Donor-Acceptor Molecular Complexes in Alternating Copolymerization

6 Donor-Acceptor Molecular Complexes in

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 21, 2015 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch006

Alternating Copolymerization and in the Polymerization of Metal Halide-Complexed Vinyl Monomers NORMAN G. GAYLORD and AKIO TAKAHASHI Gaylord Associates Inc., Newark, N. J. 07104

Polar monomers containing pendant nitrile or carbonyl groups complex with Lewis acids with a resultant increase in their electron-accepting ability relative to that of the uncomplexed monomer. Complexed monomer participates in a one-electron transfer reaction with uncomplexed monomer or another electron donor monomer—e.g., olefin or conjugated diene—to form a charge transfer complex, which is a dipolar, diradical species analogous to the proposed intermediate in certain Diels-Alder reactions. The charge transfer complex may open spontaneously—e.g., olefin-acrylonitrile—alkylaluminum halide, styrene-methyl methacrylate— alkylaluminum halide, isoprene—acrylonitrile—zinc chloride —or under the influence of free radicals—e.g., olefin— or allyl monomer-acrylonitrile-zinc chloride. The process involves spontaneous or radical-initiated homopolymerization of a Lewis acid activated, diradical charge transfer complex.

' T ^ h e free radical initiated polymerization of polar monomers containing -** pendant nitrile and carbonyl groups—e.g., acrylonitrile and methyl methacrylate—in the presence of metal halides such as zinc chloride and aluminum chloride, is characterized by increased rates of polymerization (2, 3, 4, 5,10, 30, 31, 32, 33, 34, 53, 55, 65, 66, 75, 76, 77, 87). Imoto and Otsu (30, 33, 34) have attributed this effect to the formation of a complex between the polar group and the metal halide. The enhanced reactivity of the complexed monomer extends to copolymerization with uncomplexed monomers, such as vinylidene chloride, which are readily responsive to 94 In Addition and Condensation Polymerization Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

6.

GAYLORD AND T A K A H A S H i

Donor-Acceptor

Complexes

95


free radical polymerization, wherein Q and e values and the reactivity ratio for the complexed monomer are increased (22, 23, 30, 32). H o w ever, monomers such as allylic and olefinic compounds, which are poorly responsive i n free radical initiated polymerizations, are more readily copolymerized with the complexed than the uncomplexed polar monomer (35, 72, 73). In this paper, the contribution of donor-acceptor interactions in copolymerization to the reactivity of metal hailde-complexed monomers is examined. Donor—Acceptor

Interactions

in

Polymerization

The effect of polarity on vinyl monomer copolymerization has long been recognized and is a major factor in the Q, e scheme and copolymerization theory. Mayo, Lewis, and Walling tabulated a number of vinyl monomers into an "average activity" series and an electron "donor—acceptor series (62). The activity series showed the effect of substituents on the ease with which an ethylene derivative reacted with an average radical and on stabilizing the radical which was formed thereby. The electron donor-acceptor series indicated the ability of the substituents to serve as donors or acceptors i n radical-monomer interactions. It is significant that in both series the dominant factor is the radical—monomer interaction. It was further indicated that copolymerization of two monomers which were close together in the donor-acceptor series gave a random copolymer, while monomers which were well separated in the series had a marked tendency to give an alternating copolymer. In contrast to the radical-monomer interaction in the transition state proposed by Mayo and Walling (62, 63), the formation of a molecular complex between the electron donor monomer and the electron acceptor monomer—i.e., monomer-monomer interaction—has been proposed as the contributing factor in the free radical alternating copolymerization of styrene and maleic anhydride (8) as well as sulfur dioxide and monoor diolefins (6, 9, 12, 13, 25, 41, 42, 43, 44, 61, 79, 80, 88). Walling and co-workers ( 83, 84 ) did note a relationship between the tendency to form molecular complexes and the alternating tendency and considered the possibility that alternation involved the attack of a radical on a molecular complex. However, it was "the presence in the transition state of polar resonance forms resembling those in the colored molecular complexes" which led to alternation in copolymerization (84). The significant distinction between these proposals lies in the nature of the components participating in the transition state. Thus, the radicalmonomer interaction involves a one-electron transfer from the growing radical to the electron acceptor monomer.

