 6.86 mol/l, dose rate: 1.84 x lo5 r/h.

be seen from Fig. 7, an Arrhenius plot of the polymerization rate can be approximated by two straight lines. The polymerization gave low molecular weight products such as acetaldoxime, propionaldoxime, propenylhydroxylamines and dioximes, as well as the polymer. 83 The initial phases of the polymerization were discussed on the basis of activation energies and the low molecular weight products obtained. An anionic mechanism seems to be operative above room temperature and a cationic mechanism below -23°C. The reaction in the intermediate temperature range proceeds by a competitive mechanism. 3.2.5. Crystal structure of bicyclic compounds from vinyl oxime The thermal oligomerization of methacrylaldehyde

oxime leads preferentially to two 2,4,7-trimethylperhydroisoxazolo[2,3-a]pyrid~e-2,7-dic~b~dehyde dioximes, which are conformationally isomeric with each other. The a-form is monoclinic with the space group P2Ja. The cell dimensions are a = 7.858(3), b = 13.186(6), c = 17.259(6), I3= 130.38(3), and 2 = 4. The molecules are linked by OH---O and OH..-N intermolecular hydrogen bonds, with O---O and O.--N distances of 2.794(6) and 2.760(5), respectively. 84 On the other hand, the B-form is monoclinic with a space group P2r/c, a = 13.274(11), b = 8.652(7), c = 11.618(14), B = 96.73(8), and Z = 4. The molecules are linked by two N---O hydrogen bonds, with distances of 2.824(5) and 2.897(5), respectively.85 Figures 8-11

TAUTOMERS AS MONOMERS AND INITIATORS

575

Fig. 8. A perspective drawing of the bicyclic trimer (a-form) from methacrylaldehyde oxime, by ORTEP. 86

show schematic drawings and orthogonal projections for the crystal structure of the CY-and S-forms. The thermal polymerization of 3-methyl-3-buten-Zone oxime gives preferentially the crystalline cyclic dimer, 8-hydroxy-1,4,5,7-tetramethyl-6,8-diazabicyclo[3.2.l]o~-6-ene 6-oxide.87 Its space group is P2r/n with a = 9.459(3), b = 10.335(3), c = 11.352(4), l3 = 105.50(2), and 2 = 4.

0, Carbon; l, Nitnqm;

0, Oxygen

Fig. 9. Orthogonal projection of the crystal structure of the a-form viewed along the u-axis.

576

SEIZO MASUDAand KEIJI MINAGAWA

Fig. 10. A perspective drawing of the bicyclic trimerdg-form) from methacrylaldehyde oxime, by ORTEP.

3.3. Preparation of polyoximes The bulk polymerization and copolymerization of diacetone acrylamide oxime was studied at 120°C in the presence of different initiating systems.74 The rate of polymerization of diacetone acrylamide oxime with AIBN as the initiator was much higher than with peroxides. However, in the case of perester initiating systems, cobalt salts promoted the polymerization rate markedly. Diacetone acrylamide oxime readily formed copolymers with trimethylol propane trimethacrylate as a crosslinking agent and N-vinylpyrrolidone as a reactive diluent. The copolymers exhibit good mechanical properties and high temperature behavior. The radical copolymerization of acrylamide with 4-(aldoximido)-N-[[4’-(3-acryloylaminopropyl)4’,4’-dimethylamino]butyl]pyridium dibromide and 2-methyl-N-(phenacyl-

Fig. 11. Orthogonal projection of the crystal structure of the g-form viewed along the u-axis.

