synthesis, structure and thermal degradation of diphenyl-based friedel-crafts resins

Synthesis, Structure and Thermal Degradation of D i pheny I - Based Friedel- Crafts Resins Bryan Ellis and P. G. White* Department of Ceramics, Glasses and Polymers, University of Sheffield, Elmfield, Northumberland Road, SI 0 2TZ

Phillips type Friedel-Crafts resins have been synthesised from biphenyl V by reaction with dichloromethylbenzene 111, Mark I resins, without the specific addition of catalysts. Mark I1 resins have been prepared from biphenyl and p-xylylene glycol dimethyl ether IV, using stannic chloride as the Friedel-Crafts catalyst. The structures of these two polymers are similar but have differences of detail. The structures have been established by using methods including elemental analysis, molecular weight determinations and infra-red and n.m.r. spectra. Thermal degradation was studied using differential thermal analysis (d.t.a.) and stepwise and isothermal degradation procedures. The amount and composition of the volatile fraction were determined by mass spectroscopy. The fraction consisted of benzene, toluene and xylene together with traces of methane and, from the Mark I1 resin, methanol. The volatile product was a small fraction of the sample. The residue was the major part and was analysed by determination of its molecular weight and ultra-violet and n.m.r. spectra. It was concluded that during thermal degradation the main reaction was intramolecular scission. Subsequent reactions led to the formation of substituted 9-phenylfluorene units. These uolvmers had hkh thermal stability and degradation processes were very slow below about 45dOC.

-

1. INTRODUCTION

During the last decade there has been considerable research into the synthesis of thermally stable organic polymers and it is now well established that for high thermal stability the presence of aromatic or heteroaromatic structures is essential.’ Resins with direct linkage of aromatic rings, I, have

(11 been prepared using the Kovacic reaction in which both Friedel-Crafts and oxidation catalysts are used. Kovacic and Koch studied the polymerisation of benzene Bilow and Miller have prepared branched polyphenylenes by reaction in the melt to obtain polymers with good thermal stability. Aromatic amines have also been polymerised using ferric chloride 4 or aluminium chloride-cupric chloride as the catalyst-oxidant system and thermally stable prepolymers have been ~ r e p a r e d . ~ More work has been done on synthesising resins in which aromatic (or heteroaromatic) monomers are linked together by bridge units, X in I1 which must be thermally and oxidatively stable.

and

( 11) The intrinsic thermal stability of the linkage in 11 is lower than that of the direct aromatic linkage I; the thermal stability decreases in the order benzene > phenyl-phenyl bond

*Present address British Celanese Ltd., P.O. Box 5, Spondon, Derby DE2 7BP {Manuscript received 2 I June I 9 74 and accepted 2 7 August I 9 76)

> aromatic ether > aromatic amine > phenoxysilane and benzophenone > diphenylmethane (Hamman-Dale series).6 Generally it is found that chains of repeating units have lower stability than low molecular weight model compounds. However, this effect may be due to the presence of anomalous groups with low thermal stability from which thermal degradation is initiated. For instance, the thermal degradation of p~lybenzyl ,~ which has the repeat unit I1 with X 3 -CH2-, is initiated from anomalous substituted anthracene units. Thus, attainment of the intrinsic thermal stability of I1 can only be achieved by elimination or at least minimising the number of thermally labile groups adventi- tiously introduced during polymer synthesis. Although the methylene bridge unit has the lowest stability of those given in the Hamman-Dale series Phillips * has shown that it is possible to synthesis resins with good thermal stability in which the bridge unit is a methylene group, and these resins have been evaluated in carbon fibre composite^.^ Resins prepared by this method are used commercially; their properties been determined and they have been shown to be a useful addition to the range of thermosetting resins. lo Phillips used Friedel-Crafk catalysts together with difunctional reactants such as p-xylylene dichloride III, for “Mark I resins”, and p-xylylene glycol dimethyl ether IV for Mark I1 resins. It has been shown that many different aromatic monomers can be used for the synthesis of Phillips type Friedel-Crafts resins.s The Friedel-Crafts catalyst that has been found to be most suitable for the preparation of these resins is stannic chloride which is relatively mild and thus allows facile control of the reaction. With very reactive monomers addition of stannic chloride is un- necessary (see below). The purpose of this paper is to describe the synthesis, structural characterisation and thermal degradation of Phillips type resins prepared from biphenyl V by reaction with either 111, Mark I resin, or IV, Mark I1 resin.

THE BRITISH POLYMER JOURNAL, MARCH 1977 15

The reaction between V and III is facile and the use of stannic chloride catalyst is not required; the reaction pro- ceeds readily with evolution of hydrogen chloride when the temperature is raised above 130°C. The reactivity of V with IV is lower and a higher reaction temperature is required, up to 175"C, together with the use of stannic chloride catalyst, for the preparation of Mark I1 biphenyl Friedel-Crafts resins. Whilst the polymer formed by reaction of V with I11 or TV yields the same type of product there are detailed differences in their structures. These structural differences lead to differences in thermal stability which have been evaluated by determination of the composition of the solid residue. The Mark I resin has higher thermal stability than the Mark I1 polymer, both of which have higher thermal stability than polybenzyl prepared using similar methods. It is convenient to compare the thermal stability of these biphenyl resins with the polybenzyls because the latter may be regarded as the prototype of the Friedel-Crafts polymers and have been extensively studied previously using similar experimental method^.^

Table 1. Properties of Biphenyl Friedel-Crafts Polymers

2. EXPERIMENTAL METHODS 2.1 Preparation of the polymers The Mark I resins were prepared in a reaction vessel (500 ml split level flask) which was placed in an isomantle and fitted with a glass stirrer, thermometer pocket, gas inlet and condenser. The condenser outlet was connected to a grade one porosity sintered glass bubbler which was clamped near the bottom of a 6-1 beaker filled with distilled water and fitted with an efficient stirrer. During reaction dry nitrogen was passed through the reaction mixture and the hydrogen chloride evolved was carried over and dissolved in the water. The extent of reaction was determined by titration with 3N sodium hydroxide using methyl red as the indicator. To en- sure efficient absorption of the hydrogen chloride the water was stirred and kept slightly alkaline throughout the reaction. The reactionvessel was charged with 123.2 g of biphenyl V and 105 g of III, a molar ratios of 4 : 3, and the temperature gradually increased, until at 130°C the reaction mixture turned pink and hydrogen chloride was evolved. Therefore

