-
New aspects of tetramethyleneinitiation in polymer
chemistry.
Item Type text; Dissertation-Reproduction (electronic)
Authors Clever, Hester Ann.
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Order Number 9024500
New aspects of tetramethylene initiation in polymer
chemistry
Clever, Hester Ann, Ph.D.
The University of Arizona, 1990
U·M·I 300 N. Zceb Rd. Ann Arbor, MI 48106
-
" .'f
NEW ASPECfS OF TETRAMETHYLENE INITIATION
IN POLYMER CHEMISTRY
by
Hester Ann Clever
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF CHEMISTRY
In Partial Fulfillmeut of the Requirements For the Degree of
DOCfOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
1990
-
THE UNIVERSITY OF ARIZONA GRADUATE COLLEGE
As members of the Final Examination Committee, we certify that
we have read
the dissertation prepared by Hester A. Clever
entitled New Aspects of Tetramethylene Initiation in
Polymer Chemistry
and recommend that it be accepted as fulfilling the dissertation
requirement
for the Degree of Doctor of Philosophy
/I. J(, flrc~1. J -'. Date
'3 J:/ztJ , 3-7-90
Date
3/t11D Date
iMitJ ? I J 717 Date J
7 !J7a'I--Ce:. 9iJ Date
Final approval and acceptance of this dissertation is contingent
upon the candidate's submission of the final copy of the
dissertation to the Graduate College.
I hereby certify that I have read this dissertation prepared
under my direction and recommend that it be accepted as fulfilling
the dissertation requirement.
Dissertation Director Date
-
STATEMENT BY AUTHOR
This dissertation has been submitted in partial fulfillment of
requirements for an advanced degree at The University of Arizona
and is deposited in the University Library to be made available to
bonowers under rules of the Library.
3
Brief quotations from this dissertation are allowable without
special permission, provided that accurate acknowledgement of
source is made. Requests for permission for extended quotation from
or reproduction of this manuscript in whole or in part may be
granted by the head of the major department or the Dean of the
Graduate College when in his or her judgment the proposed use of
the material is in the interests of scholarship. In all other
instances, however, permission must be obtained from the
author.
-
4
DEDICATION
To my parents, Donald and Marilyn Clever, who made this all
possible.
-
ACKNO~DGEMENTS
The author would like to thank Dr. H. K. Hall, Jr. for giving me
the
opportunity to wolk in his laboratory and for all of his advice
and encouragement
through out my graduate career. I would also like to thank Dr.
Anne Padias for all of
her advice, encouragement and patience.
I would like to acknowledge Dr. Tzepei Tien for his helpful
discussions
concerning the molecular orbital calculations, and Dr. Michael
Barfield and his NMR
class for running many of the decoupled spectra required for
analysis of the substituted
cyclohexenes.
5
-
-.
6
TABLE OF CONTENTS
Page
UST OF ll..LUSTRATIONS
UST OF TABLES
ABSTRACT
9
11
13
1. INTRODUCTION • • • • • • 15 The Bond Forming Initiation
Theory •••• • • 16 An Organic Chemist's Periodic Table. • • • • • •
18 Applications of the Bond Forming Initiation Theory 20
Trapping of a Zwitterionic Intermediate Using Cationic
Polymerization • • • • • • • • • 20
Trapping of a Zwitterionic Intermediate Using Anionic
Polymerization • • • • • • • • • • • • 22
Trapping of Diradical Intermediates • • • • 22 Borderline
Tetramethylenes ••••••••••• 26 Reactions with p-Leaving Group
Olefins 27
Application of the Bond Forming Initiation Theory to [4+2]
Systems ••••••••••••••••• 29
Proposed Research ••••••••••••••••• 35
2. REACTIONS OF l-ARYL-l,3-BUTADIENES WITH INSUBSTITUTED
ELECTROPHll..IC OLEFINS ••••
Introduction ••••••••••••••••••• Background •••••• Results • • •
• • •
Control Reactions • • • • • • • • • • • • • • • Reactions of
l-AryI-l,3-Butadienes with Tri-
substituted Electrophilic Olefins • • • • • Experimental
Observations ••••••••• Stereochemistry of the Cycloadducts • • • •
•
Reactions with Olefins Containing p-Leaving Groups •
Experimental Observations ••••• Homopolymer Cycloadduct • • • • • •
• • • •
Discussion • • • • • • • • • • • Conclusion • • • • • • • • • •
• • • • • •
3. REACTIONS OF l-METHOXY-l,3-BUTADIENE • Introduction •••••• •
• • • Background ••••• Results • • • • • • • • • • •
Control Reactions
37 37 38 39 39
41 41 44 45 45 45 47 49 53
55 55 55 S8 S8
-
4.
5.
6.
7
TABLE OF CONTENTS - Continued
Page
Reaction of I-Methoxy-l,3-Butadiene and Maleic Anhydride. • • •
• • • • • • • • • • • 58
Reaction of I-Methoxy-l,3-Butadiene and Methyl
3,3-Dicyanoacrylate • • • • • • • • • • • • • • • •• 60
Reaction of I-Methoxy-l,3-Butadiene and Dimethyl Cyanofumarate •
• • • • • • • • • • • • 62
Reaction of I-Methoxy-l,3-Butadiene and Tri-methyl
EthylenetricaIboxylate • • • • • • • • 62
Reactions of I-Methoxy-l,3-Butadiene and Acrylonitrile • • • • •
• • • • • • • • • • • • • •• 67
Reactions of I-Methoxy-l,3-Butadiene with Olefins . ~ontaining
p-Leaving Groups • • • • • • • . • 67
DIscussIon • . • • • • • . • • • • • • • • • • • • 67 Conclusion
• • • • • • . • • • . • • • • • • • • 69
REACTIONS OF ALKYL BUTADIENES AND ACRYLONITRll..E Introduction •
• • • • • • • • • • • • Background • . . • • • • • • • • • • • • •
Results •.•••.•••••.••••.• • • • •
Control Reactions •••••••••••••••• Reactions of Cyclopentadiene
with Acrylonitrile Reactions of Cyclohexadiene with Acrylonitrile
Reactions of Isoprene with Acrylonitrile • • • Reactions of
1,3-Butadiene with Acrylonitrile • Reactions of trans-Pipenylene
with Acrylonitrile Reactions of 2,3-Dimethyl-l,3-Butadiene with
Acrylonitrile • • • • • • • • . • • • Reactions of Other Dienes
with Acrylonitrile • Reactions of Dienes with Methyl Acrylate •
Discussion • • • • • • • • • • • • • • • • • • • • Conclusion •
• • • • • • • • • • • • • • •
A COMPARISON OF THEORETICAL AND EXPERIMENTAL DATA ••
Introduction. • • • • • • • • • • • • Background Results Discussion
• • • • • • • • • • • • • • • Conclusion •
APPLICATION OF THE BOND-FORMING INITIATION THEORY TO GROUP
TRANSFER POLYMERIZATION • • • •
Introduction/Background • • • • • Mechanism Studies. •
Results and Discussion • • • • • •
71 71 71 73 73 73 73 73 75 77
77 80 80 83 85
86 86 86 87 94
102
103 103 106 111
-
8
TABLE OF CONTENTS - Continued
7. CONCLUSIONS • • •
Page
114
8. EXPERIMENTAL... 116 Instrumentation Solvents • • • • • • • •
• • • • 116 Dienes •••• • • • • • • • • • • 116
Reagents • • • • • • • • • • • • • • • • • • • • • • 117
Synthesis of 1-Phenyl-1,3-Butadiene. • 117 Synthesis of
p-Anisyl-1,3-Butadiene. • 118
Olefins •••••••••••••• • • • • • • 119 Synthesis of Methyl
3,3-Dicyanoacrylate • • • • • 119 Synthesis of Methyl Glyoxylate
Methyl Hemiacetal. • 119 Synthesis of Dimethyl Cyanofumarate • • •
• • • • 120 Synthesis of Trimethyl Ethylenetricarboxylate. 120
Synthesis of 2,2-Dicyanovinyl Chloride. • • • • • 120 Synthesis of
2,2-Dicyanovinyl Iodide • • • • • • • • • • • 121 Synthesis of
2,2-Dicyanovinyl Tosylate. • • • • • • 121 Synthesis of
2-Carbomethoxy-2-Cyanovinyl Chloride 121 Synthesis of
2-Carbomethoxy-2-Cyanovinyl Iodide 122
Synthesis of 4-Cyano-l-Methylcyclohex-l-ene • • • • • 122
Control Reactions. • • • • • • • • • • • • • 123
Stability of p-Anisyl-l,3-Butadiene • • • • • • • • • • 123
Triethyl Borane/Oxygen Initiation •••••• 123 Typical Procedure for
Reactions of I-Phenyl-1,3-
Butadiene and p-Anisyl 1.3-Butadiene with Tri-substituted Olefms
3-5 and p-Leaving Group Olefins 7-11, p-Anisyl-l,3-Butadiene with
Maleic Anhydrides I-Methoxy-l,3-Butadiene with Olefins 3,4,7,10. •
• • • • • • • 124
Typical Procedure for the Reactions of I-Methoxy-l,3-Butadiene
with Trimethyl Ethylenetricarboxylate, Reactions of
I-Methoxy-l,3-Butadiene, 1,3-Butadiene, Isoprene, trans-Piperylene,
2,3-Dimethyl-l,3-Butadiene, cis-Piperylene,
2,5-Dimethyl-2,4-Hexadiene, Cyclo-pentadiene and 1,3-Cyclohexadiene
with Acrylonitrile and Reactions of Isoprene and Piperylene with
Methyl Acrylate ••• • • • • • • • • • • • • • • • • • •
Group Transfer Polymerization. • • • • • • Group Transfer
Polymerization Procedure Photopolymerizations • • • • • • • Data
for Polymers and Cycloadducts • • • •
APPENDIX A ••
REFERENCES .•
124 125 125 125 126
134
140
-
LIST OF n.LUSTRATIONS
Figure/Schemes
Scheme 1
Figure 1
Scheme 2
Scheme 3
Bonding FOnning Initiation Theory •
An Organic Chemist's Periodic Table' •
Trapping of a Zwitterionic Intennediate by Cationic
Polymerization • • • • • • • • • • • • • •
Trapping of a Zwitterionic Intennediate by Anionic
Polymerization • • • • • • • • • • • • •
Scheme 4 Trapping of a Diradical Intennediate I. •
Scheme 5 Trapping of a Diradical Intennediate IT
Scheme 6 Mechanism in Trapping a Diradical Intennediate with
TEf'dPO • • • • • • • • • •
Scheme 7 Effect of Solvent on Product Distribution • •
Scheme 8 BFI Theory Using p-Leaving Group Olefins
Scheme 9 Reactions of Cyclohexadiene with Cis- and
Trans-2-butenes • . • • • • • • • . • • • • • •
Scheme 10 Reactions of l-Cyclopropyl-l,3-Butadiene and TCNE.