TAUTOMERS AS MONOMERS AND INlTIATORS

577

oximido)-5-vinylpyridium bromide hydrochloride was kinetically investigated. 88 The oxime-containing monomers had an inhibiting effect on the polymerization. The inhibition was attributed1to an inactivation of the macroradicals by the oximes and to the presence of nitroso compounds which were produced as a result of tautomerization. Park and Jung89’9oprepared poly(arylene-1,4-oxadiazole)s bearing flexible, linear side chains by solution cyclodehydration of their precursors, poly(arylene acrylamide oxime)s, which were synthesized by solution polycondensation of 2,5-bis(n-alkoxymethyl)terephthalamide oximes with terephthaloyl dichloride. A reduction of the phase transition temperatures and an enhancement in the solubilities of the polymers were greatly affected by the length of the side chains in these systems.

cy-Cloro-o+isonitrosoacetonazine was condensed with aromatic diamines to give amidoxime group-containing polyketazines. 91Altunas and Bekaroglu92 prepared a novel polymer composed of 18-crown-6 and azoquinoxaline units from (18~crown-6)bis (quinozalin-Zone oxime). In connection with the concentration and separation of metal ions, the oximation of polymers was performed. Annenkova et aZ.93prepared polyacrolein using various catalysts and the resulting polymer was oximated. The most suitable polyacrolein for oximation was that obtained using K2S20a-Fe2 + catalyst, which gave a polymer with a sufficiently uniform composition, resulting in a low tendency toward side reactions. Acrolein was grafted onto electon-beam irradiated polyethylene films in order to make functional polymers containing oxime, hydrazone and oxyacid. 94 The conversion of the aldehyde group to oxime, hydrazone and oxyacid was small when the grafting yield of acrolein was high. Acrylonitrile was grafted onto a hollow fiber type polyethylene by radiation-induced graft polymerization. 95When the grafted resin was soaked in hydroxylamine, the cyan0 group was converted into an amide Fxime group. The pore radius, which was initially dis$ibuted broadly from 500 to 10 000 A for the base polymer, was changed to about 1000 A with a narrow distribution by the grafting. The pore volume and specific area increased and decreased, respectively, with an increase in the grafting degree. The amount of uranium adsorbed on the resin was 64% of the amount of nitrogen introduced in the amidoximation.

578

SEIZO MASUDA and KEIJI MINAGAWA

3.4. Oximes as polymerization catalysts The photopolymerization of MMA was studied in the presence of benzyl and phenylpropanedione monoxime esters. The polymerization is intitiated by photoinitiators in the excited single state.% Oxime ester derivatives were obtained by treating diacetyl monoxime and 3-oximino2,4_pentanedione, which were prepared from methyl ethyl ketone and acetylacetone, respectively, with aromatic acid chlorides. Esterification of diacetyl monoxime and 3oximino-2,4-pentanedione with methacryloyl chloride gave 2,3-buanedione O-methacryloyl oxime and 2,4-pentanedione 0-methacrlyloyl oxime, respectively. 97UV-irradiation of the oxime esters produced radicals capable of initiating the polymerixation of MMA. The copolymerization of 2,3-butanedione 0-methacryloyl oxime and 2,4_pentacedione Omethacryloyl oxime with MMA was carried out, and acrylonitrile was photografted onto the resulting copolymers. Other acetyl monoxime esters were also synthesized by the reaction of diacetyl monoxime and acyl halides (benzoyl, p-tert-butylbenzoyl, p-methoxybenzoyl, dimethoxybenzoyl, and acetyl derivatives), and the photopolymerization tendency with MMA was discussed.72798 1,2-Diphenyl-1,2-ethanedione-2-0-acryloyloxime was homopolymerized or copolymerized with (-)-menthyl acrylate, and the polymers were used as polymeric photoinitiators for mixtures of 1,6-hexanediol diacrylate and butyl acrylate. The photoinitiated capacities of the polymeric photoinitiators were compared to 1,2-diphenyl-1,2-ethanedione-2-O-acetyloxime. 71 The system of an oxime and a metal ion also has the ability to initiate polymerizations. Wu et al. 7oreported the kinetics of the aqueous polymerization of MMA initiated by copper polypropylene-based polyamidoxime-sodium sulfite systems. The overall rate of polymerization and a dependence of induction period on the concentrations of sodium sulfite, polymer-supported copper, and monomer were observed. The polymerization was initiated by a primary radical generated from the redox reaction rather than a coordination-proton transfer mechanism. The catalytic effect of molybdenum(II1) chloride and different oximes on the polymerization of MMA was studied.73 A radical mechanism for the polymerization MoC13-oxime catalysts was suggested by polymer tacticity which was identical to that for radical-polymerized MMA. 4. INTERNAL