Physical properties Molecular weight Softening point Appearance

Chlorinated solvents Benzene Ether Hexane Methanol

c , % H, % CI, % Total Ratio C/H

Infrared spectra (Films on KBr disks)

Sol u b il in/

Elemental analysis

Nuclear magnetic resonance spectra (in carbon tetrachloride solution at room temperature)

Mark I resin Mark I I resin

2470 ?; 40

White solid 70 - 78°C

Soluble Soluble Insoluble Insoluble Insoluble

92.5 6.12 1.56

100.18 15.1 1

2180 ?; 50

White solid 80 - 86OC

Soluble } Dissolution time longer Soluble Insoluble Insoluble I nsol u ble

than for Mark I resin

92.82 6.29 0.55

99.66 14.76

See Fig.1 for Mark I resin. Typical of aromatic compounds with diarylmethane- CH2-band at 7.03 p. Strong bands at 14.26 and 13.23 p, medium band at 12.25 p. Bands characteristic of biphenyl uni t 6.25, 6.4,6.6 and 6.7 p. Disubstituted aromatic rings indicated by bands at 15.6, 16.27 and 18.1 p, Band a t 2 0 . 7 5 ~ assigned to 1,2,4 trisubstituted aromatic ring. 5-6 p region - weak bands a t 5.1 5, 5.25, 5.55 and 5.75 1.1. The Mark II resin has a very similar spectrum but with a slightly stronger band at 12.25 p. The positions of the bands in the 5-6 p region are somewhat different - 5.15, 5.25, 5.58, 5.74and 5 . 9 ~ .

Aromatic CH, a broad band with Aromatic CH, a broad band with two main peaks 6 = 7.08

Aliphatic CH,, single broad Aliphatic CH,, a single broad band band 6 = 3.82

two main peaks 6 = 7.08

6 = 3.82 Ratio I C H / I C H ~ ( R , ) 3.18 R,= 2.78 Indication of end groups None

Aliphatic CH2-O-CH3, a small band at the base of the main -CH2- resonance. 6 = 3.24

16 THE BRITISH POLYMER JOURNAL, MARCH 1977

no catalyst was added, After gradually increasing the temperature to 160°C the mixture gelled after 83.5% of the ‘theoretical’ hydrogen chloride had been liberated. A soluble polymer is required for study of the effect of thermal degradation and this was prepared as above but the reaction was stopped after 70 min when 78.8% of the theoretical yield of hydrogen chloride had been liberated. The product was a colourless ‘gum’ and was purified by dissolving in 1,2-dichloroethane, filtering the solution to remove any residues and precipitation by adding the solution dropwise to ether. The precipitated polymer was filtered off and dried in a vacuum desiccator for two days. The Mark 11 resin was prepared in a similar way to that described for the Mark I except that catalyst was required and the extent of reaction was estimated from the amount of methanol collected. The reaction vessel was charged with 123.2 g of V and 89.6 g of N, a molar ratio of V to N of 4 : 3, and the temperature gradually increased to 100°C when the reactants formed a melt and then 10 ml of a 10% solution of stannic chloride in 1,2-dichloroethane was added, that is 3.85 x melt turned pink in colour and gradually darkened to a deep red. At 150°C a steady stream of methanol distilled off and the temperature was raised gradually to 175OC to maintain a steady reaction. The’reaction was stopped after 65 min when 77% of the theoretical yield of methanol had been collected. The Mark I1 polymer was purified in the same way as the Mark I polymer.

moles of catalyst to 0.8 mole of V. The

L 0

*= 80-

g 6 0 - ul 9 4 0 -

s 20-

2.2 Characterisation and thermal degradation of the polymers The structures of the polymers were determined using spectroscopic methods similar to those described for the characterisation of p~lybenzyl .~ Three types of thermal degradation experiment were used to assess the thermal stability of these resins: t.g.a. and step-wise and isothermal degra-

h - 100r

0 I 1 1 1 1 1 1 1 1 J

3 4 5 6 7 8 9 1 0 1 1

c 0

R 0 UI

.- Y

L

n a s

Wavelength m p A i l

l O O r

12 13 14 15 16 17 18 19 20 21 22 Wavelength m p

Fig.1 Infra-red spectrum of the Mark I biphenyl resins. The polymer film was ca 0.001 inch thick and was obtained by casting from 1,2dichloroethane solution (10 g/l) onto a potassiuiu bromide disc.

dation and these procedures have been described previously.’

3. STRUCTURE OF POLYMERS

The analytical and spectroscopic data used to characterise these polymers are summarised in Table 1.

3.1 Mark I resin The structure of the Mark I resin will be considered first and a tentative structural assignment made from the infra- red spectrum, Fig.1. The absorption in the 12-15 p region has three main bands which are characteristic of a p-substituted biphenyl unit, two very strong bands at 14.3 and 13.3 p and a medium band at 12.25 p. The positions of these bands agree reasonably well, with those of the model compound 4-methylbiphenyl given in a comprehensive study of the infra-red spectra of aromatic compounds.l The relative intensity of the band at 12.25 p is less than that of the related 4-methylbiphenyl, so substitution may not be ex- clusively in the p-position. However, from the infra-red spectra it appears that biphenyl is essentially monofunction- a l in these reactions. Other bands characteristic of the biphenyl unit are at 6.25 and 6.4 p 6.6 w and 6.7 s and several aromatic bands between 9 and 10 p.

