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Irregular structures in polyvinylchloride II. Unsaturated structures

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Die Angewandte Makromolekulare Chemie 23 ( 1 9 7 2 ) 173-187 ( N r . 311)
From the Slovak Technical University, Department of Physical Chemistry,
Bratislava, Czechoslovakia
Irregular Structures in Polyvinylchloride I1
Unsaturated Structures
By L. VALKO
and I. T V A R O ~ K A *
(Eingegangen am 19. Juli 1971)
SUMMARY :
I n the present paper, considerations are made on the mechanism of the dehydrochlorination of PVC, mainly from information summarized from studies with unsaturated model compounds. A semi-empirical evaluation of the activation energy has
been carried out for the monomolecular elimination of hydrogen chloride involving
unsaturated structures in PVC. The semi-empirical method proposed by VEDENEYEV
was used to study the C-Cl, C-H and C-C bonds in PVC. Activation energies for
elimination of hydrogen chloride were calculated on the basis of theadditivity of bond
energies of the four-centre activated complex for various irregular structures in the
polymer chain such as branching, head-to-heador tail-to-tail addition, and double
bonds. From a number of possible structural abnormalities controlled by polymerization conditions only a few give a contribution to the initiation of the thermal dehydrochlorination but internal ally1 type chlorine and branched structures having a
tertiary chlorine appear to have significant effect as the initiators of the decomposition, while abnormal structures at the chain ends are rather stable. It should be
pointed out that increasing delocalization or resonance stabilization in the case of
unsaturated internal-irregular structures will lower the activation energy of elimination of hydrogen chloride in consequence of decreased thermal stability of PVC.
ZUSAMMENFASSUNG :
I n der vorliegenden Arbeit uber den Mechanismus der Dehydrochlorierung von
I’VC werden die Folgerungen ZusammengefaBt, die erhalten wurden auf Grund der
Informationen, die bei Studien an niedermolekularen Modellverbindungen erzielt
worden waren. Die Aktivierungsenergie der Abspaltung des Chlorwasserstoffes aus
umgesattigten Strukturen in PVC wurde mit semiempirischen Methoden ausgerechwurde dieEnergie zur Dissonet. Mit Hilfe der semiempirischenVENEDEYEV-Methode
ziation der Bindungen C-Cl, C-H und C-C in PVC berechnet. Die Aktivierungsenergie
der Abspaltung des Chlorwasserstoffes wurde auf der Basis der Additivitat der Bind.ungsenergien des vier-zentrischen aktivierten Komplexes fiir verschiedene unregelmaBige Strukturen in der Polymerkette (Verzweigungen, Doppelbindungen,
Kopf - Kopf- bzw. Schwanz - Schwanz-Strukturen) ermittelt. Von den erwahnten
* Institute of Chemistry, Slovak Academy of Sciences, Bratislava,Czechoslovakia,
to whom correspondence should be addressed.
173
L. VALKOand I. TVARO~KA
Fehlstellen, deren Konzentration von Bedingungen der Polymerisation abhiingt,
nehmen die meisten an der Initiation der thermischen Dehydrochlorierungdes PVC
nicht teil. Dagegen sind kettenstiindige ungesiittigte Strukturen vom Allyl-Typ und
verzweigte Strukturen mit tertiiirem Chlor wirksame Initiatoren der thermischen
Zersetzung, wiihrend ungesiittigte Endgruppen vom Allyl-Typ etwas stabiler sind.
Introduction
The previous paper1 dealt with the difference in thermal stabilities between
the ideal structure and branched structures of PVC. This part presents some
interpretations of the thermal stability of terminal and internal abnormal structures in PVC having one and more double bonds. The thermal stability of PVC
is greatly dependent on the polymerization process, so this effect should be discussed from the basic standpoint, relating to the species and location of unsaturated irregular structures. According to BENGOUGH
and NORRISH~,
chain transfer to monomer is considered to be a predominant termination process in the
polymerization of vinyl chloride. Many authors suggested that the unsaturated
chain end is the site for initiation of thermal dehydrochlorination, but from present results4-6 it seems unlikely that the initiation site of the dehydrochlorination is the @-chlorinein the chain end which is formed by the disproportionation
reaction.