In Addition and Condensation Polymerization Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

96

ADDITION

H

H

I

II


R—CH H

H

-

—

I

PROCESSES

// >

C H

C( ^

0

H H C=C H

, +

ο

c

I

R—CH H

/

POLYMERIZATION

H

c

>

C — C H

o/

CONDENSATION

/ ?

c— c

Φ—c-

AND

I

ο H H R—C—C H I

H C

H C

H H C—C · H I

I I

Ο

Ο

Ο

(!)

In contrast, the donor monomer-acceptor monomer interaction involves a one-electron transfer from the donor monomer to the acceptor monomer to form a charge transfer complex. The latter undergoes homopolymerization through a radical mechanism to give an alternating copolymer. H

φ—c

H

c —

IUII

HC H

H φ__0+ I HCH

C H

J, ο

H

φ—c

+

^

C \

ο

H Λ° -C C( ι C H— c C T

>

c ·

! I

HCH

C • H

r

H

φ—c — I

HCH

-Ο

H H H H C—C C- n Φ C C

H

H

Ο

H

H H —r

r

>^0 c —

I

>

C H

H H —c-

{

]

1

//\/\

Ο

Ο

Ο

Thus, the radical-monomer interaction involves a monoradical spe cies which successively adds monomer units i n an alternating copolymeri zation, while the monomer-monomer interaction involves the homopolymerization of a charge transfer complex through a diradical coupling



6.


Donor-Acceptor

97

Complexes

mechanism—i.e., alternating copolymers result from complex homopolymerization and not through copolymerization. Iwatsuki and Yamashita (46, 48, 50, 52) have provided evidence for the participation of a charge transfer complex in the formation of alter nating copolymers from the free radical copolymerization of p-dioxene or vinyl ethers with maleic anhydride. Terpolymerization of the monomer pairs which form alternating copolymers with a third monomer which had little interaction with either monomer of the pair, indicated that the polymerization was actually a copolymerization of the third monomer with the complex (45, 47, 51, 52). Similarly, copolymerization kinetics have been found to be applicable to the free radical polymerization of ternary mixtures of sulfur dioxide, an electron donor monomer, and an electron acceptor monomer (25, 44,61,88), as well as sulfur dioxide and two electron donor monomers (42, 80). The diradical nature of the intermediate in the copolymerization of monomers through a charge transfer intermediate has been suggested by Zutty et al. (88) as a result of studies on the copolymerization and ter polymerization of monomer systems containing bicycloheptene and sulfur dioxide. The third monomer apparently enters the copolymer chain as a block segment, while the donor-acceptor monomer pair enter the chain in a 1:1 molar ratio, irrespective of the ratio present i n the monomer mixture. Similar results have been noted in terpolymerizations involving the p-dioxene—maleic anhydride (49, 51, 52) and vinyl ether-maleic an hydride (45, 49) and vinyl ether-fumaronitrile (49) monomer pairs. Iwatsuki and Yamashita (46) concluded that the molecular complex formed between p-dioxene and maleic anhydride is attacked on the p-dioxene side by a radical to yield the maleic anhydride radical which is considered to be the main growing radical. Thus, a monoradical propagation step is considered operative.

ο Η / \ / HoC

C+

HoC

Ç-

Η C•C Η

V

O ο

Η Η R—C — C / \

Ο

Η C I

,c

o

\

c—c

/

Η C1

ο

//\

(3)

c

ο

/ \ ο

Ho Ho Actually, the diradical and monoradical species in Reactions 2 and 3, respectively, are the same since both involve the participation of the complex as a diradical species. The termination and propagation steps i n these reactions are related in that the interaction between two radicals



98

ADDITION

AND CONDENSATION

POLYMERIZATION

PROCESSES

is a propagating step when coupling results and a terminating step when disproportionation occurs. Thus, i n contrast to conventional free radical polymerization mechanisms, charge transfer mechanisms involve propagation not only by polymer radical-monomer or polymer radical-complex interactions but also by polymer radical-polymer radical interactions. This leads to the conclusion that while solvents play a role in determining the participation of the third monomer i n a ternary polymerization (49), they may be involved to only a limited extent i n the termination step. This is confirmed by the absence of chlorine i n the terpolymers of p-dioxene-maleic anhydride-acrylonitrile prepared in chloroform solution (49) and the ability to prepare methyl vinyl ketone-maleic anhydride copolymers with benzoyl peroxide i n nitrobenzene solution (74). The participation of diradical species in charge transfer reactions has been demonstrated in the most widely recognized example of donoracceptor interaction—i.e., the Diels-Alder reaction. The two-step nature of this reaction has recently been proposed i n the isolation of both fourand six-membered ring products i n the thermal and photochemical reaction of butadiene and a-acetoxyacrylonitrile (14, 56).

