The formation of crosslinked polymers during the anionic polymerization of lactams

]452

T . M . FRUNZE et al.

4. V. V. KORSHAK, Metody vysokomolekulyarnoi organicheskoi khimii, t.1 (OrganicChemical Methods for High Polymers, Vol. 1). Izd. Akad. Nauk SSSR, 486, 1953 5. V. V. KORSHAK, S. V. VINOGRADOVA, M. M. TELPYAKOV and Yu. A. CHERNOMORDIK, Dokl. Akad. Nauk SSSR 147: 1365, 1962 6. M. iYl. TEPLYAKOV, S. V. V1NOGRADOVA and V. V. KORSHAK, Dokl. Akad. l~auk SSSR, Seriya khim., 334, 1964 7. V. V. KORSHAK, T. i)I. FRUNZE and V. F. PETROVA, Izv. Akad. bTauk SSSR, Otd. khim. nauk, 217, 1958 8. N. S. YENIKOLOPYAN, J. Polymer Sci. 58: 1301, 1962 / 9. N. YENII~OLOPYAN, V. I. IRZHAK and B. A. ROZENBERG, Uspekhi khim. 35: 714, 1966 10. F. R. MAYO and F. M. LEWIS, J. Am. Chem. Soc. 66: 1594, 1944 11. O.A. PLECHOVA, V. V. IVANOV and N. S. YENIKOLOPYAN, Dokl. Akad. Nauk SSSR 166: 905, 1966 12. P. J. FLORY, Trans Faraday Soc. 51: 848, 1955 13. L. MANDELKERN', Chem. Revs. 56: 903, 1956 14. Sb.: Sintezy organicheskikh preparatov (Coll. 4, In: Syntheses of Organic Preparations)• p. 305, Izd. inostr, lit., 1953 15. R. BENSON and T. CAIRNS, J. Am. Chem. Soc. 70: 2115, 1948

THE FORMATION OF CROSSLINKED POLYMERS DURING THE ANIONIC POLYMERIZATION OF LACTAMS* T, M. FRUI~ZE, V. V. KURASHEV, V. I. ZAITSEV, R. B. SHLEIF1KAI~ a n d V. V. K O R S ~ K Organometallic Compounds Institute, U.S.S.R. Academy of Sciences (Received 5 J u l y 1972)

The Anid G-669 has been used in model reactions with initiators having different functionalities to show that branched and partly crosslinked polyamides (PA) can be produced by anionic polymerization of e-eaprolactam (CL) due to the participation of acyl lactam end groups present on the macromolecules. Imide bonds which are easily hydrolyzed were found to form chiefly as a result of these reactions. :EARLIER studies [1, 2] h a d shown t h a t , depending on the f u n c t i o n a l i t y of the initiator used, the anionic p o l y m e r i z a t i o n of lactams will yield polymers possessing different properties. F o r example, p o l y c a p r a m i d e (PCA) p r o d u c e d in the presence of a m o n o f u n c t i o n a l initiator such as N - a c e t y l - e - c a p r o l a c t a m (ACL) is quite soluble in the n o r m a l solvents used for this t y p e of p o l y m e r (cresol, Vysokomol. soyed. A16: No. 6, 1257-1264, 1974.

Formation of crosslinked polymers during anionic polymerization of laetams

1453

formic or sulphuric acid) (other initiators used have been abbreviated as follows: IPCL = N,N'-isophthalyl-bis-e-eaprolaetam, TMCL = N,N'N"-trimesinyl-tert.-ecaprolactam, MACL=N-methaerylyl-e-eaprolaetam). Polyamides (PA) synthesized in the presence of polyfunctional initiators will contain a large amount of insoluble product which can be up to 70% w/w under some conditions. Polymerization over polyfunctional initiators also yields PCA having a high impact strength. The specific impact viscosity of such polymers is also 3-4 times larger than that of a PCA produced over a monofunctional initiator. There are also some differences of macromolecular structure when the polymer is produced over such initiators [2]. This study aimed at clarifying the chemistry of the formation crosslinked P A by anionic polymerization of lactams over activating compounds. RESULTS The mechanism of artior~io polymerization of lactams can be written formally as the insertion of laetam units into the initiator molecule according to modern theories [5, 7]: CO

