Urea-Formaldehydeand MelamineFormaldehyde Condensations MECHANISMS H. P. WOHNSIEDLER Stamford Research Laboratories, American Cyanamid Co., Stamford, Conn.
A
MINO resin chemistry has developed coincident with high polymer chemistry in general. From some oft he early confusion in this field several theories emerged to account for the particular properties of this group of resins. These have been largely structural, dealing with the types of reactions involved in molecular growth and cross linking, nature of end groups, and likely polymer size. Urea was considered to have the carbamide structure, and it was recognized very early t h a t through reaction with formaldehyde two fairly well defined addition products formed-monomethylolurea and dimethylolurea (9, IO). Later, when the p r e p aration of the urons was described ( l 7 ) , it became obvious t h a t all of the hydrogens in urea could be replaced by methylene radicals, In the case of melamine all of the hydrogens were found replaceable with methylol groups and tri- and hexamethylolmelamine have been described (IS,16,S I ) as substantially pure compounds. Compositions approximating di-, tetra-, and pentamethylolmelamine have also been prepared.
The methylol derivatives are crystalline according to x-ray diffraction patterns. Their melting points are not well defined, because of their reactivity a t their melting temperatures, especially in the presence of traces of impurity. However, their chemical analysis, x-ray pattern, and other properties confirm their identity as discrete individuals. They are the established monomers in the two polymeric systems. FOUR PRIMARY REACTION MECHANISMS
Four principal mechanisms have been advanced to account for the first stage in polymer formation from monomeric derivatives:
0 where R represents HzN-C-
’
,
I/
HzN-C
@-\ NI
i-
or I iJH2
/NHCHzOH
c=o \NH2
I. METHYLENE BISAMIDE
\NHCH20H
RNHCHzOH Monomethylolurea (Hydro.xymethyl)urca
Dimet hyloluren 1,3-Bis(hydroxymethy1)urea
+ HzNR +RNHCH2NHR + H2O
11. METHYLOL METHYLENE BISAMIDE OH
OH
CH2
CHz
I
RNHCHZOH
I
I
+ H N - R +RNHCHJ& - R + HzO
111. ETHER RNHCHzOH IV.
+ HOCHzNHR +RIVHCHIOCH~NHR+ HzO
AZOMETHINE
RNHCHzOH +HzO
Dimethylolmelamine
+ RS=CHz
, CM2,
RN
N2,N4-Bis(hydroxymethyl) melamine
K““ I
dN-””a =O \N-CH/
I
Methyl01 derivatives may be modified by reaction with various compounds. These lead to the formation of ionic complexes or modified structures which adapt the polymers for specific uses, The introduction of such groups introduces new factors into the general polymerization mechanism.
CH2
I
RNHCHzOH
0 Hexamethylolmelamine Hexakis( hydroxymethyl ) melamine
I
?N/””R
I
CHz
NR
I
AHs RNHCHiOH
N,N’-Bis(methoxymethy1)uron
+ R’OH*RNHCH2OR’
+ R‘\ Rf/
2679
XH
+RNHCHzN/R’ \R’
INDUSTRIAL AND ENGINEERING CHEMISTRY
2680
1
KH2 HOCH2iYH-C
P\C-SHCH20H
I1
N
I
SH
NH2+I C1I
0
b
HOCHJYH KHCHZOH
+ KaHS08 --+
d"
HOCH2NH NHCHZSO,
Ka
+
DEVELOPMENT O F STRUCTURAL THEORY
Carothers (6) in his classification of polymers distinguished between addition and condensation polymers. On the basis of his classification urea-formaldehyde and melamine-formaldehyde polymers are condensation polymers. I n their formation, however, a participating addition mechanism is not precluded. Carothers also took the view t h a t "C (condensation) polymerization involves t h e use in a multiple fashion of t h e typical reactions of common functional groups." I n the reaction of formaldehyde with t v o molecules of an amide to form a methylene bisamide linkage there exists a clearcut prototype of a participating reaction in the formation of urea-formaldehyde polymers. The methylene bisamide may be considered to be an amino formal analogous to an alcohol formal. The analogy may be carried further to include the similarity of methylol amide to the alcohol hemiformal:
