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Selective catalytic degradation of polyolefins
Prog. Polym. Sci., Vol. 15, 193-215, 1990 Printed in Great Britain. All rights reserved.
SELECTIVE
0079-6700/90 $0.00 + .50 © 1990 Pergamon Press plc
CATALYTIC DEGRADATION POLYOLEFINS
OF
S. R. IVANOVA,* E. F. GUMEROVA,* K. S. MINSKER,* G. E. ZA1KOV** and A. A. BERLIN**
*Bashkirian State University, Ufa-450074, Frunze str, 32 **Institute of Chemical Physics, Academy of Sciences of the U.S.S,R. 117334, Moskow, U.S.S.R.
CONTENTS 1. Introduction 2. Uncatalyzed thermal degradation of polyolefins 3. Catalytic degradation of polyolefins 3.1. Catalysts for the degradation of polyolefins 3.2. Mechanism of catalytic degradation of polyolefins 3.3. Kinetic peculiarities of the catalytic degradation of polyolefins 3.4. Evaluation of the effective kinetic parameters 3.5. Effect of the polymer structure 4. Conclusions References
193 194 195 195 199 204 208 212 214 214
1. I N T R O D U C T I O N
The theory of chain depolymerization of polyolefins has been interpreted in terms of the thermal decomposition of polymers by a radical mechanism) Evaluation of two main kinetic parameters - kinetic chain length and chain transfer rate constant - was considered important3 The first publications, as a rule, dealt with kinetic schemes of the thermal degradation of polyolefins based on experimental data. 3-7 These schemes deal with variations in the molecular mass of the starting polymer, the composition of the products formed and the rate of thermal degradation of the polymer to products. Electrophilic catalysts cause marked changes in the kinetics of the degradation of polyolefins. There is a change in the gross mechanism, accompanied by a considerable increase in the rate and selectivity of the process.8'9 In both the uncatalyzed~°-17and the catalyzed ts-2~ thermal degradation of polyolefins, one may single out two mechanisms of initiation: (a) initiation of depolymerization through the end groups of a polymer chain; (b) random chain cleavage at any bond in the chain. It was earlier believed impossible to determine the relative contributions of these two initiation mechanisms with existing theoretical and experimental approaches to study the reaction kinetic parameters. 2-6 A method to determine the effective initiation constants and kinetic chain lengths of the 193
194
S.R. IVANOVA et aL
concurrent processes in the degradation of polyolefins in the presence of electrophilic catalysts was proposed. 22 This enabled us to come closer to a general theory of the processes of the selective degradation of polyolefins. 2. U N C A T A L Y Z E D
THERMAL
DEGRADATION
OF POLYOLEFINS
The uncatalyzed thermal degradation of polyolefins first attracted attention at the end of the 1940s. Initial interest was focused on selective recovery of valuable gaseous hydrocarbons from the polymeric raw material?3-z9However, thermal degradation of polyolefins and similar vinyl polymers usually gives only a low yield of starting monomer, exceptions being rare (poly-ct-methyl styrene, polymethyl methacrylate3°). For the majority of polyolefins the formation of a wide spectrum of low-molecular weight products - from gaseous (CI-Cs) to liquid and waxy ones - is typical.3t Available data suggest that the most important degradation processes are: (1) random chain cleavage (with hydrogen atom transfer) leading to a broad variety of degradation products, and (2) depropagation of a chain end to monomer. 3t-33These conclusions are based on molecular weight determinations of partially degraded polymer, kinetic studies and analyses of volatile and nonvolatile degradation products? t'34'35As often as not the degradation process may be accompanied by side-reactions, for example, cross-linking of the macromolecules)6 Also, especially in the degradation of branched polyethylene (PE), the active sites may be formed at branching points with subsequent termination of the side chains, leading to no net decrease of the polymer molecular mass. The system of consecutive and parallel reactions which comprise the thermal degradation pathways of polyolefins is extremely complex. 37 However, the two processes outlined in the above paragraph appear to be usually the most important ones in polyolefin degradation. 7 As a rule, chain scission is associated with presence of various defects in the polymer chain. These may include weak bonds which are formed, for example, if the polymerization is conducted in the presence of oxygen.38'39Other "defects" may include structural irregularities, for example, bonds in which monomer units are linked in the "head-to-head" manner; 34 side substituents and internal double bonds of various types3~ and the like. The initial macroradicals formed are consumed in the various reactions of chain depropagation. For example, they undergo fl-cleavage to yield olefins substituted according to Rice's rule. 4° They also undergo chain transfer and termination reactions which result in the formation of heavier hydrocarbons. It is typical that in thermal decomposition of polymers the role of chain transfer to polymer is appreciable, t This is a decisive factor in the formation of compositionally heterogeneous reaction products. Thus, from all the possible reactions involved in the thermal degradation o f polyolefins, one may single out three of the most significant reactions which
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
195
determine the distribution of products: (1) those of radical decay - the degradation of macromolecules; (2) those proceeding with hydrogen transfer; (3) those of radical substitution (chain transfer to polymer). Depending on the structure of the polyolefin, the relative contribution of these processes may be different. For instance, comparing the degradation processes of PE and polyisobutylene (PIB), one may note that for linear PE the three reaction pathways are equally probable. 4~ However, for PIB, which contains two methyl substituents at every second carbon atom in the chain, only reactions (1) and (3) are typical. The result is that in the degradation of PE very little ethylene is formed (Table l) while in the degradation of PIB, the amount of isobutylene formed is about 20% of the weight of the starting polymer (Table 1). It has also be experimentally established (Table l) that if internal double bonds are present, as in polybutadiene, the role of hydrogen transfer reactions again increases. Consequently, the yield of monomer appears to be insignificant. Chain transfer reactions with separation of the hydrogen atom (l) from the substituent (in the case of PE, short branches - mainly ethyl and n-butyl groups - should be considered as substituents 55) and (2) fl'om the carbon atoms, located within the chain are equally probable for PE and butyl rubber (BR). Chain transfer reactions with separation of the hydrogen atom from internal carbon atoms, which result mainly in the formation of hydrocarbons with isomeric structures, are more typical for PIB (Table 1) On the whole, one may state that the thermal decomposition of PE, PIB and BR, as well as that of most other polyolefins, results in the formation of such a broad spectrum of reaction products that the very notion of selectivity relative to the process of the thermal degradation of polyolefins becomes meaningless. 3. C A T A L Y T I C
DEGRADATION
OF POLYOLEFINS
The polymerization of isobutylene readily proceeds by the cationic mechanism, with an activation energy close to zero and a relatively low heat of polymerization (Q = 53 kJ/mol). The depolymerization of polyisobutylene and some other polyolefins effectively proceeds also through carbocationic active sites. 2°'43
3.1. Catalysts for the degradation of polyolefins The electrophilic catalytic degradation of polyolefins in presence of A1C13 (Table 1) proceeds with a higher yield of gaseous products than the uncatalyzed thermal process and at markedly lower temperatures, but with poor selectivity to monomer. Evidently, in these strongly acid systems, not only are the processes of ionic degradation intensified, but also isomerization of the products. The formation of primary polymer carbocations from polyethylene in the presence of aluminium chloride (active, evidently, in the form of H + [A1C13 • OH]-44) proceeds with random hydride abstraction, sometimes followed by chain
PE PIB IIR PE PlB IIR PE PIB IIR PE PIB IIR
Polyolefin*
673 653 653 643 643 643 643 583 573 643 573 573
Temperature K
40.8 75.9 50.4 47.6 62.7 85.0 84.8 47.9 38.8 88.2 93.8 75.4
Gas yield, mass %
52.9 1.9 2.7 5.7 1.5 14.3 24.8 I. l 16.4 15.0 1.4 1.7
CI-C 3 18.7 2.1 traces 22.4 2.1 59.0 54.9 7.8 23.6 42.5 24.9 2. I
i-C4Hlo 7.5 -0.5 0.8 traces 0.4 6.7 0. l 0.2
n-C4Hlo 0.1 3.0 0.1 traces 0.1 -0.3 0.5 -0. l 0.5
~-C4H 8 1.1 16.0 2.9 15.7 19.1 0.6 19.7 88.8 39.0 21.8 73.4 93.8
i-C4H 8 19.8 0.9 30.3 2.7 1.0 0.2 0.3 2.0 12.4 14.0 0. l 0.6
fl-C4H 8 47.1 19.1 36.7 40.9 22.2 59.9 75.5 98.9 62.6 85.0 98.6 95.9
C4H 8 + C4Hlo
Yield of individual gaseous products,** mass %
*PE = polyethylene, PIB = polyisobutylene, I I R = isobutylene-isoprene rubber. **Traces of hydrogen have been identified in the gaseous products.
AICI3 A1C13 AICI3 NaAICI4 NaAIC14 NaAlCl4 MgCI2 • AICI3 MgCl2 • AICI3 MgCl2 • AtCl 3
-
-
Catalyst
TABLE l.Yield of the gaseous products from the thermal and catalytic degradation of polyolefins
l.l
7.7
79.0 60.6 53.4 76.3 25.8
Cs and higher
oz<
SELECTIVECATALYTICDEGRADATIONOF POLYOLEFINS
197
cleavage: R-CH2-CH:-CH2-R' + H+[AICI3 " OH]+ R-CH2-CH-CH2-R' [AICI3 • OH]- + H2
I
CH3 or R-CH2-CH-CH2-R' + H + [A]- ~ R-CH2-CH-CH2-R' + H2 + [A]-
I
I
CH3
CH2 +
+
--~ R - C H 2 + C H 2 - C H - C H 2 - R ' .
However, degradation processes are probably more commonly initiated at defects in the polymer chains, such as vinylidene double bonds: +
RR'=CH2 + H + [AICl3 • OH]- -~ RR'CH-CHE[AICI3 • OH] . The primary ion is isomerized with formation of a more stable tertiary carbocation"
R' + RR'-CH2[AICI3 • OH]- ,
I , R-C + [AICl 3 • OH]-.