The common intermediate has appreciable diradical structure, and the products result from the alternative modes of its collapse through different transition states.

Although the Diels-Alder reaction of a conjugated diene, such as butadiene or isoprene, with maleic anhydride, has been known to yield tetrahydrophthalic anhydride, it has recently been shown (81, 85) that alternating copolymers are prepared under the influence of ionizing radiation (81) or free radical initiators (81, 85). The participation of the charge transfer complex as a common intermediate in both adduct



6.

GAYLORD AND

TAKAHASHi

Donor-Acceptor

Complexes

99

and alternating copolymer formation is confirmed by the greater than 74% cis-1,4 microstructure of the copolymer, whereas conventional free radical polymerizations (e.g., homopolymerization of butadiene or copolymerizations of butadiene with styrene, acrylonitrile, dimethyl maleate, or fumaronitrile ) yield polymer containing less than 25% cis-1,4 unsaturation (85). The proposed reaction mechanism (81) invokes a charge transfer complex which converts to a resonance stabilized diradical. The latter leads to a Diels-Alder adduct as a result of intramolecular coupling. However, when the diradical intermediate is attacked by a free radical, it is opened, and an alternating copolymer results from intermolecular coupling.

The participation of Diels-Alder type intermediates in polymerization was considered by H i l l et al. (26) in 1939 as a result of the elucidation of the structures of the butadiene homopolymer and the butadiene-methyl methacrylate copolymer resulting from thermal polymerization in emulsion. The considerable amount of alternating 1,4 and 1,2 structures in the homopolymer and the predominantly 1,4 structure of the butadiene in the copolymer which contained more than 50% alternating units of butadiene and methyl methacrylate led to the proposal that the reaction proceeded through a Diels-Alder "dimer complex" or "activated complex." Chain initiation involved a thermal reaction in which the activated com-



100

ADDITION

AND CONDENSATION

POLYMERIZATION

PROCESSES

plex opened " i n an environment which favored chain growth rather than ring closure and chain propagation involved adding on the complex to the growing end of the chain. , , Gindin, Abkin, and Medvedev (21) observed several of the characteristics of copolymerizations involving charge transfer intermediates i n the benzoyl peroxide-catalyzed copolymerization of butadiene with acrylonitrile and methacrylonitrile. Irrespective of the initial concentrations, polymerization ceased when one of the components was consumed. The butadiene-acrylonitrile copolymer had a 67% alternating structure, while the butadiene-methacrylonitrile copolymer had an 80% alternating structure. Although donor-acceptor interactions were recognized as playing an important role i n copolymerization reactions during the period when the basic principles of copolymerization theory were being formulated, except for isolated instances, monomer-monomer interactions were not considered until recently. The formation of copolymers containing a greater than 1:1 ratio of donor—acceptor components is a function of conversion and the homopolymerizability of the monomers. Thus, 2:1 and 3:1 styrene-maleic anhydride copolymers apparently result from the interaction of a growing polystyrene chain with the 1:1 monomer pair, the coupling of a growing polystyrene chain with a growing alternating copolymer chain, or the addition of styrene monomer to a chain end containing a 1:1 monomer pair. The former are the more probable interactions i n view of the failure to obtain copolymers containing excess maleic anhydride and the formation of block copolymers i n the ternary compositions containing bicycloheptene-sulfur dioxide or p-dioxenemaleic anhydride. Influence

of Complex

Stability

on

Copolymerizability

The charge transfer complex resulting from the one-electron transfer from the electron donor monomer to the electron acceptor monomer has a stability which varies as a function of the internal resonance stabilization. The degree of stabilization apparently determines the ease with which the diradical complex opens, and consequently the stability of the complex determines whether the copolymerization occurs spontaneously or under the influence of heat, light, or free radical attack. Spontaneous 1:1 copolymerization has been noted when sulfur dioxide was bubbled through bicycloheptene at — 40 ° C . (88), when isobutylene was bubbled through methyl a-cyanoacrylate (54), when 1,3-dioxole was mixed with maleic anhydride (17), and when vinylidene cyanide was mixed with styrene (20), the latter reactions at room temperature. None of these monomers undergoes homopolymerization under the same experi-


6.