In the longer wavelength region l2 there is a band at 16.6 I.( which is present in biphenyl and is characteristic of a monosubstituted aromatic ring. Disubstitution of the aromatic ring is indicated by the bands at 15.6, 16.27 and 18.1 1.1. The band at 16.27 p can also be present in monosubstituted aromatics and so it is not useful for structural assignment. The bands a t 15.6 and 18.1 p occur with 1 :4 disubstitution and this confirms the interpretation of the 12-15 p region that there is monosubstitution of the biphenyl probably in the p-position. The band at 20.75 p is assigned to 1,2,4- trisubstitution due to 111 having a functionality of 3, which will be discussed later. The aromatic absorption bands between 5 and 6 p have been used for the assignment of substitution pattern l 3 and although relatively weak they are resolved clearly enough to exclude the substitutions: tri-l,3,5, tetra, penta and hexa.13 From the band positions recorded in Table 1 the most satis- factory assignment for the substitution is mono, dip- or di-o- and tri-1,2,4. Thus, the spectral assignments of the 5-6 p region agree with those at longer wavelength except for the possibility of the presence of the di-ortho. From these considerations the basic structure of the Mark I biphenyl Friedel-Crafts resin is VI.

CH2 I

8 THE BRITISH POLYMER JOURNAL, MARCH 1977 17

In this structure 111 has reacted with two biphenyl units, V, A and C and each biphenyl unit has reacted only once. In general 111 does not react with itself when more reactive aromatic centres are present l4 although the self conden- sation of V does occur under suitable ~0ndi t ions . l~ How- ever, once both chloromethyl groups have reacted the aromatic ring B becomes reactive to electrophilic substitution. This is similar to the reaction of 111 with benzene catalysed by stannic chloride; the benzene is monofunctional and 111 is effectively trifunctional.16 Similar behaviour occurs with the present reactants even in the absence of added catalyst (see later), and the aromatic ring D in VI has reactive positions indicated by arrows. Reaction of a chloromethyl group of III at one of these positions will give a 1,2,4- trisubstituted aromatic ring. Further substitution is poten- tially possible but can only occur to a small extent because the presence of tetra substitution is not indicated in the infra-red spectral bands in the 5-6 1.1 region. Steric factors would limit the degree of substitution and in polybenzyl it was concluded that the polymer contained mainly trisubstituted aromatic rings.' For structure VI to be acceptable it must be consistent with the other data given in Table 1 and also must be modified to allow for the presence of chlorine in the molecule, since chloromethyl end groups are not present. If present in suf- ficient concentration chloromethyl end groups would have a n.m.r. methylene resonance at 6 = 4.50 which is not detected, Thus, it is concluded that ring chlorination has occurred in the Mark I polymer; such ring chlorination is common in Friedel-Crafts reactions and also occurs in the Mark I1 polymers which would not have chloromethyl end groups. Such ring chlorination also occurred in the synthesis of both Mark I and I1 type p~lybenzyls.~

a

9

A polymer with structural units VI is a linear polymer containing n repeat units W, together with an end group WI. Also, x of the structural units will have a chlorinated ring IX and one repeat unit will have an aromatic ring of type D VI. The structure of the average molecule may be calculated from equations (1) and (2)

M , = n * M v n + x (35.5 - 1) + M v m t 1 (1)

where M v n and Mvrn are the molecular weights of units W and VIII and 1 is added for the structural unit containing the D, VI ring.

x 35.5 - 100 C1% = -

Mn

Insertion of appropriate values from Table 1 into equations (1) and (2) yields:-

n = 8.9 x = 1.08

hence the average degree of polymerisation of the Mark I biphenyl polymer is 8.9 and the average molecule contains one ring chlorinated unit. Thus it may be that growth is ter- minated when a ring chlorinated unit IX is formed and the reactive ring D in VI is not present. From structure W with appropriate values of n and x it is possible to calculate both the carbon-hydrogen ratio and the ratio of aromatic to aliphatic protons, RH, from equations (3) and (4)

c ( n * 1 2 * 2 0 ) + 1 2 . 1 2 - -- H ( n . 1 6 ) + 9 - ~ + 1 (3)

(4)

Using p? = 8.9 and x = 1 yields C/H = 15.06 and RH = 3.2, both of which are in good agreement with the experimental determination of these parameters, Table 1. From similar calculations it becomes clear that if the functionality of the repeat unit VII is > 2 it is not possible to obtain agreement between calculated and experimental C/H and RH ratios. This does not exclude the possibility of the presence of a small number of anomalous units which will be discussed later.

3.2 MarkIIresin The structure of the Mark I1 polymer is essentially similar to that of the Mark I polymer, but with some differences which affect its thermal stability. There is a small band at 5.88 1.1 in the infra-red spectrum which is probably due to the presence of a carbonyl group formed by oxidation of a methyl ether group." The presence of MeO- groups in the Mark I1 polymer is established by the presence of a methyl group n.m.r. band at 6 = 3.24. Thus, the structure of the Mark I1 polymer can be represented by X, where for the average molecule n is the number of repeat structural units, which are the same as for the Mark I resin W, y is the number of units with unreacted methyl ether groups and x is the number of ring chlorinated units IX.

The following relationships hold for structure X

Mn = n * M ~ ~ tx(35.5 - 1) +MvnI + l+yM-,,,(la)


where M-OMe is the molecular weight of a unit with a ter- minal methyl ether unit

12n t 3y - x t 9 + 1 4n + 7y

R H =

Table 2. Therrnogravirnetric Analysis of Biphenyl and Polybenzyl Resins

To tat Activation weight Tt energy

Polymer loss, % 'C* kcalhote A

Using the analytical data given in Table 1 and solving the set of equations (la), (2) and (4a) simultaneously yields the following values for the average molecule n = 7 . 4 , ~ = 1 and x = 0.34. Insertion of these values into equations (la) and (4a) establishes the self consistency of this solution. Also, it is possible to calculate the C/H ratio from equation (3a); on substitution in

C n(12 20) + (12 12) +y(9 12) H 1 6 n + 9 - x + l +lOy (3a) - - -

the above values for n, y and x the calculated C/H ratio is 14.69 which is in good agreement with the experimental value, see Table 1.