Vol. 44, No. 11
troducing as it does a functional group which can readily lead to the cross linking of linear or multiring type chains, it deserves special mention. I n this connection. some early work of Einhorn has not received t h e emphasis it deserves. This author prepared a number of niethglol methylene amides which are listed in Table 11. These were regarded as more stable than the methylol derivatives. I n the methylol methylene amides both hydrogens attached to one of the nitrogens are replaced by radicals. Assuming t h a t in polymers of the amino resin type the nitrogen atoms are linked through all three valences, a precedent for this type of bonding may be found in the uron structure and in the tetramethylol to hexamethylol melamines where available hydrogens are progressively replaced by methylol groups. I n some early experimental work, Walter (33)examined various precipitates and fractions obtained by reacting solutions of monomethylolurea and dimethylolurea at different pH values. Bv chemical analysis, determination of end groups and molecular weights in formic acid, he assigned t h e following structures to several derivatives: H~N-CZN 1,2-MethyIeneurea 1 1 (insoluble monomer) 0-CHI
0 I'
Methyleneurea (polymerizable monomer)
H2S-d--T\'=CH2 XH2
I
C=O I
,,,,k--CH2 CHz \N-CH2/
\o
3,5-Dicarbamyltetrahydro1,3,5-oxadiazine (slightly soluble ring-type dimer)
I
b=O $H2 These derivatives are characterized by methylene and ether linkages and terminal azomethine groups. Walter considered
HEMIFORMALS
(Form under neutral or alkaline conditions) Alcohol Type
CHZO
+
/OH HOR ---f CHs
TABLE I. TYPICAL METHYLEKE BISAMIDES AXD METHYLEKE BISUREaS
\OR Amino Type
CH20
N.P., C. 142-143 196 219 195-197 185-186
N X'-Methylenebisformamide N:iV'-Methylenebisaoetrtmide h',N'-Methylenebisbenzamide N,N'-Methylenebissalicylamide N,N'-Methylenebiaacrylamide 1 , l'-Methylenebis(l,3-dimethylurc :a) 1 , l'-Methylenebis(3-ethylurea) 1,I. '-Methylenediurea
+ HZNCOR +CH2/OH \"COR
FORMALS
TABLE rr.
(Form under acid conditions)
METHYLOL
References
149-161
115-116 21 8
METHYLENE
DERIVATIVES
OH AH*
Alcohol Type
(RCONHCH2ACOR)
R M.P., ' C . Reference A'-Methylol-N,N'-methylenebisbenzamide CBHS 182-185 (8) h. N-Methvlol-N,N'-methylenebispropionamide CZH5 76.77 (9) c. l-Methylol-l,l'-methylenebis(3ethylurea) C9H5SH 168- 170 (8) Einhorn han pointed out that dimethylolbenzamide is unstable and decomposes t o the monomethylo1 derivative. The methylolmethylenebisbeneamide ( a ) gradually breaks down in solution to the methylenebisbenzamide. /CHzOH CBH~CON CsHsCONHCHzOH 4- CHzO \CH*OH CeHsCONH CeHsCONH >CHz \CHz CHzO CeHsCONCHzOH CeHsCONH/
a.
H
Amino Type
" A large number of methylene bisamides and methylene bis(alkylureas) have been reported. l,l'-h/IethyIenediurea was f r a t prepared by Kadowaki ( 1 7 ) . Table I lists these various derivatives. T h e reaction between two methylol amides to give a methylol methylene bisamide (Table 11) is an important possibility in the urea-formaldehyde and melamine-formaldehyde systems. In-
--+
+
+
Methylolmethylenebispropionamide ( b ) was distinguished from monomethylolpropionamide b y solubility in ether, benzene, and chloroform and greater solution stability. Einhorn was u.nable t o prepare & monomethylo1 derivative of ethylureo under the usual conditions b u t isolated the methylol methylene derivative ( c ) .