I n the present communication, the mechanism of the dehydrochlorination of
PVC is mainly described from information summarized from unsaturated model
compound studiese. We believe that this treatment should ultimately give results
of accuracy sufficient for the prediction of reactivities of unsaturated irregular
structures in PVC. I n this paper we deal only with problems of the mechanism
of elementary reactions taking place in the course of the degradation from unsa-,
turated abnormal structures.
Estimation of Dissociation Energies
of
C-C, C-Cl and C-H Bonds
A simple equation permitting calculation of bond dissociation energies with
remarkable degree of accuracy was deduced empirically by VEDENEYEV7*8. The
dissociation energy QR-X of a bond in a molecule R-X, can be expressed as follows :
where EA-x is the specific energy for a given type of bond, i.e., G C , GCl, and
C-H. etc., i ( j ) are the number of bonds of type i which are bonded with the m-th
(n-th) carbon from the bond under consideration. cci ( q )is the specific interac174
Irregular Structures i n Polyvinylchloride 11
lion parameter of the stabilization due to resonance interaction of the free radical with the given a-bonds type in the molecule, and exp (- w ) is a coefficient
indepedent of the type of bond in the molecule, having the value 0.4 found from
experimental data of bond dissociation energies.
In the last equation, a (a = 1,2 or 3) (b) is the number of conjugated groups
adjacent to the bond under consideration (free radical is a t the p-position to the
7~-bond)and Econjis the resonance or stabilization energy imparted to a free radical by a conjugated group. The resonance energy of the G C n-bond is calculatjed, and for this purpose it is assumed, as is reasonable, that all the carbon
atoms of the radicals are in sp2 state of hybridization and that the carbon
mskeletons are planar. The resonance energy of the ally1 radicals have been cqloulated by ROBERTS
and S K W N E R
by~the MO LCAO method (Econj= 19,4 kcal,
Table 1).
From eq. (1) it follows that the bonds five or six atoms far from the bond in
question do not have much influence on the value of QR-x. Values Ek-x and other
interaction parameters, determined from experimental data on the dissociation
energies of the bonds in simple hydrocarbons, are shown in Table 1. Most values
are taken from the paper of SHIBASAKI~.
The values of bond dissociation energies, calculated by VEDENEYEV'S
equation for relatively complicated hydrocarbons, are in agreement with the observed value within about 3 kcal mole-1.
Hence, it seems to be reasonable to apply VEDENEYEV'S
semi-empirical method
t o calculate the dissociation energies of G C , G C l and C-H bonds in unsaturated
irregular structures of PVC. For this estimate the assumption has to be made
that the bond energies given by VEDENEYEV'S
equation ( 1 ) applied to the solid
state of PVC.
Constant
Value *
0.4
10.0
9.8**
11.0
127.8
109.3
145.3
19.4
*
**
All energetic parameters are expressed in kcal per mole.
By using the bond dissociation energy, Q C H ~ C ~ - H = 98 kcal per mole, the
value was determined as QCH~CI-H
= E'c-H - Orc-CI - 2°C-H.
~ C - C
175
L. VALKOand I. TVARO~KA
Activation Energies
Electronic states different from the ground state may occur either through
simple electronic excitation in a fixed equilibrium nuclear framework or through
the occurence of a nuclear configuration different from the stable equilibrium
configuration of the molecule. More conveniently, the potential energy function
is used only for the purpose of determining the configuration of the transition
state to obtain its partition function for statistical calculation of the rat,e with
the use of an appropriately assumed value of the transmission coefficient. This is
quasi-classical absolute rate theorylo. I n the theory of
the well-known EYRINC
absolute reaction rates, the rate constant k is given by eq. (2)
k (T) = xe (kBT/h) exp (AS */R) exp (-E/RT)
(2)
where the factor x is known as the transmission coefficient, (kB T/h) is a universal frequency dependent only on temperature and independent on the nature of
the reactants and the type of reaction, A S’ is the entropy of activation and
means the difference in entropy between the transition state (symbol* ) and the
reactants all referred to their standard states and E is the energy of activation.
