Thermal degradation of polyisobutylene: effect of end initiation from terminal double bonds

Po&mer Degradarron and Smhiliry 54 (19%) 33-48 0 19% Elsevier Science Llmited Printed

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Thermal degradation of polyisobutylene: effect of end initiation from terminal double bonds T. Sawaguchi Department

of Industrial

& M. Seno

Chemistry, College of Science and Technology, Tokyo 101, Japan

(Received 25 January

Nihon University, Kandasurugadai,

Chiyoda-ku,

1996; accepted 9 March 1996)

The effect of terminal double bonds on the thermal degradation of polyisobutylene was examined by using a structurally well-defined sample (M.rl = 7.09 X 10’) prepared by hydrogenation. The end initiation reactions from a terminal trisubstituted double bond (TTD) and a terminal vinylidene double bond (TVD) lead to the formation of a primary terminal macroradical (R;) and a tertiary terminal macroradical (R;), respectively. It is expected from kinetic analysis that the extent of occurrence of R; is 2-3 times higher than that of R;, depending on the functionalities flTD and fTVD and M, of the reacting polymer, in addition to the random initiation from the scission of skeletal C-C(C),, and that the stationary ratio [R;]/[R;] becomes slightly lower by hydrogenation. The effect of hydrogenation was found to be a decrease in the overall rate of degradation, owing to a marked decrease in the end initiation rate. However, the integrated concentration ratio [R;]/[R,.], which is estimated from the composition ratio of the interesting components, does not decrease, but increases. This result shows that this concentration ratio scarcely depends on the initiation reactions. A similar relationship between the ratio [R;]/[R,*] and M, of the molten polymer matrix was found for the degradation of samples with and without TVD and TTD. This result supports the view that [R;] decreases markedly with decreasing M, during the degradation, owing to a marked depression of the regeneration rate of R; resulting from an increase in the rate of diffusion-controlled termination. 0 1996 Elsevier Science Limited

NOTATION Primary (p) terminal macroradical Tertiary (t) terminal macroradical Volatile small radical Primary on-chain macroradical near the end formed by the back-biting of R; Secondary on-chain macroradical near the end formed by the back-biting of

R oph2’

R lxbl R oth2’

R,,1.

.

Roi**

(TTD),

R,. Primary on-chain macroradical near the end formed by the back-biting of R,. Secondary on-chain macroradical near the end formed by the back-biting of R,. Primary on-chain macroradical near the

(TVD),

(TTD),

33

center formed by the intermolecular hydrogen abstraction of respective radicals Secondary on-chain macroradical near the center formed by the intermolecular hydrogen abstraction of respective radicals Terminal mono-olefin having a TTD and a t-Bu formed via the back-biting of R; followed by p scission Terminal mono-olefin having a TVD and a t-Bu formed via the back-biting of R;* followed by p scission Terminal mono-olefin having a TTD and a i-Pr formed via the back-biting of R,. followed by /3 scission

34 (TV%

t-Bu i-Pr TTD TVD NT-I’D P SH m 0

T. Sawaguchi, M. Seno

Terminal mono-olefin having a TVD and a i-Pr formed via the back-biting of R; followed by p scission Tert-butyl end group, or polymer having t-Bu iso-Propyl end group, or polymer having i-Pr Terminal trisubstituted double bond, or polymer having TTD Terminal vinylidene double bond, or polymer having TVD Non-terminal trisubstituted double bond, or polymer having NTTD Polymer molecules Volatiles and semi-volatile oligomers Monomer, or its molecular weight Volatile oligomers, or their average molecular weight

1 INTRODUCTION

In this series of present studies we try to elucidate the mechanism of the thermal degradation of polyisobutylene from the correct structural and kinetic analyses of the products.1-7 It was made clear that the degradation proceeds via chain-scission reactions consisting of typical elementary reactions of primary (p) and tertiary (t) terminal macroradicals (R; and R,.) and volatile small radicals S. (see the Notation for meanings of the symbols): that is, (a) initiation; (b) depolymerization (direct /3 scission); (c) intramolecular hydrogen abstraction (backbiting) followed by /? scission; (d) intermolecular hydrogen abstraction followed by /3 scission; and The reactions are strongly (e) termination. affected by the molecular weight of the polymer matrix forming the reaction medium during the degradation.‘v5 The effect of molecular weight is clearly seen in the changes in the composition ratios of the main components in the products.2-5 These ratios can be successfully simulated by a reaction model including diffusion-controlled It would be expected that a termination.6 decrease in molecular weight of the matrix leads to decreases in concentrations of R;, R; and S., owing to an increase in the rate of self-diffusion controlled termination. The increase in the rate of termination with decreasing molecular weight is expected to be in the order; S. >R; >> R,.. Moreover, it was found that the rate constant of end initiation at the terminal double bond is

about lo3 times larger than that of the random initiation by the scission of main chain.’ Assuming the same value for the frequency factor of the scission,’ this difference is estimated to correspond to that of about 32.8 kJ mall’ in the activation energy, which is roughly consistent with the dissociation energies of allylic C-C bond and the skeletal C-C(C), bond.8 The terminal double bonds are formed by the p scission of the main chain and their concentration becomes larger as the molecular weight decreases.e Thereby, the end initiation rate of degradation increases and the radical concentration increases as the reaction proceeds. However, as described above, the marked decrease in the stationary concentration of respective radicals is caused by an increase of disappearance rate by the termination reactions larger than that of formarate by the initiation during the tion degradation.“,‘.’ In this paper, the effect of terminal double bonds on the degradation behavior is examined based on the detailed results of the thermal degradation of the structurally well-defined polyisobutylene.

2 EXPERIMENTAL 2.1 Sample, apparatus

and procedures

The polyisobutylene sample and the experimental procedure are described in detail elsewhere.’ The molecular weight characteristics of the purified polyisobutylene are as follows; M, = 2.5 x lo5 and M,JMn= 2.50. A polyisobutylene sample (M,” = 7.09 X 10’) was prepared from the polymer residue obtained by the thermal degradation of the original polyisobutylene, and hydrogenated.” Its characteristics are shown in Table 1.” This table shows clearly that the thermally prepared polyisobutylene sample has functional groups such as t-Bu, i-Pr, TVD, TTD and NTTD, and that TVD and TTD were completely converted to i-Pr by hydrogenation. The hydrogenated sample has no terminal double bonds but is slightly NTTD and the functionality The LB” remains unchanged by hydrogenation. functionalities of respective functional groups are explained elsewhere.One gram of the sample was used for each degradation experiment at 300 or 320°C. After the degradation reaction, the volatile oligomers

35

EfSect of terminal double bonds on thermal degradation Table 1. Characteristics of molecular weight and chemical structure of samples of polyisobutylene and its hydrogenate Functionality’

Composition’. mol%

M,,;’ X lO-3 2~ M h

4

7.09’ 7.0’)“’ ____ _.