I
CH3 The decomposition of carbocations leads to a wide spectrum of products. The gaseous fraction contains species ranging from CH 4 (separation of methyl branches) to C4H8 (skeleton isomerization of the ion with subsequent fldecomposition). Liquid and solid fractions contain C5 hydrocarbons and higher (Table 1). Typically, in the degradation of polyethylene, the yield of monomer is not high. End double bonds are also of significance in forming initial carbocations. Because of the high content of internal double bonds and other defects in polyethylene, however, relatively little of the degradation is thought to originate from endgroups. Increase in the content of relative to end double bonds, as in PIB and isobutylene-isoprene rubber (IIR) increases the probability that they will be sites of carbocation formation. However, a rise in selectivity to monomer is not observed (Table l). Evidently, random chain transfer to polymer plays a very important role in the degradation of polyolefins in the presence of strong acid catalysts. Side reactions in the electrophilic processes are usually suppressed with decrease in the acidity of the catalysts.45 In particular, complexes of the type of M a n " AICl3 or M(AIC14)n (M - L i , Na, K, Mg, Ca, Ba; n = 1-2) are useful in this regard. 46-48 Both the high activity of the catalysts of this type and their
198
S.R. IVANOVA et aL
TAaLE2. Catalytic degradation of polyolefinsin the presence of some metal chloride hydrates Catalyst
Temperature
Gas yield, mass %
Monomer yield, mass
K
% PIB
IIR
PE
PIB
IIR
PE*
MgCI a • n H 2 0
583
12.4
27.4
8.7
96.2
96.0
76.5
(n = 1 to 2)
603 623 583 603 623 583
81.9 100.0 11.4 36.2 62.0 3.8
92.0 100.0 19.7 31.6 35.0 4.0
24.5 33.9 21.4 2.2
99.5 99.5 90.1 92.4 98.6 23.5
96.4 97.0 93.1 92.0 97.0 19.0
77.1 78.3 73.0 53.2
MgC12 • 4H20
BaC12• 2H20
*Total yield of C4-hydrocarbons.
high selectivity in the degradation of polyolefins have been experimentally established. The gaseous reaction products contain up to 90-95% of C4hydrocarbons (Table l). These metal tetrachloroaluminate melts are known as ion media. 49"5° The drop in the acidity of the metal tetrachloroaluminate catalysts compared with that of A1C13 decreases the ratio of chain transfer to chain depropagation during polyolefin degradation. This, in turn, raises the selectivity of the process to C4-hydrocarbons. It is interesting that MgC12 hydrates give extremely high activity and selectivity to isobutylene monomer in the degradation of polyisobutylene and isobutyleneisoprene copolymers (Table 2). In the formation of catalytically active magnesium chlorides an important role is played by the crystal waterJ 1 Of the magnesium chloride hydrates, MgC12 • H20 and MgCI:. 2H20 are the most effective. Increase in the degree of hydration results in the partial hydrolysis of magnesium chloride with formation of MgOHC1, inactive as a catalyst for polyolefin degradation. Quantum-chemical calculations confirmed the possibility of the activating effect o f the crystal water (Table 3). The high catalytic activities of hydrates of MC1 • A1C13 (or MA1CI4) and of MgCI 2 are caused by the high positive charges on the hydrogen atoms in the hydrates. The stability of the hydrate of MA1C14, evaluated by the energy of complexing with water, is somewhat lower than that of the hydrates of magnesium chloride. Therefore, irrespective of the higher charges on the hydrogen atoms, the activity of the hydrates of MAIC14 salts as catalysts for polyolefin degradation is somewhat lower than that of MgCI2 hydrates (Table 3). It should also be stressed that the quantum-chemical calculations are validated by the experimentally established series of MAIC14 catalyst activity for polyolefin degradation: LiA1CI4 > NaA1CI4 > KA1C14. The acidity Pk~ and the value of
SELECTIVE CATALYTICDEGRADATIONOF POLYOLEFINS
199
TABLE3. Quantum-chemical calculations for catalysis of polyolefin degradation (LCAO-MO method with CNDO[2 approximation) Catalyst
Pk,
qn+
Complexing energy, kJ/mol
Gas yield, mass % (603 K) PIB
LiCI • H 2 0
10.5
MgOttCI MgCI2 • H20
LiA1C14
23.0 12.7 15.4 15.4 I0.0 13.7 12.5 5.6
NaAICI4
4.5
KAIC14
-
MgCI2 • 2H20 MgCI2 • 4H20
0.182 0.182 0.070 0.166 0.185 0.146 0.185 0.158 0.165 0.253 0.217 0.226 0.229 -
BR
PE
131
9.1
21.2
-
265
8.5 100.0
23.4 I00.0
49.5
260
100.0
100.0
45.0
216
59.5
45.2
21.4
88
61.1
96.5
57.4
249
55.0
46.7
55.9
-
6.3
18.0
12.3
the positive c h a r g e qn+ on the h y d r o g e n a t o m s o f the c o m p l e x e s decrease with increasing r a d i u s o f the cation. In s u m m a r y , the w e a k l y acid c o m p l e x catalysts o f the type M C I , • A1C13 a n d M g C I 2 are highly active a n d selective in the c a t a l y t i c electrophilic d e g r a d a t i o n o f polyolefins.
3.2.
Mechanism of the catalytic degradation of polyolefins
T h e m e c h a n i s m s o f the t h e r m a l a n d c a t a l y t i c d e g r a d a t i o n o f polyolefins are diverse. T h e u n c a t a l y z e d t h e r m a l d e c o m p o s i t i o n o f the p o l y m e r s is generally t h o u g h t to o c c u r by a r a d i c a l m e c h a n i s m . 3-7 D e g r a d a t i o n in the presence o f electrophilic catalysts, h o w e v e r is ionic. ~'52-54 In earlier w o r k s o n the c a t a l y t i c d e g r a d a t i o n o f polyolefins in the presence o f MAIC14 salts, the c a t i o n i c m e c h a n i s m o f the process was i n t e r p r e t e d in terms o f the single-electron o x i d a t i o n o f t h e r m a l l y g e n e r a t e d p r i m a r y radicals: 8'9 (a) R a n d o m t h e r m a l d e c o m p o s i t i o n , p r e f e r e n t i a l l y o c c u r r i n g at the weakest b o n d s , e.g. CH3
CH 3
I I "~ C H 2 - C - C H 2 - C H 2 - C I I CH3
CH 3
CH 3
I
CH 3
I I
CH2-C-(~H2 + (~H2-C~
/
CH 3
CH 3
.
200
S.R. IVANOVAet
al.
(b) A redox reaction with formation of the polymer carbocation (a mechanism which would seem plausible only if M ÷ is relatively easily reduced), CH 3
I "~ CH2-(~ + I
CH3
I
M + [AICI4]- _~0, ~ CH2_C+ [AIC14]_ .
/
CH 3
CH3
(c) Depolymerization of the macroions with formation of monomer: CH3
CH3
I ! ,-~CHE-C-CH2-C + [AICI4]I I CH3
CH3
CH3
I
I I
~ -,~CH2-C + [AIC14]- + CH2 = C
/
CH3
CH3
CH3
(d) Chain transfer to polymer with further depolymerization of the macroions starting from end groups: CH3
I ~ CH=C + I
R
CH3
I
I I
R + [AICI,]- ~ ,,~ CH--C + [A1C1,]-
CH 3
CH 3
CH3 -~
CH3
I
~CH~-C+[AICI,]
/
-
CH3
I I
I
CH3
I I
+ R - C H = C - - , ~CH2-C+[AIC14] - + CH2=C
CH 3
/
CH3
CH 3
CH3
In catalysis by the hydrates of MAICI4 salts, the polyolefin degradation process is initiated at the active site H + [MA1CI4 • OH]- having the structure CI CI.~I,DCI Na
CI
H
The hydrate can initiate the degradation both randomly along the chain and preferentially at end groups. The interaction of the catalyst with the weak bonds of the polymer chain is determined by the structure of the polyolefin. In PE the degradation process is initiated mainly at vinylidene and trans-vinylene internal double bonds. The main initiation sites along the chain in butyl rubber are the coupling points of the isobutylene fragments with the isoprene units. End group initiation proceeds through the end double bonds.
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
201
On the whole, one may state that the processes of initiation of electrophilically catalyzed polyolefin degradation may be expressed by the following general scheme: (a) End group processes: R
R
I
,-~ C H = C + H + [A]
I
~ ,-~ CH2-C + [A]-,
I
I
R
R
where R denotes C H 3 and H for (polyisobutylene, butyl rubber) and PE, respectively. (b) Random hydride abstraction followed by chain cleavage: cn
3
I
CH2-C~
CH 3
÷
I
+ H +[A]- ~ ~ C H - C ~
I
[A]
+ H2
I
CH 3
CH 3
or proton addition followed by chain cleavage (which is more probable for isobutylene-isoprene copolymer and polyethylene): R
R
I
I
CCH2-C=CH-CH2~
+ H+[A]
~ ~ C - C H 2 - C+_ C H 2 ~ [A]
I
I
I
I
R
R
R
R
-~
R
I ",~CH2-C + [A]- + C H z = C - C H 2 ~ •
I
I
R
R
The reactions of chain depropagation in the degradation of the macrocations may differ depending on the chemical nature of the polymer involved. For instance, in polyisobutylene and isobutylene-isoprene copolymer, the carbocations formed chiefly undergo//-elimination of monomer (isobutylene) by the following scheme: R
R
R
I
I
I
C H 2 - C - C H 2 - C + [A]- ~ ~ C H 2 - C + [A]
R
I
+ CH2=C.
I
I
I
I
R
R
R
R
+
The linear carbocation R - C H 2 formed in the degradation of PE reacts via two main schemes:
202
S . R . I V A N O V A et al.
(1) Skeleton isomerization with formation of a tertiary carbocation, followed by its fl-decomposition to yield isobutylene: +
+
'~ CH2-CH2-CH2-CH2-CH2 --~ ,~ C H 2 - C H 2-CH-CH3
+ ~ CH2-C-CH3
CH3 I ~
I
CH2--C
+
+ ,-,C H 2 - C H 2
•
I
CH3
CH3
(2) Isomerization o f the macroion with transfer of excessive positive charge to the sixth carbon atom via cyclization of "flexible" methylene sequences: +
+ CHT--CH2~ C H 2 - C H 2-CH 2 - C H 2 - C H 2-CH 2 ~ ,,, CH 2 CH 2 ~CH~--CH2 / /./CH~--CH 2 --) ~ CH21
+ C H 2 ~ ,,~ C H - C H 2-CH 2-CH 2-CH 2-CH 3.
fl-decomposition of the above carbocation, with subsequent isomerization and deprotonation, yields isobutane: +
+
,,, C H - C H 2 - C H 2 - C H 2 - C H 2 - C H 3 --> ,,~ C H = C H 2 + CH2-CH2-CH2-CH3 CH 3
I CH3_C+ I
CH 3
CH3
I I
RH) CH3_CH + R + . CH3
This reaction departs from the classical scheme of depolymerization and may be interpreted as degradation with subsequent chain transfer. In the catalytic degradation of branched PE (usually not more than 30 methyl branches per 1000 chain carbon atoms), carbocations may be formed at the tertiary carbon atoms. The subsequent decomposition o f the macrocation proceeds via two reactions:
SELECTIVE CATALYTIC D E G R A D A T I O N OF POLYOLEFINS
203
(1) cleavage of the main chain; (2) elimination of a side chain: +
R~-CH2-CH2 -F CH2=C-R2
I I
CH 2
+
Rl-CH/-CH2-CH2-C-R2 - -
R3
/
CH2
L R3 RI-CH2-CH2-C-R2 + R~ .
II
CH2 In both cases a new carbocation (which undergoes further decomposition) and a "dead" polymer molecule with a vinylidene double bond are formed. In summary, the electrophilic degradation of different polyolefins is initiated by a few common pathways. Degradation may start either at doublebond end groups or at weak bonds along the chain. The path of the chain degradation reaction is determined by the structure of the polyolefin. More highly substituted chains, such as polyisobutylene, tend predominantly to depropagate directly to monomer. Less substituted chains tend to form carbocations which isomerize before depropagating. Linear polyethylene undergoes degradation by a mixture of carbocation depropagation and chain transfer to polymer.
3.3. Kinetic peculiarities of the catalytic degradation of polyolefins The difference in the form of catalyst used (ion melt, hydrate, etc.) determines the particular reactions which proceed and, consequently, the peculiarities of the kinetic scheme of the process. In the ion melt the kinetic scheme is as follows: p Wrr 2 R ' } P + cat
k , 2R"
(random chain cleavage)
R" + cat k~°~ R+ P + cat k~.~ R+
(end group attack)
R,+ kaop, R;_.. + zM R~, kdoo, R~,_: + z M
204
S.R. IVANOVA et al.