The transition state “is the most unstable nuclear configuration through which
the molecules of reactants must pass on their way to becoming products”.
Large and important areas in the field of reaction rate theory apparently
whould be accessible if means were available for calculating or successfully
estimating activation energies empirically. An empirical but very successful
method was devised by M o 1 ~ 1 1 Mom
.
extends the picture of the additivity of
bond energies to the active bonds in activated complex configuration.
It has generally been assumed12 that the dehydrochlorination reactions occur via a four-centre activated complex
H C1
c- c-
, so that elimination is a pro-
cess, involving substituents on adjacent carbon atoms.
The values of the activation energy of E , per mole a t absolute zero have been
calculated with the help of additivity rules as given by Mom
E=zQi i
zQ:
(3)
i
(cleavage bonds) (wtive bonds)
which have been checked for known compounds. The various forms for the activated complex of reaction may be postulated, and the activation energy estimated for each.
I n reactions such as the elimination of hydrogen halides from alkyl halides, the
transition state will probably have considerable polar character. This idea has
~ ~ by BENSONl3, who used a “semibeen developed particularly by M A C C O L Land
ion pair” model to estimate activation energies for such polar reactions.
176
Irregular Structures in Polyvinylchloride I I
For calculational purposes, following BENSON13, four-centre transition states
have been represented in terms of one electron bonds, as illustrated for internal
unsaturated irregular structures :
.* *-CH2-CH=CH-CHCI--..
+
H-CI
The longer distances pictured between the reacting H and C1 atoms are meant
60 implay that the interactions a t these points are relatively weak. Thus, the
transition states employed in the calculations are pictured as strongly polarized
species which differ from the linear conformations by the weak interactions a t
the reacting ends. They therefore may be termed very loose cyclic structures.
Four-centre transition states are energetically feasible. During the dehydrochlorination, one G-Cl and one C H bond are broken and one C=Cn- bond and
N-C1 bond are formed. Thus, the activation energy for the elimination of hydrogen chloride a t 0°K is
+ Qc-H
E = Qc-CI
- (Q
&+
Q2-H + Q&
+Q
&I+
CO *)
(5)
where UJ * is a parameter which is zero for saturated hydrocarbons and has different values for unsaturated compounds with the double bond located inside the
chain ( w * = /3zrc);and a t the end of the chain (w" = &LC).
MOIN empirically found the active bond dissociation energies with remarkable accuracy from inspection of the great number of experimentally determined
activation energies. The dissociation energies of the active bonds in simple hyclrocarbons are shown in Table 2. The dissociation energy Qc-H of the C-H
active bond for particular hydrocarbons was calculated by the equation7
Q$-H
= 0 , 8 3 ( Q ~- ~45) kcale mole-'
( 6)
The values of the constants in VEDENEYEV'S
equation and average values of
active bond dissociation energies, together with the parameter in the treatment
are listed in Tables 1 and 2. We have tested the above set of bond energy pararneters by comparing the calculated and observed activation energies of a number of saturated and unsaturated model compounds of PVC and the agreement
i3 satisfactory.
The thermal stability of PVC is greatly dependent on the polymerization process, so this effect should be discussed from the basic standpoint, relating to the
species and location of abnormal structures,
177
L. VALEOand I. TVARO~KA
Table 2. Average values of active bond dissociation energies at 0 ° K .
I
Type of Bond
C,C1 primary chlorine
C,-Cl secondary chlorine
Ct-C1 tertiary chlorine
a-C-C in saturated hydrocarbons
n-C-C
in unsaturated hydrocarbons
H-C1 hydrogen chloride
correction to the double bond located at the
a;=
end of the chain
/3zr correction to the double bond located inside
the chain
Active Bond Energy*
Q * Kcal/mole
19.5
21.3
24.5
28.0
17.0
40.0
- 17.0
-
3.8
* Average values of active bond dissociation energies and correction parameters are
calculated from model compound studiesl.