1.60 1.6’) ~~

[i-Prl”

[t-Bu]’

[TTDI’

[TVDI’

[NnDlh

I.00 74.21

15.43 17.41

50.84 0.00

20.44 0.00

12.29 8.38

L,

/I-au fmo

0.023 0.352 1.16 1.6200.380 0.00

f,,,,

L7.1.1~ J?

[t_Bul [i - Pr]

LEEl [TVD]

15.4 0.23

2.49

‘-I&~,~“1

fT!+

.r:.

0.466 0.280 1.63 1.91 O.OW 0.183 0.00 0.18

3.0’)

-

4.14 ILCW, ._

” By limiting viscosity number measurements. ’ Heterogeneity index of molecular weight distribution detrrmmed by GPC measurements. ' l(W) x [each CH, peak intensity/total CH,a peak intensity (i - Pr + t - Bu + TTD + TVD + NITD)]. ” Iso-propyl: (CH,),CH-. “Tert-b;tyl; (CH,),C - ’ Terminal trisubstitutrd double bond: (CH,),C = CH-. ‘Terminal vinylidene double bond: CH, = C(CH,)-. ‘I Non-terminal trisubstituted double bond: -(CH,)C = CH-. ’ Average number of each functional group per molecule; f = 2 X [each functional group peak intensity]/[rotal terminal peak intensity (I ~ Pr + t - Bu + TTD + TVD]. ’ Average number of terminal double bonds per molecule:f = 2 X ([TTD] + [TVD])/([i-Pr] + [t-Bu] + [TTD] + [TVD]). * Average number of total doublr bonds per molecule; 1;, = 2 X ([‘ITD] + [TVD] + (NTTD])/([i-Pr] + [t-Bu] + [TTD] + [TVD]). ’ Oligomer samples prepared by thermal degradation of polyisobutylenr (M, = 2.50 x IO ). “I Hydrogenated polyisobutylene prepared from the polyisobutylene sample (M, = 7.09 X 10).

recovered into a liquid fraction trap chilled with liquid N, were dissolved in acetone for analysis. The polymer residue in the reaction flask was and the dissolved in 10 cm3 of chloroform solution was reprecipitated by dropping into SOcm” of acetone to remove a small amount of the semi-volatile oligomers with a relatively low volatility. The reprecipitates were termed the nonvolatile oligomers and analyzed after vacuum drying under heating. The volatilization which represents the rate of formation of volatiles was obtained from the following equation: 100 x (weight of sample - weight of polymer residues)/ (weight of sample). 2.2 Analysis GC of the volatile oligomers was recorded on a Shimadzu GC-8A gas chromatograph equipped with a flame-ionization detector and a fused silica capillary column (50 m X 0.35 mm id.) containing OV-1. The instrumental conditions are described elsewhere.2 The composition ratio for respective oligomers is represented by a relative ratio of peak intensities, which was measured with a digital integrator without calibration. TG and DTG curves were obtained on a Seiko TG/DTA 220 in a stream of N, gas. Conditions of measurement were as follows: sample weight, ca. 10 mg; temperature, initially 3O”C, programmed at 25.O”C/min up to 2OO”C, and at S.O”C/min from 200 to 5OO”C, and then maintained for 5 min. The 400 MHz ‘H NMR spectra were measured with a Jeol JNM-GX400 spectrometer operating at 399.65 MHz and at room temperature with an internal lock. The sample concentration was approximately lO%(w/v) in chloroform-d,. Tetramethylsilane (TMS) was used as an internal

standard and a 5 mm-diameter sample tube was used. Spectral width was 4.5 kHz and 65536 data points were accumulated in a JEC 32 computer. In the quantitative measurement,4 a pulse angle of 90” (approximately 11.7 ps) and a pulse repetition time of 37.281 s were adopted. A typical measurement was performed for about lo-45 h. The signal intensities of the spectra were measured by a weighing method. The compositions of functional groups were determined from the intensities of signals of methyl protons.4 The molecular weight distribution (M,/M,) was measured by an analytical GPC (Toyo Soda HLC-802 UR) using a stainless-steel column of TSK-GEL (2eHMG6 + H4000HG8 + H2OOOHG8). The data were calibrated with standard polystyrenes. The number-average molecular weight M,, was determined by the following equation’ using the limiting viscosity number measured at 30°C in toluene: [q] = 3.71 X 10m4 P”.“, where [T] is the limiting viscosity number (1g-l) and P is the number average degree of polymerization determined by the osmotic pressure method.

3 RESULTS AND DISCUSSION 3.1 Reaction process for formation of main products In the preceding papers,‘.4.6.7 we have proposed reasonable elementary reactions for the formation of the main products, which consist of the volatiles including volatile oligomers and the polymer residue including nonvolatile oligomers. These products are formed via the reactions of R;, R; and S. in the depropagation step. These radicals are firstly formed by two types

36

T. Sawaguchi,

M. Seno

of initiation reactions as follows:

cH,

CH, -c-cH,-C-CH,

H,C=C-CH,

CH,

I CH,

CH,

CH,

CH,

-c-CH,-C-CH,

H,C-C=cH

; (TTD)

C%

I

-CH?-c-cH2-c-cH2-~

k

kie

)

So + R,*

(1)

S*+Rp*

(2)

I CH,

I CH3

CH,

CH,

; (TVD) A

CH,

I

I

Equations (1) and (2) are the end initiation reactions at allylic bond from TVD and TTD, respectively. The former reaction gives Se and R,., and the latter leads to Se and R;. On the other hand, eqn (3) is the random initiation reaction by the scission of skeletal bond of the main chain and gives R; and R,. with equal ratio 5,6,10.1I The volatiles consist of monomeric commainly isobutylene,7”0 and volatile pounds, oligomers ranging from dimers to dodecamers.’ Isobutylene monomer is formed by depolymerization (direct p scission) of R; and The reaction processes for formation of R,.. 7,8~‘o~” the volatile oligomers and the nonvolatile oligomers are shown in Scheme 1. The volatile oligomers consist mainly of four types of mono-olefins (TTD),, (TVD),, (ITD), and (TVD),, which are formed by the intramolecular hydrogen abstraction (back-biting) of R; and R,. and the successive /? scission at the inner position of the main chain.‘,8,‘0,” Depending on the position of the hydrogen abstraction, (TVD), and (TTD), are formed from R; (eqns (4) and (5)), and (TVD), and (TTD), from R,. (eqns (6) and (7)), respectively. The functional groups, t-Bu, i-Pr, TVD, ‘ITD and NTTD, of the nonvolatile oligomers are formed by the elementary reactions given by eqns (S)-(16);” that is, the intermolecular hydrogen abstraction of respective radicals gives two types of on-chain macroradicals, R,,,,. and Roi2., depending on the position abstraction, and CH,) of hydrogen (CH,