•
Scheme 11 Reactions of Cis and Trans Dienes. • • • •
Scheme 12 The BFI Theory Applied to [4+2] Systems. •
Scheme 13 General Mechanism of Reactions of Dienes with
p-Leaving Group Olefins • •
Figure 2
Scheme 14
Scheme 15
Figure 3
Scheme 16
Major Isomer • • • • • •
Fonnation of Hexatriene • •
Concerted Mechanism • •
Stepwise Mechanism • •
Butler's Proposed Mechanism for the Spontaneous Copolymerization
of l-Ethoxy-l,3-Butadiene and Acrylonitrile • • • • • • • • • • • •
•
9
Page
16
19
21
23
23
24
25
26
28
32
33
34
36
38
46
47
51
52
57
-
Scheme 17
. Figure 4
LIST OF ll..LUSTRATIONS - Continued
Possible Mechanism for the Fonnation of Black Interactable
Product. • • • • • • • • •
Isomeric Cyclohexenes from the Reactions of Piperylene and
Acrylonitrile ••••••
Scheme 18 Group Transfer Polymerization • • •
Scheme 19 Reversible Dissociative Mechanism. • •
Scheme 20 Irreversible Dissociate Mechanism
Scheme 21 Associative Mechanism • . • • •
Scheme 22 Webster's Proposed Mechanism • •
10
Page
69
77
103
107
107
109
110
-
Table
1.
2.
LIST OF TABLES
Bartlett's Results
Control Reactions • •
3. Reactions of l-AryI-l,3-Butadienes with Trisubstituted
11
Page
31
• •• 40
Olefins • • • • • • • • • • • • • • • • • • • • • • • • • •
42
4. Reactions of l-AryI-I,3-Butadienes with p-Leaving Group
Olefms • • . • • • • • • • • • • • • •• • • • • • • • 43
5.
6.
7.
8.
9.
10.
Isomer Ratios of [4+2] Cycloadducts in Reactions of
p-Anisyl-l,3-Butadiene • • • • • • • • • • •
AryI-l,3-Butadienes with p-Leaving Group Olefins •
Reactions of I-Methoxy-l,3-Butadiene and Maleic Anhydride
Reactions of I-Methoxy-l,3-Butadiene • • • • • • •
Reactions of I-Methoxy-l,3-Butadiene with Trimethyl •
Ethylenetricarboxylate
Molecular Weight of MeOBD/I'NE Copolymers • •
11. Molecular Weight of MeOBD/I'NE Copolymers as a Function
• 46
• • • • • 48
•• 59
• 61
.64
• • • • • 65
of Time . • • . • • • • • • • • • • • • • • • • • • • 66
•• 74 12. Reactions of Isoprene and Acrylonitrile. • • • • • •
•
13. Feed Ratio Effects of Spontaneous Polymerizations of
Isoprene and Acrylonitrile • • • • • • • • • • • • • • • • • • •
76
14.
15.
16.
17.
Reactions of 1,3-Butadiene and Acrylonitrile. • •
Reactions of trans-Piperylene and Acrylonitrile. •
Reactions of 2,3-Dimethyl-l,3-Butadiene and Acrylonitrile
Reactions of Other Dienes and Olefins in Bulk
• • • • • 78
• • • 79
81
82
-
Table
18.
19.
20.
21.
22.
23.
24.
25.
26.
LIST OF TABLES - Continued
Calculations for Tetrasubstiwted Olefms · · Calculations for
Trisubstituted Olefins
Calculations for p-Leaving Group Olefins. . . Calculations for
Maleic Anhydride and Derivatives • · · Calculations for
Tetrasubstiwted Quinodimethanes
Calculations for Mono and Disubstiwted Olefins •
Comparison of Calculated LUMO Values and Experimental Reduction
Potentials of Trisubstiwted Olefins
Comparison of Calculated and Experimental Data for
Tetrasubstituted Oletins . . . . . . . . . Comparison of Calculated
and Experimental Values Calculations for Increasing Cyano
Substituted Olefins
· ·
12
Page
· · · · · 88 90
· · · · 91 · · · · 92
· · · · 93 9S
· · · · . . 96
· · · · · . . 98
100
27. Results of the Mechanism Swdy of Group Transfer
Polymerization. • • • 101
28. Polymerization Reaction of Methyl Acrylate and Styrene
Monomers • 112
-
13
ABSTRACI'
Hall's Bond Forming Initiation Theory, derived for [2 + 2]
systems, was applied
to [4 + 2] systems. Polymerizable electron-rich and
electron-poor olefins were reacted
to obtain evidence for reactive intermediates using
polymerization as a trap.
A diradical intermediate was successfully trapped in reactions
of I-methoxy-l,3-
butadiene with methyl 2,2-dicyanoacrylate, dimethyl
cyanofumarate, trimethyl
ethylenetricarboxylate, maleic anhydride and acrylonitrile.
Diradical intermediates were
also trapped in reactions of isoprene, 1,3-butadiene,
2,3-dimethyl-l,3-butadiene, cis and
trans-piperylene and 2,S-dimethyl-2,4-hexadiene with
acrylonitrile. Copolymers were
obtained in all cases. Copolymerization was accompanied by [4 +
2] cycloaddition.
A zwitterionic intermediate was successfully trapped in the
reactions of p-anisyl-
1,3-butadiene and l-phenyl-I,3-butadiene with various leaving
groups in the P-position.
Homopolymer of the arylbutadiene was obtained in all cases.
Polymerization was
initiated from a cationic species which arose from the
elimination of the leaving group
from a zwitterionic hexamethylene species. Polymerization was
accompanied by [4 + 2]
cycloaddition.
Reactions of p-anisyl-l,3-butadiene and l-phenyl-I,3-butadiene
with methyl 2,2-
dicyanoacrylate, dimethyl cyanofumarate, trimethyl
ethylenetricarboxylate gave only the
[4 + 2] cycloadduct. No polymer was formed in this reaction and
no evidence for a
reactive intermediate was obtained.
The BFI theory was also applied to Group Transfer Polymerization
reactions to
determine whether the reaction mechanism involved a
tetramethylene diradical
intermediate. No styrene was incorporated into the methyl
acrylate polymer, and so no
evidence for a radical intermediate was obtained.
-
14
LUMO energies calculated by AMI were compared to reduction
potential and UV
data for tetrasubstituted and trisubstituted electrophilic
olefins. The trends exhibited by
the calculated LUMO energies agreed well with the experimental
data, implying that
calculations may be a reasonable method of predicting
reactivity.
-
CHAPTER 1
Introduction
15
Throughout the years, organic chemists have been doing their
best to ignore the
polymeric byproducts in their reactions. They use reaction
conditions that favor the
small molecule products such as high reaction temperature, low
c.oncentration and highly
substituted reactants. If the reaction conditions still allow
polymer formation, they add
copious amounts of inhibitor to prevent polymerization. Polymer
chemists, on the other
hand, have done their best to ignore the small molecule products
they often obtain in
their polymerization reactions. They use low temperatures and
high concentrations to
favor polymerization. They immediately add initiator to the
reaction mixture without
first seeing if the polymerization may be spontaneous. Organic
chemists often ignore
the polymeric products found in the bottom of their flasks,
while polymer chemists
often throwaway the small molecule products. Both the organic
chemists and the
polymer chemists are losing mechanistic information by ignoring
each others products.
In efforts to explain the spontaneous polymerizations that often
accompany [2 + 2]
cycloadditions and to unify the organic chemistry and the
polymer chemistry, Hall
proposed the Bond Forming Initiation Theory shown in Scheme 1.1
According to the
Woodward-Hoffmann rules,2 [2+2] cycloadditions are thermally
forbidden, concerted
reactions. When an electron-rich olefin reacts with an
electron-poor olefin. a
tetramethylene intermediate, called such because it contains
four carbons, is formed.
This intennediate closes to form the cycloadduct. Both
zwitterionic or diradical
tetramethylenes have been proposed as intermediates.'"
-
D
=' - c(D A
1
ceD
A
--
Scheme 1 Bond Forming Initiation Theol)'
16
~ D A
~ D
According to Huisgen, this tetramethylene lies on a continuous
scale between
zwitterionic and diradical structures, and may be regarded as a
resonance hybrid of the
two extreme forms. The predominant nature of the tetramethylene
intermediate is
determined by the terminal substituents.5.6 If they are strong
donors and acceptors such
as alkoxy, amino, or cyano, a tetramethylene with zwitterionic
character is favored. If
the termini are substituted with weaker donor and acceptor
groups such as alkyl, phenyl,
~D A
c(D A
-
17
or ester groups, a predominantly diradical tetramethylene is
favored.
The termini of the tetramethylene can interact with each other
by through-bond
interaction in both the gauche and the trans forms. Maximum
interaction is achieved in
the trans configuration by the carbon p-orbitals adopting a
90°-90° conformation. The
zwitterionic intermediate should exist primarily in the gauche
form because of
Coulombic attraction between the oppositely charged termini. The
diradica1 intermediate
should exist in the trans form, as the attractive forces between
the ends are not as
strong.
~D A
gaucho trans
.,- • -,- ., +.-
The Bond Forming Initiation Theory
The Bond Forming Initiation TheoI}' extends the Huisgen
hypothesis and proposes
that these same tetramethylenes are the true initiators for the
observed spontaneous
polymerizations that often accompany [2 + 2] cycloadditions, and
that this concept is
valid for both ionic and radical polymerizations.