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1952
t h e azomethine group as an important element in polymerization mechanism and formulated this mechanism in the following manner. HOCH2-NH*CO-N=CHz
,
HOCH2-NH-CO-NHCHZ-hl-CO-NH-CHz-f-CO-NH-CHz~
CH~OH
H2OH
I
-N-CO-N-CHZ-N-CO-N-CH~-N-CO-N-CH~-
I
I
I
I
I
N-CO-N-CHz-N-CO-N-CHz-N-CO-N
,
I 7 HZ
I
I
I
I
I
I
I
I
NH-CH~-N-CHZ-N-CHZ-N-CH*-N-CH~-N-CH~OH
I n 1941 Thurston (31) proposed a new theory for the polymerization of both urea-formaldehyde and melamine-formaldehyde resins. Reasoning t h a t in common with the type of reaction which certain amines and formaldehyde undergo, melamine and formaldehyde, for example, form a Schiff's base, which then trimerizes. A polymer would then be composed, except for terminal groups, exclusively of triazine rings linked with fused hexahydrotriazine rings in a pattern such as the following: HOCH~-NH-CO-NH-CH~OH
1 HOCHZ-NH-CO-N=CH~
I
4 H2
Marvel (,94), somewhat later, proposed a modification of this theory. Considering t h a t urea is an amino acid amide, it was deemed likely t h a t i t has both an amino and an amido function acting as such a t t h e same time. In this capacity of both a n amine andan amide the two-NH2groups would behave differently toward formaldehyde. T h e amine group would form the characteristic Schiff's base, this in turn trimerizing, whereas the amido group would undergo the more usual bonding of an amide such as formation of methylene bis linkages. Marvel conceived his iype polymer t o have a form such as: tyH
co
-
N
"ZP' -CHz-NHCO-N
'7"z
2681
Marvel's principal evidence for this theory is derived from analogous reactions with various amino acid amides. Both glycinamide and e-aminocaproamide actively formed gels in reaction with formaldehyde and these in turn formed resinous bodies. From glycinemethylamide, NHzCH,CONHCH,, a substituted amino acid amide and therefore protected against amide reaction with formaldehyde, it was possible to isolate a trimer. The trimerization reaction was carried out under more or less neutral conditions. Urethane also yielded a trimer in the presence of strong hydrochloric acid. Reconsidering the various urea-formaldehyde intermediates which have some reasonable experimental basis for existence as discrete entities, they may be aligned according to the conditionsacid or alkaline-used for their preparation (Table 111). This outline of low molecular intermediates suggests t h a t p H regions are favorable to different structures. It is not implied t h a t these structures form with equal ease. Moreover, this outline appears incomplete in the acid region and the three lowest members shown need confirmation. All the configurations in Table I11 have been assigned on t h e strength of the analyses of solid or liquid materials which have been isolated from the reaction mixtures. Vogel (32)has claimed some unusually high molecular weights, 10,000 to 20,000 for colloidal urea-formaldehyde condensates based on osmotic measurements, No reaction mechanism was proposed for such a polymer. I n the polycondensation reaction of melamine-formaldehyde, Gams, Widmer, and Fisch (12) have attached particular importance t o the ether linkage and have assigned a minor role to the methylene bridge as a polymer-forming linkage. This conclusion was drawn after an introductory study of the possible resinifying linkages in a hexamethylolmelamine and several other condensates. Admitting the complexity of the configurations which can possibly form through t h e condensation of the methylol derivatives in the two systems, urea-formaldehyde and melamine-formaldehyde, it seems highly likely t h a t several basic reaction mechanisms participate as t h e monomers progress through the various intermediate stages of polymerization to the final stage designated as the cured stage. Until the time when new experimental approaches clarify this situation, one cannot write with any certainty the structures of such soluble melamine-formaldehyde polymers as have been prepared and estimated by Kohler (20) to have molecular weights of approximately 5000. Flory (11)has recently reviewed t h e subject of t h e reactivity of large molecules involved in condensation polymerization. H e has defended the view t h a t t h e intrinsic chemical reactivity of a functional group is independent of the size of the molecule to which it is attachedexcept in unusual circumstances. Experimental confirmation of this theory in connection with amino condensations would certainly be desirable. The various theories of polymerization proposed to date generally emplriasize one mechanism to the exclusion of others, Moreover, no satisfactory theory has been built around the important influence-the hydrogen ion concentration. As indicated in Table I11 for the urea-formaldehyde system, this variable exerts a directive effect on the course of condensation. I n addition to the hydrogen ion concentration variable, current theory must take into account modern structural theory for urea and melamine. I n its crystalline form, urea is regarded structurally as a resonance hybrid embracing t h e following structures (3, 21, 26):
/NHz
c=o \NH~
YNGz
c-o-
\NHz
/NHz
c-0; ~ N H ,
Hughes (16) made a n intensive study of t h e structure of melamine in t h e crystalline state. According to his work the posi.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2682
Vol. 44, No. 11
Other less symmetrical structui es may also make contributions one set being: (1 g. in 3 0 0 c c . w a t e r ) 25O
-
+lH2
/'
-N
8
/e\
\
N
CI & HA-/\N/\KH~
-"*
-SHz I'
I1
.C\
54
s-
cI
c
f i x
di!