CH, k -

CH, - lr

R;

+ R;

(3)

I

independently of radical type (eqns (8)-(13)). On the other hand, the hydrogen abstraction of R;, R,. and So yield t-Bu, i-Pr and SH (volatiles), respectively (eqns (8-13)). The /3 scission of Roil* occurs only at the main chain and results exclusively in the production of TVD and R; (eqn (14)). The j3 scission of Roi*. produces TTD and NTTD and methyl radical, and R;, respectively, depending on the position (main chain and side methyl group) of the scission (eqns (15) and (16)). Thus, a feature of the elementary reactions in the depropagation step is to regenerate R; by successive p scission after the back-biting and the intermolecular hydrogen abstraction of respective radicals. Termination reactions are given as follows, k

2Rp

PI 2

b

l

k tt 2 2R,* ___)

Rp*

S

P (or 2P)

P(or2P)

kptt

+ R;

( 17)

(18)

P(or2P)

(19)

k l

(melt phase) \tY

S* (gas phase)

(20)

Equations (17)-( 19) represent bimolecular termination reactions of respective macroradicals and eqn (20) is a quasi-termination by vaporization of Se. It was made clear in a preceding paper6 that these reactions are controlled by self-diffusional motion of respective radicals.

37

Effect of terminal double bonds on thermal degradation Intermolecular

hydrogen

abstraction

(Back-biting)

R opbl’

-

R opb2’

-

R otbl’

-

R,.

RolbZ.

-

k \pb?

U-WI,

+

U’TD),

t

(TVD),

t

(TTD),

+

RP.

RP.

(4)

(5)

RP.

(6)

-

1oL,,Z Intermolecular

hydrogen

k \tbZ

(CH,)kp,

+

RP.

(7)

abstraction

I

I Rp.

k \pbl

p scission

k \Ibl

cCH,Jk,b, *

and subsequent

*

t - Bu

+

R,,,,.

(8)

)

t - Bu

+

Ro12.

(9)

i-Pr

+

R,,,.

(IO)

i - Pr

+

Ra12.

(II)

SHk

+

Rw,.

(12)

SHI

+

RoiZ.

(13)

TVD

+

Rp.

(14)

TTD

+

Rp.

(15)

NTTD

+

CH,.(S;)

(16)

PH ___/

(CH2)kpi2

KH&, R,.

+

PH +Z-:

(CH3)k,i,

s’

+

Subsequent

Scheme

1. Reaction

PH

+z-:

f3-scission

process for formation of four types of terminal mono-olefins in the volatile oligomers functional groups of the nonvolatile oligomers.

3.2 Molecular weight dependence of respective elementary reactions The rates of end initiation reactions (eqns (1) and (2)) depend on the concentrations [TVD] and [TTD], which are equal to [PI& and [P]f&,, respectively.[P] is expressed by p/M.‘2~‘” The density p is almost constant during the degradation, because the specific volume of polyisobutylene at 217°C is nearly constant in a

and respective

range of molecular weight A4 from 3540 to 115000.‘4 On the other hand, the random initiation (eqn (3)) and the intermolecular hydrogen abstraction (eqns (8) (10) and (12)) occur at any position of the main chain and, therefore, [P] should be replaced by the concentration [N] of monomer unit of the polymer. [N] is expressed by ~/vz’**‘~ and its value is also set to be nearly constant, independent of the value of M of the polymer matrix.

38

T. Sawaguchi, M. Seno

Under these conditions, the rate equations of the reactions (l), (2) and (3), (8) and (9) (10) and (ll), and (12) and (13) are expressed as follows, respectively: KC!= ki,fPlM

(21)

V* = k,,[Nl

(22)

Vpi= k,i[R,*][N]

(23)

K = kli[R,*][N]

(24)

6.i

=

ksi[S’][N]

(25)

Thus, the rate of end initiation is inversely proportional to M, and the rate of random initiation does not depend on M. On the other hand, the rate of intermolecular hydrogen abstraction depends only on the concentration of respective radicals. The rate equations of termination reactions (17)-(20) are written as follows: VP,,= ~,,[R;l*

(26)

K,, = MRt.12

(27)

&dR;IRl

(28)

vl,,”= kt”[Sq

(29)

VP,,,=

If the termination reactions (17)-(19) are activation-controlled, the radical concentrations would increase with a decreasing M during the degradation, owing to the molecular weight dependence (M-‘) of the end initiation given by eqn (21).’ However, this is inconsistent with the experimental results.‘*5l6 In fact, the rates of bimolecular terminations (eqns (17)-(19)) are controlled by diffusional motion of terminal and the rate constant k, is macroradical@ governed by the frequency of encounter of two macroradicals” and, therefore, the diffusion coefficient D, of macroradicals, which is related to some power of molecular weight; that is, k, = 4nRDN,

= KM-”

(30)

where R is a radius of reaction sphere and N, is Avogadro’s constant. The value of 2.1 was assigned for the exponent LZ.~,‘~‘~ Similarly, if the rate of vaporization of Se (eqn (20)) is also controlled by diffusional motion of Se, the rate constant k,,, could be governed by the diffusional process through the matrix.6 Thus, in terms of the diffusion coefficient D, of Se, k,,, = 4rRD,N,

= K, M-h

(31)

The molecular weight of Se is smaller than that of the matrix and, thereby, its diffusional motion may be similar to the tracer diffusion rather than the self-diffusional motion of macroradicals.‘h-‘X The value of 2.0 was assigned for the exponent b. 6.16IX

3.3 Kinetic expression of main components of the product It was verified in the previous papers,3.’ that hydrogen abstraction followed by p scission occurs under steady-state conditions where the concentrations of respective on-chain macroradicals are kept low and constant and, therefore, the rate of these reactions could be represented by only the rates of the back-biting (eqns (4)-(7)) and the intermolecular hydrogen abstraction (eqns W-(13)). For the volatile oligomers,’ the molar concentrations of the mono-olefins formed for a given reaction time are given as follows: [TVD], = j- k,,, [R;]dt +

[wDl,u

= k,,/,,[&,.l

(32)

P-W, = j k,,,tR,W + [TTDl,o = kph&*l

(33)

[TVDI,= j- k,,[R,*ld~+ [TV% =

kth,[R,-1

(34)

[J-J-D],= j kh,[R,-]df + P-W,, = k/&-l

(3%

where [RIP01= 1

K~ldt and [R,.] = 1 [R;]dt

Accordingly, the molar ratios of the mono-olefins formed are represented by eqns (36)-(39), respectively:

[~Dl, _ b2[Rp*l b2 (TVDI,- kpdRp.l- k,,,

(36) (37)

