Growing chain macroradical recombinations during the heterogeneous polymerization of tetrafluoroethylene

GROWING CHAIN MACRORADICAL RECOMBINATIONS DURING THE .HETEROGENEOUS POLYMERIZATION OF T E T R A F L U O R O E T H Y L E N E * V. P. M~J~'X,lmov,A. I. MI~rAmov, A. M. ~_ARKEVICHand I. YE. VOX,OKHONOVICH Chemical Physics Institute, U.S.S.R. Academy of Sciences (Received 10 June 1975) The ESR method has been used to study the kinetics of decay of growing chain macroradicals in the y-irradiated PTFE during the polymerization of tetrafluoroethylene (TFE). The change of the state of radicals in the matrix is found to be the result of consecutive acts of monomer additions to the radical. The recombination kinetics are studied on a lattice model. They are described by a single constant, namely the rate constant of chain propagation. The radical recombination data axe used to determine the product of the rate constant of chain propagation with that of the monomer solubility in the polymer. The good agreement of the results confirms the validity of such an examination.

IN~rESTIGATIOI~S of t h e m e c h a n i s m s of changes of s t a t e of t h e radical are o f f u n d a m e n t a l i m p o r t to a n u n d e r s t a n d i n g of t h e kinetics of n u m e r o u s radical r e a c t i o n s in solid p o l y m e r s . T h e synthesis of a solid p h a s e p o l y m e r is k n o w n t o affect t h e radical p o l y m e r i z a t i o n kinetics [1]. A large n u m b e r of radicals are occluded in t h e solid p h a s e p o l y m e r ; these p a r t i c i p a t e in t h e process a n d h a v e a n a n o m a l o u s l y long life as a result of t h e stabilizing action of t h e p o l y m e r m a t r i x . One of t h e m o s t i m p o r t a n t questions to be a n s w e r e d is h o w to assess t h e occluded radicals, a n d also t h e m e c h a n i s m of t h e i r decay. This process is associated in t h e l i t e r a t u r e w i t h a diffusion of low mol. wt. radical f r a g m e n t s f r o m t h e liquid p h a s e to t h e m a c r o r a d i c a l in the. p o l y m e r , followed b y a d d i t i o n or d i s p r o p o r t i o n a t i o n reactions. T h e g r a f t p o l y m e r i z a t i o n of T F E on ?-irradiated p o l y - T F E ( P T F E ) is used here to s t u d y t h e kinetics of t h e d e c a y of radicals stabilized in t h e solid p o l y m e r d u r i n g t h e process, i.e. t h e chain t e r m i n a t i o n . EXPERIMENTAL

The experiments were carried out with P T F E powder irradiated with a 100 Mrad dose coming from a Co 6° source (KU 150,600). The ESR spectra were recorded on a EPR-21~ radiospectrometer produced by the Chemical Physics Institute, U.S.S.R. Academy of Sciences. The graft polymerization temperature was 20-160°C for samples placed in the resonator of the ESR spectrometer which was connected to a vacuum. This made it possible to simul* Vysokomol. soyed. AI8: No. 8, 1721-1725, 1976. 1968

Growing chain macroradical recombinations

1969

taneonsly observe the growth of the polymer chains from the monomer consumption under isobaric conditions, and the behaviour of the radicals. The TFE polymerization was initiated in this case by the radicals stabilized in the PTFE during irradiation. Complications of the EPR spectral picture were avoided by converting the "central" NCFz--CF--CFa~ type radicals into "terminal" CF~--CF2 by photolysis of ~heir peroxides [2, 3]. RESULTS

The T F E polymerization which is initiated by the radicals stabilized in t he P T F E matr ix is accompanied b y a shift of the reactive centres in this matrix, a n d this finally results in their getting ,closer together and decaying b y recombination (Fig. 1). Dimension r is used in the lattice t heory of bimolecular solid phase reactions [4-7], or volume v * ~ r 3, in which two particles can react. This reaction in volume (space) is determined by the monomolecular rate constant kl, and th e act of e n t r y into the cell, i.e. their capacity to move, b y the effective diffusion coefficient D. The radical movements can be associated in solid matricea with diffusion, a rapid transfer of the free valency, chain oxidation, etc.