If the tetramethylene is diradical in nature, it can initiate
radical copolymerization.
If it is zwitterionic in nature, it can initiate cationic
homopolymerization of the electron-
-
rich olefin and/or anionic homopolymerization of the
electron-poor olefin. It can also
simply close to give the [2 + 2] cycloadduct (see Scheme 1).
18
Polymerization is a very sensitive technique for detecting
diradical or zwitterionic
intermediates.1 It offers two main advantages over conventional
trapping techniques: 1.
A high conversion of reactant is obtained from a minute amount
of reactive intermediate
in the form of polymer, which is easily isolated and identified.
2. The nature of the
intermediate can be determined from the structure of the
polymer. Isolation of
copolymer is indicative of a diradical intermediate, while
homopolymer is indicative of a
zwinerionic intermediate (see Scheme 1).
Note that only homopolymer can be obtained from a zwitterionic
intermediate.
Copolymerization cannot occur here since that would require
attack by a cationic center
on an already electron-poor moiety unable to stabilize a cation,
or an anionic center on
an already electron-rich moiety unable to stabilizp. an anion. A
diradical center can
propagate with either olefin, since the radical can be
stabilized by either a donor or an
acceptor group. The propagation is governed by the polarity of
the radicals and
monomers and an alternating tendency should result.
An Organic Chemist's Periodic Table
In a survey of reactions of donor and acceptor olefins, Hall
developed the Organic
Chemist's Periodic Table, shown in Figure 1.1 The electron-poor
olefms are listed
across in order of increasing electrophilicity. The
electron-rich olefins are listed down
in order of increasing nucleophilicity. Reactivity increases as
one goes down and to the
right. The table shows areas of mechanistic change from
diradicals to zwitterions and
from zwitterions to ion radical pairs. Weak donors and acceptors
favor a diradical
intermediate. Strong donors and acceptors favor a zwitterionic
intermediate and very
strong donors and acceptors favor radical ion pair
intermediates.
-
19
Incr.ling eccaplor ability _
::-"c,,1!s ,-
-
20
Keeping in mind Huisgeu's idea of the nature of the hybrid
tetramethylene being
dependent on the tenninal substituents and the trends shown in
the Organic Chemist's
Periodic Table, Hall chose N-vinylcarbazole and p-methoxystyrene
as donor olefins
because of their ready polymerizability. He chose tetra- and
trisubstituted electron-poor
olefms with various combinations of ester and cyano groups for
various reasons. First,
they readily will stabilize a reactive center (radical or
anionic). Second, they are either
solids or high boUing liquids at ambient temperature, unlike
many reactive olefins which
are gases. This allows the reactions to be done under conditions
that do not require
high pressure glassware and extremely low temperatures. Third,
these olefins do not
homopolymerize, but they readily copolymerize. This makes
isolation of two different
polymers in cases of zwitterions (no easy task) WUlecessary, as
only homopolymer of
the electron-rich olefm will be obtained.
Applications of the Bond Forming Initiation Theory
Trapping of a Zwitterionic Intermediate Using Cationic
Polymerization
A zwitterionic intermediate was successfully trapped using the
polymerization
criterion in the reactions of N-vinylcarbazole and dimethyl
2,2-dicyanoethylene-l,l-
dicarboxylate.' Cyclobutane, poly(N-vinylcarbazole) and a
I-butene derivative were
obtained as products (see Scheme 2). The formation of
homopolymer in this reaction is
indicative of a zwitterionic intermediate. The olefms initially
form a brightly colored
electron-donor-acceptor (EDA) complex which collapses to fonn
the zwitterionic
tetramethylene. At equimolar concentrations and dilute solution
the latter closes to form
the cyclobutane. This is the result of a cage effect, where the
intermediate is
surrounded by a cage of solvent molecules. The trans
tetramethylene is unable to
initiate polymerization since no monomer is nearby, so it either
undergoes a proton shift
to give the I-butene or rotates about the C;-c, bond back to the
gauche tetramethylene
where it simply closes to the cyclobutane. At higher
concentrations or in excess N-
-
NCz
=' -E CN-)=(
E CN
, • COOCH,
NCI • M-C.rblll'r'
EDA
Complox
_ r-!NCZ
E--j\CN
E CN
1 NCz
--
NCr-(:! _ NC E
1 'k'yJ:
Nez
HC. r='NCZ
N~E H E
Scheme 2 Trapping of a Zwitterionic Intermediate By Cationic
Polymerization
vinylcarbazole, the trans tetramethylene initiates cationic
homopolymerization of the N-
vinylcarbazole.
21
Note that the cyclobutane formation is reversible. If this
cyclobutane is put in an
excess of N-vinylcarbazole, poly(N-vinylcarbazole) is obtained.
No color is observed in
this reaction, indicating that the BDA complex does not
participate in the initiation and
that the zwitterion is the true initiating species.
Only the cyclobutane in which the two cyano groups are a to the
N-carbazolyl
group is formed in this reaction. Cyano groups stabilize anions
more effectively than
ester groups. Hence, maximum stabilization of the zwitterionic
intermediate is obtained.
This is yet more evidence supporting the zwitterionic
intermediate. This zwitterion can
also be trapped using methanol to give the adduct shown
below.
-
22
Trapping of a Zwitterionic Intermediate Using Anionic
Polymerization
Another zwitterionic tetramethylene was trapped using an
e1ectton-poor olefin
capable of anionic polymerizadon (see Scheme 3).' Isobutyl vinyl
ether and methyl a-
cyanoacrylate reacted at room temperature in equimolar amounts
to give a dihydropyran
and poly(methyl a-cyanoacrylate). The two olefins react to fonn
a zwitterionic
intennediate which can initiate anionic homopolymerization of
the methyl a-
cyanoacrylate or close through the ester carbonyl to give the [2
+ 4] cycloadduct. This
cycloadduct is not stable. It isomerizes to methyl
2-cyano-S·isobutoxypent-4·enoate. In
dilute solution, only a trace of homopolymer is fonned, the
dihydropyran being the
major product. The zwinerion was trapped using methanol. The
results here are similar
to those obtained in the reactions of N-vinylcarbazole and
dimethyl 2,2-dicyanoethylene-
I,l-dicarboxylate. The presence of poly(methyl a-cyanoacrylate),
the cage effect, and
the methanol adduct are all evidence for a zwitterionic
intennediate.
Trapping of Diradical Intermediates
Diradical intermediates were also successfully trapped using
Hall's polymerizadon
criterion. p-Methoxystyrene and styrene both reacted with methyl
a-cyanoacrylate to
give 1:1 alternating copolymer. A double Diels-Aldel'
(Wagner-lauregg) adduct was also
formed as shown in Scheme 4.9
The system of p-methoxystyrene and dimethyl cyanofumarate was
studied
extensively.'o These two olefms reacted to give 1:1 alternating
copolymer at high
concentrations regardless of the feed ratio. Copolymer is
indicadve of a diradical
intermediate. At lower concentrations dihydropyran formadon
began to compete with
the copolymerizadon. Addition of the tetramethyl piperidine
N-oxyl radical resulted in
the adduct shown in Scheme S. A proposed mechanism for this
adduct fonnadon is
shown in Scheme 6.
-
23
~ C+'" ~o'" OIBu Dc NC + CN =< CN COOCH 3 H3 COOC COOCH 3
1
!J!
OIBu
~OCH' 0rf. H3 00C eN CN
Scheme 3 Trapping of a Zwitterionic Intermediate by Anionic
Polymerization
~
~: ~.' Ar
D
E
=< CN CN NC
\ !J1 NC
E f;yr{. E ArE eN
Scheme 4 Trapping of a Diradical Intermediate I
-
ow. y
Scheme S
E
/~ lo oddltlon eye
~~CH3
COOC¥OCH 3 H3 CN
rodlco
Diradical Intennediate IT Trapping of a
Iy morlzotlon I copo E
-
25
The kinetics of this system were studied at high concentrations
so that dibydropyran
formation did not play any role. The rate of polymerization was
found to be second
order in monomer concentration (first order in donor and first
order in acceptor since
the polymerization is strictly alternating). The derivation also
took into account the
propagation by the BOA complex. This term, which was third order
in monomer, was
found to be zero. This is evidence that the BOA complex plays no
role in the
propagation step. The kinetic data as well as the trapping
experiment support a
diradical intermediate.
s)'" ~. :2t. - Ct ~ . :. E R CN E CN
OCH.
:& - ~:' q :' Scheme 6 Mechanism of Oiradical Intermediate
Trapping with TEMPO
-
26
Borderline Tetrnmethylenes
The Organic Chemist's Periodic Table shows an area of
mechanistic change. The
donor olefin may predict a zwitterionic intermediate, while the
acceptor olefin may
suggest a diradical intermediate and vice versa. Hall and Padias
call these borderline
cases "schizophrenic tetramethylenes."ll The nature of the
tetramethylene can be altered
by varying the solvent polarity. p-Methoxystyrene and methyl
2,2-dicyanoacrylate react
to give copolymer as the major product in nonpolar solvents like
carbon tetrachloride
and dichloroethane. Dipolar aprotic solvents (DMSO, DMAc) favor
a double Diels-
Alder adduct and polar protie solvents (methanol,
hexafluoroisopropanol) favor the
cyclobutane adduct (see Scheme 7).
Scheme 7
ji'" - +
$0 h,,,,
OUSO, OU&<
r=
-
27
In polar solvents, the diradical tetramethylene exhibits more
polar character due to
solvation effects. In other words, it becomes more
zwitterion-like. A more polar
intennediate will exhibit increased Coulombic attraction between
the tennini favoring the
gauche tetramethylene and hence the cyclobutane adduct. In
nonpolar solvents, the
diradical intennediate is free to rotate about the ~-c, bond,
favoring the trans
tetramethylene which initiates copolymerization. Polar aprotic
solvents stabilize the 8+
charged end of the tetramethylene, leaving the 8- end free to
attack the phenyl ring,
thus favoring the double Diels-Alder adduct The tetramethylene
intennediate in this
example is not a zwitterion as it does not react with methanol,
but is believed to be a
polar diradical.