I n the empirical reactivity theory we have proceeded with attention to the activation energy. It is well known that a knowledge of the so-called “entropy term”
in addition to that of activation energy is required in order to obtain a theoretical information of the reaction rate. We have already learned in the previous
part of this work1 that with the theory of absolute rates we are able to calculate
the entropy term by quantum mechanics. But we may proceed without such
laborious treatments provided that we are interested in a series of similar reactions in which the “shape” of the potential energy function near the transition
state resembles to each other or otherwise has a certain correlation with its value to satisfy a sort of compensation relation between the change of the activation entropy and that of the activation enthalpy. In such cases we are allowed to
discuss, in both parts of this series the relative rate in terms of activation energy
only.
Terminal unsaturated structures
A study of the dehydrochlorination of organic chlorine compounds of low molecular weight has been carried out by BAUMand WART MAN^, and it has been
suggested that the thermally unstable structures in PVC are /3-chloro-unsaturated structures located a t the chain end of the polymer and that tertiary chlorine
exists in PVC a t the junction of branches. However, only /3-chloro-unsaturated
structures existing a t the chain end of the polymer have been considered as the
most unstable abnormal structures until further informations about the thermal
stabilities of chloroalkene compounds were reported4-6.
A consideration of the polymerization mechanism shows that thermally unstable structures are to be expected in the free radical polymerization of vinyl
178
Irregular Structures in Polyvinylchloride I 1
chloride. As pointed out earlier, the unsaturated chain ends may have an a-chloro
or p-chloro structure aepending on whether they were formed by disproportionation of radicals or by chain transfer through the monomer. Abnormal structures in the polymer chain such as allylic chlorine and already studied tertiary
chlorine can be produced, by chain transfer to dead polymer during the polymerization of vinyl chloride.According to BENGOUGH
and NORRISH~,
chain transfer to monomer is the predominant molecule termination process in the polymerization of vinylchloride. A p-chloro-unsaturated structure would be expected
from this mechanism2139 143 15 :
...-CHz-Ey
-
t
fy=CH2
(A)
1
2
3
CH=C-CH-CH-CH-CHCI-
...
The calculated values in kcal mole-1 are :
&1=97.59, Q 2 ~ 7 7 . 0 4 ,Q 3 ~ 7 3 . 6 7 Q4=73.51,
,
Q5=91.81,
Q’ix85.29, &’2-61.32, &’3=81.79, E1=45.16, E11=48.16.
Although unsaturated chain end structures were expected from termination
reactions, it is unlikely that the “zipper”-type dehydrochlorination of PVC takes place from these defects a t the chain end. Therefore, we carried out calculations in order to get some clue to the thermal stability of PVC in relation to the
various chain end structures.
Termination by disproportionation233,15916 :
is much less common than by chain transfer to monomer. Therefore, the 8chloro-unsaturated groups formed by disproportionation would be present a t a
low concentration. Both mechanisms produce unsaturation a t the chain end.
The calculated energetic quantities are :
&1=79.04, &2=95.67, Q3=55.10, &4=92.09, &5=74.90,
&’1=84.31, Q ’ 2 ~ 6 1 . 3 3 &’3=
,
79.68, E1=46.85, E11=49.65.
179
L. VALKO
and I. T V A R O ~ A
A /3-chloro-unsaturated structure (B) formed by disproportionation has been
generally considered as the unstable site in PVC. From our results, it seems unlikely that the site for initiation of dehydrochlorination is the /3-chlorinea t the
chain end which is formed by disproportionation. While it is not possible to
estimate the relative concentrations of tertiary chlorides and unsaturated
chain ends accurately, the work of BENGOUGH
and NORRISH~
previously referred to certainly indicates that tertiary chlorides are present a t much higher
concentration than unsaturated chain ends. These structures are of approximately equal stabilityo, it would be expected that most of the hydrogen chloride
would be lost from the tertiary chlorides since, as pointed out in paper 17, these
groups appear to be about forty times more numerous than unsaturated chain
ends. BRAUN
and THALLMAIER17investigated the thermal degradation of copolymers and from their investigations follows that tertiary chlorine, if present in
PVC, would be more unstable than allylic chlorine.