R" k ~ , R~, + R+ k ~ , R :
R" + P
Rm
+ Rm
ktr' R" + 2P
R ÷ + p klr R+ + 2P R+ k,~., p where k~n,kin, k~,, kd~, k~eg,kd~, k~r, ktr, kt©rare the rate constants of initiation, depolymerization, degradation of polymeric cations and radicals, chain transfer for ionic and radical sites, and chain termination, respectively. For polymer degradation by random chain cleavage, the variation in the concentration of the macrocations JR] ÷ is described by the equation: d[R + ] = k~n[cat][R'] - k,,dR+]. dt
(1)
In steady state, [R +] =
kin[Cat]JR']
where [R']
=
-
kin[cat] / ( kin[cat] ~2 W, 2k----~ 4- V \ ~ / + k'-~
/4k~(Wth+kt[cat])
kin[cat] I 2k, ~/
(--~-a~ ~
+ 1 -
]
1 .
(2)
Here W~ = Wth + k~[cat] is the sum of the rates of radical formation (1) resulting from thermal homolysis of the polymer chain and (2) promoted by the catalyst. The two extreme cases have been considered: 8 (a) There is little catalyst in the system, i.e., only a small proportion of the radicals formed is converted into macrocations. This agrees with the condition: 4k~(Wth + k~[cat] (kin[cat]) 2 >> 1. Then, [R']
=
~h 4- kl[Cat] kl
(3)
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
205
and the expression for the rate of the statistical decomposition will assume the form: v~ =
(k~ep + k~og)
kin[cat] x/Wth + kl[cat] k,o----T_ k,
+ (kd~ + kd.g)~/Wt" + klkl [cat]
(4)
In this case, the interaction reaction between the catalyst and the radical is the limiting stage of the process. (b) There is much catalyst in the system, i.e., all the radicals formed are converted into carbocations: 4kl(Wth + k' [cat])
(kin [cat]) 2
,~ 1.
In this case, the concentration of the macroradicals in the system is determined by the correlation, [R'] =
Wth + k'[cat]
(5)
kin[Cat]
and the rate of the process may be represented in the form, V~ = (k~ep + kdeg)
W~h + 2k'[cat] kt~r
(6)
When the depolymerization of PIB is initiated at end groups, the concentration of the active sites R + is determined by the expression: d[g + ] = ki'~[cat]Cn - k~or[R+], dt where Cn = Ao/Pn is the concentration of double bonds in the polymer, A0 is the concentration of monomer units in bulk polymer (or, alternatively, in polymer solution) and Pn is the degree of polymerization of the polymer. The rate equation for end-group-initiated depolymerization of PIB has the form: //2 = k'p[R+ ]
(k~lep + k~teg)kin[cat] =
kter
(k~ep + Cn
=
k'd,g)kinAo[cat] kter pn
(7)
Then the total depolymerization rate is described by the equation: V =
kin[Cat] x/Wt, + k'[cat] V~ + V2 = (k~p + k~eg) k~r __ k, +
(kdo~ +
kdeg)
/W~h + k'[cat]
4
kl
(k~, + k~e,)k[nA0[cat] +
kter/3n
(8)
206
s.R. IVANOVA et al.
TABLE 4.
Polyolefin
Gross parameters of the catalytic degradation of polyolefins Catalyst
Temperature
Z + 10%
k i • 107, s- i
Ea, kJ/mol
583 603 623 583 603 623 583 603 623
10 8 6 I0 7 6 4 3 3
0.6 1.7 4.3 0.3 0.8 2.4 0.4 1.1 3.7
100
NaAICI4
573
40
0.7
54
MgCI 2 • AICI3 BaCI z • AICI3
573 573
55 62
1.2 2.0
55 48
LiAIC14
573 603 623 643 573 603 623 643 573 603 623 643
8 19 26 31 10 12 17 20 7 9 12 19
0.4 1.2 3.9 5.8 0.4 1.3 3.6 5.0 0.1 0.5 1.2 2.7
118
K
Polyisobutylene
LiAIC14
NaA1C14
KA1C14
Butyl rubber (isobutylene-isoprene copolymer)
Polyethylene
NaAICI4
KAICI4
105
140
124
151
This equation takes account of the fact that depolymerization proceeds simultaneously by ionic and radical mechanisms. However, it does not permit one to evaluate the relative contributions of these mechanisms from observed monomer yields. Catalyst selectivity may be expressed in terms of Z ("zip length"). This is the kinetic chain length which characterizes the average number of monomer molecules released per rupture of the macromolecular chain: Z = k~ep[R+] + k~eg[R'] + kdeg[R+] + kd~[R] W,h + k'[cat] + k,[R +] + kj[R +]
(9)
It has been shown that as catalyst activity decreases along the series LiA1Cl4 > NaAIC14 > KA1C14, the values of Z and the gross rate of the degradation of polymer products also decrease (Table 4). With increasing temperature, the length of the kinetic chain of depolymerization of polyisobutylene and isobutylene-isoprene copolymer decreases for each catalyst in the series, while the rate constant of depolymerization increases. This is associated with the growth of the contribution of thermal (radical) mechanisms, which increases the
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
207
probability that chain degradation will occur in a random manner and also that chain transfer and termination will occur. In the degradation of PE the yield of monomer is very small and the concept of the kinetic chain length may be applied to the process of elimination of C4 hydrocarbons rather than to that of ethylene. However, in contrast to the behavior of polyisobutylene and isobutylene-isoprene copolymer, Z rises with increasing temperature in polyethylene degradation (Table 4). This phenomenon is, evidently, associated with an increase in chain degradation with subsequent chain transfer. This enables increased formation of one of the main products of the catalytic degradation of PE - isobutane. The values of the gross activation energy for the catalytic degradation in all the cases considered are lower than activation energy values for thermal degradation. This finding is accounted for by the decrease in both the initiation energy and the energy of degradation of the polymer chain due to the cationic nature of the active site. The above kinetic scheme characterizes the gross processes of degradation which proceed in the presence of electrophilic catalysts. It may be successfully employed for the description of the gross kinetics of the degradation of polyolefins using catalysts which are active as proton donors. It is necessary to take into account that the rate constants for the reactions between the catalyst and radicals and those for the radical decomposition of the polymer become very small. The depolymerization reaction rate will essentially be determined by the ionic processes of chain initiation and propagation. In this case, the kinetic scheme of catalytic degradation may be represented as follows: P + cat k~° R +
R+
kdep
- initiation both by random chain scission and at end groups.
R + z + Z M - chain propagation.
Ri ÷ P,n ktr~" Ri + Rm + R+ k~or P
- intermolecular chain transfer. - chain termination.
The rate of change of the concentration of the polymer carbocations formed at random may be expressed by the following equation: d[R + ] dt
k,'.[cat] [P] - k~e~[R+].
If the concentration of active sites [R ÷ ] is held constant, we obtain: [R ÷ ]
k~'n[P] [cat] kter
208
s.R. IVANOVA
et aL
For formation of active sites at end groups, the rate of change of the concentration of active sites is given by the equation, d[R + ] = k~[cat]C~ - kter[R+], dt (where Ce is the concentration of end double bonds). Accordingly, for steadystate conditions, we have: [R ÷] =
ki"n[Cat]Ce kter
The equations for the depolymerization rate will assume the following form: Vl = kd,p[R + ] =
kdepki',[P][cat] kter
i/2 = ko~p[R+] _
(random chain scission) kd,pki"n[cat]Ce kter
(depolymerization initiation at end groups) If we assume that the above two processes (random chain scission and end group attack) proceed independently, we obtain the following expression for the gross rate of degradation: V = kd¢p[cat](ki'~[P] + ki'nCe) ktcr Thus, the gross rate of degradation of polyolefins is determined primarily by the initiation processes and, in particular, by the values of the effective rate constants of initiation by random chain scission and by end group attack. This shows the necessity of evaluating the kinetic parameters of the process of initiation via different mechanisms. 3.4. Evaluation of the effective kinetic parameters The catalytic electrophilic degradation of polyolefins (for example, PIB) may be described by a general equation summing the contribution of random chain scission and end group attack: dP = ZekieCco + kirZrP dt
(10)
where P is the mass of polymer present; kit and kie are the rate constants of initiation of the polymer degradation process by random chain scission and end group attack, respectively; Zr and Ze are the kinetic chain lengths for depropagation following random chain scission and depropagation following end group attack, respectively; and C, is the concentration of the end double bonds. If we
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
209
assume that the end groups capable of initiating the depolymerization are consumed exponentially during the degradation process according to a relationship Ce = Ceoexp ( - kie. t) (C~o is the initial content of end double bonds), we have: P
=
kieZeCe°
e -kJr' +
kie -
(e kio, __ e k~rZr').
(11)
kirZ r
The numerical values of kie, Z~, kit and Zr may be determined at the initial stages of the process, when P ~ 1, t ~ 1/kie and t ,~ 1/k~Zr. Experimentally, such is possible when the process is run at the temperatures of 200-300°C, where the rates of chain termination are relatively low. Then, P
=
1 - kirZrt -
kieZetCeo.
(12)
The change in the molecular mass of PIB may be determined from the accumulated number of chain rupture points. The total number of macromolecules is n = no + k j and the number-average molecular mass is determined by the expression: P
1 -
kir Z r t -
no 1 + - -
kie Z e Ceo t
n0 /
1 _ k i r Z r t _ kicZ~C~o t kirt'~ (13) no / k where ~ o = 1~no = 1/Ceo (every primary macromolecule of PIB is assumed to contain one end double bond). The relative degree of polymerization will vary during degradation as follows: ~_ ~ o (
P"
po
=
1 -
k~rZ~t -
k~rP2t
k~eZ~t
po
(14)
Then it is possible to calculate the change in the average degree of polymerization with increasing progress of the degradation reaction de = AP. Referring to eq. (12) we have AP = (kirZ~ + kieZeCeo)t. And as a result we obtain: kie Ze kirZr + kir ]50 + - -
dL/po dot
p,o
-
k~Ze ki~Zr + - -po
(15)
If we assume that the kinetic chain length Zr is many times less than the initial degree of polymerization po, i.e. Zr ~ /50, eq. (15) may be simplified: k~ po + k~cZ____~
d/5o//5o dot
po -
kirZr +
ki~Ze
/50
(16)
210
S. R, IVANOVA et al.
i.o
o
0.8
•
,el
0120
0.10
0.30
FIG. 1. Dependence o f relative degree o f polymerization on the fraction of weight loss ct for polyisobutylene samples with the following initial degrees of polymerization: 1 - 15, 2 - 17, 3 - 20, 4 - 49, 5 - 360, 6 - 1790, 7 - 3570; catalyst - NaA1C14; T e m p e r a t u r e = 473 K.