39

Effect of terminal double bonds on thermal degradation

[TVDl, _ kpbl[Rp-l

[TVDI, - ‘h, k-1

(38)

[TTDl, _ kpb2[Rp-l --

(39)

[WD],

km[Rt’]

Thus, the ratios of the rates of abstraction of different types of hydrogens (CH, and CH3) from the same type of macroradicals, [TTD],/TVD], and [TTD],/[TVD],, are given only by the rate constant ratios without the terminal macroradical concentration ratio, as shown in eqns (36) and (37). These ratios should remain unchanged during the degradation. The ratios of the rates of abstraction of the same type of hydrogen from different macroradicals, [TTD],/[TTD], and are expressed by a product of WDl,/]TVDl,, the rate constant ratio and the macroradical concentration ratio ([R;]/[R;]), as shown in eqns (38) and (39). Thus, a change in observed values of these composition ratios reflects the change in the ratio of macroradical concentrations during the degradation. On the other hand, the molar concentrations of functional groups of nonvolatile oligomers are given as follows:’ [t - Bu] = (k,i, + kp12)[Rp*][N]+ [t - Bu], [i - Pr] = (k,i, + k,,J[R,*][N] + [i - Pr],

(40) (41)

[TVD] = (k,i,[R,*] + k,il[R,-] + k,,,[S*])[N] f

[TW,,

(42)

k [TTD] = (k,,z +s;riLm) (kdRd + k,Z[Rt*] + kvi,[S*])[N]

[NTTD] =

+ [~DIo

(43)

(k,i,y;si2m) (k,i2]Rp-1

+ k,,,,[R,*I + k .i,[S*I)]N] + [NTTDI”

(44)

Accordingly, the molar ratios between respective functional groups are given as follows: [t - Bu] = (k,i, + kp~2)[Rp*I[Nl + [t - BUIO(45) [i - Pr] (k,,, + kd[R*l[Nl + [i- Prlo

(Pw

+ wm) [TVDI (kpi,[Rp*] + kt,[R*]+ Rwz[S*])[N] + [T-fD],+ [NnDlo = (kpil[Rp*] + ktil[Rt*l + kvi,[S*])[N] + [TVDI” (46)

[TTD] _ Lz

[NmDl

1471

\ “/

k2m

The ratio [t-Bu]/[i-Pr] is expressed by a product of the ratio of the rate constants (sum) and the macroradical concentration ratio ([R,*]/[R,*]), when the initial values are neglected, as shown in eqn (45). Thus, a change in the observed value of this composition ratio reflects the change in the ratio of macroradical concentrations during the degradation. On the other hand, the ratio ([TTD] + [NTTD])/[TVD], which is related to the double bond distribution, corresponds to that between the abstraction rates (sum) of different types of hydrogens (CH, and CH,) of R;, R; and S.. The ratio [T’TD]/[NTTD] is given only by the rate constant ratio of competitive p scissions at different positions of Ro,2.. 3.4 Molecular weight dependence

of radical

concentrations

The effect of molecular weight of the matrix on the reaction was elucidated from the results of the thermal degradation of the original polyisobutylene (M,,, = 2.50 X 10’) and six samples of polyisobutylene with different molecular weights.3.5 The ratios [TfJDJ,/[TVD], and [TTD],/[TVD], of the volatile oligomers remain constant, independent of the molecular weight of the molten polymer matrix. Evidently, the experimental result shows that the rate constant ratios of eqns (36) and (37) are kept constant during the degradation, and this result confirms the validity of the assumption of the present kinetic analysis; that is, the back-biting is of a uni-molecular type and its rate depends only on local motion of the reacting end, independently of the molecular weights of the matrix and the reacting molecule itself.’ On the other hand, the observed values of ([TTD] + [NTTD])/[TVD] increase gradually with increasing time of degradation. It is deduced from the kinetic expression given by eqn (46) that the ratios between the abstraction rates of the same type of hydrogen (CH, or CH,) of respective radicals differ from one another. Although the ratio [‘fTD] I [NTTD] corresponds to the rate constant ratio between p scission rates of Ro,2. (eqn (47)) the observed values decrease gradually with reaction time. On the other hand, both the values of and [TTD],/[TTD], of the PJDI,IITW,

40

T. Sawaguchi,

volatile oligomers (eqns (38) and (39)) and the value of [t-Bu]/[i-Pr] of the nonvolatile oligomers (eqn (45)) clearly decrease with decreasing molecular weight during the degradation. These results demonstrate that the decrease in molecular weight of the matrix leads to a decreasing concentration ratio of [Rp*]/[R,-]3-5The molecular weight dependence of the ratio [R;]/[R,-] could be estimated from the composition ratios [TVD],/[TVD],, [TTD],/[mD], and [t-Bu]/ [i-Pr], and expressed as follows:3.5

pPJ (y P--w, [R,.]

(y

F-w,

[TVD],

[mD]t

&laP-B4CyMn [R,.]

[i - Pr]

*

cy

M. Seno

reasonably interpreted by a reaction model consisting of the radical chain reactions of R;, R; and Se. The marked decrease in radical concentrations does not result from a weaker molecular weight dependence (M- ‘) of the rate of end initiation, but from a stronger molecular weight dependence (M-‘) of the rate of diffusion-controlled termination with a decreasing n/l, during degradation.’ 3.5 Degradation of polyisobutylene with different end group distributions

M”

n

(48) (49)

The observed value of exponent IZ is ca. 1.04 at 300°C and ca. 0.88 at 320°C for volatile oligomers, on average,3 and 1.18 at 300°C and 0.72 at 320°C for nonvolatile oligomers.’ Thus, the value of y1observed for the volatile oligomers is roughly consistent with that for the nonvolatile oligomers. A set of rate constants could be determined by fitting the observed values of many variables such as molar concentrations and their ratios, and functionalities of respective components to the calculated values. The molecular weight dependencies of concentrations of respective radicals are given as follows: [R;] (YM;

(50)

[R,.] a M:

(51)

[Se] (YM:

(52)

The values of the exponents were determined from the observed values of respective radical The II value for [R;]/[R;] concentrations.6 corresponds to a difference between the o and p values for [R;] and [R;]. In the integrated ratio [R;]/[R,-1, the n values observed for the nonvolatile and the volatile oligomers agree with the calculated values. The order of the molecular weight dependencies is as follows: [Se] > [R;] >> [R,.]. It is clear that marked decrements of [R;] and [Se] are caused by depression of the regeneration rate of R; and S* in the depropagation step. This is due to a larger decrease of kinetic chain length (KCL) with an increasing rate of termination.’ On the other hand, [R,.] decreases only slightly in spite of a large decrease of KCL. These results could be

TG and DTG curves of polyisobutylene samples before and after hydrogenation (Table 1) are shown in Fig. 1. The temperature of onset of weight loss of the thermally prepared polyisobutylene sample is somewhat lower than the hydrogenated sample, owing to a larger rate of end initiation (eqns (1) and (2)) at the thermally labile bonds from TVD and TTD than the random initiation (eqn (3)) at the skeletal C-C(C), bond. The results of thermal degradation of these samples are shown in Figs 2 and 3, respectively, where the volatilization and M, of nonvolatile oligomers are plotted against reaction time. The effect of hydrogenation is found clearly at 3OO”C, where the volatilization decreases and M, increases on hydrogenation. Since the hydrogenated sample has not TVD and TTD, this result is due to a decrease in the rate of end initiation reactions (1) and (2). For this reason, the stationary concentrations of respective radicals decrease and, therefore, the overall rate of the

25

z ._ E 2

100

0

_;j % 3 S

:

-25

:

------

I

-50

Fig.