Internal unsaturated structures
However, the stability of a t!Cchloro-unsaturated structure can be affected
greatly by their position, whether in the main chain or on the chain end of the
polymer. I n the case of the chain end structure, the ally1 effect might be reduced
somewhat, compared to the displacement of chlorine by hydrogenla.
Allyl-type chlorine atoms acting as the weak sites in the polymer chain are
distributed randomly a t the polymer chains. Infrared spectroscopy could not
show the presence of double bonds in the undegraded polymer due to their quite
low content. In spite of the uncertaintylg, from the spectral changes can be suggested that PVC has two kinds of double bonds: (i) double bonds a t the chain
ends, (ii)internal unconjugated double bonds.
There are many possibilities for the rise of internal double bonds in polymer
chains: (i) Copolymerization with acetylene which is as an impurity in vinyl
chloride. Acetylene inhibits the polymerization process but causes the rise of
irregular structures in the polymer chain and so decreases the stability of polymer. (ii)Copolymerization with butadiene which rises by recombination of the
vinyl radicals produced by chain transfer to monomerl4. (iii)Dehydrochlorination occurs simultaneously with chain transfer to a polymer. The result is
a branched PVC having a tertiary hydrogen a t the junction of branches, formed
by chain transfer termination to the monomer20.
We consider (C) internal unsaturated structures :
180
Irregular Structures in Polyvinylchloride 11
Sor which the calculation yields the following bond dissociation and activation
energies :
Qi=73.35, Q 2 ~ 7 3 . 1 3 &3=93.79,
,
Q4=54.54, &5=91.89, Q6=71,79
($'1=80.16, Q'2=60.78, Q'3=81.86, &'4=60.81, E1=44.80, &1=33.01, E 1 1 1 ~ 4 6 . 4 2 .
The ally1 type chlorine which is located in the main chain, but not a t the end
of the polymer is more unstable than all kinds of abnormal structures present a t
the end of the polymer chains. If dehydrochlorination occurs in a zipping mechanism like a chain reaction, a long conjugated bond system should be formed
easily. The evidence obtained from a study on the physical properties of the
products supports this proposition. But with regard to the high reaction rate a t
Sairly low temperatures and the ease of dehydrochlorination a t the sites where
ihe bond energy is lower, due to the formation of unsaturated bonds a t the adjacent carbon atoms in comparison with other sites in the molecule, it may be
suggested that the reaction occurs a t random in the beginning to form unsaturated bonds a t several arbitrary sites by a zipping mechanism with a considerable
high rate. As the length of the unsaturated carbon-carbon chain increases one
might expect the conjugation effect to be diminished.
Now we consider the internal unsaturated structures with two or more double
bonds in the polymer chain. The calculated activation energies for the internal
unsaturated structure (D)
' . -CHCl-CH~-(CH=CH)n-CH-CH-CHCI-CH~
I
t
C1
-. .
'
(D)
H
with (n = 2, 3, 4,5 , 6) double bonds in the polymer chain are:
E11=33.27, E I I I = E I V = E V = E ~.I .=. =33.29.
Discussion
ASAHINA'and OZUNUKA~,
CHYTR+ and coworkers5 studied the heat stabilities of model compounds which contained branches, double bonds, or both. The
branches and double bonds were expected to initiate the thermal degradation of
I'VC. These structures might be produced by the attack of radicals such as those
derived from the decomposing catalyst or the growing chains on the polymer
chains during the polymerization. ENOMOTO
and A S A H I N Apreviously
~~
studied
the mechanism of vinyl chloride addition by use of partially or completely deutarated monomers and infrared spectroscopy. They reported that the B-hydrogen atom is more active than the a-hydrogen atom or the chlorine atom in the
transfer reaction.