To simplify the calculation of the values of the kinetic parameters of each of the concurrent reactions, the following extreme cases should be considered: (a) With a low fraction of the degradation reactions proceeding through the end groups, i.e. with kieZe/P ° ~ kirZ, ~ kirP° the expression for the reduced relative degree of polymerization has the form:
dP./L
po -
d~
Zr "
(17)
(b) With a low fraction of the degradation reactions proceeding by random chain scission we have: d~/dt ~- kie Z ~ / P . The value of/5, during such a process (including for the initial most probable molecular mass distribution, (Pw/Pn = 2)) is practically constant, i.e. dP,/P°/d~ ~_ O. This agrees with the experimental data (Fig. 1, curves 1-3). Eq. (16) in this case does not apply since the main assumption made in its derivation - that the kinetic chain length is short relative to/50 _ is not true. (c) Where random chain scission and end group attack are both important, i.e. at kir/50 >)> kieZ¢//5o >> kirZr, we have: dL//Sn
dot
kir ( / 5 ° ) 2 --
kieZe
(18)
The three variants (a), (b), (c) correspond to the three sections on the curve of the calculated value of d/5,//5°/dot vs/50, which have been obtained from the slope tangents of the corresponding experimental curves (Fig. 1). With increasing degree of polymerization, each of the three possible variants (b), (c), (a) is consecutively realized (Fig. 2).
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
IOF
I
-
211
4
4 50 I%C ~
F~
too
Jso
~ /
•I ,.I
I
20
I0
I I
2
5
4
FIG. 2. Variation in d#,//5~/d~ in the process of degradation of polyisobutylene using the catalyst NaAICI 4 at the following temperatures; 1 - 473 K, 2 - 493 K, 3 - 513K, 4 - 533K.
The values of Zr and kir/kieZc may respectively be calculated from the slope tangents of sections (a) and (c) on the d P ~ / P ~ / d ~ vs/s,0 curve (Fig. 2). The value of k~r may be calculated considering that the decrease in the polymer molecular mass occurs mainly due to random chain scission. This corresponds to the dependence dP n/po/dt = - klr/5o, i.e. the value of kir may be evaluated from the slope tangent. The values of Zr and k~r obtained from the experimental data may be used for evaluation of the numerical values of kic and Ze by eq. ( 11 ). The values obtained for the kinetic parameters - the initiation rate constants and the kinetic chain lengths - in the polymer depolymerization, using the example of PIB degradation in the presence of NaAIC14 (Table 5) testify to the prevailing role of end group attack in initiating the catalytic degradation under low-temperature conditions. With increase in temperature in the reaction zone the correlation of the values of k~, amd k~e decreases and under 260°C their values become comparable. The nature of the catalysts used in the polyolefin degradation process has been studied for the alkali and alkaline-earth metal salts NaC1. AIC13 and MgCI2 • AICI3. The complex salt MgC12 • A1CI3 is characterized by a somewhat higher acidity than NaC1 • A1CI3. This factor probably causes the increase in the activity of this catalyst in the polymer degradation process. In the degradation of PIB (Table 5) it has been shown that in the presence of MgCI2 • A1CI3 (in comparison with NaC1 • A1C13), both the total yield of gaseous products and
S.R. IVANOVA et al.
212
TABLE 5. Parameters of the reactions of initiation of the catalytic degradation of polyisobutylene by random chain scission and end group attack using different catalysts Catalyst
NaAICI 4
MgCI2 • AICI3
Ze
kir • 106
Zr
kit/kit
Temperature K
kie. 106
473 493 513
1.6 1.7 1.9
0.2 0.5 i.3
0.2 0.3 0.4
8.1 6.2 4.7
8.1 5.7 4.8
533 473 493 513 533
2.7 3.3 3.4 3.8 5.4
5.7 0.3 0.6 1.4 5.1
0.7 0.3 0.5 0.6 i.I
3.7 8.4 7.1 4.9 3.7
3.8 10.8 7.6 6.4 5.1
Eir
Eic
kJ/mol
k J/tool
60
47
70.7
45.8
that of isobutylene itself are higher. As one may see, the change in the catalyst acidity influences the selectivity of the process in terms of the yield of monomer also. Kinetic studies show that the increased selectivity to monomer is caused by increased contribution of end group attack to the total degradation process (Table 5). The correlation coefficient o f the rate constants of initiation by random chain scission with that of initiation by end group attack in the degradation process using MgC12 • AIC13 is higher than in the case using the catalyst o f lower acidity NaCI • AIC13 (Table 1). However, the correlation coefficient still drops with increasing temperature. The above approach permits one to determine the effective kinetic parameters of the different processes of initiation of degradation not only for PIB, but also for other polyolefins.
3.5. Effect of the polymer structure The laws derived above describe the correlation of the parallel reactions in the degradation of PIB, which is characterized by both a high concentration of end double bonds and by the presence of internal labile bonds. However, other polyolefins contain double bonds not only as end groups, but also at internal positions along the chain. They may also possibly, contain other "defects" (as in the case of polyethylene and butyl rubber). Here, the correlation of the reactions of initiation by random chain scission and by end group attack, in the total picture of polymer decomposition will change. Calculation of the kinetic parameters of the catalytic degradation of PE in the presence o f N a C l • A1CI 3 has shown that the values of the random initiation rate constants are commensurate with those of the rate constants of the sum of the different polymer degradation processes (Table 6). This testifies to the predominant statistical initiation of the degradation process and to the insignificant contribution of reactions proceeding by end group attack.
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
213
TABLE 6. Values of the gross rate constants and of the rate constant of initiation in the catalytic degradation of polyethylene by random chain scission (NaA1CI4) Temperature, K 573 593 613 633 653
kir"
107, s
0.25 0.21 0.95 1.31 1.71
k • 107, s- i
Eit, kJ/mol
Ea of gross process, kJ/mol
0.93 1.50 2.80 6.90 8.70
79
92
The shape of the curves of degree of polymerization vs fraction weight loss (in particular, a sharp decrease in t5,/P ° even at low extent of weight loss and the independence of the process rate with respect to po) qualitatively confirms the prevailing role of the random initiation reactions (Fig. 3). At the same time, similar data obtained for PIB are characterized by a pronounced dependence of the predominant degradation reactions on the molecular mass of the sample degraded. For instance, the low-molecular-weight polyisobutylenes (M, = 800 to 2000) with a high content of end double bonds undergo polymerization chiefly by attack on end groups, and the relative degree of polymerization changes slowly with weight loss (Fig. 1, curves 1-3). With increasing PIB molecular mass (Fig. 1, curve 4) the concentration of end groups (and end group double bonds) drops accordingly. This results in a marked growth of the relative contribution of random initiation reactions. As the initial degree of polymerization increases further (Fig. 1, curves 5-7), the end group double bond 1.0
0.8
0.6
\
0.4
0.2
I 0.05
" T "°'6
I
O.lO
0.15
I
0.20
I
0.25
FIG. 3. Variation in the relative degree of polymerization P, with weight loss fraction ct for polyethylene samples with different initial molecular mass; 1 - 2 • 03, 2 - 2 . 104, 3 - 6 . 104 , 4 - 1.45- 105 , 5 - 2.5- 105 , 6 - 2.7- 105; catalyst NaA1Cla, T = 633 K.
214
S.R. IVANOVA et al.
concentration drops still more and random chain scission becomes the predominant degradation mechanism. Hence, in the general case one may assert that the respective concentrations of internal labile groups and end group double bonds determine the character of initiation of the catalytic degradation of polyolefins. As a consequence, these factors markedly affect the numerical values of the effective rate constants of initiation. 4. CONCLUSIONS Use of electrophilic complex catalysts increases selectivity to monomer in the degradation of polyolefins (in comparison with the uncatalyzed thermal process). The catalyst acidity promotes depropagation to monomer at the expense of side reactions. The catalytic complexes may be activated by crystal water through formation of the corresponding proton donor compounds - the crystalline hydrates. They may also initiate the polymer degradation process via singleelectron oxidation of the macroradical with formation of the polymer carbocation. Although the specific active sites are different in uncatalyzed and electrophilically catalyzed degradation of polyolefins, in both cases the process proceeds by a mixture of random chain scission and attack on end groups. Initiation of polyolefin degradation by catalysts of medium acidity is determined both by the catalyst nature and by the polymer macrochain structure. The chain depropagation reactions depend on the structure of the particular polyolefin. The evaluation of the effective kinetic parameters permits one to determine the relative contributions of the concurrent reactions of random chain scission and end group attack. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. I0. 11. 12. 13. 14. 15.
R.Y. SIMHAand L. A. WALL, J. phys. Chem. 56, 707-715 (1952). E. KIRAN and J. K. GILLHAM,J. appl. Polym. Sci. 20, 2045-2068 (1976). R. SIMHA, L. A. WALL and P. BLATZ, J. Polym. Sci. 5, 615-623 (1950). R. SIMHAand L. A. WALL, dr. Po[ym. Sci. 6, 39-45 (1951). N. GRASSIEand J. SCOTT, Polymer Degradation and Stabilization, p. 246, Translation from English, M. : Mir (1988). R. SIMHAand L. A. WALL, J. chem. Phys. 29, 894-899 (1958). D.W. BROWN and L. A. WALL,J. phys. Chem. 62, 848-859 (1958). AL. AL. BERLIN, K. S. MINSKER, YU. A. SANGALOV and Yu. A. PROCHUKHAN, g~esokomolek. Soedin. A25, 1458-1463 (1983). Yu. A. SANGALOV, K. S. MINSKER, AL. AL. BERLIN and Yu. A. PROCHUKHAN, gysokomolek. Soedin. A25, 1451-1457 0983). M. SEEGERand R. I. GRITTER, J. Polym. Sci., Polym. Chem. Edn 15, 1393-1402 (1977). W.J. BAILEY, SPE Trans. 5, 59-64 (1965). M. SEEGERand H.-J. CANTOW, Polym. Bull. 1,347-354 (1979). L.A. WALL and S. STRAUS,J. Polym. Sci. 15, 313-323 (1960). W. BAILEYand L. J. BACCEI,Am. Chem. Soc. Prepr. 12, 313-318 (1971). N.A. SLOVOKHOTOVA, M. A. MAGRUPOV and V. A. KARGIN, VS,sokomolek. Soedin. A6, 1974-1979 (1964).
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54.