2

"

'

"

2

I 30 60 Time, rnin

Fie. 1

90

50 Time, rnin

I 100

FIG. 2

FIG. l. The kiue¢ic curves of radical decay and Che changes of sample weight during polymerization: /--changes of radical numbers in the sample, N/No; 2--radical concentratior~ [R]/R]0; 3--sample weights PIPe. FIG. 2. Linear transformations of the kinetic curves of radical decay at monomer pressures~. of (mmHg): •--683; 2--530; 3--420; 4--210. The mo v em e nt of radicals is a chain process in this case. I t is natural to assume the m o v e m e n t stage to be limiting from amongst the two stages ".mvolved~ namely the move m ent and the monomolecular reaction of radicals in the cell..

1970

V . P . M~L'~rn~ov eta/.

I n accordance with the Waite [6] reaction one can therefore write

(1) in which cA, cB are the reagent concentrations, ]CD=4u(DAJrDB)~-constant determining the number of collisions between reagents during diffusion; r--distance between particles in the cell; DA, DB--effective diffusion coefficients of ~nr'l 0~, mir/-!

o.10 I

0"05

r

I

i

300

I

I

]

600

I

PTFE,rnmH~ FIG. 3. The dependence of tan = on monomer pressure. l~zCicles; z,,,r=/D--lffe time of the particle in the cell; for the recombination of the radicals at t>>z we have

d[R]/dt------kdR] =,

(2)

in which [R] is the radical concentration in P T F E , particles/era 3. When the monomer adds on as a growing chain radical at frequency kp [M], in which lop--rate constant of chain propagation, [M]- actual monomer concentration, the radical will move by a distance which equals two lengths of the C--C bond present in PTFE, i.e. by 2 ~ 2 . 6 A [8]. Using the Einstein formula, one can then write the effective diffusion coefficient as 1 ~)kp[M]

(3)

and afterwards ~=

4

~ .~[M]

(4)

The system of differential equations describing the polymerization process and the behavoiur of the radicals during the process, which also considers the

Growing chain macroradical recombinations changes

1971

in polymer volume and equ. (4), will look as follows [9]: dt - -

~ ~,~'+vM

(5)

kJM][R] 3

dig] - (1 - vM[n]) ~p[M] [R], dt

(6)

in which [//] is the concentration of freshly forming polymer inside the total, T F E molecules/cmS., VM--~0"75X 10 -33 cm3--volume of monomer unit in P T F E . Equation (5) describes the decay of the radicals and its form, after solving, will be [1%]-°[R3= 1 +

1+

4~13r\ -3T--v-v~MvM)VMkp[M][R]ot

(7)

The kinetic curves of the radical concentration changes must be straight lines according to eqn. (7) in [R0]/[R]--t coordinates, and their slope tan a=(4u2~r/3 + v . ) kp [M] [R] o. Assuming t h a t the monomer concentration in the polymer will follow the rule [M]=ap, in which a is the constant of the monomer solubility in the polymer; p - - m o n o m e r pressure, one can write for tan g

tan ~= (v~+ : ~3r) kpap[R]o

(8)

One can see in Figs. 2 and 3 t h a t the behaviour of the radicals is described well by eqn. (7). The cell dimension r slightly increases with temperature according to earlier findings [9]; it will be r~2.2-3-8 tlx in the range 20-95°C. The kinetic data o f T H E PRODUCT OF THE RATE CONSTANTS OF CHAIlq P R O P A G A T I O N

AND

TH]~ T F E

SOLUBILITY IN P T F E

T , °C

25 25 25 25 25 25

~gTFE~ mmHg 683 530 420 210 630 630

kp O"(min-1/mmHg) calculatod by using equation 7* 10' '0:21 0.23 0.22 0-19 0-22 0.13

0-22 0.22 0.20 0.15 0-21 0-12

T, °C

40 58 72 72 92 92

~TFE,

mmHg 560 600 495 630 630 460

kp G (min-*/mmHg) calculated by using equation 7* 10t 0.27 0.18 0.20 0-18

0"16 0"30 0"17 0"21 0"15

0.09

* Fromthe radical recombination kinetics. t Fromthe polymerizationkinetics. macroradical recombinations from eqns. (7) and (8), together with the earlier found cell dimension [9] yielded the product of the rate constants of chain propagation and monomer solubility in the polymer; these are given in the Table below.

1972

V.P. MEL'NIKOVet aZ.