Reactions with ~-Leaving Group Olefins
If an electron-rich olefin is reacted with an electron-poor
olefin containing a p-
leaving group, a tetramethylene with zwitterionic character will
be fonned. This
tetramethylene can close to give the cyclobutane or the
~-leaving group can be
eliminated to give the cationic species shown in Scheme 8. This
species with a less
nucleophilic counterion can initiate cationic homopolymerization
of the electron-rich
olefm more effectively than the zwitterion itself.
p-Methoxystyrene was reacted with l,2,2-tricyanovinyl
trifluoromethane sulfonate,
2,2-dicyanovinyl chloride, 2,2-dicarbomethoxyvinyl chloride,I2
2,2-dicyanovinyl iodide,
and 2,2-dicyanovinyl tosylate,u All successfully initiated
cationic homopolymerization
of p-methoxystyrene. Both 2,2-dicyanovinyl tosylate and
2,2-dicyanovinyl iodide still
initiated polymerization in the presence of a radical inhibitor,
bis(3-t-buty14-hydroxy-S-
methylphenyl) sulfide, illustrating that the reaction did not
proceed by way of a radical
mechanism. Polymerization also proceeded in the presence of a
proton trap, 2.4,6-tri-t-
butylpyridine. This indicates that no proton transfer is
involved in the reaction. A
-
D ~
A
r=< X A
X. 01f, OT •• I. CI
'AA X A I
dA X A
D /
~~+ A :;1
I D A, r-{. )=' . x-
Scheme 8 BFI Theory Using P-Leaving Group Olefins
28
bimodal molecular weight distribution was obtained from this
polymerization - polymer
resulting from free ion and ion pair propagation. Ion pair
propagation is slower and the
molecular weight of the polymer is lower.
Olefms containing the triflate leaving group were extremely
reactive. The reactions
could not be controlled. Tricyanovinyl chloride and dicyanovinyl
chloride were less
reactive and hence less efficient at initiating polymerization.
This is to be expected as
triflate is a much better leaving group than chloride.
Polymerization was accompanied
by butadiene formation. As the initiator concentration
increased. so did the amount of
butadiene formed in the reaction. Polymer yield also increased.
but molecular weight
decreased due to termination by chloride ion.
Dicarbomethoxyvinyl chloride reacted
very slowly. but it also initiated homopolymerization of
p-methoxystyrene.
Reactivity correlated well with leaving group ability. The
triflates were the most
reactive. while the chlorides were the least reactive. Tosylate
and iodide fell in
-
29
between. Polymer yield and molecular weight also correlated well
with leaving group
ability. More polymer of higher molecular weight was obtained
with the olefins with
the best leaving. groups (tritlate, tosylate). Less reactive
olefins are more subject to
premature termination by the gegenion. In the case of the
chloride initiators, butadiene
derivatives are also formed.
These results illustrate how changing the substituent on the
reactants affects the
character of the tetramethylene. p-Methoxystyrene has been shown
to copolymerize with
electron-poor olefins, indicating initiation by a diradical
intermediate. By introducing a
leaving group, the nature of the tetramethylene is changed to
zwitterionic. Huisgen's
hybridized tetramethylene and Hall's Bond Forming Initiation
Theory are further
supported.
Application of the Bond Forming Initiation Theory to [4 + 21
Systems
Hall has successfully trapped both zwitterionic and diradical
intermediates using his
polymerization criterion. The Bond Forming Initiation Theory
appears to explain the
spontaneous polymerizations observed in reactions of
electron-rich olefins and electron-
poor olefms. Perhaps this theory could also be applied to
reactions of electron-rich
dienes with electron-poor olefins.
According to the Woodward Hoffmann Rules, a [4 + 2]
cycloaddition reaction is an
allowed, concerted, electrocyclic reaction. This reaction can,
however, be a stepwise
reaction. Many reviews have been written on the mechanism of
cycloaddition reactions,
and many contain examples of [4 + 2] cycloadditions that are not
concerted.14,15.16,17 The
arguments of Huisgen and Firestone" seem to typify the state of
affairs as to the
reaction mechanism.
Bartlett studied the reactions of chlorotluoroethylenes with
l,3-butadiene.'~ The
results are shown in Table 1. The [2 + 2] cycloadduct is the
major product in the
reaction with l,l-dichloro-2,2-ditluoroethylene, while the [4 +
2] cycloadduct is the
-
30
major product with vinylidine fluoride. The chlorine is better
able to stabilize the
radical than the fluorine. A stepwise intermediate is favored
over the concerted
mechanism, hence the higher yield of cyclobutane. In the cases
of trifluoroethylene and
vinylidene fluoride, the concerted mechanism may play a larger
role.
Little proposed a diradica1 intermediate in the reactions of
a-acetoxyacrylonitrile
and butadiene.:W The cyclohexene/cyclobutane ratio remained
roughly the same
regardless of the solvent or the temperature. Since it is
unlikely that competing
mechanisms (concerted [4 + 2] cycloaddition and stepwise [2 + 2]
cycloaddition) depend
on the same reaction conditions, in this case differing solvent
polarity and
temperature.the two products may have originated from a common
intermediate, namely
a diradical intermediate.
Further studies by Banlen and Schueller using 2,4-hexadiene and
ct-
acetoxyacrylonitrile seemed to refute these conclusions.21 Only
two isomeric forms of
the [4 + 2] adduct were obtained. in which the stereochemistry
of the methyl groups
was retained. In reactions of 2,4-hexadiene with ethylene, the
stereospecific [4 + 2]
cycloadduct was the only product. Both [4 + 2] and [2 + 2]
adducts were obtained in
the reactions of 1,3-butadiene and ethylene. the latter in only
0.02% yield.22 These
results lead to the conclusion that the diradical addition
becomes twenty times slower
and the concerted reaction becomes ten times faster on
substitution of the diene. This
supports the theory of two competing mechanisms: the thermally
allowed concerted [4 +
2] and the stepwise [2 + 2]. Substitution hinders initial attack
to form the diradical.
Stewart proposed a zwinerionic intermediate in the reactions of
piperylene and
tetracyanoethylene in which he obtained both vinylcyclobutane
and cyclohexene
adducts.23 Eisch and Husk also proposed a zwinerionic
intermediate in their
cycloaddition reaction of 1,1-diphenyl-l,3-butadiene and
tetracyanoethylene.:U
-
Table 1 Banlen's Results
Olefin
F C I
>=< F C I
==
-
32
Van Mele and Huybrechts obtained some rather convincing evidence
for a diradical
intennediate in the cycloadditions and the retro-Diels-Alder
reactions of cyclohexadiene
with the cis and trans 2-butenes shown in Scheme 9.25
Stereospecificity of the starting
diene was lost in each case and both isomeric
bicyclo[2.2.2]octenes were fonned.
SimUar results were obtained in the thennolysis reactions of
disubstiwted
bicyclo[2.2.2]octenes.26
Endo-cis-S-cyano-6-methyl-bicyclo[2.2.2]octene and exo-trans-
S-cyano-6-methyl-bicyclo[2.2.2]octene showed loss of
stereochemistry after undergoing
retro-Diels-Alder reactions. According to the principle of
microscopic reversibility, the
forward reaction should proceed in the same manner as the
reverse reaction.
Nishida, et al. studied the reactions of l-cyclopropyl
substiwted 1,3-butadienes with
tetracyanoethylene (see Scheme 10).27 When the
E-l-cyclopropyl-l,3-butadiene was
used, both vinylcyclobutane and cyclohexene adducts were formed.
The product ratio
o o
Scheme 9
+
!t.". -- ~'" I tH. tH. ).~".~ ~ ~~J ~:J
" CH, Reactions of Cyclohexadiene with Cis- and Trans-
2-butenes
was dependent on solvent polarity; increasing solvent polanty
ravored the cyclobutane.
This is characteristic in reactions involving zwitterions. The
vinylcyclobutane
isomerized to the cyclohexene. A zwitterionic intennediate was
proposed to explain
these results. The vinylcyclobutane was fonned as the kinetic
product and the
cyclohexene as the thennodynamic product. According to Houk, the
best known
examples of stepwise Diels-Alder reactions involve either
nonpolar partners or highly
-
33
unsymmetrical polar partners.28 Nonpolar cycloadditions have
little preference for the
concened mechanism over the stepwise mechanism, while polar
Diels-Alder additions do
show a high preference for the concened mechanism. With highly
polar diene-
dieneopile combinations, in which at least one addend is highly
unsymmetrical, stepwise
reactions involving zwitterionic intermediates take place. This
seems to be a relatively
correct assessment in the case of polar diene-dienophile
combinations.
However, Bartlett has come to the conclusion that alkyl
substituted dienes favor the
concerted mechanism (alkyl substituted and still relatively
nonpolar dienes).29 In
thereaction of butadiene with ethylene, the ratio of cyclohexene
to vinyl cyclobutane
was found to be 99.98 to 0.02. Reactions of any substituted
diene would yield a
virtually undetectable amount, if any cyclobutane. This is
supported by the fact that no
H
~ + NC CN
)=( NC CN ~
H+ CN - CN
I CN CN
dtf;H CN
eN CN
eN
\
~ CN I eN CN eN
Scheme 10 Reactions of l-Cyclopropyl-l,3-Butadiene and TCNE
-
'.
vinylcyclobutane is obtained in the reactions of 2,4-hexadiene
with ethylene.
Despite the examples of these apparently nonconcerted reactions.
the general
consensus is that [4 + 2] cycloadditions are concerted. and that
there are a few
exceptions to the rule. These are governed by the following
criteria:'o
1. Substituents at the 1.6 tennini that stabilize the biradical
or the zwitterion can
substantially lower the energy level of the intennediate.
34
2. The concened pathway requires an s-cis 1.3-diene. The smaller
the s-cis content in
the confonnational equilibrium. the higher the chance of the
two-step reaction.
3. The demands on the order of the transition state are less
stringent for the fonnation
of the intennediate than for the concerted pathway.
4. The substitution of the diene is such that the concerted
transition state is hindered.