GAYLORD AND

Donor-Acceptor

TAKAHASHi

101

Complexes


mental conditions. Apparently, the electron-donating characteristics of bicycloheptene, isobutylene, and 1,3-dioxole and the electron-accepting characteristics of sulfur dioxide, methyl α-cyanoacrylate, and maleic an hydride, respectively, are sufficiently strong that the one-electron transfer reaction occurs readily, and the resultant complex is unstable.

S0 2 -

ÇHs \t _

CH3

Γ

CH3—C

H

· CH3—C +

CH2

+

H

H2C

1

' I I~

Ç-COOCH, C

CH.)

H

CH3

N

c

.

(?)

.c_coOCH3 CN

CN

I I • CH9C —CH.>C · CH3

H x

H '

c ' N

H Ο — C

COOCH8

Ο

H

H

C—C

H + 11 ; o

O — C H

C — H

C ' ^

H

\.y

O — C H

H /O "C — c ( I Ο C— c{ H %

H

Ο

Ο Ο

C

c — c — cH

I

X

Q

,Cv Ο

H Ο — C *

V

H '

(8)

H

H

Ο C CH


(9)

102

ADDITION

H ψ—C

*

AND CONDENSATION

CH2

Il + H

H,C

„

C—CN

H φ—C*

ι

H,C-

I

"


CN

POLYMERIZATION

PROCESSES

CH,

I

C—CN

I CN

H

•CHQC —

CN

I

CHoC*

(10)

CN

Szwarc (54) has postulated that isobutylene and the a-cyanoacrylate interact to give a highly polar dimer, and the resultant copolymer should have a head-to-head, tail-to-tail structura H e has further suggested that this would exemplify a case intermediate between polymerization of diradicals and polymerization of zwitterions. As shown in Reaction 8, the electron-donating substituents on the β-carbon of the isobutylene convert the latter to a stronger donor than the α-carbon. While the electron-withdrawing substituents on the β-carbon of the a-cyanoacrylate convert it to an acceptor, the electron deficiency is partially compensated by a shift of electrons from the α-carbon, converting the latter to an electron acceptor. The resulting charge transfer complex leads to the normal head-to-tail, tail-to-head structure. The two-step ion coupling and diradical opening is actually the intermediate case proposed by Szwarc. Another case of essentially spontaneous copolymerization is the low temperature reaction of trifluoronitrosomethane with tetrafluoroethylene to give a mixture of cyclic adduct and polymer (7). F2C=CF2 + N=0 -» F2C—CF2 + —N—O—CF2CF2— CF3

Ν—Ο

CF3

(11)

I CF3 Considerable evidence has been presented i n support of a diradical mechanism for this reaction (11). A n interesting aspect of this reaction is the proposal of two different diradical species, one favoring chain growth and polymer formation and the other favoring intramolecular coupling and adduct formation.


6.

Donor-Acceptor


F2C

Ν—CF3

F2C

ο

Complexes

F F C—C—Ο—Ν F F I CF3

103

F F -C—C—Ο—N— F F I CF3


II + II

F

Ο·

Ν

I

F

F -CF

F FC

F F •C—C—Ν CF3

Ο (12)

CF.

The interactions of α-olefins or styrène with sulfur dioxide (16) or α-olefins (24, 58, 78), irans-stilbene (64), styrene (1,63), p-dioxene (52), 2,2-dimethyl-l,3-dioxole (17), or alkyl vinyl ethers (J, 63) with maleic anhydride yield charge transfer complexes which are stable and generally readily detectable either visually or by their ultraviolet absorption spectra. However, under the influence of a sufficiently energetic attack in the form of heat or free radicals, the diradical complexes open, and alternating copolymers are formed.

H X—C

H X—C+

H2C

H2C-

||+so2->

ι

X RI SO2-*-CH2C—so2H

(13)

X = alkyl, C 6 H 5

H X—C Y—C

H

* ° C — C ( C - C '

H H%

HH^O X—C+ Y-C-

C —C:

ÏÎHH « 1 " H H^ J« J

Δ or R

HH

0 ~ C (

ο

ο

Ό

Χ = alkyl, C 6 H 5 , O-alkyl Y = H X = Y = C6Hn XY=

Ο

Ο CHXH2

Ο / HHC

Ο C

\ CH,


(14)

104

ADDITION

AND

CONDENSATION

POLYMERIZATION

PROCESSES


The intermediate case where a complex is formed but opening occurs spontaneously to only a limited extent is illustrated by the reaction of butadiene or other conjugated dienes with sulfur dioxide (16). When the two components react, a cyclic adduct and a linear alternating copolymer are produced simultaneously.