H,s/\s~\YH?
HJ/*S
-'\sH~
I n view of the possiblllty of the occuirente of tautomerism also in melamine, four additional qtructures h a w received consideration ($6)' H
---
Water a l o n e
Urea
I
I
I
I
I
I
I
NHz tions of the atoms correspond to a molecule which is a resonance hybrid chiefly between structures:
NHz
+ " 2
I
Sormal or amino form
"2
Diamino-imino form
/"\
e
-N
C=KH
HS=C 1
HN
/ \
N
H /"\
H
I
KH
NKFI Is0 form Which pi
TABLE111.
!'
CONDITIONS FOR P R E P A R A T I O K O F
CHzOII
CHaOCHa
, c=o 1
1
+ c=o
CHzOH
I
Product Properties N,iV'-Bis(methoxy- B . P . 82-83' methyl) uron 0 1 inin
CH30H ,S-CI&
/N-CHzOH
-4lkaline
UREA-FORMALDEHYDE IKTERYEDIATES
CHzOCHa Tetraiiiethylol dimethylenetri urea
Crystallizable f r o m alcohol
Dirnetliylolurea
b1.p. 128' (reactive) prisms from alcohol
1,l'-Methylene diurea Trimethylenetetra-urea
Needle rrystals m.p. 218' White powder froni nater
HzXCN=CH?
Nethyleneurea
HzX--C=N A--CH~ I
1,Z-Methylene urea
Soluble, golymerizable Insoluble nonpolymerizable
HOCHzNH-C-SHCHzOH PH7
,NH-CO-SI-I2 HzC
\NH-co--?u" 'CH, ,SH-CO-SH/ HzC Acid medium
\NH-CO-A-H~ 0
I1
S Hz
I
3,5-Dicarbamyl tetrahydro-l,3,5-oxadiazine
i!=O
1
,S-CHz\ CHz \S-CHr'
I c=o I
I
NHz
0
Slightly soluble
structure best ex-
~ 5 s e sthe properties of mel-
amine is a problem in itself. T h a t melamine undcigoes qtructural change in acid versus neutral or alkaline solution has been denionstrated by ultraviolet absorption spectroscopy (5, 7 , 18). ,In the light of this a o r k and the high melting point of melamine ae. well as heat resistance of resins derived from it, the heneenoid structure is favoied n ith probably the diaminoimino form existing as a second or exclusive structure in acid solution I n aqueous solution urea ir a more nearly neutral iiioIecule than melamine. Certain investigators have proposed t h a t i t exists as an inner salt or zwitterion (SO), although this theory has not been generally accepted, probably because of inadequate means of confirming it. In Figure 1 neutralization cuives are given to demonstrate the relative basicity of urea and melamine. It 1s obvious that melamine, unlike urea, can combine n ith acids in stoichiometric
INDUSTRIAL AND ENGINEERING CHEMISTRY
2683
amount in aqueous solution. Dimethylolmelamine is a slightly weaker base than melamine itself. These considerations have made i t possible t o derive from melamine and formaldehyde a series of acid resins existing in solution as “acid colloids” (37). I n this case, the resin acts as a hydrophilic colloid invariably showing a blue Tyndall haze to its solutions. Melamine acid colloids have been widely used in the paper industry for making wet-strength paper (93). T h e process involves the addition of the resin colloid directly to the slush stock in the paper beater. The structure of the monomer and its polymers which are involved in the making of the acid colloid may be represented as follows: Lu OF1
c1-
h
4
2
h
3
J
10-20
Dimethylolmelamine hydrochloride may be considered as essentially the starting monomer. The assumption is made that the polymerization mechanism is t h a t involving “methylene bis” linkages, and t h a t under these conditions of pH the diaminoimino tautomeric form of melamine is present to a considerable extent. The specific structure given represents the polymer and its molecular size a t a time when it is still in a dispersed and ungelled form preparatory to use in the paper beater. The polymer carries a net positive charge, and this as well as its size is considered important for its adsorption by the negatively charged cellulose ( 6 ) . The point requiring emphasis is t h a t in this particular condensation in the pH region of 0.5 to about 3.5 the polymer exists in a dissociated and ionic form in which it carries a positive charge, A somewhat similar situation is brought about when a ureaformaldehyde of melamine-formaldehyde intermediate is combined with sodium bisulfite (f, 3). I n this case, as shown above, dissociation gives rise to ions carrying a negative charge (1). In view of the large difference in the characteristic of basicity of urea and melamine, the question may be raised as to whether the same polymerization theories will be applicable to their formaldehyde condensates. A strong likelihood exists t h a t urea would undergo the general reactions and electrokinetic behavior of an amino amide to a lesser extent than melamine. EXPERIMENTAL
