Structure formation in SKF-26 fluorine-containing rubber using pentaerythritol derivatives containing phosphorus

Polymer Science U.S.8.]g. 17ol. 22, No. 6, pp. 1544--1558, 1980

Printed in Poland

0032--$950/80/061544-10507.50/0

C 1981PergamonPre~ Ltd.

STRUCTURE FORMATION IN SKF-26 FLUORINE-CONTAINING RUBBER USING PENTAERYTHRITOL DERIVATIVES CONTAINING PHOSPHORUS* L. P. KARASF.'V, V. P. BUKALOV, fix. M. OGlEr?, A. P. Tus~.vEV and E. YE. N~ANT'YEV All-Union Scientific Research and Design Institute for the Rubber Industry V. I. Lenin State Institute of Pedagogics, Moscow Volgograd Polytechnic

{Received 15 May 1979) It was established that hexafluoropropylene-vinylidenefluoridecopolymers (SKF26 fluorine-containing rubber) form structures with pentaerythritol derivatives contaming phosphorus. A polymer network is formed as a result of alkylation of phosphites with C--F rubber fragments according to the Arbuzov reaction mechanism. Physical and mechanical properties, heat and chemical stability of vulcanisates were examined. Derivative thermogravimetry and high temperature continuous chemical stress relaxation were used to study thermal oxidative breakdown of the structures obtained, which showed separate st:ages in the degradation mechanism. b r a n c h e s of t e c h n o l o g y p r e s e n t ever increasing r e q u i r e m e n t s f o r increase o f technical a n d operational p r o p e r t i e s of elastomers, including r u b b e r based o n S K F - 2 6 fluorine-containing rubber; existing m e t h o d s o f v u l c a n i z a t i o n of this r u b b e r [1] are far from being able t o e x h a u s t all t h e possibilities available. A s t u d y is m a d e o f s t r u c t u r e f o r m a t i o n of SK_F-26 b y b i s - p h o s p h o r y l a t e d d e r i v a t i v e s o f p e n t a e r y t h r i t o l (PSP), t h e p r o p e r t i e s o f which are shown in T a b l e I. Synthesis o f p h o s p h i t e s of this series has been studied in detail [2-4]. T h e s e p h o s p h i t e s h a v e p r o p e r t i e s of highly effective stabilizers o f p o l y m e r i c m a t e r i a l s [5-7], fire-proofing agents [8], etc. W e established t h a t P S P e x a m i n e d are able to r e a c t w i t h S K F - 2 6 w i t h n e t w o r k f o r m a t i o n ; fluorine-containing rubber, in t h e same w a y as fluorinec o n t a i n i n g m o n o m e r s [9], shows properties o f r e a c t i v e alkyl halides. T h e m o b i l i t y o f fluorine a t o m s in S K F - 2 6 is increased a t t e r t i a r y c a r b o n a t o m s c o m b i n e d w i t h e l e c t r o n e g a t i v e CF3 radicals a n d in allyl fluoride f r a g m e n t s f o r m e d as a consequence o f the d e h y d r o f l u o r i n a t i o n of rubber, in which the C - - F b o n d is weakened b y t h e effect o f conjugation. DEVELOPING

Unfilled rubber mixtures based on SKF-26, containing magnesium oxide as HF accepter (15 parts by wt. per 100 parts by wt. rubber) and a vulcanizing agent were selected for investigation. * Vysokomol. soyed. A22: No. 6, 1404-1412, 1980. 1-544

SKF-26 fluorine-containing rubber

1545

The samples were tested according to general requirements for the determination ~f physical and mechanical properties of vuleanizates. They were prepared in accordance with GOST 269-66. PSP was pllrified b y vacuum distillation or recrystallization. I R spectra of materials were obtained using a UR-20 device. Rubber films for spectra were prepared from a polymer solution in o,thylacetate directly on polished NaCI she¢,ts by evaporation of solvent. 3~I) NMR of modified rubber samples was determined in ~m ethylacetate solution using a Brucker HX-90 device at a frequency of 36.4 Hz and 85% Ha PO, was tile external stando.rd. An AKhM-8MD chromatograph with a cohnnn 3 m long and 4 mm in diamete,' was used for the analysis of gas liberation, 5A molecular sieves were used as adsorbent, the rate of the carrier gas (helium) was 50 l./min and temperature was 50 °. The degree of crosslinking of vulcanizates was determined by the inverse value of equilibrium swelling in acetone [10]. Thermogravimetric studies wBrc carried out using a MOM derivatogTaph (Hungary) at a rate of 6 deg/min. High temperature chemical stress relaxation was determined usiug an MRS-500 machine with a heated chamber. Temperature in the chamber was m a i n t a i n e d automatically with an accuracy of :i: 2°. Results of relaxation were a~lalysed after the experimental curve had been expanded to elementary exponents using the Brukhauov-Tobol'skii method [ i l l .