Thermally degraded polyisobutylene Its hydrogenate

100

1. TG polyisobutylene

and

I

I

200

300

400

Heating

temperature

DTG

curves

and

its hydrogenate

of

(“C) thermally (M,,,

=

prepared

7.09X I@).

Effect of terminal double bonds on thermal degradation

: : 320 “C

4 : 300 “C

40 t

0

30

60

90

Time (min)

Fig. 2. Changes in volatilization with the thermal degradation of polyisobutylene at 300 and 320°C before (open marks) and after (filled mark) hydrogenation.

degradation reaction decreases on hydrogenation. At 320°C the effect of hydrogenation is found only at an earlier stage of reaction before 30 min, owing to an increase of the overall rate of

a t

.L

0

4 : 300

I 30

oc

:

I 60

: 320 “C

I 90

Time (min)

Fig. 3. Changes in M, of the nonvolatile oligomers with the thermal degradation of polyisobutylene at 300 and 320°C before (open marks) and after (filled marks) hydrogenation.

41

degradation reaction started from the end initiation in addition to the random initiation. Changes in the composition ratios of interesting volatile oligomers obtained by the degradation of polyisobdtylene before and after hydrogenation are summarized in Tables 2 and 3, respectively. Figures 4(a) and (b) show changes in the composition ratios for trimers with the reaction time for the degradation at 300 and 320°C respectively. The ratios [TTD],/[TVD], and [TI’D],/[TVD], h ave different values from trimers to heptamers, but remain nearly constant during the degradation and unchanged by hydrogenation. This result agrees fairly well with the kinetic expectation (eqns (36) and (37)). On the other hand, the observed values of the ratios and [TVD],/[TVD], given by [~Dl,/ rmt eqns (38) and (39) are different for respective oligomers. Although a marked change on hydrogenation is not found at 320°C these ratios decrease clearly with reaction time. At 300°C these ratios become higher after hydrogenation, although a decrease in initiation rate results in a decrease in regeneration rate of R,; in the depropagation step.3.3.6 Table 4 shows changes of compositions and functionalities of the nonvolatile oligomers obtained under the same conditions as Tables 2 and 3. As shown in Table 4, for the thermally prepared polyisobutylene (part l), &+ increases, f TVD and jr; decrease, and &,,, fNTTD and f;& change in a complicated way with the degradation time. For the hydrogenated polyisobutylene (part 2), f&,, fTVD,h and A, increase markedly with decreasing M, during the degradation, owing to an increase in the average number of scissions (MJM, - 1) of main chain. However, fl_,+ slightly decreases and h-B” is almost unchanged. Table 5 shows changes in the composition ratios between respective functional groups with the reaction time. For the double bond distribution, the observed values of the ratios ([TTD] + [NTTD])/[TVD] and [TTD]/[TVD] are not changed by hydrogenation at both temperatures. This shows that the rate ratios hydrogen (eqn (46)) of th e intermolecular abstraction of respective radicals followed by p scission depend not only on the stationary radical concentrations but on the initial double bond distribution. Moreover, the observed value of the ratio [TT’D]/[TVD] means that the rate of end initiation from TTD (eqn (2)) is 2-4 times larger than that from TVD (eqn (1)) and, therefore, the

T. Sawaguchi, M. Seno

42 Table 2. Changes

in composition

ratios

of volatile oligomers with the thermal (IV”= 7.09 x 103) (Part 1)

Temp. (“C)

Time (min)

M,” X lo-’

Volatilization wt%

degradation

______.~__

Trimers

WDI, 300

60

3.71

16.0

2.14

320

90 90 1.5 30 60

3.68 3.30 4.46 3.99 2.92

22.1 19.1 14.4 24.6 31.0

2.14 2.15 1.97 2.19 2.23

Tetramers

[TTDI, [TV’& V’VDI, PDI,

[TTDI,

of polyisobutylene

I-rrDl, WDI,

1.14

12.4 13.4 12.6 14.6 12.4 I1 .x

6.61 1 .09 6.80 1.Ou 5.w 1.21 x.95 1.12 6.38 1.10 5.80 ______~~

[TTDI, [TVDI,

P”-Dl, P’DI,

W’DI, VW,

[TTDI, P’DI,

4.56 3.50 3.88 3.46 3.91 3.6X

4.29 4.61 4.42 4.35 4.27 4.07

1.47 I .56 1.91 I .45 1.6X I .47 1.77 1.41 1.55 1.42 1.41 1.40 ~.~.____~

~

(Part 2) Pentamers

Temp (“C)

300

320

-. I’ By measurements

.___

Hexamers

[TTDI,

m

[TVDJ,

ITTDI,

[TTDI,

m

LTVDI,

[TTDI,

[TTDI,

[=Dl,

PDl,

P’DI,

WDI,

P-V,

WDI,

[TVDI,

PDI,

WDI,

P’DI,

PDI,

P’DI, P’DI,

2.06 2.27 1.66 2.23 2.06 1.96

1.26 1.60 1.44 1.53 1.18 1.20

2.42 2.31 2.18 2.06 2.15 2.09

1.43 I.28 1.38 1.32 1.34 1.26

2.26 2.17 2.20 2.01 2.0X 2.05

I .30 1.35 1.44 1.12 1.45 1.30

1.X3 2.13 2.2x 2.11 2.10 1.89

of limiting

viscosity

Table 3. Changes

2.28 2.76 3.88 2.73 2.33 2.62

3.72 3.91 4.48 3.99 4.05 4.26

.~~

Heptamers

4.05 3.71 3.6X 4.19 3.43 3.56

4.27 2.05 2.34 2.67 2.14 2.14

[TTDI, WDI, 3.20 3.42 3.48 3.78 3.02 3.00

number.

in composition

ratios

of volatile oligomers with the polyisobutylene (M. = 7.09 x l@)

thermal

degradation

of hydrogenated

(Part 1) Temp.