d
u
)
m
r
-
The importance of the hydrogen ion concentration in the condensation reactions of melamine and formaldehyde is generally known. In order to demonstrate this in a systematic way a series of reactions, controlled in pH by the addition of suitable pH buffers, was studied. These were carried out a t 50% concentration and 6 O O i n such a manner as first to produce trimethylolmelamine in solution. The p H value was then altered by addition of buffer and reaction continued for a short interval a t the new pH value. The type of product which formed under the different pH conditions was of particular interest.
2684
pH 11.8
DH10 5
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 11 tion W M develppcd in t'he same n-ay as in preparation 1-that is, by heating 20 minutes a t 60". Two h u n d r r d a n d eighteen rams of the 50% solution equivalent t'o 0.5 mole of melamine) were diluted a t once with 56.5 nil. of water followed by 35.4 g r a m s of 3 6 % h y d r o chlori$acid, held 2minutes at550 , and then cooled. During reaction with the acid in preparation i as well as 6 the concentrabion of melamine plus formaldehyde was 35%.
f '
A d e t a i l e d physicochemical study of the type of productwhich forms under the conditions used for pH 8 9 preparations 6 and 7 has been published (6) and reference to the cationic form of the polymer has been made above. They are inrluded here because of their general relationship to the remainder of the $3 p H 7.4 region and particularly to the region of pH 6.3 to 11 .6, to which attention is 110G drawn. In this region precipitates in large yield were obtained instead of the colloidal solutions pH 6.4 characteristic of the pH range of 0.5 to 3.5. Rlethods used in the examinationof the precipitates and results obtained are outFigure 2. X-Rag Diffraction Patterns of ~Irlaniirie-FormaldehydePrecipitates lined in Table IT'. There will be noted a siniilarity P R O C E D U R E F O R R RE PA RATIOS 1 (Table IV). l'araformaldeill all the products of reactions at, pH 7.6 to 11.6 in crystallinity hyde (91.8 gramp, equivalent to 3 moles of formaldehyde) and acrording to x-ray diffraction patterns (Figure 2), composition, 173 ml. of water were heated to 60" in a flask equipped with a and such properties as fusion point and solubility. These prodstirrer and reflux condenser, and 2.0 ml. of 0.5 .V sodium hydroxide were added. The temperature fell and nhen it w w uets resembled trimethylolmelaniine. At pH 6.3 the product restored to 60' within 4 minutes, the paraformaldehj-de dissolvid 11 as entirely different. It 71-as amorphous, devoid of fusibility, almost completely. One hundred and twenty-six grams (1 mole) and insoluble in boiling water, and its high values for nitrogen of melamine were added, follox-ed by 3.0 nil. of 0.5 LV sodium and formaldehyde (as regenerated by phosphoric acid distillahydroxide, and heating I+-ascontinued for a 20-minute interval tion) indicated t,hat condensation had taken place. This product a t 60". After the first 10 minutes the melamine dissolved. Thi: p H value measured after the full interval x-ith a glass electrotfeh a s the properties and composition of a Ion polymer. type pH meter (Leeds and Sorthrup S o . 