The diene must be in the s-cis fonn. which is present only in
minute
concentrations. for a concerted reaction to occur. The s-trans
fonn of the diene is the
0 0 0
~ (A j - f_ -1
~A • A
0
~D 0 erA + erA
-A
Scheme 11 Reactions of Cis and Trans Dienes
-
3S
predominant fonn, and can also be expected to react. However,
due to the geometry
constraints, it will react in a stepwise manner. Addition of the
olefin to the tenninus of
the diene can lead to either diradical or zwitterionic
intennediates as shown in Scheme
11. The identity of the intennediate will be dependent on the
substituents present
Rotation about the olefin C-C bond will result in loss of
stereospecificity. Rotation
about the allylic bond is unlikely since this would disrupt the
allylic stabilization.
A stereospecific reaction is expected when the HOMO of the diene
and the LUMO
of the dienophile or vice versa is small and the steric
conditions allow a cisoid
confonnation of the diene and thus a smooth reaction in the 1,4
position. Large
HOMO-LUMO separation slows down the rate of the synchronous
reaction to such an
extent that perhaps a slower two-step reaction becomes
possible.
Cis I-substituted dienes will favor the transoid configuration
of the diene by
sterically hindering the cisoid confonnation. A nonconcerted
mechanism should be
favored. Substituents capable of stabilizing intennediates will
also favor the two step
mechanism.
Proposed Research
The Bond Forming Initiation Theory, derived for [2 + 2] systems,
could also be
applied to [4 + 2] systems. When a transoid electron-rich diene
reacts with an electron-
poor olefm, an intennediate may be fonned as shown in Scheme 12.
The resonance
fonn where the active center is adjacent to the donor group
should be favored because
of extra stabilization. As Huisgen suggested for the
tetramethylene intennediate, the
hexamethylene intennediate may also exist as a hybrid between
the zwitterionic and
diradical extremes. The nature of the intennediate should also
be substituent-dependent
in the same way. Strong donors and acceptors should favor the
zwitterionic
intennediate and weak donors and acceptors should favor a
diradical intennediate.
-
36
This hexamethylene intermediate in the extended conformation can
initiate
polymerization. If the hexamethylene intermediate is diradical
in nature, it could initiate
copolymerization. If it is zwitterionic in nature, it could
initiate either cationic homo-
polymerization of the electron-rich diene and/or anionic
homopolymerization of the
electron-poor olefin. TIle intermediate in the cisoid
conformation can simply close to
form the cyclohexene. This theory does not rule out the
possibility that the cycloadduct
can also be formed via a concerted mechanism. It only suggests
that if an intermediate
is present. polymerization may be a useful means of identifying
it
A cisoid diene should react via a concerted mechanism to give
only the
cyclohexene. The transoid diene should react via an intermediate
to give either the
cyclobutane or the cyclohexene. More cyclobutane than
cyclohexene is expected from a
diradical intermediate because rotation about the bond that is
part of the allylic system
is unlikely. At higher temperatures. more cisoid diene is
present and the amount of [4
+ 2] cycloadduct should increase.
D
j + •
1 I; &A c(
A
D
Scheme 12 The BFI Theory Applied to [4+2] Systems
~ D A
~ D
k'y1 A
-
Introduction
CHAPI'ER 2
Reactions of l-AryI-l,3-Butadienes With Trisubsdtuted
Electrophilic Olefins
37
In order to apply the Bond Fonning Initiation Theory discussed
in the previous
chapter to [4 + 2] systems, reactants must be chosen so that an
intermediate will be
favored. According to Houk, stepwise Diels-Alder reactions are
favored when the
reactants are highly unsymmetrical and polar.n It seems logical
that unsymmetrical,
polar reactants with donor and acceptor substituents on the
diene and the dienophile
respectively, will favor an intermediate. The choice of
substituents will depend on the
type of intermediate desired for study.
According to the Organic Chemist's Periodic Table (Figure I),
phenyl substituents
on the diene should favor a diradical intermediate. The
trisubstituted electrophilic
olefms methyl 3,3-dicyanoacrylate, dimethyl cyanofumarate, and
trimethyl ethylenetricar-
boxylate were used in the olefin-olefin reactions and excellent
results were obtained.
These olefins could also be used in the [4 + 2] cases.
The isolation of polymer on reactions of the dienes with the
olefms would indicate
the presence of a reactive intermediate. If a copolymer were
obtained, the reaction
would be proceeding by way of a diradical intermediate. A
homopolymer would
indicate that the reaction is proceeding via an ionic
intermediate.
Huisgen's hybrid tetramethylene theory can also be applied to
hexamethylene
intermediates. By introducing a leaving group at the (i-position
of the olefin, the
character of the hybrid is shifted towards zwitterionic. As the
leaving group is expelled,
the character of the intermediate should become more and more
zwinerionic. The result
-
38
is a cationic intennediate that could initiate
homopolymerization of the diene as shown
in Scheme 13.
The dienes selected for this study were I-phenyl-l,3-butadiene 1
and p-anisyl-l,3-
butadiene 2. The trisubstituted elecuophilic olefins methyl
3,3-dicyanoacrylate 3,
dimethyl cyanofumarate 4, and trimethyl ethylenetricarboxylate 5
were chosen to study
2:'(~ J -~ k/~
C-l/ x
o
A
Scheme 13 General Mechanism of Reactions of Dienes with
p-Leaving Group Olefms
the diradical intennediate. The olefms with p-leaving groups
selected to study the
zwitterionic intennediate were 2,2-dicyanovinyl chloride 7,
2,2-dicyanovinyl iodide 8,
2,2-dicyanovinyl tosylate 9, 2-cyano-2-carbomethoxyvinyl
chloride 10, and 2-cyano-2-
carbomethoxyvinyl iodide 11.
Background
I-Phenyl-l,3-butadiene has been polymerized by anionic and
cationic means.32..""",33,36
No mention of radical polymerization of I-phenyl-l,3-butadiene
or p-anisyl-l,3-
butadiene has been found. Appropriate control experiments would
have to be perfonned
in order to ascenain the polymerizabUity of these monomers. The
trisubstituted olefins
-
used in this study do not homopolymerize. Thus only a
homopolymer of the
arylbutadiene would be obtained if the reactive intermediate
were a zwitterion.
Results
Control Reactions
39
Control experiments were done to determine if the chosen systems
would
polymerize. The results are shown in Table 2. The reactants were
deliberately initiated
using both cationic and radical initiators. p-Toluenesulfonic
acid and boron trifluoride
etherate successfully initiated cationic homopolymerizations of
I-phenyl-l,3-butadiene to
give poly(phenylbutadiene). No radically initiated
homopolymerization of the
arylbutadienes was successful. Radical copolymerization
reactions of the arylbutadienes
and the trisubstituted olefins were also unsuccessful. The
reaction of p-anisyl-l,3-
butadiene with trimethyl ethylenetricarboxylate is a much slower
reaction than reactions
with methyl 3,3-dicyanoacrylate and dimethyl cyanofumarate.
Initiation with AIBN at
85°C gave only the [4 + 2] cycloadduct. Due to the rapid
reaction rates of the
spontaneous reactions between p-anisyl-l,3-butadiene and both
methyl 2,2-
dicyanoacrylate and dimethyl cyanofumarate, no deliberate AIBN
initiated
polymerizations were successful.
Triethyl borane (in THF) and oxygen were used as the initiating
system at -50°C.
p-Anisyl-l,3-butadiene was dissolved in toluene and the triethyl
borane, oxygen, and
methyl dicyanoacrylate were added sequentially. A dark orange
charge transfer complex
formed immediately on addition of the olefin. After 16h at
-50°C, a precipitate had
formed. This precipitate proved to be the [4 + 2] cycloadduct.
No polymer was
obtained. Because of limited solubility of the diene in toluene
at -SO°C, the reaction
was repeated at -3SoC in dichloroethane. The results were the
same - no copolymer
was obtained. The same procedure was repeated using dimethyl
cyanofumarate as the
-
40
Table 2 Control Reactions
Monomers Initiator Solvent Time Temp. Polymer (h) (OC)
Yield(%)
1 p-TsOH DCE 0.03 25 81
1 BF3 "Et 2O CH 3N02 0.02 25 79
1 AIBN Bulk 48 70 0
2 + 3 Et3B/02 CH 3 16 -50 0
2 + 3 Et3B/02 DCE 16 -35 0
2 + 4 Et3B/02 DCE 16 -35 0
2 + 5 AIBN Bulk 16 85 0
2 + 29 AIBN Bulk 72 80 0
2 + 5 AIBN 29 41 80 0
DCE = D~chloromethane
-
electrophilic olefin. The same results were obtained; the [4 +
2] cycloadduct was the
only product formed. No copolymer was formed.
41
AIBN initiated polymerization of acrylonitrile in the presence
of the p-anisyl-l,3-
butadiene/trimethyl ethylenebicarboxylate system was
unsuccessful. No polymer was
obtained. AIBN initiated polymerization of acrylonitrile was
inhibited by the presence
of p-anisyl-l,3-butadiene. The diene appears to act as a radical
polymerization inhibitor.
According to Mulzer, I-phenyl-l,3-butadiene does not
spontaneously
homopolymerize.37 p-Anisyl-I,3-butadiene was heated at 175°C for
IS hours. No
decomposition, Oligomerization, or other reactions occurred.
After 96 hours, there
appeared to be some dimerization. The diene does not
spontaneously homopolymerize
with itself.
Reactions of l-Aryl-l,3-Butadienes with Trisubstituted
Electrophilic Olefms
p-Anisyl-l,3-butadiene reacted in less than a minute with maleic
anhydride 6 to
give an essentially quantitative yield of the [4 + 2]
cycloadduct 18 (see Table 3).
Reactions of the electrophilic olefms 3-5 with the
arylbutadienes 1-2 in dichloroethane
and nitromethane gave exclusively [4 + 2] cycloadducts.
Two stereoisomers were formed from endo and exo additions.
Isomer ratios were
determined by O. C. and are reported in Table 4. No [2 + 2]
cycloadduct was
observed in any case, nor was any polymer observed.