(“C)

300

320

Time (min)

60 90 90 15 30 60 90

M,” x 10m3

4.21 4.27 4.17 6.75 3.68 2.74 2.90

8.4 12.5 12.0 7.6 18.4 31.5 35.7

Tetramers

Trimers

Volatilization wt% [TTDI,

[TTDI,

[TVDI,

[TTDI,

m

ITVDI,

P’DI,

P’DI,

P’DI,

[TV”],

1.17 1.17 1.07 1.24 1.19 1.14 1.18

9.52 7.41 7.91 10.5 6.7X 5.35 5.83

17.2 14.2 16.4 19.6 12.9 10.7 10.7

I .69 1.x1

2.12 2.24 2.21 2.31 2.27 2.29 2.17

VDI, P’DI,

P’DI, IT=‘],

[TTDJ, VDlt

5.52 4.04 3.93 5.92 4.64 3.42 3.17

6.06 5.17 5.40 6.64 4.92 3.67 3.57

1.54 1.42

1.98

I .44

1.79 1.58 1.54 2.01

1.59 1.49 1.43 1.79

(Part 2) Pentamers

Temp. (“C)

300

320

“By measurements

Heptamers

Hexamers -

[TTDI, P’Dln-.

I-rrDl, P-VW,

[TVD1, WDl,

[TTDI, P-W,

[TTDI, P’DI,

2.02 2.18 2.03 2.19 2.10 1.90 2.35

1.70 1.63 1.63 1.x1 1.61 1.36 1.68

4.11 3.00 3.75 4.27 3.21 2.57 1 .x3

4.89 4.01 4.65 5.17 4.19 3.59 2.56

2.46 2.11 2.16 2.36 2.23 2.04 1.94

of limiting

viscosity

number.

E P-W, 1.68 1.30 1.47 1.48 1.52 1.28 1.67

[TVDI,

m

m

m

ITVDI,

WDI,

WDI,

W’DI,

2.74 2.66 3.06 2.82 2.51 2.01 2.10

4.03 4.32 4.47 4.52 3.69 3.19 2.43

2.34 2.05 2.15 1.74 2.08 2.02 1.51

1.58 1.45 1.54 1.17 1.50 1 .Ol 1.04

[TVDI, [TTDI, PDI, U-W, ________~ 2.33 2.X4 2.90 3.05 2.47 I.38 1.96

3.44 4.00 4.06 4.54 3.43 2.77 2.84

43

Effect of terminal double bonds on thermal degradation (a)

ratio [R;]/[R,*] has a value of 2-4. The observed value of [TTD]/[NTTD] of the hydrogenated polyisobutylene is slightly lower than that of the thermally prepared polyisobutylene and increases gradually with time. On the other hand, the observed value of the ratio [t-Bu]/[i-Pr] deand remarkably on hydrogenation, creases increases slightly with degradation. Thus, the observed values of compositions [t-Bu], [i-Pr], [TTD], [TVD] and [NTTD] include the contribution of larger initial values ([t-Bu],,, [i-Prlo, [TTD],, [TVD], and [NTTD],), as shown by eqns (40)-(44). The concentrations of functional groups formed by the reactions (8)-(16) are evaluated in the next section.

300 “C l

0 0:

TTD p hTd

A _ TWj AmITVD q

.

: TTD ’

fTd sp f : TTD t Id 1,

0

.

. . A

A

A

3.6 Effect of the terminal double bond 8

8

a I

I

30

60

8 90

Time (min)

20 (b;

320 “C

z: TTD p

hd

A. TVD; . . tTva

IY

0: TTD .

hd

$:

TTD t kvd 1,

fp

0 0

a

As described in the preceding section, the molecular weight dependence of the ratios [R;]/[R,*] could be estimated from the molecular weight dependencies of respective radical concentrations obtained by the simulation.’ Figures 5(a) and (b) show plots of the composition ratios for trimers against M, of the nonvolatile oligomers obtained by the degradation of thermally prepared polyisobutylene and its hydrogenate at 300 and 32O”C, respectively. The ratios [TTD],/[TVD], and [TTD],/[TVD], remain constant, but the ratios [TTD],/[TTD], and [TVD],/[TVD], decrease with decreasing M,. Similar relationships between these ratios and M, were obtained for the thermally prepared sample and its hydrogenate at both the temperatures, despite different degradation behaviors before and after hydrogenation (Figs 2-4). The concentrations (mol/cm’) of respective functional groups are expressed as follows:4-h [t -

4

0 0

I

I

I

30

60

90

Time (min)

.. ^^ . . Fig. 4. Changes m tne composltlon ratios ot tour termmal mono-olefin trimers with the thermal degradation of polyisobutylene at (a) 300 and (b) 320°C before (open marks) and after (filled marks) hydrogenation.

= /&u/M

(53)

Prl= [f’]_L = p.LlM

(54)

D”W = [J’l.fQ-m = pfm”IM

(55)

WDI = [PlfTw = pfTv,lM

(56)

[NmDl= [PlfNm = pf~n,,IM

(57)

[i -

P

W = [PIL

Assuming 0.75 g/cm’ for the value of p, the molar concentrations given by eqns (53)-(57) could be calculated from the observed values of functionalities and M, (Table 4). Plots of the calculated values of the molar concentrations of respective functional groups at various degradation

1.60 1.62 1.89 1.81 1.52 1.49 1.83

MWh r ”

1.69 1.69 1.80 1.85 1.65 1.53 1.68 1.52

7.09 3.71 3.68 3.30 4.46 3.99 2.92

M,,” x1O-j

7.09 4.21 4.27 4.17 6.75 3.68 2.74 2.90

0 60 90 90 15 30 60

Time (min)

0 60 90 90 15 30 60 90

300

Temp. (“C)

300 0.68 0.66 0.70 0.05 0.93 1.59 1.44

M”0 K-1’

0.91 0.92 1.15 0.59 0.78 1.43

M,,, M-1’ n

of respective

groups,

15.43 15.07 19.67 18.39 15.04 16.23 18.98

1 .oo 2.62 3.76 3.34 1.99 2.34 3.70

[T-TDI” 0.00 22.77 28.71 27.74 21.67 31.03 37.40 37.03 ~_________..