215
D . K . LAM, Div, Polym. Chem., Am. Chem. Soc. Prepr. 16, 50-56 (1975). R. McGUCHAN and I. C. McNEILL, Eur. Polym. J. 4, 115-122 (1968). J. KODAIRA,Z. OSAWAand H. ANDO, J. Chem. Soc. Japan, Chem. Ind. Chem. 12, 1892-1898 (1977). C. VASILEand P. ONE, Acta Polym. 36, 543-550 (1985). T. VAMAGUCHI, T. KAMIGUCHI, T. 1TO et al., J. Chem. Soc. Japan, Chem. Ind. Chem. 1171-1174 (1976). A. LuccI-I~SI, G. MASCHIOand P. GLUST*, lmballaggio 32, 215-222 (1981). AL. AL. BERLIN, E. F. GUMEROVA, S. R. IVANOVA and K. S. MINSKER, VS'sokomolek. Soedin. B29, 816-819 (1987). Y. KITAOKAand K. MURATA, J. Fuel. Soe. Japan 50, 791-799 (1971). K. MURATA, K. SATO and H. TESHIMA, Kagaku kogaku ronbunshu 8, 279-284 (1982). R.C. POLLER, J. chem. Technol. Biotechnol. 30, 152-160 (1980). K. MURATAand T. MAKINO, J. Chem. Soc. Japan, Chem. Ind. Chem. 12, 2414-2420 (1973). T. SAWAGUCHI,T. KUROKI and T. Ih'EMURA,Bull. Japan. Petrol. Inst. 19, 124-130 (1977). K. MURATA and K. SATO, Kagaku kogaku ronbunshu 7, 64-70 (1981). T. CROMPTON and L. W. MYERS, Plast. Polym. 36, 205-212 (1968). N . M . EMMANUEL,Zadachi Fundamental'nykh Issledovanii v Oblasti Stareniya i Stabilizatsii Polimerov, P. 1-25, M. : Khimiya (1970). S.L. MADORSKY,Thermal Degradation of Organic Polymers, Interscience, New York (1964). S.L. MADORSKY, S. STRAYSet al., J. Polym. Sci. 4, 639 (1949). D.V. VAN KREVELEN,Svoistva i Khimicheskoye Stroyeniye Polimerov, M. : Khimiya (1976). N. GRASSlE, The Chemistry of the Polymer Degradation Processes. Translation from English, M. : Izd. In. Let. (1959). AL. AL. BERLIN and S. VOLFSON, Kineticheskii Metod v Sinteze Polimerov, M. :Khimiya (1973). T.J. HENMAN (ED.), Controlled Cross-linking and Degradation, Degrad. Stab. Polyolefins, pp. 29-62, London, New York (1983). CH. TENFORD, Physical Chemistry of Polymers. Translation from English, M. :Khimiya (1969). H.H. JELLINEK, J. Polym. Sci. 3, 850-858 (1948). H.H. JELLINEK, J. Polym. Sci. 4, 13-25 (1949). A. KOSSIAKOFEand F. O. RICE, J. Am. chem. Soc. 65, p. 590 (1943). M. SEEGERand H.-I. CANTOW, Makromolek. Chem. 176, 1411-1425 (1975). R . G . J . MILLER and H. A. WILLIS, J. Polym. Sci. 14, 569 (1959). K. DAHL and F. JOHNSON, Polym. Left. Ed. 13, 651-655 (1975). Yu. A. SANGALOV,K. S. M1NSKER et al., Teor. i Eksperim. Khirniya 18, 535-542 (1982). F. EIZINGER, Khimiya i Tekhnologiya Monoolefinov. Translation from German, Gostoptekhizdat. (1960). S.R. IVANOVA,I. YU. PONEDEL'K1NAet al., V~sokomolek. Soedin. A28, 266-271 (1986). S.R. IVANOVA,T. V. ROMAMKOet al. , V.~sokomolek. Soedin. A27, 244-250 (1985). S.R. IVANOVA,1. Yu. PONEDEL'KINAet al., V.~sokomolek. Soedin. A28, 1217-1221 (1986). A.A. FURMAN, Neorganicheskiye Khloridy: Khimiya i Tekhnologiya, p. 416 M.:Khimiya (1980). C. CHAMBERS,J. W. LARSENet al., Ind. Engng Chem. Process Des. Dev. 23, 648-654 (1984). J. KENNEDY, The Cationic Polymerization of Olefins, p. 109. Translation from English, M. : Mir (1978). A. AYAME, T. YOSH1DAand H. KANOH, Bull. Japan Petrol. Inst. 17, 226-231 (1975). A. AYAME, Y. UEMADU, T. YOSHIDA et al., J. Japan Petrol. Inst. 22, 280 287 (1979). X. UEM1CH1,A. AYAME,T. YOSHIDA et al., J. Japan Petrol Inst. 23, 35-43 (1980).
SELECTIVE
0079-6700/90 $0.00 + .50 © 1990 Pergamon Press plc
CATALYTIC DEGRADATION POLYOLEFINS
OF
S. R. IVANOVA,* E. F. GUMEROVA,* K. S. MINSKER,* G. E. ZA1KOV** and A. A. BERLIN**
*Bashkirian State University, Ufa-450074, Frunze str, 32 **Institute of Chemical Physics, Academy of Sciences of the U.S.S,R. 117334, Moskow, U.S.S.R.
CONTENTS 1. Introduction 2. Uncatalyzed thermal degradation of polyolefins 3. Catalytic degradation of polyolefins 3.1. Catalysts for the degradation of polyolefins 3.2. Mechanism of catalytic degradation of polyolefins 3.3. Kinetic peculiarities of the catalytic degradation of polyolefins 3.4. Evaluation of the effective kinetic parameters 3.5. Effect of the polymer structure 4. Conclusions References
193 194 195 195 199 204 208 212 214 214
1. I N T R O D U C T I O N
The theory of chain depolymerization of polyolefins has been interpreted in terms of the thermal decomposition of polymers by a radical mechanism) Evaluation of two main kinetic parameters - kinetic chain length and chain transfer rate constant - was considered important3 The first publications, as a rule, dealt with kinetic schemes of the thermal degradation of polyolefins based on experimental data. 3-7 These schemes deal with variations in the molecular mass of the starting polymer, the composition of the products formed and the rate of thermal degradation of the polymer to products. Electrophilic catalysts cause marked changes in the kinetics of the degradation of polyolefins. There is a change in the gross mechanism, accompanied by a considerable increase in the rate and selectivity of the process.8'9 In both the uncatalyzed~°-17and the catalyzed ts-2~ thermal degradation of polyolefins, one may single out two mechanisms of initiation: (a) initiation of depolymerization through the end groups of a polymer chain; (b) random chain cleavage at any bond in the chain. It was earlier believed impossible to determine the relative contributions of these two initiation mechanisms with existing theoretical and experimental approaches to study the reaction kinetic parameters. 2-6 A method to determine the effective initiation constants and kinetic chain lengths of the 193
194
S.R. IVANOVA et aL
concurrent processes in the degradation of polyolefins in the presence of electrophilic catalysts was proposed. 22 This enabled us to come closer to a general theory of the processes of the selective degradation of polyolefins. 2. U N C A T A L Y Z E D
THERMAL
DEGRADATION
OF POLYOLEFINS
The uncatalyzed thermal degradation of polyolefins first attracted attention at the end of the 1940s. Initial interest was focused on selective recovery of valuable gaseous hydrocarbons from the polymeric raw material?3-z9However, thermal degradation of polyolefins and similar vinyl polymers usually gives only a low yield of starting monomer, exceptions being rare (poly-ct-methyl styrene, polymethyl methacrylate3°). For the majority of polyolefins the formation of a wide spectrum of low-molecular weight products - from gaseous (CI-Cs) to liquid and waxy ones - is typical.3t Available data suggest that the most important degradation processes are: (1) random chain cleavage (with hydrogen atom transfer) leading to a broad variety of degradation products, and (2) depropagation of a chain end to monomer. 3t-33These conclusions are based on molecular weight determinations of partially degraded polymer, kinetic studies and analyses of volatile and nonvolatile degradation products? t'34'35As often as not the degradation process may be accompanied by side-reactions, for example, cross-linking of the macromolecules)6 Also, especially in the degradation of branched polyethylene (PE), the active sites may be formed at branching points with subsequent termination of the side chains, leading to no net decrease of the polymer molecular mass. The system of consecutive and parallel reactions which comprise the thermal degradation pathways of polyolefins is extremely complex. 37 However, the two processes outlined in the above paragraph appear to be usually the most important ones in polyolefin degradation. 7 As a rule, chain scission is associated with presence of various defects in the polymer chain. These may include weak bonds which are formed, for example, if the polymerization is conducted in the presence of oxygen.38'39Other "defects" may include structural irregularities, for example, bonds in which monomer units are linked in the "head-to-head" manner; 34 side substituents and internal double bonds of various types3~ and the like. The initial macroradicals formed are consumed in the various reactions of chain depropagation. For example, they undergo fl-cleavage to yield olefins substituted according to Rice's rule. 4° They also undergo chain transfer and termination reactions which result in the formation of heavier hydrocarbons. It is typical that in thermal decomposition of polymers the role of chain transfer to polymer is appreciable, t This is a decisive factor in the formation of compositionally heterogeneous reaction products. Thus, from all the possible reactions involved in the thermal degradation o f polyolefins, one may single out three of the most significant reactions which
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
195
determine the distribution of products: (1) those of radical decay - the degradation of macromolecules; (2) those proceeding with hydrogen transfer; (3) those of radical substitution (chain transfer to polymer). Depending on the structure of the polyolefin, the relative contribution of these processes may be different. For instance, comparing the degradation processes of PE and polyisobutylene (PIB), one may note that for linear PE the three reaction pathways are equally probable. 4~ However, for PIB, which contains two methyl substituents at every second carbon atom in the chain, only reactions (1) and (3) are typical. The result is that in the degradation of PE very little ethylene is formed (Table l) while in the degradation of PIB, the amount of isobutylene formed is about 20% of the weight of the starting polymer (Table 1). It has also be experimentally established (Table l) that if internal double bonds are present, as in polybutadiene, the role of hydrogen transfer reactions again increases. Consequently, the yield of monomer appears to be insignificant. Chain transfer reactions with separation of the hydrogen atom (l) from the substituent (in the case of PE, short branches - mainly ethyl and n-butyl groups - should be considered as substituents 55) and (2) fl'om the carbon atoms, located within the chain are equally probable for PE and butyl rubber (BR). Chain transfer reactions with separation of the hydrogen atom from internal carbon atoms, which result mainly in the formation of hydrocarbons with isomeric structures, are more typical for PIB (Table 1) On the whole, one may state that the thermal decomposition of PE, PIB and BR, as well as that of most other polyolefins, results in the formation of such a broad spectrum of reaction products that the very notion of selectivity relative to the process of the thermal degradation of polyolefins becomes meaningless. 3. C A T A L Y T I C
DEGRADATION
OF POLYOLEFINS
The polymerization of isobutylene readily proceeds by the cationic mechanism, with an activation energy close to zero and a relatively low heat of polymerization (Q = 53 kJ/mol). The depolymerization of polyisobutylene and some other polyolefins effectively proceeds also through carbocationic active sites. 2°'43
3.1. Catalysts for the degradation of polyolefins The electrophilic catalytic degradation of polyolefins in presence of A1C13 (Table 1) proceeds with a higher yield of gaseous products than the uncatalyzed thermal process and at markedly lower temperatures, but with poor selectivity to monomer. Evidently, in these strongly acid systems, not only are the processes of ionic degradation intensified, but also isomerization of the products. The formation of primary polymer carbocations from polyethylene in the presence of aluminium chloride (active, evidently, in the form of H + [A1C13 • OH]-44) proceeds with random hydride abstraction, sometimes followed by chain
PE PIB IIR PE PlB IIR PE PIB IIR PE PIB IIR
Polyolefin*
673 653 653 643 643 643 643 583 573 643 573 573
Temperature K
40.8 75.9 50.4 47.6 62.7 85.0 84.8 47.9 38.8 88.2 93.8 75.4
Gas yield, mass %
52.9 1.9 2.7 5.7 1.5 14.3 24.8 I. l 16.4 15.0 1.4 1.7
CI-C 3 18.7 2.1 traces 22.4 2.1 59.0 54.9 7.8 23.6 42.5 24.9 2. I
i-C4Hlo 7.5 -0.5 0.8 traces 0.4 6.7 0. l 0.2
n-C4Hlo 0.1 3.0 0.1 traces 0.1 -0.3 0.5 -0. l 0.5
~-C4H 8 1.1 16.0 2.9 15.7 19.1 0.6 19.7 88.8 39.0 21.8 73.4 93.8
i-C4H 8 19.8 0.9 30.3 2.7 1.0 0.2 0.3 2.0 12.4 14.0 0. l 0.6
fl-C4H 8 47.1 19.1 36.7 40.9 22.2 59.9 75.5 98.9 62.6 85.0 98.6 95.9
C4H 8 + C4Hlo
Yield of individual gaseous products,** mass %
*PE = polyethylene, PIB = polyisobutylene, I I R = isobutylene-isoprene rubber. **Traces of hydrogen have been identified in the gaseous products.