4. THERMOGRAVIMETRIC ANALYSIS

Although many methods have been used for the analysis of thermogravimetric experiments the relatively simple method of Newkirk l 8 proved to be effective for the determination of activation energies for the thermal degradation of poly- b e n ~ y l s . ~ This method has been used to analyse the thermal degradation of these biphenyl resins and the appropriate graphs are given in Fig.2; also included is that for a polybenzyl prepared from benzyl ~ h l o r i d e . ~ The polybenzyl has a lower triggering temperature than these biphenyl resins, see Table 2, which also lists the kinetic parameters obtained from the t.g.a. experiments. Inspection of Fig.2 and Table 2 shows that these biphenyl resins have much better thermal stability than polybenzyl. This can be attributed to the absence of substituted anthracene units which are present in polybenzyl and are 'weak links' from which degradation is initiated. Thus, for polybenzyl there is a low temperature degradation process which has the lower activation energy, this does not occur with the biphenyl resins and hence a single activation energy is ob-

-1.0 -

y -1.5- 0 0 A

-20 -

-25 I I I I I I

1.25 1.30 1.35 140 1-45 1.50 1.55

Fig.2 Tga Newkirk plots for Mark I biphenyl resin. 8 Mark I1 biphenyl resin.

0 Polybenzyl prepared from benzyl chloride. Sample size: cu 25 mg temperature programme:- linear rate of increase l"C/min from 25 to 650°C

95.2 435 57.2 1.8 2.5 x 10l6 Mark I (Ill and V)

Mark II 91.9 420 64.0 f 6.4 3.3 x 10'8 ( IV and V)

Polybenzyl 89 372 El = 30.9 f 2.4 4.8 x lo7 (chloride (up to 40% Mark I) weight loss)

E,, = 74.5 3.2 1.1 1023

*Tt is the triggering temperature and is equal to the temperature at which dwldt is greater than 0.25% per min. after loss of low molecular weight material.

tained using the Newkirk method to analyse the t.g.a. experiments. The Mark I biphenyl resin has higher thermal stability than the Mark 11. This is similar to the higher thermal stability of polybenzyl prepared from benzyl chloride compared with those synthesised from benzyl alkyl ethers.7 The detailed differences in degradation of the Mark I and Mark I1 biphenyl resins will be discussed later following presentation of the results of stepwise and isothermal degradation experiments.

4.1 Stepwise thermal degradation In the stepwise thermal degradation experiments the polymer is held at successively higher temperatures for 1 h, the temperature being increased in convenient steps of 50°C. This procedure is very similar to that used by Madorsky and Strauss,19 but with a slightly different time temperature programme, and has been discussed previously for the degradation of polybenzyls.' In these experiments the total

Table 3. Volatile Fraction Obtained with Stepwise Degradationt

Degradation temp.

Composition of V R in mole %

Polymer 'c Benzene Toluene Xylene

Mark I

Biphenyl resin (I I I and V)

Mark II

Biphenyl (IV and V)

Polybenzyl (chloride)

400

450 500 400

450 500 200 250 300 350 400* 450*

68.6

78.2 22.7 28.8

49.2 18.2 0 0 1.4 1.6 8.0 26.2

14.5

13.2 36.4 21.9

35.5 39.0 100 1 00 98.6 98.4 90.2 69.0

16.9

8.6 40.9 49.3

15.3 42.1 0 0 0 0 1.8 4.8

t Sample heated from 200°C upwards at 50°C intervals each temperature was held constant for 1 h Methane was observed in the volatile fraction obtained after degradation at these temperatures but in too low a concentration to be determined quantitatively.


6.0

5.0

4-0 - 3.0 -

I I I I I I 1 200 250 300 350 400 450 500

Temp. O C

Fig.3 stepwise degradation of:-

Mark I biphenyl resin; 8 Mark I1 biphenyl resin; 0 Polybenzyl prepared from benzyl chloride; 0 ,Polybenzyl prepared from benzyl methyl ether

Percentage volatile fraction, vR.25, vs temperature from

amount of the degradation products volatile at 25"C, Fig.3, and also the composition of this fraction, Table 3, was determined by mass spectroscopy. For these biphenyl Friedel-Crafts resins the volatile fraction V ~ . 2 5 is very small at temperatures less than 400°C (8.016% for Mark I) and even at 500°C it is only 2.8% for the Mark I resin. The Mark I1 polymer is somewhat less thermally stable, V R . ~ ~ obtained at 500°C is 3.2%, Fig.3. These volatile fractions are very much less than those obtained on thermal degradation of polybenzyl, see Fig.3. For an equivalent VR .26 the degradation temperature has to be approximately 150 higher for the biphenyl polymers than for polybenzyl, see Fig.3. The composition of the volatile fraction obtained from the biphenyl resins is quite different from that evolved from the polybenzyls, see Table 3. At lower degradation temperatures the percentage of benzene in VR .25 is high and decreases at 500°C. In the V ~ . 2 5 obtained from the polybenzyls the yields of benzene at degradation temperatures below 250°C were too low to be measured quantitatively; however with increasing temperature the relative amount of benzene increases monotonically. It is important to note that although the percentage of benzene in V R . ~ ~ is large the absolute amount is small. Thus for the Mark I biphenyl resin the amount of benzene formed at 400°C is only about a tenth of that formed during the degradation of polybenzyl at the same temperature. This is because of the very small absolute amount of VR.Z5 (0.016%) for the biphenyl Mark I resin compared with that of polybenzyl(1.6%) at 400°C. The product distribution in V R . 2 5 will be discussed in relation to the isothermal degradation experiments. Since the volatile fraction is small the major product is the residue which is deep red in colour. It is possible to analyse the residue obtained from the isothermal experiments and hence gain further insight into the thermal degradation of these resins.

4.2 Isothermal degradation In the isothermal experiments small (0.1 g) polymer samples were sealed under vacuum and heated isothermally for specific times at controlled temperatures. Rapid initiation and quenching allowed accurate specification of the thermal

0 2 a >

- after 60 rnins.

4.0

3.5

3.0 -

Time ( m i n l Fig.4 Percentage volatile fraction VR YS time for isothermal degradation for:- Q Mark I biphenyl resin at various temperature (2) 415"C, (3) 440°C

(4) 465"C, (5) 490°C. (1) 390°C is omitted for clarity - after 1 h VR < 0.01%;

0 Polybenzyl prepared from benzyl chloride and degradation at

Polybenzyl prepared from benzyl methyl ether and degradation 440°C;

at 415°C

treatment. The volatile fraction VR was analysed by mass spectroscopy.and the results are summarised in Fig.4 for the Mark I and Fig.5 for the Mark I1 biphenyl Phillips-type Friedel-Crafts resins. By comparison with Fig3 it can be seen that the volatile fraction is about the same in both the stepwise and the isothermal experiments. However, the differences in thermal stability of the Mark I and Mark I1 biphenyl are more clearly illustrated in Fig.5. The equivalent volatile fractions obtained from polybenzyls are also shown

TO 6.08*/* After 60mins

3.5

0 %

K >

Time tmin) Fig.5 degradation for -& Mark I1 biphenyl resin at various temperatures: (2) b 41SaC, (3) 6 440"C, (4) d465"C, (5) 0 490°C. (1) 390°C is omitted for clarity - after 1 h VR = 0.011. &Mark I Diphenyl resin degradation at 465°C.