i

"1 +CH,CO~RCo

nNHRCO

/

" CHACO[

NHRCOln--N R

One therefore could expect the end groups of the 1)A produced in the presence of a monofunctional initiator, e.g. ACL, to be the acetyl and imide (acyl laetam) groups. Where a polyfunetional initiator is used, the PA will probably contain only the acyl lactam end groups shown below: CO\

/CO OC

I--I R

2nHNRCO -

R

CO N[COI1NItl~--COR'CO--[--NHRCO--ln--N\

It

/

a CO

oc\ / ~ Ico

I--I

3nHNRCO

COiNnaCOl,,N/I \ co

R

,

/N[CORNHlnCOR'CO[ NHRCO],~N t/

R

The numbers of such end groups ought to correspond with those of the functional groups present in the initiator. Differences of chemical nature cud nulnbers of end groups per macromolecule will therefore be obtained when initiators with differing flmctionalities arc used. The acyl lactam groups are knowu to be quite reactive; they react easily with water [5, 8] and amines [9]. Wiehterle and co-workers [10] assumed that the following reactions take place in such a lactam polymerization, which will lead to branched polymers:

1454

T. 1~. FRUNZE et al.

,-,,,.CO.

C=O / ]

~N-

\

l-i-R ~C0

[

~

I

CO~ NH~

(1)

CO--n--~q--CO~

N--CO~

I +1

~N-

~CO--N~

~CO--N~

~

I

(2)

CO + H N ~

The above schemes show that branching will result from inter-chain reactions of the maeromolecules, and also of aeyl lactam end groups. The inter-chain reactions (eqn. (2)) are independent of the number of end groups present on the macromolecules and primarily depend on the concentration of amide ions present in the reaction system. As to reaction (1), this largely depends on the number of end groups on the macromolecule, which depends on the initiator functionality. One therefore can expect a l~rger functionality to cause more intense branching, followed by crosslin~king due to reactions in which the end groups participate. When reaction (1) takes place during lactam polymerization, the macromolecules with 2 or more acyl lactam end groups will react with the amide groups and this will result in the production of crosslinked polymers.

Clarification of the part played by the acyl lactam end groups present on PA macromolecules in the crosslinking reactions was sought b y studies of model reactions i.e. in reactions of PA with initiators having different functionalities (ACL, IPCL, TMCL, MACL) in the presence of metallic sodium as catalyst; all other conditions were similar to those of the anionic polymerization of lactams.

i

Ill~~

Ill//

:III

gI

I00

200 Fxc. 1

300 ~°0

0 04

0.3

O'J

0.7

[TMOL], mole % FIG. 2

FIG. 1. The thermomechanical curves of P A produced in the presence of (mole %): 1--1.5 TMCL; 2--0.6 TMCL; 3--0"35 ACL; 1", 2 ' - - a f t e r 20 hr boiling in water. FIo. 2. The formic acid insoluble fraction of PCA as a function of TMCL concentration: / - - o r i g i n a l samples; 2 - - a f t e r 10 hr boiling in water.

Formation of crosslinked polymers during anionic polymerization of lactams

1455

The PA used was a copolymer of type "Anid G-669" [11] stabilized with acetic acid; its molecular weight (mol.wt.) was 8700 (from end group titration [12]). This PA contained chiefly carboxyl end groups (105-9×10 -~ g-equiv./g) and few amine end groups (8.6 x l0 -6 g-equiv./g), i.e. about 8% of the total end groups present. The Anid G-669 was selected for the reactions with the initiators because it forms a melt at relatively low temperature (150-160°C) and is very soluble in organic solvents, which made the reaction and the processing of the end product much easier. The macromolecular end group reactions with the monofunctional initiator (ACL) can probably take two routes under our conditions: 0

0 U

O ~--N~ II _ a I ~C--N,.~ --"-" O--C

-,

I

+-.