The formation of branches and double bonds and some abnormal additions in
the polymerization of vinyl chloride are supported by spectroscopic methods
181
L. VALKOand I. T V A R O ~ A
described by the ENOMOTO1g. It seems probable that these formations are stimulated by the chain-transfer activity of the ,&hydrogen atom. From the above,
one might conclude that the instability of PVC could be related to the abnormal structures which could be varied by the conditions of the polymerization.
Above all, the high transfer ability of the B-hydrogen atom and the tail-to-tail
addition results in some abnormal structures (branches and double bonds) which
cause the formation of hydrogen chloride and consequently result in a decrease
of the stability of PVC.
The results indicate that whenever -(CH2-CHCl),
structural units are replaced by isolated double bonds a t any position in the polymer chain, the activation energy for elimination of hydrogen chloride is less than that of the saturated structuresl, the only exception is, that the branched structures appear to
be slightly more reactive than the unsaturated chain end-structures (A) and (B).
Three general trends can be discerned in the present results : (i) the magnitude
of the effect varies considerably with the position of the double bond structures (A), (B) and (C). (ii)The greatest increase in reactivity is observed when the
dehydrochlorination mechanism involves an internal allylic-activated elimination of hydrogen chloride from structure (C). (iii)As the number of the formed
double bonds increasis, they have a slight activating effect on the elimination of
the substituent from the neighbouring monomeric unit. The activation effect
spreads only to the neighbouring unit22.
The replacement of a carbon-hydrogen bond by a carbon-chlorine bond in the
end-irregular structure (A) and (B) appears to stabilize the polymer molecule
both due to the strengthening nonbonding interactions of the chlorine atom
with the hydrogen and to the conjugation effect from the well-known partial
double-bond character of the carbon-chlorine bond. The greater bond dissociation energies of the C-C1 bonds in structures (A)Q2 and in (B)Q1 are, compared to
other C-C1 bonds, largely due to the participation of the 3 dn orbitals of chlorine,
which are usually not taken into account in the LCAO-MO scheme. Several
authors, notably MUUIKEN 23 and HoYLAND24,have pointed out that the halogen atom C1 has d-orbitals available in the valence shell for acceptor action suggesting the possibility that the net charge release in conjugated systems is controlled by competition between the donor action of pn and the acceptor action
of dn orbitals. The purpose of this remark is to demonstrate that the halogen
dz -orbital participation in conjugation in olefinic halogen derivatives is feasible
and the apparent paradoxon in the thermal decomposition behaviour of the structures (A) and (B) can be reasonably understood through pn-dn hybridization. I n principle, a variation in the bond polarity is accompanied by a change
in the hybridization of the corresponding atomic orbitals. Donation of charge
from the C13pn orbital into the n-system of the hydrocarbon leaves the chlorine
182
Irregular Structures in Polyvinylchloride 11
atom with a formal positive charge (R- = C1+ n-bond), reducing the promotion
energy required for hybridization. pz - dn hybridization occurs because of an
increase in the C-C1 bond strength.
I n the previous papers it was postulated that several unstable type structures should be capable of initiating hydrogen chloride elimination. I n the proximity of the transition state of a chemical reaction, of whatever sort it may be,
the whole reacting system should be treated as essentially one many-electron
system like an ordinary molecule. Accordingly in such a system the electrons
will be rearranged a t the approach of reacting species so as to bring about a
new distribution of electrons which is in general different from that of original
molecules. Such an “electron delocalization” process may play a role not only
in the activated complex of chemical reactions but also in the formation of
reaction intermediates or molecular complexes.
The electron delocalization takes place to cause stabilization of the whole
system. The stabilization serves to lower the magnitude of the activation energy, so that we may discuss, with some reservations mentioned in the previous
paper6, the relative rate for unsaturated structures having terminal and internal double bonds in terms of the magnitude of this stabilization energy.