Polybenzyl prepared from benzyl methyl ether and degradation

Percentage volatile fraction VR YS time for isothermal

at 415°C


80 I

80 70 60 50 40 30

0: 70 w.. 60 * 50

40

>

- - - - - -

d n 2 3 m" 10

0 10 20 30 40 50 60 70 Time (min)

Fig.6 isothermal degradation of biphenyl Mark I resins at various temperatures. (1) -0 390"C, (3) 9 440°C, (4) Q465"C, (5) 0. 490°C, (2) 415°C omitted for clarity - similar to (3). 0 Polybenzyl prepared from benzyl chloride and degradation at

Percentage of benzene in the volatile fraction IS time for

440°C

on Figs 4 and 5 which demonstrate very clearly that the yield of volatiles from the biphenyl resins is much less than that from polybenzyl. It should be noted that the V , values are small fractions of the total weight of the polymer sample. Thus, that obtained from the biphenyl resins on degradation at 390°C is too small to show on Figs 4 and 5; it is less than 0.01% even after thermal treatment for 1 h. The volatile fraction increases with severity of thermal degradation; thus after 1 h at 490°C it is 2.1% for the Mark I and 2.78% for the Mark I1 biphenyl resins, again illustrating the lower thermal stability 'of the Mark I1 polymer. The composition of the volatile fraction was analysed by mass spectroscopy and the major products were benzene,

20

101 4-Q=-=- O C

10

320 i 0 380 420 460 500

Temp. 'C Fig.7 degradation for biphenyl Mark I resin. 9 Benzene, p Toluene, Q Xylene, 0 Benzene % in VR from the degradation of polybenzyl prepared from benzyl chloride.

Benzene in VR.ZS from biphenyl Mark I polymer with stepwise

Composition of VR vs temperature after 1 h isothermal

degradation

toluene and xylene. Minor amounts of methane, and from the Mark I1 polymer methanol, were detected but were in so, low a concentration that their amounts could not be determined. The percentage of benzene in the volatile fraction for the Mark I resin is given in Fig.6; it is higher than for the polybenzyl (chloride - Mark I), but the absolute amount of benzene formed at 440°C after 1 h is about 10 times larger for polybenzyl (chloride) than for the Mark I biphenyl resin. This is because of the much later volatile fraction formed during degradation of polybenzyl. The relative % composition of the volatile fraction changes with both time of degradation at constant temperature and with temperature of degradation. The proportion of benzene in the volatile fraction increases with duration of thermal treatment at 390°C but at higher temperatures it decreases with longer times of degradation, see Figd. At intermediate degradation temperatures (41 5-44OoC) there is little change in the relative amount of benzene formed with extent of degradation. Because the relative proportion of benzene in the volatile fraction has a maximum value of about 75% at 390"C, Fig.7, it is clear that the detailed degradation mechanism must change with temperature. In Fig.7 the data points obtained from the stepwise degradation are included and it can be seen that there is good agreement between both types of experiment. The difference in behaviour between the biphenyl and polybenzyl Mark I type resins is illustrated in Figs 6 and 7. For the polybenzyl there is a monotonic increase with degradation temperature of the relative amount of benzene present in the volatile fraction. The behaviour of the Mark I1 biphenyl resin is similar to that of the Mark I except that the relative proportion of benzene in the volatile fraction is lower, see Fig.8. Since the volatile fraction consists mainly of benzene, toluene and xylene an increase in the relative proportion of benzene must be accompanied by a decrease of the relative proportions of one or both of the other two components. Thus, for the Mark I biphenyl resin the maximum relative proportion of benzene at about 400°C is 'mirrored' by corresponding minima in the relative proportion of toluene, see Fig.7. The changes with time in the relative proportions of toluene and xylene in the volatile fractions obtained

0 C 0 N c 5

101 3 a - - - 0 lG 20 30 40 50 60 '10

Time (minl Fig.8 isothermal degradation of biphenyl Mark I1 resins at various temperatures. (1) + 390°C. (2) '0 415°C. (4) d 645°C. (3) 440°C is almost identical to (2) and is omitted for clarity. 0 Biphenyl Mark I resin degradation at 440°C.

Polybenzyl prepared from benzyl methyl ether and degradation

Percentage of benzene in the volatile fraction vs time for

at 440°C


70 80t

340

20 10

300 380 420 460 500

320 360 400 440 480 520

Temp. *C Fig.9 degradation for biphenyl Mark I1 resin. d Benzene, '0 Toluene, d Xylene.

Benzene % in VR from the degradation of polybenzyl prepared

Benzene in VR.Z~ from biphenyl Mark I1 polymer with stepwise

Composition of VR us temperature after 30 min isothermal

from benzyl methyl ether.

degradation

K 5 0 - > 0 'c 4 0 -

30 - 2 0 -

0 10- a 'ij 0 -

L, c

c

- 50 0

0 30 C

10

0 10 20 30 40 50 60 70 Time (min)

Fig.10 Percentage of toluene and xylene in the volatile fraction vs time for isothermal degradation of biphenyl Mark I1 resins at various temperatures. A Toluene, B Xylene, (1) (5) @ 490°C. For toluene (4) 465°C almost identical with (3) omitted for clarity

390"C, (2) b 415"C, (3) 6 440"C, (4) d 465"C,

from the Mark I1 biphenyl resin are shown in Fig.10. The relative proportion of xylene decreases with increase of both time and temperature of degradation. This decrease in the relative amount of xylene is also shown in Fig.9; as the yield of benzene increases that of xylene decreases. At higher temperatures the relative yield of toluene increases with increased degradation temperature, Figs 9 and 10. The molecular weight of the residue decreases with increased time and temperature of degradation as in Fig.11 and 12 for Mark I and Mark I1 biphenyl resins respectively. From the

changes in molecular weight it is possible to calculate the number of bond scissions of 'cuts' using equation (5).20

C(t) = N [l/@n@)I - [1/&(0)1 ( 5 )

where c(t) is the total number of cuts per gram occurring in time t, N is Avogadro's number, Rn(0) and M,(t) are number-average molecular weights initially and after time t .

