Influence of the stabilizer distribution on the thermal stability of poly (methyl methacrylate)

Polymer ScienceU.S.S.R. Vol. 20, pp. 1777-1782. (~) Pergamon Press Ltd. 1979. Printed in Poland

0032-3950]78]0701-1777507.50]0

INFLUENCE OF THE STABILIZER DISTRIBUTION ON THE THERMAL STABILITY OF POLY(METHYL METHACRYLATE)* G. V. LEPLYANIN, S. ]~. ~=~AFIKOV, O. I .

KOgCHEV,F .

Z. GALIN, E . G.

VaxisovA

a n d Y E . S. MALYARCHUK Chemistry I n s t i t u t e of the Bashkir Affiliated Branch of the U.S.S.R. Academy of Sciences (Received 19 September 1977)

Tile distribution of stabilizing additions in polymeric materials has a significant influence of their thermal stability. This is particularly clear in the case of polymers decomposing by the depolymerization mechanism. As an example a study was maple of how the distribution of some stabilizers containing thioadamantane structures influences the thermal stability of poly(methyl methacrylate). F o r comparison some data are given on the influence of stabilizing additions introduced as admixtures and end groups on the thermal stability of polystyrene. STABILIZING additions were introduced into the polymer before its formation: a m i x t u r e of monomer with the appropriate amount of the addition was polymerized in bulk before conversion of all tile monomer to polymer had taken place. D A A (2.85 × 10 .3 mole/1, was used as tile initiator. The polymerization was run for 10 hr, in the absence of oxygen, in ampoules placed in a t h e r m o s t a t where the water temperature was maintained at 50~=0.05°; next, postpolymerization was carried out for 5 hr at 120 °. The amount of residual m o n o m er in specimens did not exceed 0.3 wt.~o. The molecular weights (viscometry) of polymers obtained in this w a y were within limits of 10s-10 e for PMMA and 104-105 for PS. The spread in t h e MWs was determined by the influence of the additions. The m e t h o d of introducing additions into pro-formed polymer was as follows: in a 3~o solution of polymer in benzene was dissolved the required weighed portion of a stabilizing addition. Films cast from this solution were dried i n vacuo to constant weight, after which cylinders 4-5 m m in diameter were made from the films by hot pressing. Cuttings for further investigation were then prepared from these films. Monomers used for polymerization were subjected to normal purification processes to remove stabilizer [1], and similarly the methyl methacrylate (MMA) fractions with b.p. 42°]100 torr, and the styrene fraction with b.p. 70°]60 torr. D A A was reerystallized repeatedly from methanol and dried i n vacuo to constant weight at room temperature; m.p. 103.5 °. The substances used as stabilizing additions were the following thioadamantanes an d their derivatives

II3C

s)

CH3

1,3,5,7-tetramethyl-2,4,6,8-tetrathioadamantane

CH3 * Vysokomol. soyod. A20: No. 7, 1575-1579, 1978.

1777

(r), m.p. 155 °

G.V. LEPLYANIN e~ ~ '

1778

CH3 S~S H~

H3C

1,3,5,7 -tetra111ethyl -2,4, 6,8, 9,10 -hexathioadamlantane 228.0 °

(n),

In. p°

cI-/3

SS

•

3,5, 7, 9, 9'-pentamethyl - 1-mercapto- 2,4, 6,8 -tetrathioadamantane (III), 111.p. 160 °

BC

C

3,5, 7,9, 9'-pentamethyl- 1-thiobenzyl- 2,4, 6,8-tetrathioadamantane (~V), 111.p. 144 °

cHs

bis -(3,5,7,9,9'-penta111ethyl-2,4,6,8-tetra thioadammantyl-1) disulphide (V), m.p. 246247 °

Compounds I, I I a n d II]: were synthesized in accordance with procedures outlined in [2, 3]; IV and VI were prepared using potassium mmercaptide of I I I a n d benzyl chloride (for IV) or methaeryloyl chloride (for VI) and were purified b y repeated rccrystallization from methanol. Compound V was synthesized from 1TI and purified by reerystallization from ethyl alcohol.

Thermal stability of poly (methyl methacrylate)

1779

The thermo-oxidative stability of the polymer was determined with the aid of a n "MOM" type derivatograph whilst heating in air at the rate of 5 deg/min.: a test specimen weighed 100 rag. The temperature of the onset of degradation of the polymer was taken to be that determined at the point where the abscissa axis was intercepted by a tangent to the curve of weight loss, the foregoing point equal to a loss of 5 wt.%. Test specimens were in the form to portions cut from polymeric blocks. To estimate the extent to which thioadamantanes were inserted into the polymer chain we used as end groups values of the chain inhibition (transfer) constant (kz/k~,) obtained from the relation 1