7662) \vas 10.0 at 25". The fact must be st,ressed that under relatively mild condiAt the completion of this stage, considered as the trimethyloltions of heating for a very sinal1 change in hydrogen ion concenmelamine stage, the buffer was added in the form of a solution of the glycine and sodium chloride (Table IV) in 42.8 ml. of 0.5 't' tration (pH 7.6-6.3) polymerization has been initiated. An sodium hydroxide. Heating was continued for 10 minutes a t analogous sit'uation could be demonstrated for the urea-formal60" and the solution cooled promptly. On standing one day it dehyde system. In both systems a prompt responee takes place changed in form to a thick, granular paste. The concentration to a change in pH value from the neutral to the acid state. Hyof nonaqueous material was 50% by m-eight. After standing several days 150 grams of the product were mulled with 64 ml. drogen ion concentration is, therefore, the directive influence as of water to lower the concentration to 35% and the slurry was demonstrated through (1) t,he small magnitude of change restirred 30 minutes t o equilibrate it. The precipitate was collected quired, and (2) prompt response to such change. on a Buchner funnel and washed with 80 ml. of water. Drying was carried out in an evacuated desiccator over phosphorus pentoxide. IONIC R.IECHANISM The same procedure was used for preparations 2 to 5) inclusive, except that the concentration r a a closer to 50 than 55%, A number of investigators have called attention to the hydrogen as in the foregoing case, during the trimethylolinelami~iestage. ion influence and sought to explain it in terms of formation of ionic Reaction in the presence of the buffer was a t 50% i: I in all five complexes ( S 4 ) , adsorption or combination of the acid in a cocases. PROCEDURE FOR PREPARATIONS 6 A N D 7. In these two cases valent bond (M), or dissociation of the urea (29). i t was impractical t o operate a t 50% concentration throughout In visualizing how these ions play such an influential part we the reaction, because of the high reaction rate leading to gelation. may consider the likely ionic forms t h a t urea and melamine and The concentration therefore had to be Ion-ered and the reaction their methylol compounds assume in solution. time curtailed. For preparation 6 the trimethylolmelamjne solu-
.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
November 1952
Wheland (36) has proposed t h a t urea, through attachment of hydrogen ion to the molecule, exists in the following resonating ionic forms: L
"/*
/NH2
C=O +H
+ 2"/
C--OH
C-OH
\NH2
*"\
V H ,
Melamine by analogy would assume the resonating ionic forms:
I
NH1 I1
2685
Here, the activated ionic form undergoes reaction a t a group which is near the location of the charge. With formation of the dimer, charge is released with loss of a proton. This probably takes place to a major extent with charged and uncharged dimer coexisting. With further growth of the polymer additional proton is released, so t h a t in the case of the trimer a smaller proportion would exist in the charged form than in that of the dimer. Coincident with release of the proton the triazine reverts to the aromatic structure. As an alternative mechanism, reaction might take place involving the proton attached to the imino cation and t h e hydroxyl of a methyl01 group of a neutral molecule as also shown above (suggested by J. J. Levenson, reviewer). This would result in a neutral dimer with the triazine having the aromatic form due to the ejection of the proton. With the participation of ether and azomethine-type structural mechanisms a t least two other ionic mechanisms are possible. The purely molecular form of the reaction may also take place. A suggested form of a typical higher polymer is the following:
Structure I11 is considered as probably playing a major part (14). Dimethylolurea or a methylolmelamine can readily be visualized as existing in similar charged or ionic forms. Structure type I11 has been proposed for the acid form of triniethylolmelamine to explain the change in its absorption spectra in going from the neutral to the acid state ( 7 ) .