Experimental Observations
There was some experimental difficulty in doing the bulk
reactions. All of the
reactants are solid at room temperature. When the diene is
melted and then
supercooled, it stays in liquid form. The reactants can be mixed
by pouring the diene
into the dienophile. The resulting reaction mixture is very
viscous and cannot be
-
42
Table 3. Reactions of 1-Aryl-1,3-Butadienes with Trisubstituted
Olefins
Reactants Solvent Cone. Time Temp. Adduct Adduct (Ml (hI (OCI
'Yield
1 + 3 Bulk 1 65 12 87
DCE 5 4 25 12 71
DCE 1 4 25 12 73
1 + 4 Bulk 1 25 13 84
DCE 5 5 25 13 57
DCE 1 16 25 13 61
1 + 5 Bulk 96 75 14 71
DCE 5 144 70 14 86
2 + 3 Bulk 20 25 15 85
DCE 5 2 0 15 76
DCE 1 20 25 15 88
CH 3NO, 5 16 25 15 91
2 + 4 Bulk 20 25 16 67
DCE 5 2 0 16 68
DCE 1 20 25 16 77
CH 3NO, 5 16 25 16 76
2 + 5 Bulk 20 25 17 0
DCE 5 48 150 17 21
CH 3NO, 5 16 80 17 83
2 + 6 Bulk 0.03 25 18 98
DCE - Dichloroethane
-
43
Table 4. Reactions of 1-Aryl-1,3-Butadienes with P-Leaving Group
Olefins
Reactants Solvent Cone. Adduct Adduct Homopolymer (M) 'BYield 18
Yield
1 + 7 Bulk 19 54 6
DCE 5 19 43 2
DCE 1 19 47 0
CH 3N0 2 5 19 39 4
1 + 8 DCE 5 20 46 16
CH 3N0 2 5 20 34 25
1 + 9 DCE 1.5 0 37
CH 3N0 2 2 0 54
1 + 10 DCE 5 21 77 1
CH 3N0 2 5 21 80 1
1 + 11 DCE 5 22 79 1
CH 3N0 2 5 22 87 2
2 + 7 DCE 5 23 80 5
2 + 9 DCE 5 0 95
DCE 5* 0 100
2 + 10 DCE 5 24 88 8
2 + 11 DCE 5 25 74 13
DCE - Dichloroethane * Molar Ratio of
p-anisyl-1,3-butadiene/2,2-dicyanovinyl tosylate
10: 1.
-
44
stirred, hence the lower yields for the bulk reactions in some
cases. In the case of p-
anisyl-l,3-butadiene and ttimethyl ethylenetricruboxylate, the
reactants solidified on
mixing and no reaction occurred. Starting material, which was
identified in both NMR
spectra and O. C. analyses, remained in all of these reactions,
but was Dot isolated.
The reactions of dienes 1 - 2 with methyl 3,3-dicyanoacrylate
and dimethyl
cyanofumarate are very rapid and exothermic. The reactants were
mixed at O°C in
efforts to control the reaction rate. After the initial heat had
dissipated, the reaction
mixture was stirred at room temperature. The reactions of dienes
1 - 2 with trimethyl
ethylenetricarboxylate are very slow. These reactions were mixed
at room temperature
and stirred at higher temperatures.
Stereochemistry of the Cycloadducts
Reactions of the two arylbutadienes with the trisubstituted
olefins gave two isomers
resulting from endo and exo addition. The predominant isomer was
thoroughly
investigated by 'H NMR. and has the structure depicted in Figure
2. This structure was
determined by proton homodecoupling experiments. Coupling
constants are discussed
only for 4,4,5-tricarbomethoxy-3-phenyl-cyclohexene 14. The
coupling constants of
cycloadducts 12-17 were also determined and the same analysis
was again applied to
determine their stereochemistries.
Irradiation of ~ and ~ shows that H, is coupled only to these
two hydrogens.
The doublet from H, is simplified to a singlet on irradiation of
Hl , indicating that the
methyne is adjacent to the quaternary carbon and to the vinyl
proton, Ha. The
stereochemistry at C, and Cs was determined by calculating
coupling constants, ISla and
JS,60' ISla is approximately 11Hz and Is,60 is approximately
7.5Hz. This is indicative of
axial-axial and equatorial-axial coupling, respectively. Thus,
Hs is in the axial position.
Homoallylic coupling constants 131a and 13,60 were calculated to
be 305Hz and 105Hz
-
45
respectively. Comparison with Barfield's calculated coupling
constants indicates that H,
is axial."
Reactions with Olefins Containing p-Leaving Groups
Homopolymer of the arylbutadiene was obtained in all reactions
of arylbutadienes 1
- 2 with Uisubstituted olefins having leaving groups in the P
position (see Table 5).
The [4 + 2] cycloadduct was also obtained in all cases as a
mixture of two isomers,
except in the reactions using 2,2-
-
46
A
3 2 H H A
:5 H
Figure 2 Major Isomer
Table S. Isomer Ratios of [4 + 2] Cycloadducts in Reactions of
p-Anisyl-1,3-Butadiene
Olefin Ratio Olefin Ratio
3 2:1 7 3:1
4 1:1 10 1:1
5 1:1 11 3:1
-
47
backbone stereochemistry is cis-lA, or ttans-1,4. IR spectra
show no terminal vinyl
groups, so it is unlikely that 1,2 or 3,4 addition oCCUITed. The
yield increased with
increasing leaving group ability, as did molecular weight. The
molecular weights of the
polymers are low (see Table 6), which may be atUibuted to the
fact that a 1:1 molar
ratio of diene to olefin was used. Molecular weight increases
with increasing leaving
group ability. When 10 mole% of dicyanovinyl tosylate was used,
a quantitative yield
of poly(p-anisyl-l,3-butadiene) was obtained.
Cycloadduct
[4 + 2] Cycloadduct was obtained in all cases, except in the
reactions with 2,2-
dicyanovinyl tosylate. All reactions were done at high
concentration or in bulk to favor
polymerization. Two isomers of the cycloadduct were observed
(see Table 5). No
extensive NMR studies of these [4 + 2] cycloadducts were done.
However, because of
similar splitting patterns, these isomers are believed to have
the same stereochemistry as
the cycloadducts found in the reactions with the trisubstituted
olefins.
Side products of a highly conjugated nature were also observed,
as was the
methanol adduct of the olefin starting material, formed during
the work-up. The latter
was isolated and obtained in quantitative yield. The former was
present in very small
quantities and was not isolated. This side product is probably
the hexatriene shown in
Scheme 14. Analogous reactions took place in reactions of
p-methoxystyrene and these
~-leaving group olefins to form butadienes."
)' f..-r-" '17 el Ne
Scheme 14. Formation of Hexatriene
~ J' .y eN
-
48
Table 6 AryI-l,3-Butadienes with jl-Leaving Group Olefins
Reactants Solvent Cone. Adduct Adduct Homopolymer (M) \Yield ,
Yield
1 + 7 Bul'k 19 54 6
DCE 5 19 43 2
DCE 1 19 47 0
CH 3NOz 5 19 39 4
1 + 8 DCE 5 20 46 16
CH 3NOz 5 20 34 25
1 + 9 DCE 1.5 0 37
CH 3NOz 2 0 54
1 + 10 DCE 5 21 77 1
CH 3NOz 5 21 80 1
1 + 11 DCE 5 22 79 1
CH 3NOz 5 22 87 2
2 + 7 DCE 5 23 80 5
2 + 9 DCE 5 0 95
DCE 5* 0 100
2 + 10 DCE 5 24 8e S
2 + 11 DCE 5 25 74 13
DCE - Dichloroethane • Molar Ratio of
p-anisyl-1,3-butadiene/2,2-dicyanovinyl tosy1ate
10:1.
-
49
Discussion
Deliberately attempted copolymerization reactions of aryl
butadienes and olefins 3 -
5 were unsuccessful. Because of the rapid spontaneous reaction
rates of the aryl
butadiene with methyl 2,2-dicyanoacrylate and dimethyl
cyanofumarate, an initiator
capable of initiating polymerization at low temperatures was
needed. Low temperature
polymerization of lithium enolates and vinyl monomers had
previously been done with
a triethyl borane/oxygen system.40•41 This proved unsuccessful
in the aryl butadiene
reactions. Only [4+2) cycloadduct was obtained and no copolymer
was observed.
Because the reactions of the aryl butadienes with trimethyl
ethylenetricarboxylate
are much slower, AIBN was used as an initiator. Again, only [4 +
2) cycloadduct was
obtained. AIBN-initiated copolymerization of p-anisylbutadiene
and trimethyl
ethylenetricarboxylate was also tried in the presence of
acrylonitrile, a third radically
polymerizable monomer. The diradical intermediate, if present,
and the AIBN were
expected to initiate polymerization of the acrylonitrile. Again,
no polymer of any kind
was obtained. AIBN initiated polymerization of acrylonitrile in
the presence of p-
anisyl-l,3-butadiene again failed - no polymer was obtained. An
aryl radical formed in
this reaction could be stabilized by the aryl group to such a
degree that propagation is
inhibited. The arylbutadiene acts as a radical trap. Thus, no
copolymer can form and
evidence for a diradical intermediate in these reactions must be
found through different
methods.
The choice of l-aryl-l,3-butadienes as the electron-rich dienes
proved to be poor.
The reactions of the arylbutadienes with the trisubstituted
olefms were carried out, even
though it was unlikely that spontaneous polymerization would
occur. Indeed, this was
found to be the case. No copolymer was obtained, nor was any [2
+ 2) cycloadducL
The [4 + 2) cycloadducts were the only products.
-
SO
The stereochemistry of the cycloadducts can provide insight into
the mechanism of
these reactions. If the reaction is concerted, two isomeric
cycloadducts would be
formed as shown in Scheme IS. If the olefin addition is endo, a
cyclohexene where
the substituents at C, and C~ are trans is obtained. If the
olefin addition is exo, a
cycloadduct where the substituents at C, and C~ are cis is
obtained. In the reactions of
dimethyl cyanofumarate, four differenl isomers would be expected
if the reaction was
stepwise - two isomers where the stereochemistry of the ester
groups was retained and
remained trans, and two where the C4-C~ bond undelWent rotation
and the ester groups
were cis. In the reactions of methyl 3,3-dicyanoacrylate and
Uimethyl
ethylenetricarboxylate only two isomeric cyclohexenes would be
obtained regardless of
the reaction mechanism.