(2

l

8

,

i

I5

3

L

I

9

]

~

I

/5

,,tl

,

23

~

I

k

I

•

28 35. vxlO-~cm -!

Fie. 1. I R spectra characterizing structure formation of SKF-26: a: 1--SKF-26, film; 2 - - P B M P , thin layer; 3 - - S K F - 2 6 + P B M P , film; 4--sample 3 after heating (150°, 40 rain); 5--sample 4 after thermostatic control (200 °, 24 hr); b: 1--SKP-26, film; 2 - - P B E P , thin layer; 3 - - S K F - 2 6 + P B E P , film; d--sample 3 after heating (150 °, 40 rain); 5--sample 4 after temperature control (200 °, 24 hr). I R a n d N M R s p e c t r o s c o p y a n d G L C w e r e u s e d t o s t u d y t h e m e c h a n i s m of s t r u c t u r e f o r m a t i o n . T h e following b o n d s were identified in the I R s p e c t r u m of a crude mixture containing SKF-26 and pentaerythritol bis-methylphosphite (PBMP) (Fig. la): P - - O - - C (1025 cm-l), C - - F (1225 cm-1), C - - H o f f l u o r i n e c o n t a i n i n g r u b b e r (1395 cm-1); t h e b a n d a t 1480 c m -1 a p p e a r e d i n 12 P S P

1546

L.P. Kam~SEVet a/.

samples and, apparently, characterized the system of phosphorinane rings. During heating films under conditions of vulcanization ~nd temperature control a widening is observed in the I R spectrum of the band at 1225 cm -I as a result of the superimposition of vibrations of phosphoryl (1210-1250 cm -1) on vibrations of C - - F bonds; the intensity of absorption of the band at 1.025 cm -1 decreases, the peak at 1480 em -~ is retained and a band appears at 1640 cm -1 (double bond), which is evidence of the dehydrofluorination of the fluorine-containing polymer. After heating the test for trivalent phosphorus in the reagent mixture was negative. The p3_.p5 transition was also confirmed using a model system conraining very low molecular weight SKF-26 and methyl-l,3-propylenephosphite. On heating the latter with rubber the J signal characterizing trivalent phosphorus of this phosphite [12] disappears from the 3~p NMR spectrum of the mixture (128.5 p.p.m.) and a J signal appears at 18.2 p.p.m, which is typical of pentavalent phosphorus. Methyl fluoride was found in vulcanization gases by GLC. Based on the foregoing and results obtained by Kirpichnikov et al. [13] concerning the stability of spiran structures in reactions of alkyl halides with bis-alkyl phosphates of pentaerythritol, structure formation of SKF-26 with pentaerythritol bis-methylphosphite may be presented as alkylation by the Arbuzov mechanism / 2rubber -- F :- CtlzO--P \

--, :rubber--

OCII~ CII20 ~ / ~P--OCIta C /\ / ocI12 Cll~,O PBMP

heating

->

~../OCH~ / ( ; H 2 0 ~ i ~ - rubber ~ /'C~(H~0Jl OC I-I:5 0 C I t 2 " OCH3 0

()CtI~

II/

--~ rubber--P

\

CH~O 0 /

C

\ I~

p.- rubber '-2CTI3--F

"\OCH2 / \CII20 / The r u b b e r - - F structure calls for the participation in the reaction of units of two types CF, I ---C--~ and ~--C----CH--~ I I F F Interaction of SK_F-26 with pentaerythritol amidophosphites takes place by a more complex system.