II

0 o II

0 II

O=C

/

I

O----C--R--N--CO--R

\N--C--R /

~\N--C--R

(a)

4

' O=C\

-,

1

"o

co _/

(4)

I C--R 0

R

O-

depending on the type of carboxyl group present (in or outside the lactam ring) which will react with the amide group of the polymer. One can expect in both cases that N-substituted PA will form, i.e. N-aminocaproic substituents on the amide groups of macromolecules in the first case, and N-acetyl-substituted in the second. It is also possible for reactions (3) and (4) to run simultaneously. The tabulated results show that Anid G-669 yielded only soluble polymers with the monofunctional initiator (we took a 5-fold excess of ACL on polymer to get a PA to get the maximal substitution of amide groups). Reprecipitation of the reaction mixture with water from a formic acid solution produced larger polymer yields than those given b y the control, in which Anid G-669 was heated to 160°C without the initiator. An especially large yield (39~o more) was obtained in a catalyzed reaction. This is taken to be definite proof that a reaction took place between the initiator and the PA. The tabulated results also show that heating of Anid G-669 with metallic sodium gave partial decomposition of the polymer, which was increased at higher temperatures. Heating the Anid G-669 to 160°C without sodium did not alter the intrinsic viscosity. Substitution

1456

T.M.

F~U~ZE et al.

of amide groups brought about by the ACL reaction over the catalyst can thus be accompanied by decomposition reactions. However, none of these resulted in crosslinking. Other results were obtained when Anid G-669 was reacted with polyfunctional initiators (IPCL, TMCL and MACL). The polymers produced in their presence contained an insoluble fraction; the amount depended on the reaction conditions. Its formation is probably associated with a crosslinking process as described below: OC

~co~

+

CO

/\~--COR'CON/\ R

+ ~CON~ R

~CON~,~

~CONM~

I 0CRNCOR'CONRC0

-

~CON~

CO

o R'co + 2 /I I

~,C0~

\R

The Table makes it clear t h a t a bifunctional initiator gives rise to the largest amount of isoluble fraction in a 4 hr reaction at 160°C. This does not happen at 140°C because the mixture is non-homogeneous at t h a t temperature. Elevation to 200°C caused a large decrease in the insoluble fraction. Some decrease also occurs at 200°C when the PA is reacted with the terfunctional initiator TMCL. These facts are evidence of the thermal instability of bonds forming the crosslinks between macromolecules. The initiators IPCL and TMCL were used as 10% w/w on polymer during the reaction with Anid G-669. MACL yielded only a trace of insoluble fraction under these conditions. A 20% w/w MACL use resulted in a marked increase of insoluble PA fraction (about 9~o ). The difference is probably due to the initiators having differing mechanisms of crosslinking. MACL can be formally regarded as a monofunctional initiator as it contains only one activating group per molecule. Its presence during anionic polymerization of lactams caused an adduct to form between MACL and sodium caprolactam on the double bond, and this led to the formation of a polyfunctional initiator [2]. Crosslinked products of the Anid G-669 reaction with MACL seem to form as a result of the existence of N-substituted polymers containing methacrylyl groups as substituent, followed by unsaturated groups which react to form the transverse bonds between macromolecules. On the basis of model reactions we confirmed by experiment t h a t crosslinked polymers form during the initiated anionic polymerization of lactams as a result of imide end groups on macromolecules reacting with the amide groups of other PA macromolecules.