AICI3 A1C13 AICI3 NaAICI4 NaAIC14 NaAlCl4 MgCI2 • AICI3 MgCl2 • AICI3 MgCl2 • AtCl 3
-
-
Catalyst
TABLE l.Yield of the gaseous products from the thermal and catalytic degradation of polyolefins
l.l
7.7
79.0 60.6 53.4 76.3 25.8
Cs and higher
oz<
SELECTIVECATALYTICDEGRADATIONOF POLYOLEFINS
197
cleavage: R-CH2-CH:-CH2-R' + H+[AICI3 " OH]+ R-CH2-CH-CH2-R' [AICI3 • OH]- + H2
I
CH3 or R-CH2-CH-CH2-R' + H + [A]- ~ R-CH2-CH-CH2-R' + H2 + [A]-
I
I
CH3
CH2 +
+
--~ R - C H 2 + C H 2 - C H - C H 2 - R ' .
However, degradation processes are probably more commonly initiated at defects in the polymer chains, such as vinylidene double bonds: +
RR'=CH2 + H + [AICl3 • OH]- -~ RR'CH-CHE[AICI3 • OH] . The primary ion is isomerized with formation of a more stable tertiary carbocation"
R' + RR'-CH2[AICI3 • OH]- ,
I , R-C + [AICl 3 • OH]-.
I
CH3 The decomposition of carbocations leads to a wide spectrum of products. The gaseous fraction contains species ranging from CH 4 (separation of methyl branches) to C4H8 (skeleton isomerization of the ion with subsequent fldecomposition). Liquid and solid fractions contain C5 hydrocarbons and higher (Table 1). Typically, in the degradation of polyethylene, the yield of monomer is not high. End double bonds are also of significance in forming initial carbocations. Because of the high content of internal double bonds and other defects in polyethylene, however, relatively little of the degradation is thought to originate from endgroups. Increase in the content of relative to end double bonds, as in PIB and isobutylene-isoprene rubber (IIR) increases the probability that they will be sites of carbocation formation. However, a rise in selectivity to monomer is not observed (Table l). Evidently, random chain transfer to polymer plays a very important role in the degradation of polyolefins in the presence of strong acid catalysts. Side reactions in the electrophilic processes are usually suppressed with decrease in the acidity of the catalysts.45 In particular, complexes of the type of M a n " AICl3 or M(AIC14)n (M - L i , Na, K, Mg, Ca, Ba; n = 1-2) are useful in this regard. 46-48 Both the high activity of the catalysts of this type and their
198
S.R. IVANOVA et aL
TAaLE2. Catalytic degradation of polyolefinsin the presence of some metal chloride hydrates Catalyst
Temperature
Gas yield, mass %
Monomer yield, mass
K
% PIB
IIR
PE
PIB
IIR
PE*
MgCI a • n H 2 0
583
12.4
27.4
8.7
96.2
96.0
76.5
(n = 1 to 2)
603 623 583 603 623 583
81.9 100.0 11.4 36.2 62.0 3.8
92.0 100.0 19.7 31.6 35.0 4.0
24.5 33.9 21.4 2.2
99.5 99.5 90.1 92.4 98.6 23.5
96.4 97.0 93.1 92.0 97.0 19.0
77.1 78.3 73.0 53.2
MgC12 • 4H20
BaC12• 2H20
*Total yield of C4-hydrocarbons.
high selectivity in the degradation of polyolefins have been experimentally established. The gaseous reaction products contain up to 90-95% of C4hydrocarbons (Table l). These metal tetrachloroaluminate melts are known as ion media. 49"5° The drop in the acidity of the metal tetrachloroaluminate catalysts compared with that of A1C13 decreases the ratio of chain transfer to chain depropagation during polyolefin degradation. This, in turn, raises the selectivity of the process to C4-hydrocarbons. It is interesting that MgC12 hydrates give extremely high activity and selectivity to isobutylene monomer in the degradation of polyisobutylene and isobutyleneisoprene copolymers (Table 2). In the formation of catalytically active magnesium chlorides an important role is played by the crystal waterJ 1 Of the magnesium chloride hydrates, MgC12 • H20 and MgCI:. 2H20 are the most effective. Increase in the degree of hydration results in the partial hydrolysis of magnesium chloride with formation of MgOHC1, inactive as a catalyst for polyolefin degradation. Quantum-chemical calculations confirmed the possibility of the activating effect o f the crystal water (Table 3). The high catalytic activities of hydrates of MC1 • A1C13 (or MA1CI4) and of MgCI 2 are caused by the high positive charges on the hydrogen atoms in the hydrates. The stability of the hydrate of MA1C14, evaluated by the energy of complexing with water, is somewhat lower than that of the hydrates of magnesium chloride. Therefore, irrespective of the higher charges on the hydrogen atoms, the activity of the hydrates of MAIC14 salts as catalysts for polyolefin degradation is somewhat lower than that of MgCI2 hydrates (Table 3). It should also be stressed that the quantum-chemical calculations are validated by the experimentally established series of MAIC14 catalyst activity for polyolefin degradation: LiA1CI4 > NaA1CI4 > KA1C14. The acidity Pk~ and the value of
SELECTIVE CATALYTICDEGRADATIONOF POLYOLEFINS
199
TABLE3. Quantum-chemical calculations for catalysis of polyolefin degradation (LCAO-MO method with CNDO[2 approximation) Catalyst
Pk,
qn+
Complexing energy, kJ/mol
Gas yield, mass % (603 K) PIB
LiCI • H 2 0
10.5
MgOttCI MgCI2 • H20
LiA1C14
23.0 12.7 15.4 15.4 I0.0 13.7 12.5 5.6
NaAICI4
4.5
KAIC14
-
MgCI2 • 2H20 MgCI2 • 4H20
0.182 0.182 0.070 0.166 0.185 0.146 0.185 0.158 0.165 0.253 0.217 0.226 0.229 -
BR
PE
131
9.1
21.2
-
265
8.5 100.0
23.4 I00.0
49.5
260
100.0
100.0
45.0
216
59.5
45.2
21.4
88
61.1
96.5
57.4
249
55.0
46.7
55.9
-
6.3
18.0
12.3
the positive c h a r g e qn+ on the h y d r o g e n a t o m s o f the c o m p l e x e s decrease with increasing r a d i u s o f the cation. In s u m m a r y , the w e a k l y acid c o m p l e x catalysts o f the type M C I , • A1C13 a n d M g C I 2 are highly active a n d selective in the c a t a l y t i c electrophilic d e g r a d a t i o n o f polyolefins.
3.2.
Mechanism of the catalytic degradation of polyolefins
T h e m e c h a n i s m s o f the t h e r m a l a n d c a t a l y t i c d e g r a d a t i o n o f polyolefins are diverse. T h e u n c a t a l y z e d t h e r m a l d e c o m p o s i t i o n o f the p o l y m e r s is generally t h o u g h t to o c c u r by a r a d i c a l m e c h a n i s m . 3-7 D e g r a d a t i o n in the presence o f electrophilic catalysts, h o w e v e r is ionic. ~'52-54 In earlier w o r k s o n the c a t a l y t i c d e g r a d a t i o n o f polyolefins in the presence o f MAIC14 salts, the c a t i o n i c m e c h a n i s m o f the process was i n t e r p r e t e d in terms o f the single-electron o x i d a t i o n o f t h e r m a l l y g e n e r a t e d p r i m a r y radicals: 8'9 (a) R a n d o m t h e r m a l d e c o m p o s i t i o n , p r e f e r e n t i a l l y o c c u r r i n g at the weakest b o n d s , e.g. CH3
CH 3
I I "~ C H 2 - C - C H 2 - C H 2 - C I I CH3
CH 3
CH 3
I
CH 3
I I
CH2-C-(~H2 + (~H2-C~
/
CH 3
CH 3
.
200
S.R. IVANOVAet
al.
(b) A redox reaction with formation of the polymer carbocation (a mechanism which would seem plausible only if M ÷ is relatively easily reduced), CH 3
I "~ CH2-(~ + I
CH3
I
M + [AICI4]- _~0, ~ CH2_C+ [AIC14]_ .
/
CH 3
CH3
(c) Depolymerization of the macroions with formation of monomer: CH3
CH3
I ! ,-~CHE-C-CH2-C + [AICI4]I I CH3
CH3
CH3
I
I I
~ -,~CH2-C + [AIC14]- + CH2 = C
/
CH3
CH3
CH3
(d) Chain transfer to polymer with further depolymerization of the macroions starting from end groups: CH3
I ~ CH=C + I
R
CH3
I
I I
R + [AICI,]- ~ ,,~ CH--C + [A1C1,]-
CH 3
CH 3
CH3 -~
CH3
I
~CH~-C+[AICI,]
/
-
CH3
I I
I
CH3
I I
+ R - C H = C - - , ~CH2-C+[AIC14] - + CH2=C
CH 3
/
CH3
CH 3
CH3
In catalysis by the hydrates of MAICI4 salts, the polyolefin degradation process is initiated at the active site H + [MA1CI4 • OH]- having the structure CI CI.~I,DCI Na
CI
H
The hydrate can initiate the degradation both randomly along the chain and preferentially at end groups. The interaction of the catalyst with the weak bonds of the polymer chain is determined by the structure of the polyolefin. In PE the degradation process is initiated mainly at vinylidene and trans-vinylene internal double bonds. The main initiation sites along the chain in butyl rubber are the coupling points of the isobutylene fragments with the isoprene units. End group initiation proceeds through the end double bonds.
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
201
On the whole, one may state that the processes of initiation of electrophilically catalyzed polyolefin degradation may be expressed by the following general scheme: (a) End group processes: R
R
I
,-~ C H = C + H + [A]
I
~ ,-~ CH2-C + [A]-,
I
I
R
R
where R denotes C H 3 and H for (polyisobutylene, butyl rubber) and PE, respectively. (b) Random hydride abstraction followed by chain cleavage: cn
3
I
CH2-C~
CH 3
÷
I
+ H +[A]- ~ ~ C H - C ~
I
[A]
+ H2
I
CH 3
CH 3
or proton addition followed by chain cleavage (which is more probable for isobutylene-isoprene copolymer and polyethylene): R
R
I
I
CCH2-C=CH-CH2~
+ H+[A]
~ ~ C - C H 2 - C+_ C H 2 ~ [A]
I
I
I
I
R
R
R
R
-~
R
I ",~CH2-C + [A]- + C H z = C - C H 2 ~ •
I
I
R
R
The reactions of chain depropagation in the degradation of the macrocations may differ depending on the chemical nature of the polymer involved. For instance, in polyisobutylene and isobutylene-isoprene copolymer, the carbocations formed chiefly undergo//-elimination of monomer (isobutylene) by the following scheme: R
R
R
I
I
I
C H 2 - C - C H 2 - C + [A]- ~ ~ C H 2 - C + [A]
R
I
+ CH2=C.