SKF-26 fluorine-containing rubber

1547

The following bonds were identified in the I R spectrum of an unheated polymer film with pentaerythritol bis-diethylamidophosphite (PBEP) (Fig. lb): P - - O - - C (1035 cm--1), C - - F (1225 cmq), C - - H rubber (1395 cm-1), phosphorinane (1480 c m 1). During heating this film bands at 1035 and 1480 cm -1 are retained in the spectrum of the sample and the intensity of absorption of the band at 1225 cm -1 is reduced. During subsequent temperature control of the film the relative intensity of absorption of the band at 1225 cm -1 increases and t h a t of a band at 1035 cm -1, decreases. The peak at 1480 cm -~ disappears. This pattern of spectra is evidently due to the rupture of ester bonds of P - - O - - C rings, the formation of a phosphoryl group and the superposition of vibrations on vibrations of C - - F bonds. Transition during two-stage vulcanization of trivalent phosphorus to pentavalent phosphorus was confirmed analytically and by NB'IR using a model system containing SKF-26 ONq~i and diethylamino-l,3-propylenephosphatc. I t was shown t h a t in the alp NMR spectrum of a mixture of the latter with rubber signal $ (121 p.p.m.) disappears after heating, this being typical of trivalent phosphorus [12] and signal 5 (17.4 p.p.m.), appears which is typical of the pentavalent form. Results enable structure formation of SKF-26 with pentaerythritol bismethylphosphite to be presented as alkylation by the Arbuzo~ reaction mechanism with the formation at the intermediate stage of a stable quasi-phosphonium compmx tl4, 15] which, under conditions of temperature control breaks down with ring opening forming fluoroxethane structures

2 rubber -- 1"-~- (Czl[~)2N--P

//

OCH=

N /

\

/\ OCIIz

--.

CII=0 \

(:

healing "P--N((]2II~,) 2 "

../

>

(JH20

PBt-P OCH~ CIIoO */ \, / '\ + rubber --- P C P -- rubber 2bY---*

\()Ctl/

\(;II20

N(( :21It,)2

N(C.db,)2

II i !1 • rubber--I'--(JCH.--C--CIIzO--I' .... rubber !

I

N(ColI~), C H 2 F

I

N(C2Hs)z

During heating of S K F - 2 6 - P B E P mixtures the rubber undergoes dehydrofluorination, which ts confirmed by the appearance in the 1R spectrum of the film of a band at 1640 cm -1 (double bond). A broad band is formed at the same time at 3340 cm -1 (NH) evidently as a consequence of the secondary reaction o f aeidolysis of amide residues of the vulcanizing agent with hydrogen fluoride~ I t is known [16] t h a t this process represents nueleophilie attack at nitrogen.

1548

L.P. KARASEVet aL

During thermostatic control the intensity of absorption in this region decreases, which indicates the participation of amine bonds in structural conversions [1]. Finally, the formation of a band at 2380 cm -1 ( P - - H ) is most probably the result of partial hydrolysis of amidophosphite. Pentaerythrito] bis-phenylphosphate (PBPP) interacts by a similar method, with opening of phosphorinane rings with SKF-26.

Qo/qt

0.8-

~

0.6

3 I

10

l

t

I

ZO 3O Trine, days

#0

FIe. 2. Variation of the degree of structure formation in vulcanizates by the action of concentrated nitric acid on samples containing PBMP (1), PBPP (2) and PBEP (3). The stability of crosslinks in the structures obtained was determined from results of action on vulcanizates of concentrated nitric acid which, as shown by Gilinskaya et al. [17], does not involve molecular chains of the fluorine-containing polymer, but breaks down the C - - N bond of the main chain and C = N of Lrel.,% ~,t.fPa 1000-20 " R

a "1 ×2

L,e,% ~,mPa , 1000-15~ \

I 1I

•

'/~ -6oo 0.4

-

× . ~ ~ -I ~ "' ~~ -

=\ =

,o

~ [ ----If --.--11l

,1 x2

h\ . 4 ~ = : ~ ' ~ - - . ~ . . L\,;Z_ " -

1/4 600- 9

i,I

" "---'----~." --*

. "c~---~

o,_

. ~s~ ~

• 0

2

'

o., c

q

6

I

1

3

2

I

8

I J~rt5 b q ~ g - m o l e , 10 2

.....

o.3

-, 0

Fie.