The correctness of the quantitative results can be verified on the polymerization binetics. The process rate, i.e. the accumulation of the polymer in a system containing N growing chains, can be determined from

dAP

=k [M]lV

lz

=

iz

plV

(9)

in which ziP is the weight Of the freshly produced p o l y m e r ; / l = 100=tool. wt. of TFE; NA--Avogadro number. The integration of the equation yields

k ~ = zip (t~)-ziP (t~) iv__ A t.

~vhich is better presented in the form of

(Je(tt)--AP(tt))/Po. N___A t, N (t) _

[R]op .I

Sx "~0

~t

#

(10)

The lcpa obtained by processing the kinetic data according to equation (10) $,

are also given in the Table; the

JfN(No) d~ was determined by numerical integrati

tion of the kinetic curves of radical decay (Fig. 1). A description of the bimolecular reaction kinetics within the limits of the cell model [4-7] assume a random movement of the reactants within the solid matrix, while the polymerization within the polymeric matrix imposes well defined correlations, such as the irreversibflity of the addition act, namely no backward movement, also the composition of the supermolecular structure, etc. These complicated problems demand further investigation. The bpa found by various methods from the radical recombination kinetics and those of polymerization (Table) agree well at the same time, which is evidently confirmation of the validity of the examination carried out in this study. Translated by K. A. REFERENCES

1. C. BAMFORD, V. BARB, A. JENKINS and P. ONION, K i n e t i k a radikal'noi polimerizatsii vinilovykh soyedinenii (The Radical Polymerization Kinetics of Vinyl Compounds). Izd. inostr. Lit., 1961 2. E . R . KL1NSHPONT a n d R. K. MILINCHUK, Khim. Vys. Energii 1: 242, 1967 ~3. S. SIEGEL and H. HEDPETH, J. Chem. Phys. 46: 3904, 1967 4. J. F R A N K a n d E. RABINOVITSH, Trans. F a r a d a y Soc. 70: 120, 1934

Anionic polymerization of tolane and of diphenylbutadi-yne

1973

5. 6. 7. 8.

T. R. WAITE, Phys. Rev. 107: 463, 1957 T. R. WAITE, J. Chem. Phys. 28:103,'1958 Ya. S. LEBEDEV, Kinetika i Kataliz 8: 245, 1967 W. S~PPARD and K. SHARTS, Organicheskaya khimiya ftora (The organic Chemistry of Fluorine). Izd. "Mir", 1972 9. V. P. MEL'NIKOV, A. I. MTKHAILOV,A. M. MARKEVICH and V. I. GOL'DANSKII, Dokl. Akad. Nauk SSSR 224: 1115, 1975

ANIONIC POLYMERIZATION OF TOLANE AND OF DIPHE.NYLBUTADI-YNE* V. M. M_ISn~,P. P. KIS~ITSA, 1~. I. ]3OT.OlqDAYEVAand M. I. CHF~gA.q~ Chemic~al Physics Institute, U,S.S.R. Academy of Sciences (Received 16 June 1975)

Tolane and diphenylbutadi-yne were polymerized in THF. The polymerizates are soluble, coloured, paramagnetic and heat resistant, having high flow temperatures. Their spectral characteristics and electro-physical properties have been investigated. The polytolane and polydiphenylbutadi-yne have the structure of substituted polyenes. The polymerizations of diaryl derivatives of acetylenes are known to be difficult. The thermal polymerization of the diphenylbutadi-yne (DPB) in bulk requires a process temperature of 150-195°C for example; 6 1 ~ of an insoluble fraction forms even at 195°C. The macromolecules contain polyene as well as poly-acene fragments [1, 2]. The thermal polymerization of DPB in solution gives fairly good yields in tens of hours at 120°C and higher temperatures [3] .The thermal bulk polymerization of tolane is efficient only above 300°C. Above this temperature it is accompanied by a reduction of the hydrogen content in the polymers [4]. The high temperature polymerizations of diaryl acetylenes is thus accompanied by various secondary processes, primarily degradations, and these become apparent as insoluble products, as well as changes in the elemental composition. The presence of structural defects in the macromolecules of such polymers, and also of various types of chain units, makes a reliable interpretation of some of the spectral and electrophysical data almost impossible. DPB polymerization over some initiator complexes, such as iso-BuaA1--TiC14 or vanadyl acetylaceto* Vysokomol. soyed. AI8: 1~o. 8, 1726-1732, 1976.