Formation of crosslinked polymers during anionic polymerization of laetams

1457

I (~rosslinks b e t w e e n t h e p o l y m e r macromolecules form due to t h e - - C O - - N - - C O - imide groups present. As the l~tter are reactive a n d easily h y d r o l y z e d , we studied the resistance to hydrolysis of the polymers p r o d u c e d in the presence o f initiators h a v i n g different functionalities. The polymers were h y d r o l y z e d b y boiling in distilled water. The changes in T m for the polymers, the insoluble f r a c t i o n a n d the viscosities of the soluble fractions were d e t e r m i n e d on the samples d u r i n g hydrolysis. r~HE

REACTION

CONDITIONS

FOR

FUNOTIONALITIES~

POLYAI~IIDE AND

G-669

SOME OF THE

WITH

INITIATORS

PRODUCT

OF

DIFFERENT

PROPERTIES

(50/o w/w catalyst; [~] of original Anid G-669 and after 6 hr heating at 160° without catalyst or initiator=0.48 dl/g)

Activator

ACL IPCL

TMCL

Amount % w/w of polymer

temp., °C

500 500 10 10 10 10 10 10 10 20

160 200 160 160' 160 160 160 200 160 200 160 160

Reaction conditions soluble fraction* time, gel fraction amount, hr % w/w % W/w* [~/], dl/g 4'0 4"0 4-0 4'0 1'0 2"5 4"0 4'0 4"0 4"0 4"0 4"0

Nil

3.9 28.0 26.7 10-0 16-3 14-0 Traces 8.8

74'9 52"1 139"i 91 "5 55-7 60'8 69'7 56"7 75"1 78"2 69'7 70"8

0"27 0"25 0"35 0"51 0'68 0'56 0"69 0"21 0'73 0"34 0"26 0"30

* The amountof solublefractionwas determined after precipitation with water fromnil 85~ formicacid solution J" 1~o catalyst.

Isolated with water from a polymersolutionin formic acid. Analysis of the t h e r m o m e c h a n i c a l curves (Fig. 1) showed t h a t the P A prod u c e d in the presence of the ter-functional initiator h a d slightly differing viscous flow characteristics t h a n one p r o d u c e d with a m o n o f u n c t i o n a l . The same s t a r t ing t e m p e r a t u r e of softening p r e s u m a b l y m e a n s t h a t the transverse b o n d s are relatively rare in the crosslinked polymers. P A hydrolysis, w h e n TMCL was used, clearly shows t h a t its viscous flow was close to t h a t of a linear PA. The P A most affected b y hydrolysis was one p r o d u c e d over a low c a t a l y s t c o n c e n t r a t i o n when the polymeric p r o d u c t h a d a smaller n u m b e r of t r a n s v e r s e bonds, as Fig. 2 shows. The ability to h y d r o l y z e in the case of p a r t l y crosslinked p o l y m e r s w i t h r a n d o m b r a n c h i n g becomes still clearer when Fig. 3 is examined; a longer d u r a tion of hydrolysis results in the insoluble fraction diminishing in q u a n t i t y in

1458

T.M.

FRUNZE et al.

all cases. This can be most clearly seen on the polymers produced at small catalyst concentrations. An increase in the latter will require a longer hydrolysis time to get a completely soluble polymer/which also seems to be associated with an increase in ~he number of crosslinks present in the polymer. This hydrolyzing capacity appears in branched PA even after they had become soluble. Figure 4 shows the [~/] values of PA solutions as a function of hydrolysis time.

[q_z, dl/y 8,

\

80 ~

~-o/

6 \\\\\\i ~z \.. o

,.,

~II

F

%

"

eo

T/me, hp

FIG. 3

'

¢'o

3 i

/0

I

1

1

2O JO T/me, #',

v

I

4O

Fro. 4

FzG. 3. The formic acid insoluble fraction of PCA as a function of the boiling period in water after production with: 1 - 4 - - T M C L ; 5 - - I P C L , using amounts (mole %) of: 1--2.1; 2--0-6; 3--0.45; 4--0.35; 5--1-6. FzG. 4. The [~/] of PCA produced in the presence of (mole %): 1--0-35 ACL; 2--0.35 IPCL; 3--0"35 TMCL; 4--0.45 TMCL as a function of boiling period in water.

