Die Angewandte Makromolekulare Chemie 62 ( I 977) 203-21 3 ( N r . 91 I )

Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 16206 Prague 6, Czechoslovakia

Polymerization Initiated with Polymeric Compounds, 11*

FrantiSek Svec, Subhash Pundlik Vernekar, and Jaroslav KAlal

(Received 16 August 1976)

SUMMARY: The polymerization of methyl methacrylate initiated by the copolymers methacrylalde-

hyde - styrene - divinylbenzene and acrylaldehyde - ethylene dimethacrylate in the absence of usual initiators was investigated. The polymerization was found to proceed fairly readily and fast. Acceleration can be achieved by adding glycerylaldehyde. An increase in the surface of the initiating copolymer favourably influences the reaction rate; at the same time, however, physical trapping of ungraft poly(methy1 methacrylate) molecules in the macroporous initiator seems likely to occur. It was also found that only copolymers containing aldehyde groups could be used for initiation and that besides MMA some other monomers could be polymerized in this way, such as glycidyl methacry- late, acrylic and methacrylic acid, acrylonitrile, and alkyl acrylate.

ZUSAMMENFASSUNG: Die Polymerisation von Methylmethacrylat wurde untersucht, die durch Copolymere

aus Methacrylaldehyd - Styrol - Divinylbenzol und Copolymere aus Acrylaldehyd - Athylendimethacrylat in Abwesenheit ublicher Initiatoren ausgelost wird. Die Polymerisa- tion verlauft radikalisch und schnell. Eine Beschleunigung kann durch Zugabe von Glycerinaldehyd erreicht werden. Eine groBere Oberflache des initiierenden Copolymeren begunstigt die Reaktionsgeschwindigkeit, aber parallel dazu scheinen leicht Einschlusse von ungepfropften Polymethylmethacrylat-Molekulen aufzutreten. Es wurde gefunden, da13 nur Copolymere mit Aldehydgruppen als Initiatoren wirken und daB auch andere Monomere wie Glycidylmethacrylat, Acryl- und Methacrylsaure, Acrylnitril und Alkyl- acrylate polymerisiert werden konnen.

* Part I : S. P. Vernekar, F. Svec, and J. Kglal: 4th IUPAC Intern. Conf. on Modified Polymers, Bratislava, Czechoslovakia July 1975, Preprints Vol. 1, P. 54.

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F. Svec, S. P. Vernekar. and J. KBlal

Introduction

It has been observed that in the system of some natural polymers, such as starch, wool, some proteins, cellulose etc. containing various functional groups (-OH, -CONH-, -CH=O etc.) initiation of methyl methacrylate (MMA) occurs in the presence of water without a conventional initiator being present.

In 1962 Kimura et al.' observed that methyl methacrylate (MMA) could be polymerized in the presence of soluble starch without the usual radical initiator being present. Very high yields were obtained with dialdehyde starch (95 % conversion). Similar experiments were performed with a number of other natural polymers, such as silk' and and synthetic polymers like poly(viny1 alcohol)6, copolymer styrene - a~ryla ldehyde~.~ and styrene - methyl vinyl ketone'. Gaylord et al.5 found that a considerable acceleration effect on the polymerization of MMA in the presence of cellulose was brought about by the addition of glycol aldehyde and glycerylaldehyde, although the latter compounds alone were only little active. This observation is in accordance with that of Imoto6 who observed only a slow polymerization in the presence of poly(viny1 alcohol) (P = 8) terminated by the aldehyde group, and no polymer- ization at all in the presence of polyhydroxy compounds and sugars. It is clear, therefore, that the aldehyde groups play an important role in the so-called noninitiated polymerization only if they are part of a system containing a high-molecular weight compound or if they are attached to a high-molecular weight compound. The effect of water is not quite unequivocal because in some cases it greatly accelerates the p o l y m e r i ~ a t i o n ~ ~ ~ - ~ while in others its influence is small'. Besides MMA, attempts have also been made to polymer- ize and copolymerize ~ t y r e n e ' . ~ , ~ and a c r y l ~ n i t r i l e ~ ~ ~ ~ ~ ~ ~ , which however were unsuccessful in the presence of cellulose and water. A number of experiments involving inhibitors have shown7*' that the initiation of polymerization pro- ceeds via radical mechanism, without, however, any general idea of the type of the latter being available. A generally accepted view is that an important role is played by the donor - acceptor interaction between the aldehyde (donor) and MMA (acceptor), with water also participating in this interaction.