1

kz

[Z]

following the method of Mayo [4]. The mean coefficient of methyl methaerylate polymerization P was determined for our purposes from the relation [5] /3=2.3 × 103 [q]t.a:, on the basis of viscosimetrie determinations of intrinsic viscosity values of benzene solutions of PMMA specimens prepared in presence of different amounts of stabilizing additions Z at degrees of conversion of 5-6%. R e a s o n s for selection of t h e c o m p o u n d s u n d e r s t u d y were as follows: I a n d I I e n t e r t h e p o l y m e r as chemically u n c o m b i n e d admixtures; I I I , I V a n d V are b o u n d t o p a r t i c i p a t e in chain t e r m i n a t i o n d u r i n g p o l y m e r i z a t i o n , a n d will be i n c o r p o r a t e d as end groups; c o m p o u n d / I I copolymerizing w i t h ~ M A r e s u l t s in c o p o l y m e r s c o n t a i n i n g stabilizing fragment, s in side groups. T.decomp., °O 320 4 3 Z 280

2~0

1

g W [Z] " IOa, mole h'acfion

FIG. 1. Plots of the decomposition temperature of PMMA prepared in the presence of thioadamantanes (prior to total conversion of monomer to polymer) vs. concentration of addition I (1), I I (2), IV (3), I I I (4) in the monomer mixture. T h e i n c o r p o r a t i o n of a n y of t h e t h i o c y c l a n e s u n d e r s t u d y in P M M A or P S c o n s i d e r a b l y enhances t h e stability of the l a t t e r t o w a r d s t h e r m o - o x i d a t i v e 4egrad a t i o n (Figs. 1 a n d 2). H o w e v e r , m a j o r differences in the e x t e n t of ithe stabilizing effect do n o t a p p e a r t o be a t t r i b u t a b l e m e r e l y to the influence o f s t r u c t u r a l dissimilarities in t h e c o m p o u n d s in question. F o r instance, in t h e case of I a n d I I

]780

G.V. LEPLY~NIN

et a t

the more marked stabilizing effect of I I is attributable to the larger number of sulphur atoms in the molecule, though where derivatives of I are concerned, differences in stabilizing properties can be accounted only in terms of the influence which these derivatives have on the formation of macromolecules and on their distribution in the polymeric matrix. T. decomp., °C 3ZO~/o II V

T. decomp., °0

I / ~

2 8 0 ~- ' ~

IO '

~

o

,o 2

j..--'o----o

I

08

~I

1

2¢0 l

l'O 2"0 CZ]'IO 3 mole fractions

Fie. 2

L

I

,,I,

IO 20 [ Z ] =I0~ mole fractions

Fie. 3

-FIG. 2. Plots of the decomposition temperature of PS prepared in the presence of I (1) -and V (2, 3), vs. thioadamantane concentration in the initial mixture: 1 -- initiation by DAA (2.58 × 10-S mole/1.) prior to total conversion of monomer to polymer; 2, 3 -- photopolymerization to a degree of conversion of 5-7 % (2) and prior to total conversion of monomer to polymer (3). FIG. 3. Plots of the decomposition temperatures of copolymors of MMA with VI vs. amount o f units of VI in the eopolymer (1) and vs. amount of III used in the copolymerization of MMA with 7.5 × 10-a mole fractions of VI (2). Under conditions of radical polymerization of methyl methacrylate and ~styrene compounds I and I I are completely inert, influencing neither the rate nor the coefficient of polymerization. This means that polymers prepared in their presence contain the latter stabilizing additions in the form of admixtures. Compounds I I I and IV interact with free radicals leading polymerization process, and appear in the role of weak inhibitors, i.e. participate in chain termination, reducing both the rate and the coefficient of polymerization. Termination on inhibitor competes with mutual termination and leads to the polymer molecules containing a smaller number of bonds possessing the lowest stability towards 4egradative processes and formed both through disproportionation of macroradicals (terminal C : C bonds), and through recombination of the latter radicals (bonds between two tertiary carbon atoms). I t follows, of course, that the larger the share of termination on inhibitor, the smaller will be the number of "weak" b o n d s in the polymer (the larger the number of end groups containing thioa d a m a n t a n e fragments), and the lower will be the rate of initiation of degradative processes on account of "weak" bond opening (other conditions being identical). I n other words, thioadamantane compounds having high values for chain inhibition (chain transfer) constants during polymerization, will have a stronger stabilizing influence. These suppositions tally with experimental findings (see

Thermal stability of poly (methyl methacrylate)

1781

Table). Their soundness is borne out b y a study of the stabilizing influence of other chain transfer agents, as well as b y a diminution of the stabilizing effect of I I I when added to the pre-formed PMMA. Without doubt the thermal stability of PMMA prepared in the presence of I I I and IV is determined not only b y CONSTA:NTS F O R I N H I B I T I O N ( C H A I N T R A N S F E R ) B Y SOME S U L P H U R OONTA.II~II~G CO:V[POUbl'D~ DURING

MMA

P O L Y M E R I Z A T I O N A N D M A X I M A L D E C O M P O S I T I O N T E M P E R A T U R E S FOI~ P I ~ I ~ PREPARED

I N T H E I P . PI~ESENCE

(Polymerization at 60 °, [ D A A ] = 2.85 × 10 -3, mole/1.)