The difference in the ground state of the polymer chain is slightly between the
dissociation energy of the C-Cl bond a t the p-position to the double bond in irregular structures having double bonds a t the chain ends (A) and (B) and inside
the chain (C). The same is true for bonds C-H in the above mentioned structures. The clue for elucidating substantionally different instability of the internal p-chloro-unsaturated structures (C) in comparison with the structures
having double bonds a t the chain ends (A) or (B),respectively, must be looked
for indifferent total energy value of the four-centre activated complex of structures investigated. From the results summarized in this paper it follows that
the intramolecular interaction of four-centre activated complexes with the nelectrons of the double bonds in the transition state decreases the total energy
of the activated complex (a&=,< 0 ) , whereby it increases the activation
energy of dehydrochlorination. This destabilization effect in the case of the
double bond inside the chain is in evidence to a substantionally less degree
(/I
:==,=
-3.8kal) thaninthe caseof the double bond a t the end of the chain,
the results of which is that the activation energies for the elimination of hydrogen chloride are the lowest with respect to other irregular structures of PVC.
If the calculation of the bond dissociation energies of a particular type of
bond, e. g. a C-Clbondora C-H bondatthep-position to double bond it was
assumed that all carbon atoms of the radicals are in sp2 state of hybridization
and delocalization energy due to interaction of free electron with the n-bond is
Econj. This seems unreasonable for all of the activated complexes considered,
183
L. VALKOand I. TVARO~KA
because probably the ring of the four-centre activated complex is slightly out
of the plane of the n-bond. As a consequence, the resonance energy Econjwill undoubtedly be somewhat smaller than the supposed value. The extent of the reduction is known, and we have estimated it to be in the order of the values of
the a:==, and @:=c parameters.
There is one common feature of these unimolecular dehydrochlorination reactions which is different from dissociation reactions, whether polar or free radical. This difference is the rehybridization of the carbon attached to C1 and H
from sp3 in the ground state to sp2 in the final state to permit stabilization
through delocalization in the activated complex. This, of course, will lower the
activation energy over that expected in the corresponding saturated structures.
However, one would expect that in the case of terminal p-chloro-unsaturated
irregular structures, sp2 hybridization would not predominate in the activated
state. The estimate of the bond dissociation energies from eq. (1) and calculated activation energies from eq. ( 5 ) , and the measured activation energies
(Table 3 in previous paperl) could be in error if the activated complexes formed
in the solid state of PVC have a different structure and have additional stabilization from rehybridization in comparison with the model compounds studied.
The actual measured activation energy which arises from an ARRHENIUS
equation with no pre-exponential temperature dependence is an average quantity
and depends on the shape of the distribution function of defects in the polymer
chains.
The decrease of activation energies with double bonds inside the chain can
be explained moreover by the effect of hyperconjugation which is due to the
resonance interaction of the 1s orbitals of the methyl or methylene hydrogen
atoms and the Zp, orbital of the n-electrons of the unsaturated carbon atoms
in the considered molecule. I n many reactivity problems the transition state
might be said to be more delocalized than the ground state. Dissociation reactions, whether polar or free radical, have this character. A more dramatic increase in hyperconjugation energy is found for the activated complex corresponding to the internal unsaturated structures in comparison with the energy
of the activated complex of the terminal unsaturated structures. This difference
in stabilization energy following from eq. (3) in principle is equal to
E ((A) or (B))- E (C) =
7Q 7
c!13 kcal
and may not be assigned to the individual bond or bonds, but to the whole activated complex. High value of the stabilization energy (13 kcal) for internal
,8-chloro-unsaturated structures relative to the terminal unsaturated structures
reflects the influence of the allylic stabilization energy of four-centre activated
complex on the kinetics of a thermal dehydrochlorination reaction with acti184
Irregular Stmcctures in Polyvinylchloride 11
vation energies in the range of 31.30 - 33.74 kcal including also branched structures (see previous paperl) .