[t-Bu]’ 17.41 15.44 17.63 18.07 15.11 15.45 18.10 15.10

74.21 44.07 34.36 37.67 45.05 33.12 22.27 26.38

0.00 8.23 9.72 9.86 8.38 10.19 11.02 9.75

[TVD]”

mol%

[i-Pr]’

Compositiond,

polyisobutylene

20.44 17.82 16.55 17.74 19.23 16.79 15.61

50.84 51.57 48.60 48.87 51.11 50.95 49.83 _

[TVDJ”

mol%

[T-fDl”

(Part 2) Hydrogenated

(t-Bull

Composition”, [i-Pr]

nonvolatile

prepared polyisohutylene

double bonds of the (&& = 7.09 x 105)

(Part 1) Thermally ___ _

end

8.38 9.50 9.57 6.66 9.79 10.22 11.21 11.75

0.352 0.346 0.444 0.416 0.344 0.376 0.431

f;-Bu

0.466 0.409 0.374 0.402 0.440 0.389 0.354

fTw

0.280 0.297 0.258 0.264 0.289 0.317 0.270

J&r,,

Functionality’

1.16 1.18 1.10 1.11 1.17 1.18 1.13

f+rn

1.620 0.974 0.760 0.807 0.999 0.738 0.502 0.598

0.380 0.341 0.390 0.387 0.335 0.344 0.408 0.342

0.00 0.50 0.64 0.59 0.48 0.69 0.84 0.84

0.000 0.182 0.2l.5 0.211 0.186 0.227 0.248 0.221

0.183 0.210 0.212 0.143 0.217 0.228 0.253 0.266

0.00 0.69 0.85 0.81 0.67 0.92 1.09 1.06

J;”

1.63 1.59 1.47 1.51 1.61 1.57 1.49 ~~.

f;”

0.18 0.89 1.06 0.95 0.88 1.15 1.34 1.33

fin’

-__

1.91 1.89 1.73 1.77 1.90 1.89 1.75

f;n

__.

of polyisobutylenes

Functionality’

degradation

i-p, f;Lu f-w frvn fNm

0.023 0.060 0.085 0.076 0.046 0.054 0.084

12.29 12.93 11.41 11.66 12.63 13.69 11.88

PnDl’

J-p,

by thermal

[N’f-TDl’

oligomers

CH, signal intensity ” By limiting viscosity number measurements. ’ By GPC measurements. ” Average number of scission. ” 100 X [each CH, peak intensity/total 7 Terminal trisubstituted double bond: (CH,),C = CH - . ’ Terminal (i-Pr + t-Bu + TTD + TVD + NTTD)]. “Iso-propyl; (CH,),CH-. ‘Tert-butyl; (CH,),C-. = CH-. ’ Average number of each functional group per molecule: trisubstituted double bond; -(CH,)C vinylidene double bond: CH, = C(CH,) - . ’ Non-terminal f = 2 x [each functional group peak intensity]/[total terminal peak intensity (i-Pr + t-Bu + TTD + TVD)]. k Average number of terminal double bonds per molecule: + f; = 2 x ([TTD] + [TVD])/([i-Pr] + [t-Bu] + [TTD] + [TVD]). ’ A verage number of total double bonds per molecule: & = 2 X ([TTD] + [TVD] + [NTTD])/([i-Pr] [t-Bu] + [TTD] + [TVD]).

320

320

Mwh M ”

M”” x1O-’

Time (min)

and functionality

Temp. (“C)

Table 4. Composition

2 0

%

e & R 3 “*,

2

3

45

Effect of terminal double bonds on thermal degradation Table 5. Changes in composition ratios between functional groups of the nonvolatile oligomers by thermal degradation of polyisobutylenes (M,,,,= 7.09 x loj) (Part 1) Thermally prepared polyisobutylene Temp.

(“C)

Time (min)

300

0 60 60 90 15 30 60

320

(“C)

15.4 5.75 5.23 5.51 7.56 6.94 5.13

2.49 2.89 2.94 2.75 2.66 3.03 3.19

Time (min)

[t - Bu] ~ [i - Pr]

0 60 90 90 15 30 60 90

0.23 0.35 0.51 0.48 0.34 0.42 0.81 0.57

300

320

[i - Pr] - [i - Pr], = (k,i, + kti2)[Rt*][N] (59) [TVD] - [TVD],,

[NTTD] - [NTTD], = (k,, :;, X

-

V.Dl

tTVD1 -

W-‘-D1

3.09 3.62 3.63 3.41 3.31 3.85 3.95

4.14 3.99 4.26 4.19 4.05 3.72 4.19

(60)

> s12m

(kpiz[Rp*]+ kt,[R*] + kvi2[S.])ENI

(62)

[-ITD] + [N’ITD]

WQ’I -

-

[t - Bu] - [t - Bu]” = (k,i, + kpi2)[Rp*][N] (58)

= (k,,,[R,*] + k,il[R,.] + kvi,[S*])[N]

WDI WDI

-

2.77 2.95 2.81 2.59 2.93 3.39 3.80

times versus M, are shown in Figs 6(a) and (b). The concentrations, [t-Bu], [TTD], [TVD] and [NTTD] except for [i-Pr], increase with a decreasing Ikf,. However, the concentrations obtained for the hydrogenated polyisobutylene are inconsistent with those for the thermally prepared polyisobutylene, owing to different initial distributions of functional groups. The concentrations of the respective functional groups formed in the process of thermal degradation, which are named corrected concentrations, could be obtained by the following equations:

912

[TTD] + [N-ITD]

(Part 2) Hydrogeneated polyisobutylene __.~~

__~ Temp.

U-W VW __.__-_

E 1 Fi

3.92 3.94 3.49 3.15 3.91 4.41 5.00

W’DI W=‘Dl 0.00 2.40 3.00 4.17 2.21 3.00 3.34 3.15

Plots of the corrected concentrations of respective functional groups versus M, are shown in Figs 7(a) and (b). At both temperatures, the corrected concentrations obtained for the samples before and after hydrogenation are well correlated to M,,. This result shows clearly that the functional groups of the nonvolatile oligomers are formed by the intermolecular hydrogen abstraction of respective radicals (eqns (8)-(13)) followed by p scission (eqns (14)-(16)) in the depropagation step. For the hydrogenated polyisobutylene, the concentration [i-Pr] newly formed by these elementary reactions is much lower than [i-Pr], and this would lead to negative values of the corrected concentration, as shown in Figs 7(a) and (b). The molar concentrations [R;] and [R,.] formed by the initiation reactions (eqns (l)-(3)) are given as follows: [R,*] = kiep&DIMn + ki,[N]

(63)

[R;] = kc~fTv,JMn + k,[N]

(64)

The calculated values of [R;] and [R;] are listed in Table 6, where the values of k,, and k,, are set to be 1.70 X lo-’ and 1.88 X lo-‘” min’ at 3OO”C, respectively, and the values of other parameters are given in Table 4. The contribu-

46

T. Sawaguchi, M. Seno 300 “C .

(a)

,“:

.

auf

P 0. A 0 0

TTD p hd f

v :i-Pr : t-Bu A : TTD l :TVD + : NTTD

O?+++-e

I

Mn x 11Y3

o@ * J

I

I

I

I

3

4

5

6

1

1 (b)

V.

320 “C

:i-Pr 0 .:t-Bu

Mn x 10m3

: TTD 0 n :TVD 0 + : NTTD A A

20 (b)

320 “C

t

: TTD ,, hd t

l

% A

15 VA

A

01

.