2,6 00 1

2,000 1,800 1,6 0 0

c II:

2 0 0 1 I,I,II 0 15 30 45 60 75

Time (min) Fig.11 Number average molecular weight vs time for isothermal degradations at biphenyl Mark I polymer at various temperatures. (1) -0 390"C, (2) p 415"C, (3) 9 440°C, (4) 0, 465"C, (5) @ 490°C. 0 Polybenzyl prepared from benzyl chloride. x indicates magnitude of the correction for the volatile fraction

2pooL 2,400

0 10 20 30 40 50 60 70

Time (min) Fig.12 Number-average molecular weight us time for isothermal degradation of biphenyl Mark I1 polymer at various temperatures. (1) -0 390"C, (2) b 415"C, (3) 6 440"C, (4) d 465"C, (5) 0 490°C.

Polybenzyl prepared from benzyl methyl ether


f

60 50 60 30 20 10-

e, a

- - - - -

L

cn 0 J

Fig.13 biphenyl Friedel-Crafts resins and polybenzyl prepared from benzyl isopropyl ether.

Mark 1 biphenyl resin, Ecuts = 36.5 f 3.5 kcal/mole 8 Mark I1 biphenyl resin, Ecuts = 30.4 f 2.3 kcaYmole Polybenzyl, Ecuts = 28.9 f 2.2 kcal/mole

Arrhenius plots for cuts during isothermal degradation of

The number average molecular weight used with equation (5) is that of the whole sample and is calculated by correct- ing the molecular weight of the residue by allowing for the amount of the volatile fraction. This correction is very small except for degradation at longer times and higher temperatures. The rate of scission may be estimated from the inverse of the time required for an equal number of cuts, AtT, at temperature T. Then an activation energy, E, for the scission reaction may be obtained by use of the Arrhenius equation in the form (6)

log AtT = C + (E/RT) ( 6 )

This procedure was used previously to determine the activation energy for ‘cuts’ during the degradation of polybenzyls and its application is d i sc~ssed .~ Linear Arrhenius type plots are obtained, see Fig.13. The higher thermal stability of the biphenyl resins is again demonstrated and the Mark I resin is more stable than the Mark 11; at a given temperature AtT is larger for the more stable polymer. The activation energies for cuts are given in the caption to Fig.13 and are

Temp. ‘C Fig.14 Changes in n.m.r. spectra. Changes in R H , the ratio of aromatic to aliphatic protons, YS temperature during isothermal degradation of biphenyl resins. Q Mark 1 6 Mark I1

lower than those obtained from the t.g.a. measurements, see Table 1. In general, agreement between the activation energies for ‘cuts’ in the isothermal degradation and t.g.a. would not be expected because in the latter experiments it is the yield of volatile products at higher temperatures which will determine the measured activation energy. The ‘cuts’ must occur intramolecularly and since the volatile fraction is small the products of the scission reactions are not volatile; that is, different processes are measured. That quite complex changes occur in the solid residues is shown by their n.m.r. spectra, especially for the residues after degradation at higher temperatures. At all degradation temperatures there is an increase in the ratio of aromatic to aliphatic protons; R H increases with degradation for both Mark I and Mark I1 resins, see Fig.14. At the lower degradation temperatures 390 and 41 5°C the only detectable change in the n.m.r. spectra for both Mark I and I1 resins, is the relatively small increase in R H . At higher degradation temperatures the changes in n.m.r. spectra are different in detail for the two resins. For the Mark I the presence of a methyl group is detectable by the n.m.r. band at 6 = 2.28 in the residue left after degradation at 440°C. The amplitude of this methyl resonance increases with degradation temperature and the ratio of methylene to methyl protons decreases from 5.5 to 0.74 for degradation at 465 and 490°C respectively. For the residue obtained at 490°C the methyl group resonance is split into a doublet at 6 = 2.28 and 2.36 indicating that their must be two different sites for the methyl groups. There are also changes in the positions of the methylene and aromatic proton resonances. Thus, in the products obtained after degradation at 440,465 and 490°C the methylene shifts to 6 = 3.83,3.87 and 3.92 from 6 = 3.82 in the original resin. The n.m.r. band for the aromatic protons progressively broadens, indicating an increasing degree of substitution of the aromatic rings together with the concurrent loss of aliphatic protons, as shown by the increase in R H , see Fig.14. The presence of a =CH group is indicated by a n.m.r. band at 6 = 4.83 in the residues after degradation at 465°C for both the Mark I and I1 resins. With increased degradation the residue becomes darker in colour and a band at 305 mp appears in their U.V. spectra. The intensity of this band increases with severity of degradation (see Fig.15) and there is little difference between the intensity of this band in residues obtained from the Mark I

€305

Time (rnin)

Fig.15 Changes in U.V. spectra. Changes in specific absorption coefficient at 305 mp YS time for degradation of Mark I biphenyl resin at various temperatures. (3) Q 44OoC. (4) 9 465°C. (5) 0- 49OoC. The band at 305 mp was detected in the early stages of degradation at 415’C but was of low intensity. The changes in specific absorption coefficient of the Mark I1 resin are almost identical with those of the Mark 1 and data points have been omitted for clarity


and I1 resins degraded under the same conditions. The position of this band, together with the n.m.r. proton resonance due to a SCH group indicates the formation of substituted 9-phenylfluorene units XI on degradation of these biphenyl polymers.

5. DISCUSSION

Degradation at lower temperatures, that is below 400°C is slight for both Mark I and Mark I1 resins. Thus, the t.g.a. triggering temperatures Tt are well above 400°C; the reduction of molecular weight (Figs 11 and 12) after 1 hour at 390" is equivalent to one intramolecular cut per 10 molecules. The number of cuts per polymer molecule C, is calculated from equation 7.