]3

Effect of the dose of PBMP on tensile

2

[

q

I 1

pQrt~ bl~f wf:

I0 I

2

3

6

8

,q-mole, I0 z

strength o"t (1), relative elongation L~l (2) degree of structure formation 1/Q (3) of samples at~r vulcanization in a pres~ (160"o 60 mln) (I) and after temperature control at 200° (H-IV). Time of temperature controh e--24 (II) and 48hr (rn); b--12 (II), 24 (III) and 36 (IV). B.

and the

1549

SKF-26 fluorine-containing rubber

erosslinks. I t was established (Fig. 2) t h a t structures with alkyl- and arylphosphates of pentaorythritol; as regards stability towards nitric acid, are close to peroxides and are superior to structures with amidophosphites. Tile lower stability of the latter is duo to the fact t h a t in addition to phosphorus--carbon bonds in crosslinks of vulcanizates with amidophosphates, nitrogen--carbon bonds, unstable in HNOs represent a certain proportion; as noted previously, these are formed as a result of the interaction with rubber of diethylamine separated (luring acidolysis of H F amidophosphite. TABLE

I. P H Y S I C A L .*kl'CDCHEMICAIJ P R O P E R T I E S OF P S P OF T H E S T R U C T U R E

/ O C H , \ /CHzO~ R--P~ ?C. /P--R OCH~ "CH~O Molecular B.p., weight °C/10-Storr, B.p., °C

R

--

O C H

160-165 170-175 180-184

256.2 340.3 372.3 380.3 338.4 366-3 378-3

s

OC4H 9 -- 0CH~CH~0C~H~ --

-- 0C6H'~ --N(C2Hs)~ --N(CHsCH,)~O -- N H - - C e l l s

nZ0d

d~O~

1.4773 1.4689 1.4758

1.092 1-223 1.182

102-105 85-87 88-91 92-94

Three-dimensional network density 1/Q, tensile strength at and relative elongation Lrel of the structures obtained show a marked dependence on the dosage of products and conditions of vulcanization (Fig. 3); high values of l/Q, at and Lrei are only achieved by temperature control which at 200 ° is practically completed in 24 hr. Most satisfactory heat stability of rubber is achieved with a dosage of PSP close to 4 parts by wt. per 100 parts by wt. of rubber (Table 2). TABLE

2. H E A T

S T A B I L I T Y OF

SKF-26

VITLCANIZATES W I T H

200°, 30 days coefficient of ageing* k ,, [ k,

Dosage of product, p.b.w.

2.0 4.0 5.0 7.0

0-80 1.04 1.12 0.82

DIFFERENT

DOSES

Exposure 250°, 15 days coefficient of ageing* 1/Q k,, I kz

Vulcanizates with PBMP 0.62 0.24 0"55 0.73 0-27 0"60 0.27 0.63 0.31 0-12 0.49 0.44

OF

PSP

l/Q

0"24

0"26 0"29 0"34

0"08

0-43

0.25 0.47 0.33

0"24 0-30 0"35

0"66

0.70

Vulcanizates with PBEP

3.0 4.0 5.0

I I I

1.15 1.25 1.32

0-24 0-42 0.34

0.23 0.28 0.35

0-53 0"65 0.23

* g e Is the ageh~g coemcient for tensile strength, k l - - f o r relative elongation.

1550

L . P . KARASEV eta[.

Properties of vulcanizates with other P S P (Table 1) are similar to those shown in Fig. 3. The rate of structure formation of SKP-26 decreases with an increase in the length of the alkyl radical combined with phosphorus on transition to phenyl and on reducing the basicity of the amine residue. The t y p e of process during heating was studied b y differential-thermal analysis (DTA) and thermogravimetric analysis (DTG). Results using the vulcanizato with P B M P are given in Fig. 4. 330 =

AT . . . . .

d

50L

300

100

GO

20 T i m e , rain

FiG. 4. Derivatograms of SKF-26 with PBMP: a--TGA, b--DTG, c--DTA, d--temperature variation. 1--PBMP; 2--PBMP-}-magnesium oxide (1:2); 3--SKP-26+PBMP-}-magnesium oxide (20 : 1 : 3) after vulcanization in a press (150°, 60 min); 4-- sample 3 after temperature control (200°, 24 hr); 5 - - SKF-26. Two stages of decomposition are observed for pure phosphite (at 280 and 330°), which are accompanied b y exothermie effects. On adding magnesium oxide, the t y p e of heat liberation changes--three exothermic peaks are observed on the curve of DTA (at 250, 290 and 370°). A change in heat effects is due to the formation of a complex of magnesium oxide with P B M P which appears less heat-stable than pure phosphite.