One can see from the reported results t h a t the viscosity diminishes during hydrolysis when the PA were produced in the presence of a bi- or ter-functional initiator, while t h a t of a linear PA remains practically unchanged. For example, the viscosity of a polymer synthesized with 0-175 mole ~o IPCL dropped to ½ t h a t of the original sample after 30 hr hydrolysis. Where 0.120 mole °/o TMCL were used, it was after 40 hr hydrolysis only 1/6 t h a t measured after 20 hr. The largest decrease in viscosity occurred in the case of branched PA in the first few hours of hydrolysis. Summing up one can coficlude t h a t the reaction of amide groups in some macromolecules with imide groups of others play an important part in crosslinking of PA during the anionic polymerization. The additional imide groups which are thus created form the transverse bonds between the macromolecules. The high reactivity of the imide groups is the cause of the relatively easy hydrolysis of crosslinked PA which results in the disappearance of the insoluble fraction present, and afterwards in a decrease in a molecular weight.

F o rm at i o n of erosslinked polymers during anionic polymerization of lactams

1459

EXPERIMENTAL S t a r t i n g materials. The e-caprolaetam was v a c u u m dried at 15°C/16 mm; m . p . = 69.5°C (68 70°C [13]). ACL was produced as described elsewhere [14]; m.p. 125°C/17 mm, n~° 1.4893, d~° so 1.4891, d 4s0 1.095 ace. to [14],. 1.099 (125°C/17 ram, n D MACL was produced as described by Wichterle [15]; m.p. 119-121°C/3 mm, n~° 1.4998, 20 1.4996 [15]). d 420 1"0712, MD=49-58 (sale. 49-38) (b.p. 117-121°C/2 3 m m , n D IPCL was synthesized as described before [3]; m.p. 141.5-142°C (141.5-142°C ace. to the literature). TMCL was produced as described before [1]; m.p. 156"5-157.5°C (same values given in the literature). P o l y m e r i z a t i o n of e.caprolactam in the presence of initiators of different functionalities was carried out as before [1]. The unreaeted monomer was separated from the P A (about 1 g) by boiling the P A chips in distilled water (100 ml) for 10 hr. The polymer was then filtered out, dried and its yield and intrinsic viscosity were determined. H y d r o l y s i s of the P C A was carried out under the conditions described above for the removal of unreacted monomer. T h e A n i d G.669 reaction with the initiators was started by weighing the required amounts of reactants, and catalyst (metallic sodium), after which the initiator was placed in an ampoule which was several times evacuated, filled with argon and then sealed. The ampoule was placed in a silicon bath preheated to the required temperature (160 or 200°C) and then kept at this temperature for 4 hr. After cooling the ampoule was opened and the contents extracted with 85% formic acid, filtering off the insoluble polymer part, and precipitating out the other part of PA from the filtrate with water. The insoluble fraction and the filtered off soluble polymer part were washed on the filter with water, then acetone, aud v a c u u m dried at 50°C to constant weight. The viscosity of the polymer was determined in 85 ~°/o formic acid at 25°C. Translated by K. A. ALLEN