(7 )

which is obtained by rearrangement of equation (5). The number NVex of aromatic molecules of type x per polymer molecule in the voltatile fraction can be calculated from

VR is the percentage volatile fraction (Figs 4 and 5). X is the percentage of aromatic m_olecules of type x in the volatile fraction (Figs 7 to 10). M, and M, are the molecular weights of the polymer and aromatic molecule. To calculate the number of aromatic molecules per polymer molecule it is necessary to sum equation (8) over all the species present in the volatile fraction, that is:

Nv = EN"., (9)

Because other species are present only in trace amounts the summation has to include only the yields of benzene, toluene and xylene. Using the data for the degradation for 1 h at 390°C of the Mark I biphenyl resin and equations 8 and 9 the loss of aromatic molecules corresponds to one for every 300 resin molecules. This compares with about one aromatic molecule for 40 molecules of polybenzyl (chloride) after degradation for 1 h at 365°C. This again illus- trates the higher thermal stability of the biphenyl Friedel- Crafts resins. Because the scissions at 390°C that cause a decrease in molecular weight are about 30 times more common than scissions which yield monomer it is clear that cuts must occur at main chain bonds a and b in a repeat unit such as VII rather that at peripheral bonds in XII.

8 The ratio of scissions to aromatics is reduced at higher degradation temperatures; it is about 6 for the Mark I and 9 for the Mark I1 resins on degradation at 490°C for 1 h. Thus, even with the highest yields of the volatile fraction, there are many scissions which lead to lower molecular weight residues and not to the formation of small aromatic molecules. Therefore, the argument regarding preferential scission of bonds a or b in the repeat unit VII apply at all stages of the degradation. From inspection of the structural formula VII for the Mark I biphenyl resin it is clear that toluene will not be the pri- mary product of a single scission reaction. Of course, such few cuts could be due to scission of bonds between anomalous groups, but they would be present in such low concentration that only thermal degradation studies will show their presence (see latter). That major changes occur in the resin due to scission of main chain bonds a and b of the repeat unit VII is shown, not only by the decrease in molecular weight, but also by the changes in U.V. and n.m.r. spectra, Fig.14, and have been interpreted as due to the formation of substituted 9-phenylfluorene units XI. A possible route for the formation of substituted 9-phenylfluorene units requires a sequence of reactions following scission of main chain bonds a or b in the repeat unit VII. Thus scission of bond b will yield radicals XI11 and XIV.

+CH2 I

1 + a

Scission of bond a will yield similar radicals but with a slightly different substitution pattern. Radicals such as XIII and XIV formed by main chain scission will react by hydrogen abstraction. The only sites in these resins with


readily available hydrogens are the methylene groups, which will react with XIII to form XV and XVI.

Q Q + I

The presence of methyl groups in the residue is established from their n.m.r. spectra, 6 = 2.28. The relative intensity of the methyl band increases with extent of degradation. Thus n.m.r. evidence supports the formation of a species similar or related to XV. It is the presence of the radical XVI that is essential for 9-phenylfluorene to be formed by combination with an aromatic free radical. Thus, mutual combination of XVI with XIV will give XVIII from which a substituted 9-phenylfluorene XIX is formed.

Methylenic radicals similar to XVI may be formed by abstraction from different methylene sites. Also any aromatic radical combining with XVI or similar species could poten- tially lead to the formation of 9-phenylfluorene derivatives, although some would be more sterically restricted. Aro- matic radicals may also be formed by loss of hydrogen atoms which may occur at temperatures approaching 500°C.21 Such aromatic radicals could combine with XVI.

An alternative route for the formation of 9-phenylfluorene which is not directly related to a scission reaction is as follows. A substituted diphenylmethane unit XX con-

denses to form a substituted fluorene unit XXI which is subsequently converted to a substituted 9-phenylfluorene by hydrogen abstraction and radical combination with an aromatic radical.

P2\

Hydrogen abstraction followed by radical corn bination

Support for these reaction schemes is provided by the studies of the pyrolysis of diphenylmethane 22 which at 475'C yields fluorene together with other products and similar condensations XX += XJU have been observed.22 These reaction schemes will not account for the formation of low molecular weight volatile products. Benzene is not formed by scission of the bond in a biphenyl unit because this bond has high stability as discussed in the introduction. Only at higher pyrolysis temperatures, above 1000°C, is benzene formed from biphenyl pyrolysis, even then the yield is only 20%. The major product is a tar containing p ~ l y p h e n y l s . ~ ~ The presence of a few anomalous structural units due to reaction of a second dichloromethylbenzene unit with biphenyl is not excluded by the data presented to establish structure W. Such an anomalous unit would have the structure XXII, and scission of bond cy followed by hydrogen abstraction and scission of bond 0 will yield benzene and bond y toluene.

W to those of

Anomalous units such as XXII if present may not be the only source of benzene and toluene. Hydrogen abstraction by XIV followed by the same sequence of reactions as that given for XW will also yield benzene and toluene. Because of the maximum in the proportion of benzene in the volatile fraction and the corresponding minimum for toluene, Fig.7, there must be at least two different routes with different activation energies for the formation of one of these products.


Xylene may be formed from XXII if either bond y or y1 is cut initially to yield a species such as XXIII.

6 XXTV will form xylene by hydrogen abstraction and is an intermediate in the degradation of p~ly-p-xylylene.~~ An unzipping reaction occurs in the degradation of poly-p- xylylene but that is not possible during the pyrolysis of these resins. Such a reaction has been discussed in relation to the degradation of p o l y b e n ~ y l . ~ > ~ ~ The degradation of the Mark I1 resins is essentially similar to that of the Mark I polymers: there are differences of detail due to different structures of the two resins, represented by W and X. The yield of xylene from the Mark I1 resins is higher at low degradation temperatures and may be due to the presence of more anomalous units such as XXII, from which xylene is formed. Increased relative reactivity of IV with the biphenyl unit could be due to the more bulky ether group restricting reaction with reacted IV. The major degradation reactions for the Mark I1 resin are scissions leading to a reduction in the molecular weight of the residue. These proceed faster for the Mark I1 than the Mark I resins, which could be due to more facile scission of either bonds a or b in XXV compared with bonds a and b in w.