9.8 0-26 0.05 0.01

0.121 0.43[ 0.30 0.15

0.131 13.8 0.341 0-62 0.34 0.21 0.19 0.01

1.3 18 60 107

1.2 16 55 83 5.4 23 54

0.5 5.5 30 68

0.3 5.5 35 58

0.321 0.24 0.15

1.5 0.32 0.02

0.14[ 14.3 I 0-40[ 1.2 0.31 0.16 0-15 0.02

[ A(7[/¢×10~] a t 250 °

v

0.I 0.9 5.0 11

0"161 30"9 1 0"34 7 " 0 [ 0.35 0-61 0.15 0-19

Vulcanizates with P B E P 0.1 0.15 18.6 0-1 2.4 0.361 3.3 1.1 12 0.32 0.45 4.7 17 0.17 0.07 9.2

I

0.16[ 28.8 0.361 4.5 0.34 1.2 0.14 0.18

0.27[ 22.9 0.43[ 5-7 0-20 1.6 0.10 0.92

[ Ao-[k×lO' a t 270 °

0.2 0.15 20.9 I 0.1 3.1 0.38 2.0 1.3 14 0-331 0.45 6"5 25 [0.14 0.06 17

Vulcanizatos with P B P P

0.23[ 16.6 0.38[ 2-8 0.21 0.55 0.18 0.30

Vulcanizates with P B M P

v

0.251 12.3 1 0.2 0.321 1.3 3.5 0.23 0.16 14 0.20 0.11 21

a t 230 °

r I Aalk×lO~

7.3 21.1 29.5 30.8

9.1 22.5 27-4 29.2

8.6 18.0 30-2 30.3

Ea, kcal/molo

0"42 5"3,t 7"09 7"80

1"12 5"71 6"85 7"!1

0"82 3"86 7.34 7.42

logA, see -~

SKF-26

58.7 39"8 28.1 24.8

54.5 34.4 29.2 28.0

56"7 42"4 26"9 26"2

24.2 31.3 35.{J 37.6

24"7 32'2 35"5 37-0

24"9 30"0 34"3 37"6

--jS ~ , AG ~, e.un. kcal/molo

VULCANIZATES BY 5 0 ~ o

* Coaventional notaflona: T - r e l a x a t i o n time, hr; J a - proportion of the Io~s of initial stress; k - reaction rate constant, sec-'. The other notationa are given in #l~e texL

8.5 0.60 0.05 0.03

I-0 0.28[ 0.29[ 16 0.24 55 0.19 94

a t 210 °

~ I Ja[l¢×1041

K I N E T I C A N D A C T I V A T I O N P A R A M E T E R S * OF C H E M I C A L STRESS R E L A X A T I O N I N E L O N G A T I O N OF

Elementary processes

TABLE 3.

J~

O

1552

L.P. K-A~ASEVeta/.

Thermal breakdown of samples begins both after vulcanization and during heating at 270-280 °. Weight loss of the sample under temperature control (TGA curve) takes place at a lower rate than that of the sample vulcanized in a press, since during thermostatic control the system has already been stabilized to a considerable extent as a consequence of the liberation of volatile products (HF, H~O etc.). In the interval up to 300 ° breakdown is not accompanied b y sudden heat effects. Subsequently, volatile products are more vigorously liberated and weight loss begins. Pure rubber begins to decompose at 350 ° and burns above 450 ° . Analysis of the breakdown of vulcanizates ~ccording to results of the variation of continuous high-temperature chemical stress relaxation enables four stages to be distinguished (Table 3). The first (most rapid stage) shows low activation energy values (Ea~7"3-9"1 kcal/mole) and most favourable entropy values (ztS÷--54.5 - --58-7 e.un.) and is, apparently, due to the rupture of intermolecular coordination and hydrogen bonds. At the second stage Ea is 18.022-5 kcal/molo and corresponds to Ea of the thermal-oxidizing breakdown of C - - C bonds in carbon-chain polymers ( ~ 21 kcal/mole) [18, 19]. Energy values of C - - P , P - - N , P - - O bonds (65-77 kcal/mole) [20, 21] and C--C, C--O bonds (62.8 and 75 keal/mole) are practically identical [22]. Processes of thermaloxidative breakdown at this stage m a y therefore be classified as processes of decomposition of crosslinks formed b y P S P and branched residues combined with phosphorus. At the third and fourth stages Es values are 27.4--30-2 and 29.2-30.8 kcal/mole, respectively and close to indices Ea of thermal oxidative breakdown of fluoroelastomers during pyrolysis ( ~ 30 kcal/mole) [23]. The difference in the rate of thermal oxidation at these stages is most probably explained b y the existence in the polymer chain of fluorine-containing rubber of structures with different reactivities. During relaxation taking place in stages free energy zig ~ and pre-exponentia] values log A increase and there is a direct compensation relation between Ea and log A. I t follows from the parameters examined, bearing in mind the rate of thermooxidative breakdown in Stages and the overall relaxation time that the vulcanizates have heat stabilities which decrease in the order P B P P ~ P B M P ~ P B E P . These results prove new possibilities of using compounds containing phosphorus as chemical additives of polymer materials. Tranalat~