REFERENCES 1. V. V. K O R S H A K , T. M. F R U N Z E , V. V. K U R A S H E V , It. B. SHLEIFMAN and L. B. DANILEVSKAYA, Vysokomol. soyed. 8: 519, 1966 (Translated in Polymer Sei. U.S.S.R. 3: 567, 1966) 2. V. V. KORSHAK, T. M. FRUNZE, V. V. KURASHEV, V. I. ZAITSEV and T. M. BABCHINITSER, Vysokomol. soyed. AI2: 416, 1970 (Translated in Polymer Sci. U.S.S.R. 2: 475, 1970) 3. V. V. K O R S H A K , A. M. KOGAN, V. A. SERGEYEV, R. B. SI.ILEIFMAN, L. B. GUREVICH and G. B. ANDION, Sb.: Geterotsepnye vysokomolekulyarnye soyedineniya (In: Hetero-Chain High Molecular Weight Compounds). p. 24, Izd. " N a u k a " , 1964 4. J. CZERNJ, Plast. m o d . 21: 107, 1969 5. O. WICHTERLE, J. SEBENDA and J. K R A LI ~ EK , K h i m i tekhnol, polimerov 7: 39, 1961 6. EL' AZMERLI, V. V. KOItSHAK and V. A. SERGEYEV, Vysokomol. soyed. 7: 2067, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 2264, 1965) 7. J. SEBENDA and J. STEI'ILI~EK, Coll. Czech. Chem. Comm. 28: 2731, 1963 8. O. WICHTERLE, P. SITTLER and P. ~EFELIN, Chem. Listy 11: 52, 1958 9. J. ~EBENDA and J. K R A L I ~ E K , Coll. Czech. Chem. Comm. 23: 766, 1958 10. O. WICHTERLE, P. SITTLER and P. ~EFELIN, J. Polymer Sei. 53: 249, 1961

1460

YE. L. GAL'PERIN

11. V. V. KORSHAK and T. M. FRUNZE, Sinteticheskie geterotsepnye poliamidy (Synthetic ttetero-Chain Polyamides). p. 456, Izd. Akad. Nauk SSSR, 1962 12. V. A. MYAGKOV and A. B. PAKSHVER, Zh. prikl, khim. 29: 1703, 1956 13. Spravochnik po khimii polimerov (Polymer Chemistry Textbook). p. 22, Izd. "~qaukova dumka", 1971 14. R. BENSON and T. CAIRNS, J. An~. Chem. S'oc. 70: 2115, 1948 15. O. WICHTERLE and P. GREGOR, J. Polymer Sci. 34: 309, 1959

THE STRUCTURAL CHANGES PRODUCED BY y-RADIATION IN HYDROGEN CONTAINING POLYFLUOROETHYLENES* YE. L. GXL'PERI~ (Received 13 July 1972) Wide and small angle X-ray diffraction methods were used to study the structural changes in polyethylene, polyvinyl fluoride, polytrifluoroethylene, and also in the vinylidene fluoride-tetrafluoroethylene and tetrafluoroethylene-ethylene copolymers after exposure to 100-1500 iKrad doses of ?-e°Co radiation. The radiation resistance of crystalline and maeromolecular structures was found to decrease as the number of fluorine atoms increased in the polymers. All the polymers used (except polyvinyl fluoride) showed a phase transition due to chain defects when they were irradiated; this led to a larger symmetry of the unit cell. The differing behaviour of block and powder samples of H containing polyfluoroethylenes during irradiation was found to diminish with increasing content of fluorine atoms. In addition to the existence of different mechanisms of structural change in the latter polymers there were some characteristics of the polyvinyl fluoride which are explained here.

T~E structural changes which take place in crystallizing polymers exposed to ionizing radiation are still little known today, although they can greatly affect the physico-mechanical properties. Those studies which were made dealt mainly with polyethylene (PE) and the results obtained sometimes contradict each o t h e r [1-11]. Separate i n f o r m a t i o n exists a b o u t the effects o f ionizing r a d i a t i o n on t h e crystalline a n d maeromolecular s t r u c t u r e s o f p o l y a m i d e s [12-14]. Some i n f o r m a tion is available for p o l y t e t r a f l u o r o e t h y l e n e ( P T F E ) [15-17], which was f o u n d to become more crystalline after exposure to y-rays, a n d also for polytrifluoroethylene ( P F E ) [18]. B o t h these p o l y m e r s are fully h a l o g e n a t e d a n d their r a d i a tion stability is m u c h lower t h a n t h a t of P E [2] or of the h y d r o g e n - c o n t a i n i n g polyfluoroethylenes [19]. P o l y v i n y l i d e n e fluoride (PVF~) was investigated f r o m a m o n g the latter after exposure in air to y-radiation [20-22]. W h e n the l a t t e r * Vysokomol. soyed, h16: No. 6, 1265-1273, 1974.