2

2

TV

1

I

1

4

5

6

I

Mn x 1O-3

. A A

0

I 3

Fig. 6. Plots of the concentrations of respective functional groups versus M, of the nonvolatile oligomers obtained by thermal degradation of polyisobutylene before (open marks) and after (filled marks) hydrogenation at (a) 300 and (b) 320°C.

I

I

I

I

I

3

4

5

6

7

Mn x 10m3

Fig. 5. Relationships between the composition ratios of four terminal mono-olefin trimers and M, of thermally degraded polyisobutylene at (a) 300 and (b) 320°C before (open marks) and after (filled marks) hydrogenation.

tion of random initiation to [R;] and [R;] (the second term of the right hand of eqns (63) and (64)) is 0.252 X lo-” mol/cm3. The contribution of end initiation is much larger for [R;] than [R,.]. The calculated values of [R;]/[R;] remain almost unchanged during the degradation,

47

Effect o,f terminal double bonds on thermal degradation (a)

2

I? & % .s

I

300 “C

Table 6. Concentrations of macroradicals determined from eqns (63) and (64) using &o, fTvD and M. of the nonvolatile oligomers listed in Table 4 (Part 1).

V v :i-Pr 0. :t-Bu A A : TTD 0 n :TVD 0 0:NTTD

(Part 1) Thermally Temp. (“C) 300

prepared polyisobutylene ______-_.--

Time (min)

[R;] x 10” mol cm-’

(R,.] x 10” mol cm- ’

0 60 90 90

2.34 4.31 4.06 4.54

1.09 1.66 155 1.81

~~

____ Temp. (“C) 300

/ 5

4

3

1

6

(Part 2) Hydrogenated ~_~ Time (min)

[R;] x 10” molcm-’

0 60 90 90

0.252’ 1.77 2.16 2.06

I

(b)

V v 0. Ar 0 n

320 “C

” Using k,, = 1.70 X lo-’ cited from Ref. 6. ’ Equal to k,, [N].

: i-Pr :t-Bu :TTD :TVD

and

[Rp.] [R,-I __~. 2.15 2.60 2.62 251 ___-

polyisobutylene [R;] x IO” mol cm- ’

-Mn x lo-’

-~

0.252h 0.803 0.894 0.897 -__-__

k,, = 1.88 X 10. I” mini

[Rp.] LRt.1 1.00 2.20 2.42 2.30

’ at 300°C

(Figs 2 and 3) and the lower value of [R;]/[R,+] is inconsistent with an increment of the ratio [R;]/[R,*] on hydrogenation (Figs 4 and 5, and Tables 2 and 3). This shows clearly that changes in the stationary concentrations of R; and R; are not caused by the change of initiation rate. Thus, it is clear that the rates of end initiation reactions from TTD and TVD (eqns (l)-(3)) strongly affect the overall rate of degradation.

0 6:NTTD

Y

AA A

i;e

8

u

.

;al _fj 0

WV

-_________

t

I

.

4 CONCLUSION .

4-e--

--_____

I

. I 3

I 4

I

I

5

6

. I

bin x IO-3

Fig. 7. Plots of the corrected

concentrations of respective functional groups versus M, of the nonvolatile oligomers obtained by thermal degradation of polyisobutylene before (open marks) and after (filled marks) hydrogenation at (a) 300 and (b) 320°C.

depending on the observed values of f&, and fTvD and [TTD]/[TVD] (Table 5). The effects of hydrogenation are clearly found in decreasing values of [R;], [R,*l and [R;]/[R,*] on hydrogenation. The lower values of [R;] and [R;] could be interpreted by a decrease in the overall rate of degradation on hydrogenation

The effect of the initiation reactions from terminal double bonds on the thermal degradation of polyisobutylene was examined using a sample prepared by hydrogenation. It is expected from the kinetic analysis that the concentrations [R;] and [R,.] formed by the end initiation reactions from TTD and TVD and the random initiation from scission of the skeletal bond become lower and the ratio [R;]/[R;] becomes slightly lower for the hydrogenated sample. The effect of hydrogenation was experimentally found as a decrease in the overall rate of degradation, owing to a marked decrease in the end initiation rate. However, the integrated concentration ratio [R;]/[R,*] does not decrease but increases. This result means that this concentration ratio scarcely depends on the initiation reactions. A similar relationship between the ratio [R;]/[R,.] and M, of the molten polymer matrix was observed for

48

T. Sawaguchi,

the degradation of both samples before and after hydrogenation. This result supports the idea that the stationary concentrations of respective radicals decrease with decreasing M, during the degradation owing to a marked depression of the regeneration rate of R; in the depropagation step, and this decrease is caused by an increase in the rate of diffusion-controlled termination.‘,s,6 REFERENCES 1. Sawaguchi, Macromol.

2. Sawaguchi,

T., Tekesue,

T., Ikemura,

T. & Seno, M.,

Chem. Phys., 1% (1995) 4139.

T., Ikemura,

T. & Seno, M., Macromol.

Chem. Phys., 197 (1996) 215.

3. Sawaguchi, T. & Seno, M., Polym. J., 28 (1996) 392. 4. Sawaguchi, T. & Seno, M., Polymer, in press. 5. Sawaguchi, T., Ikemura, T. & Seno, M., Polymer, in press. 6. Sawaguchi, T. & Seno, M., Polymer, in press.

M. Seno

7. Sawaguchi, T. & Seno, M., Polym. Degrud. Slab., in press. 8. Mita, 1. In Aspects of Degradation and Siabilizarion of Polymers, Chapter 6, ed. H. H. G. Jellinek. Elsevier, New York, 1978. 9. Sakaguchi, Y. & Sakurada, I., Koubunski Kagaku, 5 (1948) 242.

10. Tsuchiya,

Y. & Sumi, K., J. Polym. Sci. Part A-l, 7

(1969) 813.

11. Kiran, E. & Gillham,

K. J., J. Appl. Pofym. Sci., 20

( 1976) 2045.

12. Cameron, G. G., Makromol. Chem., 100 (1978) 255. 13. Cameron, G. G., Meyer, J.M. & McWalter, I.T., Macromolecules, 11 (1978) 696. 14. Fox, T. G. & Flory, P. J., J. Phys. Colloid Chem., 55 (1951) 221.

15. Smoluchowski, M., Z. Phys. Chem., 92 (1918) 129. M., Coutandin, J. & Sillescu, 16. Antonietti, Macromolecules,

17. Green,

H.,

19 (1986) 793.

P. F. & Kramer,

E. D., Macromolecules,

19

(1986) 1108.

18. Nemoto,

N., Kishine,

Macromolecules,

M., Inoue,

26 (1991) 1648.

T. & Osaki,

K.,