(15) Under the influence of heat, free radicals, or radiation the yield of copolymer is increased, and the yield of adduct is decreased (19). This is analogous to the diene—maleic anhydride interaction i n the existence of a common intermediate in both adduct and copolymer formation. However, in the Diels-Alder case adduct formation occurs so readily that the copolymer does not normally accompany the adduct. W h e n the intermediate complex is exposed to radiation, both copolymer and adduct are formed (81). The direct analogy of the diene-sulfur dioxide reaction to the diene-maleic anhydride reaction is the greater than 75% cis-1,4 unsaturation of the copolymer i n both cases. W h e n charge transfer intermediates are formed, they are apparently capable of pursuing three different reaction paths: (a) intramolecular diradical coupling yielding a 1:1 adduct, (b) intermolecular diradical coupling yielding an alternating 1:1 copolymer, or (c) no reaction until the input of sufficient energy drives the reaction to follow Paths a or b. Catalysis

of

Complex

Formation

The interaction of a strong electron donor with a strong electron acceptor results in a spontaneous one-electron transfer and the formation of a charge transfer complex. Once the complex is formed, it may follow one or more of the paths already discussed. However, the one-electron transfer reaction does not readily occur unless the monomers are sufficiently strong i n their electron-donating or accepting character. Mayo and Walling (62) have already indicated this in their observation that random copolymers are formed when the two monomers are close together in the donor-acceptor series, while there is a marked tendency for alternation with monomers which are widely separated in the series.



6.

GAYLORD AND

TAKAHASHi

Donor-Acceptor

Complexes

105

Increasing the electron-accepting character of an electron acceptor monomer would result i n a greater separation i n the donor-acceptor relationship with a given electron donor monomer. As a result there would be an increased tendency for alternation in the copolymerization. A further consequence of the ease of complex formation may be a more rapid copolymerization, although the complex may form an adduct or await the input of sufficient energy to open. In our work on copolymerizations involving charge transfer intermediates, it has been noted that when a mixture of styrene and maleic anhydride is heated to 80 ° C . i n the presence of benzoyl peroxide, an extremely exothermic reaction occurs, and in a sealed system the temperature rises from 8 0 ° to 250 ° C . within three minutes, and the conversion is quantitative. Similarly, whereas the Diels-Alder reaction is accelerated at elevated temperatures, under polymerization conditions, the reaction of isoprene and maleic anhydride is extremely exothermic, and the relative amounts of adduct and copolymer are temperature dependent. It has been reported ( S i ) that the rate of copolymerization is very fast compared with the rate of homopolymerization of the diene or the dienophile, and the energy of activation is approximately 5 kcal./mole. Although the rate of copolymerization increases at elevated temperatures, the simultaneous adduct formation which also occurs more readily at elevated temperatures limits the maximum rate to lower temperatures. Catalysis of the diene-dienophile Diels-Alder reaction was not considered feasbile until recently. Thus, although those adducts which were difficult to form could be produced at elevated temperatures and after prolonged reaction times, it was not considered possible to catalyze the reaction so that it would proceed under milder conditions. W i t h i n the last few years, it has conclusively been demonstrated that the Diels-Alder reaction is susceptible to catalysis with Lewis acids or Friedel-Crafts catalysts such as aluminum chloride (15, 18, 36, 37, 38, 39, 40, 57, 67, 68, 69, 70, 71, 82, 86). As a result of catalysis, it was possible to change the following: ( 1 ) Time—temperature-yield relationship—i.e., increase the yield of adduct produced at a lower temperature after a shorter reaction time (2) Ratio of geometric isomers ( 3 ) Ratio of exo and endo isomers (4) Optical activity of adduct Since the latter effects result from a change in the transition state, it has been proposed that the aluminum chloride complexes with the polar group in the dienophile and influences the structure of the transition state. The catalysis can be considered to involve a greater electronaccepting character of the complexed dienophile compared with the


106

ADDITION AND CONDENSATION POLYMERIZATION

PROCESSES

uncomplexed dienophile. As a consequence, the one-electron transfer from the electron donor diene occurs more readily, increasing the yield of diradical charge transfer complex.

CH 2

c—c=o

+


Η

8

+

+

t

I OR

^

"

An

CH 2 I!

c-^c^o. Η I

W

CH 9

Donor-Acceptor Molecular Complexes in Alternating Copolymerization

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