Only two isomeric cyclohexenes were obtained in all cases. These
isomers were
those where the stereochemistry of the ester groups in the
reactions of the aryl
butadienes with dimethyl cyanofumarate remained trans. There was
no loss of
stereochemistry and no evidence that the reactions proceed via a
diradical hexamethylene
intermediate.
According to the Organic Chemist's Periodic Table, the
methoxyphenyl group and a
dicyano moiety are a strong donor and strong acceptor,
respectively. Intermediates in
the reactions of vinyl compounds having these groups as
substituents behave as
"schizophrenic" tetramethylenes. Changing reaction conditions,
such as solvent polarity,
can change the nature of the tetrarlethylene. Polar solvents
favor a zwitterionic
intermediate, while nonpolar solvents favor a diradical
intermediate. p-Anisyl-l,3-
butadiene and methyl dicyanoacrylate may behave in an analogous
manner. By
changing the solvent to more polar nitromethane, a zwitterionic
intermediate may be
favored. This intennediate could initiate cationic
homopolymerization of the diene,
-
SI
D D
~ !£ :CA A H X,A H A
j I
4 4 A H 1\1
"' A A o;'Ay: o~v:
H H H A A H
Scheme 15. Concerted Mechanism
-
S2
[
D
H
CN CN.
E E D
H H H E
Figure 3. Stepwise Mechanism
which has already been shown possible.42 In this case using the
more polar
nitromethane did not induce the zwitterionic intermediate to
initiate homopolymerization
of the diene. Only the [4 + 2] cycloadduct was obtained. No
homopolymer of the
diene was obseJVed.
According to Huisgen, the tetramethylene intermediate exists not
as a diradical nor
as a zwitterion, but a resonance hybrid of the two. Substituents
will cause the
tetramethylene to exhibit characteristics of one or the other.
The same can be said of
the hexamethylene intermediate. By introducing a leaving group
at the 13 position of
the olefm, the character of the hybrid is shifted towards
zwitterionic character. As the
leaving group is expelled, a cationic intermediate is formed
which could initiate
homopolymerization of the electron-rich aryl butadiene very
efficiently. This is indeed
the case.
-
Reactions of I-phenyl-l.3-butadiene and p-anisyl-l.3-butadiene
with nisubstituted
oleflIlS containing j3-leaving groups were run in
dichloroethane. Homopolymer of the
diene was obtained in all cases. Homopolymer is evidence that
this reaction proceeds
via an ionic intermediate.
The cycloadducts were obtained in all cases with the exception
of reactions of
dicyanovinyl tosylate. As in the previous reactions. two isomers
were formed. They
have the same structure as those previously discussed. based on
spectral data. In the
cases of the 2-carbomethoxy-2-cyano vinyl olefins. no rotation
about the olefin C-C
bond was observed.
53
Reactions of the chloro and iodo substituted olefins gave both
cycloadduct and
homopolymer. the former more adduct and less polymer than the
laner. Reactions of
the tosyl substituted olefin gave only homopolymer. The tosyl
group is the least
nucleophilic and is less likely to interfere with the
polymerization. Iodide is less
nucleophilic than chloride and would be less likely to interfere
with polymerization than
chloride. The olefins having a tosylate group would be expected
to initiate
polymerization more efficiently than iodo substituted oleflIlS.
which are expected to be
more effective initiators than chloro substituted olefins.
Molecular weights of the
polymers also increased with decreasing nucleophilicity
indicating that the tosylate was
interfering less with the polymerization than were the iodide or
chloride.
Conclusion
The cycloadducts formed in these reactions probably result from
the concerted
reaction of the cis diene. Since only two isomeric cycloadducts
were obtained instead
of four (in the cases where the substituents at C4 were
different). there is no evidence
that the cycloadduct formation was the result of ring closure of
a diradical intermediate.
Since no copolymer was formed. there is no evidence for a
diradical intermediate. The
-
54
homopolymers obtained in the j3-leaving group olefin reactions
could only have been the
result of a zwitterionic intermediate. Thus, in the reactions of
l-aryl-l,3-butadienes with
(3-leaving group olefins, a zwitterionic intermediate was
successfully trapped using the
polymerization criterion.
-
55
CHAPTER 3
Reactions of I-Methoxy-l.3-Butadiene
Introduction
The l-aryl-l.3-butadienes proved to be unsatisfactory for
studying possible diradical
intermediates. They appeared to act as radical polymerization
inhibitors. I-Methoxy-
1.3-butadiene should not be capable of stabilizing a radical to
such an extent. and it
may prove to be a suitable diene for this study.
Background
There is little mention of polymerization reactions of
l-alkoxy-l,3-butadienes in the
literature. l-Alkoxy-l,3-butadienes spontaneously polymerized in
the presence of
chloromethyl ethers.43 Flaig reports the spontaneous
copolymerization of I-methoxy-
1,3-butadiene and maleic anhydride.44 However, he was not
interested in the copolymer
and took steps to inhibit the polymerization with methylene
blue. Even so, cycloadduct
formation was still accompanied by polymerization, but to a
lesser extent (80%
cycloadduct and 14% copolymer with inhibitor compared to 35%
cycloadduct and 45%
copolymer without inhibitor).
Stepek studied the reaction of l-methoxy-l,3-butadiene with
maleic anhydride in
great detail.45 He found that these two compounds spontaneously
polymerized to give a
1:1 alternating copolymer, where the I-methoxyl,3-butadiene
copolymerized by 1,4
addition. This polymer was found to be identical with that
formed when dibenzoyl
peroxidelp-bromo-N,N-dimethylaniline was used as the initiator.
Since a copolymer was
formed, and the reactivity ratios for these two monomers are
essentially zero,
characteristic of highly alternating copolymers, he postulated a
radical mechanism.
-
The rate of polymerization was found to be second order. The
rate of
cycloaddition, measured in the presence of hydroquinone which
inhibited the copoly-
merization, was also found to be approximately second order.
56
Energies of activation for each of these processes were
measured. Activation
energies for the copolymerization reactions of
I-methoxy-l,3-butadiene and maleic
anhydride and l-ethoxy-l,3-butadiene and maleic anhydride were
found to be 14.8
kcal/mole and 16.3 kcal/mole respectively. The cycloaddition
reactions were found to
have activation energies of 12.1 kcal/mole and 11.4 kcal/mole
respectively. These
different activation energies for the copolymerization and the
cycloaddition coupled with
the fact that inhibitors had no effect on the fonnation of the
cycloadduct led Stepek to
conclude that there were two different mechanisms operating in
this reaction.
I-Methoxy-l,3-butadiene is readily polymerized by cationic
initiators such as
mineral acids, zinc chloride, boron trifluoride, and aluminum
chloride.46.47 These
reactions are very violent and are best done at low temperatures
and in dilute solution.
Butler and Chen studied the reactions of l-ethoxy-l,3-butadiene
and acrylonitrile.48
These reactants spontaneously copolymerized to yield highly
alternating copolymers
having cis-3,4, trans-3,4, cis-l,4 and trans-l,4 structures.
They proposed a mechanism
where the electron donor acceptor complex fonned ion radical
pairs which then initiated
the polymerization as shown in Scheme 16. Cycloaddition
accompanied the copolymer
fonnation.
Since there is precedence for spontaneous copolymerizations in
systems involving 1-
alkoxy-l,3-butadienes, I-methoxy-l,3-butadiene should prove a
good choice for further
investigations of diradical intennediates. On reaction with the
same trisubstituted olefins
(3-5) used in the arylbutadlene reactions, it is expected that
both copolymer and
cycloadduct will be obtained. The fonner should result from
initiation by the diradical
-
57
z u"= -~'--V
, I
1
1 1 z u
"= :z u -..... 0
+ -..... o~
Scheme 16 Butler's Proposed Mechanism
-
S8
intermediate. The later could result from either closure of the
diradical intermediate or
from a concerted reaction.
Results
Control Reactions
I-Methoxy-l.3-butadiene was heated at 66°C for seven days under
Ar. No
reactions of any kind. neither spontaneous polymerization nor
cycloaddition were
observed. I-Methoxy-l.3-butadiene was also heated at 66°C in the
presence of AIBN
for seven days. Again. no reaction occurred. The reaction of
I-methoxy-l.3-butadiene
and trimethyl ethylenetricarboxylate was initiated with AIBN.
Both 1:1 copolymer and
two isomeric [4 + 2] cycloadducts were obtained in 31% and 69%
yields respectively.
The I-methoxy-l.3-butadiene/acrylonitrile system was also
initiated with AIBN. Low
molecular weight copolymer was obtained in 12% yield. Two
isomeric cyclohexenes
were obtained in 69% yield.
Reactions of I-Methoxy-l,3-Butadiene and Maleic Anhydride
I-Methoxy-l.3-butadiene and maleic anhydride were reacted in 2M
dioxane solution
at 25°C under argon. The results are shown in Table 7. A 1:1
diene/olefin ratio was
used. A bright yellow charge transfer complex was observed on
mixing. After an hour
this yellow color was completely gone. The reaction mixtures
were precipitated into
ether containing a small amount of inhibitor. Both copolymer 30
and [4 + 2]
cycloadduct 31 were obtained.
Viscosity measurements were done to determine if copolymer
molecular weight
increased with time. Inherent viscosities were found to increase
with increasing reaction
time. This is evidence for a radical reaction terminating by
coupling.
-
59
Table 7 Reactions of I-Methoxy-l,3-Butadiene and Maleic
Anhydride
Time Cycloadduct Copolymer (h) Yield % ~1nh Yield % ( l/g)
1 69 8 3.26
2 80 10 1.99
3 72 8 2.44
4 73 7 2.97
5 74 4 3.02
-
60
Reactions of I-Methoxy-l,3-Butadiene and Methyl
3,3-Dicyanoacrylate
I-Methoxy-l.3-butadiene 28 and methyl 3.3-dicyanoacrylate 3 were
reacted in bulk
and in SM solutions of dichloroethane and methanol. The results
are shown in Table 8.