OLEFIN

TYPE TAUTOMERS

4.1. Introduction It is generally considered that internal olefins are poorly polymerized because of steric hindrance of the 1,2-dialkyl substituents. For example, 2-butene (2B) is not polymerized into poly-Zbutene (P2B). Under appropriate conditions, however, it can give poly-lbutene (PlB), i.e., 2B is polymerized after isomerization to 1-butene (1B). This kind of polymerization is called monomer-isomerization polymerization. 293

TAUTOMERS AS MONOMERS AND INITIATORS

2B

579

P2B

+t CHF

c

H

.-b

CM”3

1B

PlB

In this case, isomerization of the original monomer gives an isomer which can undergo ordinary vinyl polymerization. During the polymerization, unreacted monomers isomerize to supply new reactive monomers, and the composition of isomers in the reaction mixture approaches thermodynamic equilibrium. Therefore, the polymerization markedly depends on the reactivity of the isomers produced by the migration of the double bond within the monomer, Le. tautomerization of a monomer. Otsu and coworkers reported that many internal olefins, e.g., 2-butenes, 2-pentene, and 4-methyl-2-pentene, etc., undergo monomer-isomerization polymerization with Ziegler-Natta catalysts.2 Here we focus on the monomer-isomerization polymerization of internal olefins, mainly 2-butene, as a kind of tautomer polymerization. 4.2. Monomer-isomer&don

polymerization

4.2.1. Polymerization A typical example of monomer-isomerization polymerization is seen in polymerization of 2-butene (2B). There are two geometrical isomers in 2B, i.e., cis and trans isomers, both of which were considered to be non-polymerizable due to the steric hindrance of the two methyl groups. However, the use of Ziegler-Natta catalysts was found to be effective for the polymerization of 2B. Friedlander tirst observed the polymerization of 2B using AlEts-TiCI,,-montmorillonite as the initiator, though the structure of the polymer obtained was not identified. Symcox,lOOShimizu et aZ.,lol and Iwamoto and Yuguchiro2 independently found, at nearly the same time, that 2B can be polymerized to give high molecularweight PlB through coordination anionic polymerization in the presence of various Ziegler-Natta catalysts. For example, polymerization of 2B, at a concentration of about 6 mol/l in n-heptane, with AIEt3-TiC13 catalyst (AliTi- in molar ratio) at 80°C for 28 h resulted in the conversion of 18.0% and 17.5% for c2B and t2B, respectively. The IR and X-ray spectra, and physical properties indicated that the polymers obtained from both c2B and t2B were isotactic PIB.rol Isomerization polymerization, which includes isomerization of monomer during polymerization, was already known for the cationic polymerization of some branched

580

SE120 MASUDAand KEUI MINAGAWA

Table 5. Results of the isomerization of c2B and t2B with various catalysts in n-heptane at 8O”C, [butene] = 5.67-6.00 movL Butene

Catalyst

Catalyst cont. ( x 10’ mol/I)

c2B kzH&A (GHs)zAICI ::z)AIc12 Tic4 LiAIH4 TiCI + H20a Tic& TiQ TiCI

11.1 8.3 8.3 5.0 45.3 11.7 5.0 20.2 20.2 20.2

t2B Tic& Tic& Tic& vcl,

12.5 12.5 12.5 10.0

Time (h)