I
I
I
I
R
R
R
R
+
The linear carbocation R - C H 2 formed in the degradation of PE reacts via two main schemes:
202
S . R . I V A N O V A et al.
(1) Skeleton isomerization with formation of a tertiary carbocation, followed by its fl-decomposition to yield isobutylene: +
+
'~ CH2-CH2-CH2-CH2-CH2 --~ ,~ C H 2 - C H 2-CH-CH3
+ ~ CH2-C-CH3
CH3 I ~
I
CH2--C
+
+ ,-,C H 2 - C H 2
•
I
CH3
CH3
(2) Isomerization o f the macroion with transfer of excessive positive charge to the sixth carbon atom via cyclization of "flexible" methylene sequences: +
+ CHT--CH2~ C H 2 - C H 2-CH 2 - C H 2 - C H 2-CH 2 ~ ,,, CH 2 CH 2 ~CH~--CH2 / /./CH~--CH 2 --) ~ CH21
+ C H 2 ~ ,,~ C H - C H 2-CH 2-CH 2-CH 2-CH 3.
fl-decomposition of the above carbocation, with subsequent isomerization and deprotonation, yields isobutane: +
+
,,, C H - C H 2 - C H 2 - C H 2 - C H 2 - C H 3 --> ,,~ C H = C H 2 + CH2-CH2-CH2-CH3 CH 3
I CH3_C+ I
CH 3
CH3
I I
RH) CH3_CH + R + . CH3
This reaction departs from the classical scheme of depolymerization and may be interpreted as degradation with subsequent chain transfer. In the catalytic degradation of branched PE (usually not more than 30 methyl branches per 1000 chain carbon atoms), carbocations may be formed at the tertiary carbon atoms. The subsequent decomposition o f the macrocation proceeds via two reactions:
SELECTIVE CATALYTIC D E G R A D A T I O N OF POLYOLEFINS
203
(1) cleavage of the main chain; (2) elimination of a side chain: +
R~-CH2-CH2 -F CH2=C-R2
I I
CH 2
+
Rl-CH/-CH2-CH2-C-R2 - -
R3
/
CH2
L R3 RI-CH2-CH2-C-R2 + R~ .
II
CH2 In both cases a new carbocation (which undergoes further decomposition) and a "dead" polymer molecule with a vinylidene double bond are formed. In summary, the electrophilic degradation of different polyolefins is initiated by a few common pathways. Degradation may start either at doublebond end groups or at weak bonds along the chain. The path of the chain degradation reaction is determined by the structure of the polyolefin. More highly substituted chains, such as polyisobutylene, tend predominantly to depropagate directly to monomer. Less substituted chains tend to form carbocations which isomerize before depropagating. Linear polyethylene undergoes degradation by a mixture of carbocation depropagation and chain transfer to polymer.
3.3. Kinetic peculiarities of the catalytic degradation of polyolefins The difference in the form of catalyst used (ion melt, hydrate, etc.) determines the particular reactions which proceed and, consequently, the peculiarities of the kinetic scheme of the process. In the ion melt the kinetic scheme is as follows: p Wrr 2 R ' } P + cat
k , 2R"
(random chain cleavage)
R" + cat k~°~ R+ P + cat k~.~ R+
(end group attack)
R,+ kaop, R;_.. + zM R~, kdoo, R~,_: + z M
204
S.R. IVANOVA et al.
R" k ~ , R~, + R+ k ~ , R :
R" + P
Rm
+ Rm
ktr' R" + 2P
R ÷ + p klr R+ + 2P R+ k,~., p where k~n,kin, k~,, kd~, k~eg,kd~, k~r, ktr, kt©rare the rate constants of initiation, depolymerization, degradation of polymeric cations and radicals, chain transfer for ionic and radical sites, and chain termination, respectively. For polymer degradation by random chain cleavage, the variation in the concentration of the macrocations JR] ÷ is described by the equation: d[R + ] = k~n[cat][R'] - k,,dR+]. dt
(1)
In steady state, [R +] =
kin[Cat]JR']
where [R']
=
-
kin[cat] / ( kin[cat] ~2 W, 2k----~ 4- V \ ~ / + k'-~
/4k~(Wth+kt[cat])
kin[cat] I 2k, ~/
(--~-a~ ~
+ 1 -
]
1 .
(2)
Here W~ = Wth + k~[cat] is the sum of the rates of radical formation (1) resulting from thermal homolysis of the polymer chain and (2) promoted by the catalyst. The two extreme cases have been considered: 8 (a) There is little catalyst in the system, i.e., only a small proportion of the radicals formed is converted into macrocations. This agrees with the condition: 4k~(Wth + k~[cat] (kin[cat]) 2 >> 1. Then, [R']
=
~h 4- kl[Cat] kl
(3)
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
205
and the expression for the rate of the statistical decomposition will assume the form: v~ =
(k~ep + k~og)
kin[cat] x/Wth + kl[cat] k,o----T_ k,
+ (kd~ + kd.g)~/Wt" + klkl [cat]
(4)
In this case, the interaction reaction between the catalyst and the radical is the limiting stage of the process. (b) There is much catalyst in the system, i.e., all the radicals formed are converted into carbocations: 4kl(Wth + k' [cat])
(kin [cat]) 2
,~ 1.
In this case, the concentration of the macroradicals in the system is determined by the correlation, [R'] =
Wth + k'[cat]
(5)
kin[Cat]
and the rate of the process may be represented in the form, V~ = (k~ep + kdeg)
W~h + 2k'[cat] kt~r
(6)
When the depolymerization of PIB is initiated at end groups, the concentration of the active sites R + is determined by the expression: d[g + ] = ki'~[cat]Cn - k~or[R+], dt where Cn = Ao/Pn is the concentration of double bonds in the polymer, A0 is the concentration of monomer units in bulk polymer (or, alternatively, in polymer solution) and Pn is the degree of polymerization of the polymer. The rate equation for end-group-initiated depolymerization of PIB has the form: //2 = k'p[R+ ]
(k~lep + k~teg)kin[cat] =
kter
(k~ep + Cn
=
k'd,g)kinAo[cat] kter pn
(7)
Then the total depolymerization rate is described by the equation: V =
kin[Cat] x/Wt, + k'[cat] V~ + V2 = (k~p + k~eg) k~r __ k, +
(kdo~ +
kdeg)
/W~h + k'[cat]
4
kl
(k~, + k~e,)k[nA0[cat] +
kter/3n
(8)
206
s.R. IVANOVA et al.
TABLE 4.
Polyolefin
Gross parameters of the catalytic degradation of polyolefins Catalyst
Temperature
Z + 10%
k i • 107, s- i
Ea, kJ/mol
583 603 623 583 603 623 583 603 623
10 8 6 I0 7 6 4 3 3
0.6 1.7 4.3 0.3 0.8 2.4 0.4 1.1 3.7
100
NaAICI4
573
40
0.7
54
MgCI 2 • AICI3 BaCI z • AICI3
573 573
55 62
1.2 2.0
55 48
LiAIC14
573 603 623 643 573 603 623 643 573 603 623 643
8 19 26 31 10 12 17 20 7 9 12 19
0.4 1.2 3.9 5.8 0.4 1.3 3.6 5.0 0.1 0.5 1.2 2.7
118
K
Polyisobutylene
LiAIC14
NaA1C14
KA1C14
Butyl rubber (isobutylene-isoprene copolymer)
Polyethylene
NaAICI4
KAICI4
105
140
124
151
This equation takes account of the fact that depolymerization proceeds simultaneously by ionic and radical mechanisms. However, it does not permit one to evaluate the relative contributions of these mechanisms from observed monomer yields. Catalyst selectivity may be expressed in terms of Z ("zip length"). This is the kinetic chain length which characterizes the average number of monomer molecules released per rupture of the macromolecular chain: Z = k~ep[R+] + k~eg[R'] + kdeg[R+] + kd~[R] W,h + k'[cat] + k,[R +] + kj[R +]
(9)
It has been shown that as catalyst activity decreases along the series LiA1Cl4 > NaAIC14 > KA1C14, the values of Z and the gross rate of the degradation of polymer products also decrease (Table 4). With increasing temperature, the length of the kinetic chain of depolymerization of polyisobutylene and isobutylene-isoprene copolymer decreases for each catalyst in the series, while the rate constant of depolymerization increases. This is associated with the growth of the contribution of thermal (radical) mechanisms, which increases the
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
207
probability that chain degradation will occur in a random manner and also that chain transfer and termination will occur. In the degradation of PE the yield of monomer is very small and the concept of the kinetic chain length may be applied to the process of elimination of C4 hydrocarbons rather than to that of ethylene. However, in contrast to the behavior of polyisobutylene and isobutylene-isoprene copolymer, Z rises with increasing temperature in polyethylene degradation (Table 4). This phenomenon is, evidently, associated with an increase in chain degradation with subsequent chain transfer. This enables increased formation of one of the main products of the catalytic degradation of PE - isobutane. The values of the gross activation energy for the catalytic degradation in all the cases considered are lower than activation energy values for thermal degradation. This finding is accounted for by the decrease in both the initiation energy and the energy of degradation of the polymer chain due to the cationic nature of the active site. The above kinetic scheme characterizes the gross processes of degradation which proceed in the presence of electrophilic catalysts. It may be successfully employed for the description of the gross kinetics of the degradation of polyolefins using catalysts which are active as proton donors. It is necessary to take into account that the rate constants for the reactions between the catalyst and radicals and those for the radical decomposition of the polymer become very small. The depolymerization reaction rate will essentially be determined by the ionic processes of chain initiation and propagation. In this case, the kinetic scheme of catalytic degradation may be represented as follows: P + cat k~° R +
R+
kdep
- initiation both by random chain scission and at end groups.
R + z + Z M - chain propagation.
Ri ÷ P,n ktr~" Ri + Rm + R+ k~or P
- intermolecular chain transfer. - chain termination.
The rate of change of the concentration of the polymer carbocations formed at random may be expressed by the following equation: d[R + ] dt
k,'.[cat] [P] - k~e~[R+].
If the concentration of active sites [R ÷ ] is held constant, we obtain: [R ÷ ]
k~'n[P] [cat] kter
208
s.R. IVANOVA
et aL
For formation of active sites at end groups, the rate of change of the concentration of active sites is given by the equation, d[R + ] = k~[cat]C~ - kter[R+], dt (where Ce is the concentration of end double bonds). Accordingly, for steadystate conditions, we have: [R ÷] =
ki"n[Cat]Ce kter
The equations for the depolymerization rate will assume the following form: Vl = kd,p[R + ] =
kdepki',[P][cat] kter
i/2 = ko~p[R+] _
(random chain scission) kd,pki"n[cat]Ce kter
(depolymerization initiation at end groups) If we assume that the above two processes (random chain scission and end group attack) proceed independently, we obtain the following expression for the gross rate of degradation: V = kd¢p[cat](ki'~[P] + ki'nCe) ktcr Thus, the gross rate of degradation of polyolefins is determined primarily by the initiation processes and, in particular, by the values of the effective rate constants of initiation by random chain scission and by end group attack. This shows the necessity of evaluating the kinetic parameters of the process of initiation via different mechanisms. 3.4. Evaluation of the effective kinetic parameters The catalytic electrophilic degradation of polyolefins (for example, PIB) may be described by a general equation summing the contribution of random chain scission and end group attack: dP = ZekieCco + kirZrP dt
(10)
where P is the mass of polymer present; kit and kie are the rate constants of initiation of the polymer degradation process by random chain scission and end group attack, respectively; Zr and Ze are the kinetic chain lengths for depropagation following random chain scission and depropagation following end group attack, respectively; and C, is the concentration of the end double bonds. If we
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
209
assume that the end groups capable of initiating the depolymerization are consumed exponentially during the degradation process according to a relationship Ce = Ceoexp ( - kie. t) (C~o is the initial content of end double bonds), we have: P
=
kieZeCe°
e -kJr' +
kie -
(e kio, __ e k~rZr').