Molecular Ratios Melamine Formaldehyde Water of condensation Molecular weight
At low p H values all trimethylolmelamine molecules should assume this ionic form ( 7 ) . At a pH value of 6.3, where condensation was first noted as taking place, some of these ions will begin to form. It, therefore, seems logical to attribute the high reactivity a t pH 6.3 to these ions. One of t h e possible mechanisms for polymer growth is through the interreaction of the ionic and molecular forms of trimethylolmelamine: OH
v
HOCH~NH-C~
I1
. I
-.
,
,/
I
YCP kH CHZ OH
OH'
1
8
23 9 1536
Here, all structural mechanisms participate in what appears to be a random distribution but probably a t different stages of the polymer's growth. This would depend on circumstances favorable to one or the other type of condensation mechanism. This view is based on the fact t h a t the environment in which a polymer forms and reaches its ultimate growth mag change radically. In the case of a molding compound, for example, polymer formation may take place initially in aqueous solution a t moderate temperature and finally in a relatively dry, plastic state a t elevated temperature. ACKNOWLEDGMENT
The writer has had the benefit of many helpful discussions with and suggestions of his associates which he is pleased to acknowledge. He wishes to extend his appreciation to L. -4. Siege1for contributing the x-ray diffraction photographs and to the American Cyanamid Co. for permission to publish this paper. LITERATURE CITED
(1) Auten, R. W., Paper Trade J., T A P P I , 127, 332 (1948). (2) Auten, R. W., and Rainey, J. L., U. S.Patent 2,407,599 (1946). (3) Bell, Gillespie, and Taylor, Trans. Faraday Soe., 39, 137 (1943). (4) Carothem, W. H., J . Am. Chem. Soc., 51, 2548 (1929). (5) Costa, (3. W., Hirt, R. C., and Salley, D. J., J. Chem. Phys., 18, 434 (1950). (6) Dixon, J. K., Christopher, G. L. M., and Salley, D. J., Paper Trade J., T A P P I , 127, 455 (1948). (7) Dixon, J. X., Woodberry, N. T., and Costa, G. W., J . Am. Chem. Soc., 6 9 , 5 9 9 (1947). (8) Einhorn, A., Ann., 343, 207-310 (1905).
2686
INDUSTRIAL AND ENGINEERING CHEMISTRY
(9) ZbkZ., 361, 113-65 (1908). (10) Einhorn, A,, and Hamburgei, A , , Ber., 41,24 (1908). (11) Flory, P. J., Chem. Reas., 39, 137 (1946). (12) Gams, A., Widmer, G., arid Fisch, I\‘.,Brit. Plastics, 14, 508-20 (1943). (13) Gams, A , , W-idmer, G., and Fisch, W., H e h . Chim. A c t e , 24,302E (19411. --, (14) Hirt, R. C., unpublished work. (15) Hodgins, T. S., Hovey, A. G., Hewitt, S., Barrett, W.R., and Meeske, C. J., IXD.ESG. CHEM.,33, 769 (1941). 63, 1737 (1941). (16) Hughes, E. W., J. A m . C h e m SOC., (17) Kadowaki, H., BUZZ.Cherrb. SOC.Japan, 1 1 , 2 4 8 (1936). (18) Klotz, I. hl., and Askounis, T., J . Am. Chem. SOC.,69, 801 (1947). (19) Knudsen, P., Ber., 47, 2698-701 (1914). (20) Kohler, R., KoZZoid Z., 103, 138 (1943). (21) Kumler, W. D., and Fohlen, G. bf., J . Ana. Chem. SOC.,64, 1944 (1942). (22) Landes, C. G., and Maxwell, C . S.,Paper Trade J . , T A P P I , 121, 51-60 (1945). U. 8.Patent 2,559,220 (1945). (23) Lundberg, L. *4., Ibid., 2,475,846 (1949). (24) Marvel, C. S., Elliott, 3. R., Roettner, F. E., and Yuska, Henry, J . A m . Chem. SOC..6 8 , 1681 (1946). (25) Ostrogovich, L4,,Gazt. ital., 65, 566 (1934). \ - -
Yol. 44, No. 11
(26)
Pauling, L., Brockway, L. O., aiid Beach, J. Y., ,J, A m . Cliena.