28 28 28 28 28 28 28 28 28 100 158 28 28 100 158 100

Isomer distribution (%) 1B

t2B

c2B

0.0 0.0 0.0 0.0 0.0 0.0 0.0 4.0 3.5 3.6 3.8 0.0 1.2 3.0 3.5 0.0

0.9 0.9 0.0 0.0 0.0 0.0 0.0

99.1 99.1 100.0 100.0 100.0 100.0 100.0 86.6 81.5 47.9 35.3 0.0 3.6 10.9 15.1 0.1

9.4

15.0 48.5 60.9 100.0 95.2 86.1 81.4 99.9

a H,Ofli molar ratio = 0.05

cr-olefms. 10334 In this case, the isomerization takes place at the growing carbonium ion during the propagation step. On the other hand, the polymerization of 2B to PlB with a Ziegler-Natta catalyst was considered to be a two-step reaction, i.e., 2B was assumed to isomerize into more reactive 1B monomer before polymerization. The reaction conditions for the isomerization polymerization of internal olefins were extensively examined, and much evidence for the distinct two-step reaction has been obtained. The two processes are influenced differently by the catalysts. The effects of a variety of catalysts on isomerization and polymerization have been extensively studied. 4.2.2. Isomerization and equilibrium The equilibrium of butene isomers in the presence of each component of a typical Ziegler-Natta catalyst was examined. ’ Table 5 summarizes the results for c2B and t2B. No thermal isomerization occurred below 80°C. TiC13was the only catalyst effective for isomerization from 2B to 1B. The composition of the isomers approached the equilibrium calculated from thermodynamic stability, i.e., S.l%, 22.5% and 72.4% for lB, c2B, and t2B, respectively. The concentration of 1B in the polymerization system was expected to approach equilibrium as the polymerization progressed. Actually, an analysis of the remaining monomer composition after polymerization revealed that the concentration of 1B gradually increased and approached the equilibrated concentration. Since the equilibrium of butene isomers lies towards 2B, the concentration of polymerizable 1B is always low during polymerization. Some catalysts capable of both isomerization of butenes and polymerization of 1B do not induce isomerization polymerization, when the catalysts require large amounts of monomer because of their unstable active sites. lo5In this case, however, the addition of an effective catalyst for rapid isomerization,

TAUTOMERSASMONOMFWANDINlTIATORS

581

e.g. NiC12, induces polymerization. Therefore, proper selection of catalysts for both isomerization and polymerization is important for monomer-isomerization polymerization. 43. Po~merization catalysts 4.3.1. Transition metal compounds Various Ziegler-Natta catalysts were examined for the polymerization of butenes. The results with transition metal compounds (TiC13,TiC14,VC13,VOC13and V(acac),) and R&l (R = C2H5or iso-CdH9) indicated that Tic&-RN catalysts have high catalytic activity. lo5 VC13catalyzed neither the isomerization nor polymerization. The polymer yield was very low with TiC14-(~H5)~ and other V-based catalysts in spite of the fact that these catalysts have catalytic activity for both the isomerization of butenes and polymerization of 1B. In fact, the monomer composition approached equilibrium, indicating that these compounds catalyze the isomerization process. The low polymer yield was then explained to be due to the low stability of the polymerization site of the catalysts. The increase of polymer yield with NiC& as the cocatalyst with TiCl,&GH5)+U supported this interpretation. 4.3.2. Alkylaluminum Polymerization of 2B was carried out with R&lCl,-Tic& catalysts (R = GH5 or iso&I&,, x = l--3) to clarify the effect of alkylalwninum. lo5The polymer yield was high only in the case of x = 0, i.e. using the R&l catalyst. Both the polymer yield and monomer composition remaining after polymerization depended on the Anti molar ratio. The yield showed a maximum value at Anti ratios near 2.0-3.0, while the component of polymerizable 1B increased with increasing Al/Ti molar ratio. The optimum conditions for the polymer yield may be explained as a result of a counterbalance of alkylation and reduction of Tic4 by (GH&Al. The effect of different alkylaluminums on the polymerization provides useful information concerning the reaction mechanism’06 as is described later. 4.3.3. Ternary catalysts The addition of transition metal compounds as a cocatalyst which facilitates the isomerization can play a significant role in the monomer-isomerization polymerization. lo7 The polymerizal.ion of 2B was carried out in the presence of metal chloride, acetylacetonate (acac), and dimethylglyoxime (DMG), with Al(&H&-Tic& catalyst. It was found that NiC12, Fe(acac)3, Cr(acac)3, and Ni@MG)z facilitated the polymerization, while Al(qH5)3-Fe(acac)3, Al(C2H5)3-Ni(acac)3, and Al(&H5)3-Ni(DMG)2 induced no polymerization. NiS, Ni(acac)z, and metal halides except for NiClz retarded the polymerization. Among the transition metal compounds examined, nickel compounds are known as excellent isomerization catalysts. The catalytic properties of TiCIJ-N&-(~H&Al were studied in detail. lo8 The addition of NiC12as a cocatalyst with TiCl&&H5)&l facilitated the isomerization of c2B, which caused a change in the rate-determining step from isomerization ‘topolymerization. The polymer yield was the highest at NQ”‘Iiand AI/X molar ratios of 1 and 3, respectively. Though NiClz itself does not catalyze the isomerization, it