~ E. S~MERE

REFERENCES

1. F. A. G~LH.-OGLY, A. S. NOVIKOV and 8. N. NUDEL'MAN, Ftorkauchuki i roziny na ikh osnove (Fluorine-containing Rubber and Vulcanizates). Khimlya, 1960 2. B. A. ARBUZOV, R. P. ARSH1NOVA, S. G. VUL'FSON and E. T. MUKMENEV, Izv. AN SSSR, ser. khim., 2426, 1973 3. U.S.A. Pat. 3928505, 1974; RZhKhim., 17N135P, 1976 4. IL J. LUCAS, F. W. MITCHELL and C. N. 8CULLY, J. Amer. Chem. Sc~. 72: 5491. 1950

SKF-26 fluorine-containing rubber

1553

5. Brit. Pat. 1380449, 1975; R Z h K h i m . , 22T105P, 1975 6. German Federal Republic Pat. 2223551; Bull. izobr, za rubezhom (Bull. of I n v e n t i o n s Abroad), cl. CO8, No. 4, 55, 1975 7. U.S.A. Pat. 3794614, 1974; R Z h K h i m . , 5TI07P, 1975 8. U.S.A. Pat. 3893970, 1975; Bull. izobr, za rubezhom (Bull. of Inventions Abroad), cl. CO8, No. 18, 49, 1975 9. I. L. KNUNYANTS, V. V. TYULENEVA, Ye. Ya. PERVOVA and R. N. STERLIN, Izv. AN SSSR, ser. khim., 1797, 1964 10. P. J. FLORY a n d J. REHNER, J. Chem. Phys. 11: 512, 1943 11. G. M. BARTENEV, Relaksatsio1~nyye yavleniya v polimerakh, Khimiya, 1972 12. Yu. Yu. SAMITOV anti T. V. ZYKOVA, V sb. K h i m i y a i primeneniye fosfororganieheskikh soyedulenii (Chemistry and Application of Organo-phosphorus Compounds). Nauka, 1972 13. P. A. KIRPICILNIKO~ T, E. T. MUKMENEV, L. V. VERIZHNIKOV, Ye. I. VORKUNOVA and O. V. VOSKRESENSKAYA, V sb. Sintez i issledovaniye khimikatov-dobavok dlya polirnernykh materialov (Synthesis and Study of Chemical Additives of Polymeric Materials). Tambovskaya pravda, 1969 14. E. Ye. NIFANT'YEV ~ud N. L. IVANOVA, Zh. obshch, ldlimii 49: 1492, 1970 15. M. I. KABACHNIK and V. A. GILYAROV, Dokl. AN SSSR 96: 991, 1954 16. B. Ye. IVANOV and V. F. ZHELTUKHIN, Uspekhi khimii 39: 773, 1970 17. N. S. GIIA~SKAYA, F. A. GALIL-OGLY, G. A. GUBAI and A. S. NOVIKOV, K a u c h u k i rezina, No. 12, 7, 1964 18. A. S. KUZ'MINSKII, L. Yu. LYUBCHANSKAYA, L. G. ANGERT and G. K. MIKHAILOVA, V sb. Dostizheniya nauki i tekhnologii v oblasti reziny (Progress in Science and Technology in the Field of Rubber). Khimiya~ 1969 19. S. G. KIRYUSHKIN, V. P. FILIPENKO, V. T. GONTKOVSKAYA, L. A. LOVACHEV and Yu. A. SHLYAPNIKOV, V sb. Karbotsepnyye polimery (Carbon-Chain Polymers). Nauka, 1977 20. L. PAULING, The Nature of the Chemical Bond, Ithaca, 1960 21. I. F. LUTSENKO and M. V. PROSKURINA, Uspe -khi khimii 47: 1648, 1978 22. Spravochnik khimika, col. 1 (Chemists' Handbook), Khimizdat, 1962 23. T. G. DEGTEVA and A. S. KUZM:INSKII, V sb. Khimicheskiye svoistva i modifikatsiya polimerov (Chemical Properties and Modification of Polymers). Nauka, 1964