AI: 1 diene/olefin ratio was used. A charge transfer complex was
formed as evidenced
by the bright yellow color of the reaction mixwre on initial
mixing. As in the
arylbutadiene reactions discussed in the preceding chapter. the
bulk reaction became too
viscous at the onset of the reaction to allow adequate mixing of
the diene and olefin.
The reaction mixture was warmed at 80°C for a few minutes to
allow the stining of the
reaction mixture. Both copolymer 32 and [4+2] cycloadduct 33
were obtained in bulk
and in dichloroethane. Only [4+2] cycloadduct was obtained in
methanol.
The copolymer composition is approximately 1:1 in each monomer
according to the
elemental analysis. IH NMR spectra of low molecular weight
fractions show that the
predominant structure is 1.4. The copolymers tum yellow and
become insoluble on
exposure to air.
The cyclohexene adduct.
S-carbomethoxy-4,4-dicyano-3-methoxy-l-cyclohexene 33
was obtained as a mixture of two isomers. The isomer ratio was
determined to be 1:2
by gas chromatography. The structure of the major isomer was
determined by detailed
analysis of the NMR spectra. 15.6a was found to be 11.3 Hz and
15.6a was found to be
S.9 Hz. These coupling constants indicate that H5 is in the
pseudoaxial position and
that the ester group is in the pseudoequatorial position. If ~
were equatorial. both
coupling constants would be expected to be between 0 and S Hz.
H3 was determined to
be in the axial position by comparing Barfield's expected
homoallYlic coupling constants
with experimental data.4P
-
61
Table 8. Reactions of 1-Methoxy-1,3-Butadiene
Olefin Solvent Conc. Time Temp. Cycloadduct Copolymer CMI ChI
C'CI yield ('1 yield ('I
3 bulk 16 26 90 10
bulk 16 26 92 5
DCE 5 16 26 90 8
DCE 5 67 26 91
CH30H 5 16 26 42 0
4 bulk 16 26 59 6
bulk 47 26 57 40
DCE 5 47 26 50 20
DCE 5 67 26 84
29 bulk 72 26 57 40
DCt 5M 72 26 50 20
-
62
Reactions of I-Methoxy-l,3-Butadiene with Dimethyl
Cyanofumarate
I-Methoxy-l,3-butadiene and dimethyl cyanofumarate were reacted
in bulk and in
SM dichloroethane solution. The results are shown in Table 8. A
charge transfer
complex was formed as evidenced by the yellow color of the
reaction mixture. All of
the dimethyl cyanofumarate did not initially dissolve in the
diene when no solvent was
used. The reaction mixture was warmed at 80°C for a few minutes
to facilitate the
dissolving. Both copolymer 34 and [4+2] cycloadduct 35 were
obtained.
The copolymer composition is approximately 1:1 in each monomer
as evidenced by
elemental analysis. The copolymer is of too high molecular
weight to obtain a highly
resolved 1H NMR spectrum, so the mode of addition is not known.
(probably mostly
cis and trans 1,4) The copolymer turns yellow and becomes
insoluble on exposure to
air.
The cyclohexene adduct,
4,S-wcarbomethoxy-4-cyano-3-methoxy-l-cyclohexene 35,
was obtained as a mixture of two isomers in a 1.4: 1 ratio as
determined by gas chroma-
tography. The structure of the major isomer was determined in
the same way as that
for the adduct from methyl 3,3-dicyanoacrylate - by detailed
analysis of the NMR
spectra. J5.6& was determined to be 12.1 Hz and J5,60 to be
S.6 Hz. These values agree
with the coupling constants for axial-axial and axial-equatorial
couplings respectively.
Analysis of homoallylic coupling constants show H3 to be axial,
and thus the methoxy
group to equatorial.
Reactions of I-Methoxy-l,3-Butadiene and Trimethyl
Ethylenetricarboxylate
I-Methoxy-l,3-butadiene and trimethyl ethylenetricarboxylate
were reacted in bulk
and SM, 1M, and O.SM solutions of dichloroethane at various
temperatures. The
results are shown in Table 9. At 2SoC, both copolymer 36 and [4
+ 2] cycloadduct 37
were obtained in bulk and SM solution. In 1M and O.SM solutions
of dichloroethane at
-
63
room temperature, only cycloadduct was fonned. At 70°C, and at
longer reaction times,
both copolymer and cycloadduct were obtained. At 1:1 feed
ratios. copolymer yield
increased with increasing temperature. The molecular weight of
the copolymer was
quite high as shown in Table 10.
The molecular weight of the copolymer was studied as a function
of time. The
results are shown in Table 11. At 28°C, copolymer yield
increased with increasing
reaction time. Inherent viscosity, measured at 32.SoC in
chlorofonn showed no increase
in molecular weight with time. Reactions were done at a 2:1 feed
ratio of I-methoxy-
1,3-butadiene to trimethyl ethylenetricarboxylate. No difference
in product composition
was noted. However, the molecular weight of the copolymer
decreased.
Reactions were also done in acetonitrile and methanol. Only 1 %
copolymer was
isolated from the reaction on acetonitrile. The molecular weight
of this polymer was
significantly lower than the polymer from reactions done in
dichloroethane. No
significant difference was noted in methanol.
The copolymer composition was 1:1 according to the elemental
analysis. The
copolymer appeared to be hygroscopic. Analyses not done
immediately after heating at
100°C gave erroneous results. The IH NMR was inconclusive as far
as copolymer
structure was concerned. At high concentrations the copolymer
appears to have either
trans- 1,4 and/or cis-l,4 structure. At lower concentrations, at
higher temperatures and
in methanol solution, 3,4 addition competes with the 1,4
addition. Unlike the
copolymers obtained in reactions of methyl 2,2-dicyanoacrylate
and dimethyl
cyanofumarate, these copolymers did not tum yellow and crosslink
on exposure to air.
The [4 + 2] cycloadduct was obtained as a mixture of two
isomers. These isomers
could not be resolved by gas chromatography, but analysis of the
IH NMR spectrum
showed the isomers present in equimolar amounts.
-
64
Table 9 Reactions of 1-Hethoxy-1,3-Butadiene vith Trimethyl
Ethylenetr!carboxylate
HeOBD/TriE Solvent Conc. Time Temp. Cycloadduct Copolymer ratio
(H) (h) ('C) yield It) yield It)
1:1 bulk 16 25 57 50
DCE 5 16 25 77 22
DCE 1 16 25 81 0
DCE 0.5 16 25 59 0
DCE 5 63 70 50 40
2:1 DCE 5 16 25 79 12
DCE 5 16 57 74 10
1:1 CH3CN 5 13 25 80 1
CH30H 5 13 25 70 20
-
65
Table 10 Molecular Weight of HeOBD/TriE Copolymers
Feed Ratio Solvent Temp. Copolymer Molecular 'C yield It) Weight
Hn
1:1 DCE 25 22 1,231,000
2:1 DCE 25 12 320,000
1:1 DCE 57 25 364,000
2:1 DCE 57 10 286,000
1:1 CH3CN 25 1 87,000
1:1 CHJOH 25 20 1,130,000
-
Table 11 Molecular Weight of MeOBDtrNE Copolymers as a FWlction
ofThne
Time Copolymer :J 1nh (h) Yield % (al/g)
5 7 2.45
6 10 1.53
8 16 2.06
13 47 1.57
66
-
Reactions of 1.Methoxy·l,3.Butadiene with Acrylonitrile
1·Methoxy·l,3-butadiene and acrylonitrile were reacted in bulk
and in SM
dichloroethane solution at room temperature for 72 hours. The
results are shown in
Table 8. Both copolymer and [4+2] cycloadduct were obtained in
all reactions.
Polymer yield decreased with decreasing concentration as
expected.
67
The copolymer 38 appears to be predominantly cis-l,4 and
trans-l,4. The 3,4
structure appears to be present also. The copolymers tum yellow
and become insoluble
when exposed to air.
The cycloadduct 39 was obtained as a mixture of two isomers as
detennined by
gas chromatography. In the bulk reactions the isomer ratio was
approximately 1:1. In
SM solution, it was 1 :3. These isomers are those where the
cyano group is adjacent to
the methoxy group and are either cis or trans to it, analogous
to the cyclohexenes
discussed previously.
Reactions of l·Methoxy·l,3·Butadiene With Oletlns Containing
p-Leaving Groups
I-Methoxy-I,3-butadiene was reacted with 2,2-dicyanovinyl
chloride and 2·
carbomethoxyvinyl chloride in SM dichloroethane solutions. In
both cases, traces of [4
+ 2] cycloadduct were obtained. The major product of these
reactions was a black,
insoluble solid substance. Attempts to dissolve it in acetone,
chlorofonn, DMSO, and
DMF were unsuccessful. The reactions were repeated using only
10% of the olefin.
The results were the same. An intractable black solid was the
major product.
Discussion
I-Methoxy-l,3-butadiene proved to be a better diene for the
study of diradical
intennediates in the spontaneous polymerizations of
electron-rich dienes with electron-
poor olefms. I-Methoxy-l,3-butadiene was reacted with three
trisubstituted electrophilic
-
olefms and acrylonitrile at varying concentrations and reaction
conditions. Copolymer
and [4+2] cycloadduct were obtained in the reactions with all
four olefins. Since
copolymer is formed. there is evidence for a diradical
intermediate.
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The copolymer composition was approximately 1: 1 for all
reactions. This is to be
expected if the hexamethylene mechanism is operating in this
reaction. The polar
diradical should favor the alternating structure.
The copolymers have a predominantly 1,4 structure. The
copolymers containing
cyano groups are readily oxidized in air. Care should be taken
to insure that polymers
be stored in sealed ampules.
The cycloadducts were again formed as a mixture of two isomers.
No loss of
stereochemistry was observed in the reactions using dimethyl
cyanofumarate. Therefore.
there is no evidence that the cycloadduct is formed from the
diradical intermediate.
The reac