582

SEIZO

Table 6.

MA!XJDAand KEJJIMINAGAWA

Apparent rate constants for isomerization andpolymerization. ‘09[c2B] -4.0 moV1,[Tic&] = 50 mmol& Al/X - 3.0

Rate constant I&”

Numerical value (s-l) TiCVA&H+

4

1.1 x lo5

1.7 x 1o-4

: k, 4 4, k,

4.5 3.2 1.4 4.7 4.3 3.8

7.3 5.5 2.0 7.5 9.3 6.7

x x x x x

10” 1o-3 1o-5 lo5 1O-4

x lOA 10-j x lOA x lo4 x 10q

’ kxy(x, y; 1, t, c, p) are the rate constants indicated in the scheme.

forms a bimetallic compound composed of Ti, Ni, and alkylaluminum, which has high activity for isomerization. The effects of each component in the various combinations of catalysts have provided much information on the reaction mechanism for monomer-isomerization polymerization of internal olefins. 4.4. Kinetics and mechanism In order to clarify the reaction mechanism, the kinetics of the monomer-isomerization polymerization of c2B was studied in the presence of Tic&-NiCl,-(GH&Al or TiC13(&H&Al catalyst. lo9 The apparent activation energies were estimated from polymerizations performed at temperatures between 50 and 90°C. The apparent activation energies for polymerization with TiQ-(C,H&Al and with Tic&-NiClz-(CzH&Al were estimated to be 25.6 and 17.2 kJ/mol, respectively. These values are lower than those reported for 1B with the VC13/A1(C2H&catalyst; 46.1 kJ/mol. This difference is related to the isomerization from 2B to 1B. The apparent rate constants shown in the following scheme were determined from eqns (l)-(4) by use of computer simulation, and the results are provided in Table 6. CH3 CH3\c=C/

H’

‘H

c2B kC*

II

CH2=CH

ktc

I CH2CH3

CH3\c=C/H H’

‘CH

kpb

1B

PlB

3

t2B

d[c2B]/dt = kr,[lB] + &[t2B] - (k,r + k,)[c2B]

(1)

TAUTOMERSASMONOMERSANDINITL4TORS

583

d[t2B]/dt = k,,[ lB] + k,[c2B] - ($r + &)[t2B]

(2)

d[lB]/dt = kCl[c2B] + k&2B] - (k,, + kl, + $J[lB]

(3)

d[PlB]/dt = $,[lB]

(4)