Variations in the activation energies of dehydrochlorination of various unsaturated structures arise mainly from the following causes : (i)differences in stabilization energies of the four-centre activated complexes in terminal and internal @-chloro-unsaturatedstructures due to conjugation or hyperconjugation ;
~(ii)
steric strain in the activated complexes. If, therefore, the delocalization is
roughly the same in the ground state of the terminal and internal @-chloro-unsaturated structures, but different in the four-centre activated complexes of
structures investigated, then those activated structures with the greater resonance stability will be favoured in the dehydrochlorination if no other influences predominate. It should be pointed out that increasing delocalization or
resonance stabilization will lower the activation energy of elimination of hydrogen chloride in consequence of decreasing thermal stability of PVC. For chlorounsaturated structures, resonance may be entirely responsible for the activation energy for elimination of hydrogen chloride in comparison with the saturated
structures.
I n the case of unsaturated irregular structures of PVC, the influence of the
side groups and the length of the polymer chain on the G-C bond dissociation
energies were estimated from eq. (1). As an example, calculated C - C bond
dissociation energies for several irregular structures (A) to (D) are introduced.
The bond dissociation energies as a rule are not known for these structures,
hence they cannot be used to establish the relationship between QIand activalion energies of C-C bonds scission. There are few data for comparison with
the present work. The only similar investigation is that of first other present
authors25 who have found the activation energy of cleavage the G C bonds 57
lical mole-1, which in principle is equal to the G C bond dissociation energy Q,'
in @-positionto the double bond for example in the irregular structure (A). There
is one major path of breakdown: cleavage of the C-C bond beta to the double
bond. Scission of the beta C-C bonds leads to a alkyl and ally1 or polyenyl type
radical.
We have seen in the case of model compounds for PVC6 that it has been possible to establish rather complete empirical formulas which, in principle, permit
the calculation of the activation energies of the thermal dehydrochlorination of
these molecules. Obviously it is not possible to use elaborate formulas to discuss
dehydrochlorination reactions in which polymer chains are involved. However,
&hegeneral empirical formalism obtained in the case of model compounds has
been extended to more complex reactions in such a way that it has been possible
to determine what are the main factors which are responsible for the chemical
reactivity of such large molecules with the irregular structures. The factors are
185
L. VALKOand I. T V A R O ~ K A
numerous. Fortunately, however, in certain cases some of these factors are predominant and others can be neglected. Therefore,rather rough calculations appear
t o be helpful when we are able t o select the important factors. I n fact it turns out
t h a t even very rough procedures permit t o determine correctly the irregular
structures which are responsible for the thermal degradation of PVC, for which
in accordance with BRAUNand BENDER^^ we postulated a unimolecular elimination mechanism.
By carrying out similar empirical or quantum-chemistry semi-empirical calculations, also for radical-chain process for PVC degradation, allow t o elucidate the nature of reactions taking place in thermal destruction of PVC in inert
atmosphere.
I Khis
The authors would like t o express their gratitude t o Mr. P. K O V A ~ ~for
assistance in computing and in gathering of the calculated data.
L. VALKOand I. T V A R O ~ KJ.APolym.
,
Sci. (to be published).
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and R. G. W. NORRISH,
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and M. OZONUKA,
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B. OBEREIGNER,
and D. L ~ MInt.
, Conf. Chem. Transformation of
Polymers, Bratislava, June 1968.
6 L. VALKO
and I. TVARO~KA,
Eur. P,olym. J. 7 (1971) 41.
7 N. N. SEMENOV,
Some Problems in Chemical Kinetics and Reactivity, Princeton
Univ. Press. Princeton, New Jersey 1958,
8 Y. SHIBASAKI,
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9 J. S. ROBERTS
and H. A. SKINNER,
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a n d P . J. THOMAS,
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and G. R.
HAUGEN,
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14 G.M. BURNETT
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15 G. A. RAZUVAEV.
G. G. PETUCHOV,
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~M.
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1
2
186
Irregular Structures in Polyvinylchloride II
S. ENOMOTO
and M. ASAHINA,J. Polym. Sci. A 4 (1966) 1373.
T. KELEN,G. BALINT,G. GALAXBOS,
and F. TUDOS,Eur. Polym. J. 5 (1969) 597.
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and R. F. BENDER,Eur. Polym. J.-Supplement (1969) 269.
21
22
187
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