~decomp. Compound

k,lkp

(max. value),

Compound

kz/k,

oC

°C I IV

III

0 0.012 0.610

280 290 310

Tde¢omp. (max. value),

III* n-CI2H~sSH S,

0.016 0.100

280 295 290

* Added to PSIMA.

thioadamantane fragments entering macromolecules as end groups, b u t also b y those failing to react during polymerization. In "pure" form the stabilizing effect of thioadamantane structures as end groups was investigated in the case of thermo-oxidative degradation of PS prepared b y photopolymerization in the presence of V acting in this instance as photoinitiator and inhibitor [6]. It can be seen from Fig. 2 that whereas a mechanical addition of I has practically no effect on thermal stability, the addition of thioadamantane structures to chain ends raises the decomposition temperature of PS. The insertion of thioadamantane structures into side groups of the macromolecule likewise results in increased thermal stability (Fig. 3, curve 1), although the stabilizing effect is considerably less marked than in cases where the foregoing structures are inserted as end groups (Fig. 1, curve 4). I f stabilizing fragments are inserted not only into side groups, but also as end groups, the thermal stability of PMMA is increased, though it remains below the level observed when compound I I I is used b y itself (Fig. 3, curve 2). The foregoing results are seen as evidence fully corroborating the view expressed in [7] that degradative effects in PMMA stem from the pre:~ence of weak bonds formed during mutual termination. Thermal degradation* is initiated b y opening of the latter bonds occurring in the temperature region 250-300 ° . :Processes of random bond opening play a role at 300-320 °, and determine the thermal stability of the macromolecules. The most significant rise in decomposition temperature of PMMA are obtainable when the polymerization is run in t h e presence of stabilizing additions acting as polymerization inhibitors: chain * Although the results of derivatography whilst heating ill air were used ia the iuvestigation, the role of oxidative processes is only very slight, as we showed on a previous

occasion [8].

1782

B. S~. ~ T n ¢

and L. A, VOL'~

termination on inhibitor leads to a reduced number of weak bonds in the macromolecules, resulting in a corresponding increase Jn the thermal stability up to temperatures at which processes of random bond opening occur. Stabilizing additions introduced as mechanical admixtures do not alter the thermal stability of the macromolecules, and have only an insignificant effect on the latter when incorporated as side groups: in such cases weak bond opening is the factor determining the thermal degradation. Translated by R. J. A. HENDRY REFERENCES 1. 2. 3. 4. 5.

V. V. K O R S H A K (Ed.), Sbornik. Monomery (Monomers). vols. l, 2, 1951, 1953 S. S. CHANG and E. F. WESTRUM, J. Phys. Chem. 66: 524, 1962 K. OLSSON a n d S. Q. ALMQUIST, Arkiv kemi 27: 571, 1967 F. R. MAYO, J. Amer. Chem. Soc. 65: 2342, 1943 S. R. RAFIKOV, S. A. PAVLOVA and I. I. TVERDOKHLEBOVA, Metody opredeleniya m o l e k u l y a r n y k h vesov v polidispersnosti vysokomolekulyarnykh soyedinenii (Methods of Determining the Molecular Weights a n d Polydispersity of High Polymers). Izd. A N SSSR, 1963 6. G.V. LEPLYANIN, S. It. RAFIKOV, E. G. VARISOVA, O. I. KORCHEV and F. Z. GALIN, Vysokomol. soyed. A18: 597, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 3, 680, 1976) 7. S. MADORSKII, Termicheskoye razlozheniye organicheskikh polimerov (Thermal Degradation of Organic Polymers). Izd. "Mir", 1967 8. G. V. LEPLYANIN, S. R. RAFIKOV, Yu. N. YEGOROV, G. M. PROKHOROV, R. V. ZAINULLINA and O. I. KORCHEV, Vysokomol. soycd. BI8: 22, 1976 (Not translated in P o l y m e r Sci. U.S.S.R.)

Polymer ScienceU.S.S.R. Vol. 20, pp. 1782-1789. (~)Pergamon Press Ltd. 1979. Printed in Poland

0032-3950/78/0701-1782507.50/0

ON THE POLYMERIZATION MECHANISM OF LACTAMS* B. Sm KnXITI~ and L. A. VOL'F S. M. K i r o v Textile and Light I n d u s t r y Institute, Leningrad

(Received 20 September 1977) A s t u d y has been made of the electronic structure a n d of the p o l a r i t y of t h e amide bond in lactam molecules, a n d the reactivity of laetams has been investigated in relation to the number of ring members and the ts~pe and position of substituents. On the basis of the results a polymerization mechanism is proposed for laetams * Vysokomol. soyed, A20: No. 7, 1580-1585, 1978.