The polymerization rate constant is about 40 times higher than the isomerization rate constant from c2B to 1B. In the presence of NiC12,the polymerization rate constant is not significantly affected, while the isomerization rate constant increases and these constants become very similar. The roles of TiC13, alkylaluminum and NiClz during isomerization and polymerization are very complicated. It has been considered that there are different active sites for isomerization and polymerization, and the isomerization site involves a Ti-H bond. Three possible routes were considered for the formation of Ti-H bond.ro6 (i) Direct hydrideexchange reaction between titanium compounds and diethylaluminum hydride contained as a small amount of impurity in triethylaluminum. (ii) Decomposition by B-hydrogen elimination of the a-alkyltitanium complex obtained from an alkyl-exchange reaction of titanium compounds and triethylaluminum. (iii) Termination reaction by g-hydrogen elimination. Polymerization results with different alkylaluminums, i.e., (iso-C&&U, (CzHML (&H&NH, and (CH&Al, have suggested that the g-hydrogen elimination mechanism is the main route for producing the isomerization site involving Ti-H bonds. This reaction is shown in the following schemes.‘m (C2H5)+l + Ti%, &H5Ti%1,

- C,H5Ti(“)Cl, _ 1 + (C,H5)2AlC1 _ 1 - HTi%l,

(5)

_ I + C&H,

(6) In this B-hydrogen elimination, the oxidation state of the titanium complex does not change, and the reaction product contains only the corresponding alkenes. On the other hand, in another possible path, i.e., reductive dealkylation, the oxidation state changes, and an equal amount of alkane and alkene are produced. 2qH,Ti(“)Cl, _2 --+2Ti(“- ‘)Cl, _2 + C21&+ C&&

(7) The analysis of reaction products revealed that 2B isomerizes via a-alkylmethyl complexes formed by the insertion of c2B into the Ti2’-H bond produced from the catalyst components. The polymerization site is carried by the Ti3+ complexes. 4.5. Copolymerizahonand applications Copolymerization between internal olefins with Ziegler-Natta catalysts which catalyze monomer-isomerization polymerization yields corresponding 1-olefin copolymers. The monomer re.activity ratios for various monomer-isomerization copolymerization systems were reported, e.g., 2-butenes with 2-pentene, 2-hexene, 2-heptene, 3-heptene, and 2octene. The monomer reactivity ratio for each monomer is an apparent value because the reactivity is dependent on both isomerization and polymerization. The reactivity ratios

SE120 MASUDA and KEIJI MINAGAWA

584

for several combinations of internal olefins and corresponding l-olefins are reviewed in literature. 3 A well-designed monomer-isomerization polymerization process gives a homopolymer comprised only of the 1-olefin. The required characteristics of the catalyst for obtaining a homopolymer is the selectivity of both the polymerization of the 1-olefin and no polymerization of the other internal olefins even if many isomers are mixed in the reaction system. When catalyst induces no selective polymerization of 1-olefin from a mixture of isomers, the polymerization gives a copolymer of 1-olefin and 2-oletin. For example, the polymerization of 2B using VCl,-(C,H&Al-Ni(DMG)2 catalyst gave a copolymer containing both 1B and 2B units.ll’ This result shows that participation of the internal olefin in the copolymerization is possible when the catalyst is effective for isomerization but is less active for the polymerization of the 1-oletin. By use of different activities of various catalysts, it is possible to obtain different polymers from the same monomer. A significant example is seen in the case of propenylbenzene (PB).l’l With Tic&-(CZH&Al (A?Ti > 2), PB gave polyallylbenzene (PAB) through isomerization to allylbenzene (AB) monomer before polymerization. When TiC13(C2H&UCl-NiC12 was used, the resulting polymer was a copolymer of PB and AB. On the other hand, with TiC13-C2HdC12, ordinary vinylene polymerization occurred and polypropenylbenzene (PPB) was obtained. CH=CH I

I C&C&

CH3

PPB

cPB and tPR

k-

-F”-Cf

------

CH2-7

(2)

CH3

C&C&

CH2

P(PB-co-AB)