(27) (28)
Pulvermacher, G., Ber., 25, 307-10 (1892). Scheibler, H., Trostler, F., and Scholz, E., 2 . 3nge.w. Cliem., 41,
SOC.,57, 2705 (1935). 1305 (1928).
Smythe, L. E., J . Phgs. a?id Colloid Chem., 5 1 , 3 6 9 (1947). Taylor and Baker, “Sidgwick’sOrganic Chemistry of Nitrogen,” p. 280, New York, Clarendoii Press, 1937. (31) Thurston, 3. T., Gibson Island Conference. .July 1441 (unpublished paper). (32) Vogel, R. E., Kunststofe, 31, 309 (1941). (33) Walter, G., Trans. Faraday Soc., 32, 377-95 il936); K c d o i d Z., (29) (30)
57, 229-34 (1931).
Walter, G., Trans. Faradnu SOC.,32, 396 (1936). Walter, G., and Lutwak, R., Kolloid Beihefte, 40, 158 (1934). Wheland, George W., ”Theory of Resonance.” S e w York, . Johu Wley & Sons, 1944. (37) Wohnsiedler, H. P., and Thomas, W.A I , , I-. ,5 Patents 2,345,(34) (35) (36)
543 (1944); 2,485,079, 2,485,080 (1949). RECEIVED for review April 13, 1931. ACCEPTED.June 18, 19.52. Presented before the Division of Paint, Varnish, and Plastics Chemistry, Symposium o n Urea, Melamine, and Related Resina, a t the !19th lfeeting Of the .4MERICAh- CHEMICAL S O C I E T Y , Boston, 3Iasa.
Chloromethylation of Pol-ystvrene J
J
4
GIFFIN D. JONES Physical Research Laboratory, The Dow Chemical Co., Midland, Mich.
EARLY all of the common reactions of aromatic substitution have been applied to polystyrene. The chloromethylation of styrene-divinylbenzene copolymer and subsequent amination with tertiary amines has been described ( I , 3, 4). This paper deals with the chloromethylation of polystyrene. As a rule a solvent is necessary t o carry out a reaction with a polymer. Chloromethylation a i t h chloromethyl ether is an ideal case from this point of view because both polystyrene and chloromethylated polystyrene are soluble in chloromethyl ether. The catalyst, too, dissolves in the reaction mixture even though, as in the case of zinc chloride, it may not be soluble in chloromethyl ether itself. As a Friedel-Crafts reaction chloromethylation requires less driving force than alkylation. If this were not so it would be impossible to isolate the substituted benzyl chloride that is produced in chloromethylation. The alkylation reaction is encountered as a secondary reaction in most chlorompthylations as pictured in the following equation:
Iln
, ClCH A OCH -+
Catalyst
f
a
R /
tion can be carried to a higher might per cent of chlorine at the gel point if it is carried out in a more dilute solution in chloromethyl ether. The use of a lower viscosity grade of polystyrene permits a higher degree of chloromethylation prior to gelation. This is shown in Table I where data are given for rhloromethylations carried t o the point of gelation. As a further illustration it was found th3t soluble samples of 10 t o 14% chlorine veere obtained by chloromethylating polystyrene of viscosity grade 87 in 8.7oJ, concentration or viscosity grade 2 in 22% concentration. The term “viscosity grade” is defined in Table I. A \yay of observing the progress of chloromethvlation is provided by the measurement of the viscosity rise which occurs slowly a t first and abruptly as the gel point is approached. This is shomn in Figures 1 to 3. In Figure 1 the ordinate is a measure of viscosity as obtained fiom a recordlug ytirrer-viscosimeter. The data from which this graph was plotted were obtained from an experiment carried out in a resin pot of 3-liter capacity a hich was filled with 2 liters of a 14% by neight solution of polystyrene (commercial molding grade) in chloromethyl ether. Zinc R ~ C H , C I chloride (50 grams) was added and the solution stirred by a syn__ chromotor driven electrically by a synchrogenerator that was turned a t constant speed by an electric motor. The viscosity was indicated by the current required t o drive the synchromotor. The voltage drop across
R15
14.8 15.9
F~~& given ooncentration (2.67%) of Z D C in ~ ~solution.
90
80
16.a