This paper covers the investigations undertaken by the authors on the thermal stability of the biphenyl-based resins. In many possible applications this could be less important than their oxidative resistance. Oxidative studies on thin films of resins in air at temperatures in the range 100-210°C have been carried out using infra-red methods to measure quantitatively the formation of carbonyl groups. This method is similar to that usee by Conley 26 for the oxidation of polybenzyl. Carbonyl groups are formed in both polymers due to the oxidation of the methylene groups between the phenyl rings.

0 Air . ... B g a The oxidation of the biphenyl-based Friedel-Crafts resins is faster than‘that of polybenzyl and will be discussed in detail in a subsequent paper. The thermal degradation of these resins is slow. They have good thermal stability. It would be a pity if the present detailed discussion of the mechanism of their degradation ob- scured the fact that these resins are thermally stable and with suitable choice of reactant proportions highly stable crosslinked polymers can be produced. In the present study the formation of network polymers was prevented in order to be able to analyse the pyrolysis residues. The Mark I resin is more stable than the Mark 11, but evolution of HCl during fabrication is unacceptable. However, it would be possible to prepare prepolymers using the Mark I route and use a Mark I1 route to cure the resins during moulding. The differences in thermal stability between these Mark I and Mark I1 resins are much smaller than the differences in thermal stability of polybenzyls prepared by Mark I and I1 routes. Thus, use of a Mark I1 route for curing the resins would not lead to much lower thermal stability. The high thermal stability of these resins is due to the intrinsically high stability of the biphenyl units, scission reactions occurring at methylene aromatic bonds.

6. ACKNOWLEDGEMENTS a b

8’ 6

CH2

Scission of bond b in XXV yields W I and XIVa similarly to the scission of bond b in VII.

Thanks are due to the Ministry of Defence/Procurement Executive who supported this work. We are grateful to Miss M. A. McKinnon for the elemental analysis and Mr Tyson for the n.m.r. spectra; both at the Chemistry Department. To Mr L. N. Phillips, Dr G . Wood and Dr Brenda Parker of the Royal Aircraft Establishment for their continued interest and support of this work. Crown copyright reproduced by permission of the Controller of Her Majesty’s Stationery Office.

References 1

2

Idris Jones, J. J. Macromolec. Sci., 1968, C2, 303; Chemy Britn, 1970,6, 251 Kovacic, P.; & Koch, F. W. in “Encyclopedia of Polymer Science and Technology” ed. H. F. Mark, N. G. Gaylord and N. Bikales, Interscience, New York, 1969,11,380; Noren, G. N.; & Stille, J. K. J. Polym. Sci, D. 1971,5 ,385 Bilow, N.; & Miller, L. J. J. Macromolec. Sci. 1967, A-1, 183 Bingham, A.; & Ellis, B. J. Polym. Sc i , A-1, 1969,7 ,3229 Ellis, B.; & Stevens, J. V. J. Polym. ScL, A-1, 1972, 10,553 Hamman, W. C. WADC-Tech. Rep. 1958,57-657, pp. 158- 171; ASTIA Doc. No. AD 142285; Dale, J. W.;McElhill, E. A.: O’Neill, G . J.; Scheurer, P. G.; & Wilson, G. R. WADC-Tech. Rep. 1961,59-95, Parts I , I1 and 111: ASTIA Doc. No. AD 267980

3 4 5 6


7

8 9 10

11

12

13

14

15

16 17

Ellis, B.; & White, P. G. J. Polym. Sci, Polym. Chem., Edn., 1973,11,801 Phillips, L. N. Trans. Plast. Inst., Lond., 1964,32,298 Parker,B. M. R.A.E. Tech. Rep. 1973, No. 72220 Harris, G. I.; Edwards, A. G.; & Huckstepp, B. G. Plast. Polym., 1974,42,239 Cannon, C. G.; & Sutherland, G. B. B. M. Spectrochim. Acta, 1951,4,373 Stewart, J. E. in “Interpretive Spectroscopy”, Ed. S. K. Freeman, Reinhold, New York, 1965, Chapter 3, p. 144-5; Stewart, J. E.; & Hellman, M. J. Res. Natl. Bur. Stand., 1958, 60,125 Szymanski, H. A. “I.R. Theory and Practice of Infra-red Spectroscopy”, Plenum Press, London, 1964, p.273 Grassie, N.; & GiUes, J. J. Polym. Sci. Polym. Chern, Edn, 1973,11,1531 Iovu, M.; & Tudorache, E. Makromolek. Chem., 1971,147, 101; 1973,166,51 Grassie, N.; & Meldrum, 1. G. Eur. Polym. J., 1968,4,571 Eichel, F. G.; & Othmer, D. F. Ind. Engng Chem., 1947,41, 2623

18 19

20

21

22

23

24

25

26

Newkirk,A. E. Analyt. Chem., 1960,32,1558 Madorsky, S . L. “Thermal Degradation of Organic Polymers”, Interscience, New York, 1964 Tobolsky, A. V.; Kotliar, A. M.; & Lee, T. C. P. in “Thermal Stability of Polymers”, Ed. R. T. Conley, Marcel Dekker, New York, 1970, p.94 Fitzer, E.; Muller, K.; & Schaefer, W. in “Chemistry and Physics of Carbon”, Ed. P. L. Walker, Marcel Dekker, New York, 1971,7,279 Madison, J. J.; & Roberts, R. M. Ind. Engng Chem., 1958, 50,237 Lahaye, J.; Palmer, H. B.; & Sharma, R. K. Carbon, Oxford 1968,6,419 Jellinek, H. H. G.; & Lipovac, S . H. J. Polym. Sci , A-I, 1970, 8,25 17; Jellinek, H. H. N.; & Ronel, S . H. J. Polyrn Sc i ,

Lenz, R. W.; Luderwald, I.; Montaudo, G.; Prozybylski, M.; & Ringsdorf, H. Makromolek. Chemie, 1974,175,2441 Conley, R. J. J. Appl. Polym. Sci, 1965,9,1107

A-I , 1970,8,2605


synthesis, structure and thermal degradation of diphenyl-based friedel-crafts resins

Documents