CH2=CH

AB

(1) Tick-(W&Q (2) TiCl,-(C~H&A.lCl-NiClz, (3) TiC13-QH&C12

PAB

‘k&H5

585

TAUTOMERS AS MONOMERS AND INITIATORS

Some appbcation of monomer-isomerization polymerization may be possible by using abundant data on catalysts, reaction conditions, and by utilizing the proposed reaction mechanism. The selectivity of polymerization of 1-olefins can be used for the production of useful copolymers. For example, a copolymer of branched 1-alkenes was copolymerized with 2butene instead of 1-butene. ‘I2 Since the concentration of 1B is kept low throughout the copolymerization, the reaction rate is controlled as suitable for copolymerization with the branched alkenes. As an application of monomer-isomerization polymerization, the separation of butene isomers by the selective polymerization of 1-butene was proposed.1’3 A mixture of &u-B, 1B and t2B, and c2B was polymerized with TiC13-AIR3 catalysts. NMR spectra revealed that the resulting polymer was PlB. The remaining monomer composition was dependent on the polymerization conditions. It was shown that the control of the AliTi ratio and the polymerization temperature was important for this “polymerization-separation method” of isomers. Rapid polymerization of 1B and no isomerization of 2B to 1B enables the separation to occur. 5. CIOPOLYMERIZATION

OF ACIDIC MONOMERS

AND BASIC

VINYL

5.1. Introduction The acid-,base chemistry of amphoteric compounds is usually characterized in terms of macroscopic dissociation constants, which are composites of the microscopic constants for the individual groups. Microscopic acid dissociation constants of a number of pharmacologically and biologically active compounds (for example, amino acids, ‘14,115oligopeptides , 116~117 pyridine derivatives,1’8Y119 and drugs,‘207121)were determined. In addition, a number of papers and reviews on acid dissociation equilibria of polyelectrolytes have been published. 12”Particularly, poly(acrylic acid) has been investigated in detail. 123In regard to polyampholytes, Nagasawa and Holzner presented macroscopic constants for poly(2-dimethylaminoethyl methacrylate-co-methacrylic acid).‘” A copolymer of acidic and basic vinyl monomers is an amphoteric compound and a tautomer in which a proton of the carboxyl group can migrate to a nitrogen atom of an amino group. We review here spontaneous copolymerization of acidic and basic vinyl monomers and the pH-metric determination of macroscopic and microscopic acid dissociation constants of the resulting copolymer. The central topic in this section concerns the system of acrylic acid (AA) and 4-vinylpyridine (4VP). 5.2,. Spontaneous copolymerization of acrylic acid and Cvinylpyridine At high concentrations, the mixing of AA and 4VP causes the resulting material to carbonize with a large quantity of heat generated. However, on mixing in the limited concentration range, spontaneous reactions occur without the generation of heat. The spontaneous copolymerization was carried out at a fixed ratio of monomers in the feed ([AA]/]:4VP] - 1). The rate increased with an increase in the monomer concentration.

586

SEIZO MASUDA and KEIJI MINAGAWA

Table 7. Copolymerization Additive

(mom)

None AIBN

BQ” None Water BuNHr AcOH BFs * Et* BuLi

(1:o) 1:$

of AA and 4vpB in the Presence of various additives

WI @W

[4W

1.94 1.94 1.94 2.92 2.92

1.86 1.86 1.86 2.80 2.80 2.80 2.80 2.80 2.80

2.92 2.92 2.92

boV1)

Temp. (“C)

Time (mitt)

Conv. (%)

40 40 40 0 0 0 0 0 0

30 30 30 20 20 20 20 20 20

18.8 19.8 20.6 11.3 23.0 0.0 34.6 21.0 9.0

a [AA]/[4VP] = 1.04. b BQ = p-benzoquinone.