(11)
kirZ r
The numerical values of kie, Z~, kit and Zr may be determined at the initial stages of the process, when P ~ 1, t ~ 1/kie and t ,~ 1/k~Zr. Experimentally, such is possible when the process is run at the temperatures of 200-300°C, where the rates of chain termination are relatively low. Then, P
=
1 - kirZrt -
kieZetCeo.
(12)
The change in the molecular mass of PIB may be determined from the accumulated number of chain rupture points. The total number of macromolecules is n = no + k j and the number-average molecular mass is determined by the expression: P
1 -
kir Z r t -
no 1 + - -
kie Z e Ceo t
n0 /
1 _ k i r Z r t _ kicZ~C~o t kirt'~ (13) no / k where ~ o = 1~no = 1/Ceo (every primary macromolecule of PIB is assumed to contain one end double bond). The relative degree of polymerization will vary during degradation as follows: ~_ ~ o (
P"
po
=
1 -
k~rZ~t -
k~rP2t
k~eZ~t
po
(14)
Then it is possible to calculate the change in the average degree of polymerization with increasing progress of the degradation reaction de = AP. Referring to eq. (12) we have AP = (kirZ~ + kieZeCeo)t. And as a result we obtain: kie Ze kirZr + kir ]50 + - -
dL/po dot
p,o
-
k~Ze ki~Zr + - -po
(15)
If we assume that the kinetic chain length Zr is many times less than the initial degree of polymerization po, i.e. Zr ~ /50, eq. (15) may be simplified: k~ po + k~cZ____~
d/5o//5o dot
po -
kirZr +
ki~Ze
/50
(16)
210
S. R, IVANOVA et al.
i.o
o
0.8
•
,el
0120
0.10
0.30
FIG. 1. Dependence o f relative degree o f polymerization on the fraction of weight loss ct for polyisobutylene samples with the following initial degrees of polymerization: 1 - 15, 2 - 17, 3 - 20, 4 - 49, 5 - 360, 6 - 1790, 7 - 3570; catalyst - NaA1C14; T e m p e r a t u r e = 473 K.
To simplify the calculation of the values of the kinetic parameters of each of the concurrent reactions, the following extreme cases should be considered: (a) With a low fraction of the degradation reactions proceeding through the end groups, i.e. with kieZe/P ° ~ kirZ, ~ kirP° the expression for the reduced relative degree of polymerization has the form:
dP./L
po -
d~
Zr "
(17)
(b) With a low fraction of the degradation reactions proceeding by random chain scission we have: d~/dt ~- kie Z ~ / P . The value of/5, during such a process (including for the initial most probable molecular mass distribution, (Pw/Pn = 2)) is practically constant, i.e. dP,/P°/d~ ~_ O. This agrees with the experimental data (Fig. 1, curves 1-3). Eq. (16) in this case does not apply since the main assumption made in its derivation - that the kinetic chain length is short relative to/50 _ is not true. (c) Where random chain scission and end group attack are both important, i.e. at kir/50 >)> kieZ¢//5o >> kirZr, we have: dL//Sn
dot
kir ( / 5 ° ) 2 --
kieZe
(18)
The three variants (a), (b), (c) correspond to the three sections on the curve of the calculated value of d/5,//5°/dot vs/50, which have been obtained from the slope tangents of the corresponding experimental curves (Fig. 1). With increasing degree of polymerization, each of the three possible variants (b), (c), (a) is consecutively realized (Fig. 2).
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
IOF
I
-
211
4
4 50 I%C ~
F~
too
Jso
~ /
•I ,.I
I
20
I0
I I
2
5
4
FIG. 2. Variation in d#,//5~/d~ in the process of degradation of polyisobutylene using the catalyst NaAICI 4 at the following temperatures; 1 - 473 K, 2 - 493 K, 3 - 513K, 4 - 533K.
The values of Zr and kir/kieZc may respectively be calculated from the slope tangents of sections (a) and (c) on the d P ~ / P ~ / d ~ vs/s,0 curve (Fig. 2). The value of k~r may be calculated considering that the decrease in the polymer molecular mass occurs mainly due to random chain scission. This corresponds to the dependence dP n/po/dt = - klr/5o, i.e. the value of kir may be evaluated from the slope tangent. The values of Zr and k~r obtained from the experimental data may be used for evaluation of the numerical values of kic and Ze by eq. ( 11 ). The values obtained for the kinetic parameters - the initiation rate constants and the kinetic chain lengths - in the polymer depolymerization, using the example of PIB degradation in the presence of NaAIC14 (Table 5) testify to the prevailing role of end group attack in initiating the catalytic degradation under low-temperature conditions. With increase in temperature in the reaction zone the correlation of the values of k~, amd k~e decreases and under 260°C their values become comparable. The nature of the catalysts used in the polyolefin degradation process has been studied for the alkali and alkaline-earth metal salts NaC1. AIC13 and MgCI2 • AICI3. The complex salt MgC12 • A1CI3 is characterized by a somewhat higher acidity than NaC1 • A1CI3. This factor probably causes the increase in the activity of this catalyst in the polymer degradation process. In the degradation of PIB (Table 5) it has been shown that in the presence of MgCI2 • A1CI3 (in comparison with NaC1 • A1C13), both the total yield of gaseous products and
S.R. IVANOVA et al.
212
TABLE 5. Parameters of the reactions of initiation of the catalytic degradation of polyisobutylene by random chain scission and end group attack using different catalysts Catalyst
NaAICI 4
MgCI2 • AICI3
Ze
kir • 106
Zr
kit/kit
Temperature K
kie. 106
473 493 513
1.6 1.7 1.9
0.2 0.5 i.3
0.2 0.3 0.4
8.1 6.2 4.7
8.1 5.7 4.8
533 473 493 513 533
2.7 3.3 3.4 3.8 5.4
5.7 0.3 0.6 1.4 5.1
0.7 0.3 0.5 0.6 i.I
3.7 8.4 7.1 4.9 3.7
3.8 10.8 7.6 6.4 5.1
Eir
Eic
kJ/mol
k J/tool
60
47
70.7
45.8
that of isobutylene itself are higher. As one may see, the change in the catalyst acidity influences the selectivity of the process in terms of the yield of monomer also. Kinetic studies show that the increased selectivity to monomer is caused by increased contribution of end group attack to the total degradation process (Table 5). The correlation coefficient o f the rate constants of initiation by random chain scission with that of initiation by end group attack in the degradation process using MgC12 • AIC13 is higher than in the case using the catalyst o f lower acidity NaCI • AIC13 (Table 1). However, the correlation coefficient still drops with increasing temperature. The above approach permits one to determine the effective kinetic parameters of the different processes of initiation of degradation not only for PIB, but also for other polyolefins.
3.5. Effect of the polymer structure The laws derived above describe the correlation of the parallel reactions in the degradation of PIB, which is characterized by both a high concentration of end double bonds and by the presence of internal labile bonds. However, other polyolefins contain double bonds not only as end groups, but also at internal positions along the chain. They may also possibly, contain other "defects" (as in the case of polyethylene and butyl rubber). Here, the correlation of the reactions of initiation by random chain scission and by end group attack, in the total picture of polymer decomposition will change. Calculation of the kinetic parameters of the catalytic degradation of PE in the presence o f N a C l • A1CI 3 has shown that the values of the random initiation rate constants are commensurate with those of the rate constants of the sum of the different polymer degradation processes (Table 6). This testifies to the predominant statistical initiation of the degradation process and to the insignificant contribution of reactions proceeding by end group attack.
SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
213
TABLE 6. Values of the gross rate constants and of the rate constant of initiation in the catalytic degradation of polyethylene by random chain scission (NaA1CI4) Temperature, K 573 593 613 633 653
kir"
107, s
0.25 0.21 0.95 1.31 1.71
k • 107, s- i
Eit, kJ/mol
Ea of gross process, kJ/mol
0.93 1.50 2.80 6.90 8.70
79
92
The shape of the curves of degree of polymerization vs fraction weight loss (in particular, a sharp decrease in t5,/P ° even at low extent of weight loss and the independence of the process rate with respect to po) qualitatively confirms the prevailing role of the random initiation reactions (Fig. 3). At the same time, similar data obtained for PIB are characterized by a pronounced dependence of the predominant degradation reactions on the molecular mass of the sample degraded. For instance, the low-molecular-weight polyisobutylenes (M, = 800 to 2000) with a high content of end double bonds undergo polymerization chiefly by attack on end groups, and the relative degree of polymerization changes slowly with weight loss (Fig. 1, curves 1-3). With increasing PIB molecular mass (Fig. 1, curve 4) the concentration of end groups (and end group double bonds) drops accordingly. This results in a marked growth of the relative contribution of random initiation reactions. As the initial degree of polymerization increases further (Fig. 1, curves 5-7), the end group double bond 1.0
0.8
0.6
\
0.4
0.2
I 0.05
" T "°'6
I
O.lO
0.15
I
0.20
I
0.25
FIG. 3. Variation in the relative degree of polymerization P, with weight loss fraction ct for polyethylene samples with different initial molecular mass; 1 - 2 • 03, 2 - 2 . 104, 3 - 6 . 104 , 4 - 1.45- 105 , 5 - 2.5- 105 , 6 - 2.7- 105; catalyst NaA1Cla, T = 633 K.
214
S.R. IVANOVA et al.
concentration drops still more and random chain scission becomes the predominant degradation mechanism. Hence, in the general case one may assert that the respective concentrations of internal labile groups and end group double bonds determine the character of initiation of the catalytic degradation of polyolefins. As a consequence, these factors markedly affect the numerical values of the effective rate constants of initiation. 4. CONCLUSIONS Use of electrophilic complex catalysts increases selectivity to monomer in the degradation of polyolefins (in comparison with the uncatalyzed thermal process). The catalyst acidity promotes depropagation to monomer at the expense of side reactions. The catalytic complexes may be activated by crystal water through formation of the corresponding proton donor compounds - the crystalline hydrates. They may also initiate the polymer degradation process via singleelectron oxidation of the macroradical with formation of the polymer carbocation. Although the specific active sites are different in uncatalyzed and electrophilically catalyzed degradation of polyolefins, in both cases the process proceeds by a mixture of random chain scission and attack on end groups. Initiation of polyolefin degradation by catalysts of medium acidity is determined both by the catalyst nature and by the polymer macrochain structure. The chain depropagation reactions depend on the structure of the particular polyolefin. The evaluation of the effective kinetic parameters permits one to determine the relative contributions of the concurrent reactions of random chain scission and end group attack. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. I0. 11. 12. 13. 14. 15.
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SELECTIVE CATALYTIC DEGRADATION OF POLYOLEFINS
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