However, no copolymerization took place at concentrations below 2 mol/l. An ‘H-NMR spectrum of the copolymer obtained (&In=3600, determined by GPC) revealed that the reaction proceeded by cleavage of the carbon-carbon double bond. Table 7 shows the copolymerization with or without additives. The radical initiator AIBN and the inhibitor p-benzoquinone do not accelerate or inhibit (or retard) the reaction, respectively. The spontaneous copolymerization is accelerated, retarded, and inhibited by the cationic catalyst (boron trifluoride etherate), the anionic catalyst (butyllithium), and butylamine, respectively. Though in general water is an inhibitor for anionic polymerization, it behaves like an accelerator in this case. The reaction proceeds rapidly even at 0°C. From these results, the spontaneous copolymerization seems to be cationic. Figure 12 shows a composition diagram for the spontaneous copolymerization of AA with 4VP. Copolymers with 1:l composition are obtained independently of the ratio of monomers in the feed.

[AA] in comonomer (mob%) Fig. 12. Monomer-copolymer

composition

curve of the AA-4VP copolymer.

TAUTOMERSAS MONOMERSAND INITIATORS

587

dWCHQQ -22 ‘d,,,,,b H Fig. 13. A schematic representation of macroscopic and microscopic ionization equilibria.

5.3. .Macroscopic and microscopic acid dissociations of the copolymer Figure 13 gives a schematic representation of the microscopic ionization equilibria of the alternating copolymer. Microscopic acid dissociation constants are defined as

k _ W+ID1 k2= [H+ID1 k12I [H+l[wl k21= [H+lW1 1PI [III1 [II [III The relationship between macroscopic and microscopic dissociation constants is

K _ W+l~~IIl+[II~lI+ +k 1 [IIl+[IIIl_ 1 c L 12 K= [H+][IV] k12 k2r [II

With reference to the composition data mentioned above, the alternating copolymer of AA with 4VP and the 1:l copolymer of methyl acrylate with 4VP were prepared. Macroscopic acid dissociation constants (pKr, pK2, and pK3)were determined from titration of the copolymers with potassium hydroxide.rZ In much the same way as the alternating

588

SEIZO MASUDA and KEIJI MINAGAWA

Table 8. Acid dissociation Sample

PK,

AA4VP mixture AA-4VP alternating copolymer PAA-P4VP mixture

4.52 3.98 3.75

constants and tautomeric constants

pK2

pk

pkz

pk12

pkzl

K,

5.47 11.78 6.24

4.72 4.90 3.97

4.96 4.03 4.17

5.27 10.86 6.02

5.03 11.73 5.83

1.75 0.13 1.58

neutral molecule

25-

'

2

4

6

6

10

12

14

PH Fig. 14. Distribution

curves for the microscopic

forms of the AA-4VP copolymer.

copolymer, the monomer mixture and the homopolymer mixture were also titrated. The microscopic acid dissociation constants (pkr, pk2, pk12,and pk2r), which are summarized in Table 8, were calculated by assuming k2 = K3.126 This assumption is generally accepted in the case of amino acids. The distribution of the microscopic forms of the alternating copolymer as a function of pH is of interest in connection with the pH-dependence of biologically and physiologically active amino acids and pyridine derivatives. Figure 14 shows the fractional concentrations of the respective forms. The relative concentrations were calculated using the equations shown above for microscopic constants and hydrogen ion concentration. The fraction of the protonated form, which exists exclusively in the lower pH range, decreases with increasing pH values and fully vanishes at pH N 5, while the deprotonated form begins to appear around pH = 9. The zwitterion and neutral molecule forms coexist over a wide range of pH, and the latter form is predominant regardless of the pH value. On the other hand, for systems of the monomer mixture and the homopolymer mixture, zwitterionic and neutral molecule forms coexist in a limited region of pH (- 5,) and the former is predominant. The systems of methacrylic acid4knylpyridine and acrylic acid-2-vinylpyridine were also copolymerized spontaneously. However, the copolymerization of methacrylic acid and 2-vinylpyridine required a radical initiator. Microscopic acid dissociation of the resulting 1:1 copolymers was determined. 127

TAUTOMERS

AS MONOMERS

AND INITIATORS

589

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1.

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