In this work we used crosslinked macroporous polymeric materials with a high internal surface area in the form of regular spherical particles which offer technical advantages compared to those mentioned above.

204

Polymerization Initiated with Polymeric Compounds

Experimental Part

Preparation of monomer and polymerization : MMA was dried on a molecular sieve and distilled under reduced pressure. The

distillate was heated to 90°C for one hour to remove impurities by prepolymerization; the remaining monomer was distilled in the dark under nitrogen at reduced pressure. The purified monomer was stored at -20°C in the dark several days before use at utmost.

Crosslinked polymers were prepared by suspension radical polymerization in an aqueous medium with polyvinylpyrrolidone as suspension stabilizer. Macroporous struc- ture was obtained by polymerization in the presence of a mixture of dodecanol and cyclohexanol according to a procedure published earlierg with a modified monomer mixture. The size of the specific surface area of copolymers used was 5&100mZ/g. The content of the aldehyde groups was determined by the usual oximation method”.

The polymerization of MMA was carried out in sealed glass ampoules under inert atmosphere in the dark at 80°C. Oxygen was removed from the polymerization batch by triple “freeze-pump-thaw’’ cycle. On completion of polymerization the content of the ampoule was suspended in a suitable solvent (n-butanone for MMA) and introduced into methanol. The precipitated product was washed with methanol and dried i. vac. at room temp. to constant weight.

The overall conversion was determined after subtracting the weight of the polymeric initiator and correction for the weight of the polymer obtained by thermal polymerization determined by blank test (2-3 %).The soluble fraction was extracted in a Soxhlet apparatus for 60 h. Grafting efficiency represents a percent fraction of polymer fixed in the matrix related to 1 g of matrix expressed in %.

Molecular weight of PMMA was determined from viscometric measurements in n- butanone and confirmed by light scattering and osmometry.

Results and Discussion

Works reported so far in this field and involving synthetic polymers con- cerned soluble polymers. This method is little effective if applied to the investiga- tion of mechanism and kinetics simultaneously, because it is difficult to separate a polymer having initiating activity and containing graft chains of polymerized monomer and homopolymer formed in the reaction. We therefore used cross- linked polymeric materials in the form of regular spherical particles where separation by extraction is very simple, and polymerization reaction can also be performed by a flow scheme’ ’. Preliminary experiments in the presence of a radical polymerization inhibitor (hydroquinone) did not lead to any polymerization under conditions at which in its absence the polymerization usually proceeded in high yield. This leads to the conclusion that polymer-ini- tiated polymerization has radical character as suggested also by other a ~ t h o r s ’ ~ ~ .

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F. Svec, S. P. Vernekar, and J. Kglal

Tab. 1 summarizes results obtained with the copolymer methacrylaldehyde - styrene - divinylbenzene (MA-ST-DVB) in the polymerization of methyl methacrylate (MMA). From the standpoint of mechanism it is certainly of interest that the amount of graft polymer does not differ too much from the homopolymeric (soluble) fraction, which allows a conclusion that roughly half the radicals initiating the polymerization enjoy free motion, while the other half is fixed on the surface and leads to grafts. A somewhat higher fraction of graft polymer can be accounted for by the restriction of mobility of fixed radicals. The mobility of radicals in solution makes possible their interaction and termination by a primary radical, thus reducing the amount of soluble polymer obtained.

Tab. 1. Polymerization of MMA in presence of MA-ST-DVB gel and properties of the product obtained. Reaction temp. 80"C, MA content in gel 2.30mmol/g, MMA content 0.94 g.

Gel Time Conver- M,x Free Grafting Grafting (8) (h) sion polymer (%)* efficiency

( %) (%I (%I

0.1 0.5 5.3 1.5 0.1 1 8.4 1.6 0.1 2 24.7 2.1 0.1 4 96.6 2.2 0.1 8 97.0 1.8 0.01 4 5.2 2.0 0.02 4 15.0 2.4 0.05 4 37.7 3.6 0.16 4 89.1 2.1

38.4 35.3 27.5 40.8 78.6

78.2 43.4 50.5

-

61.6 64.7 72.5 59.1 51.4

51.8 56.6 49.5

-

32.4 52.3

183.5 542.8 441.2 323.6 365.3 394.4 255.5

* Grafting is defined as: percent fraction of polymer fixed in the matrix related to the total amount of formed polymer

If polymerization proceeds in solution and not in bulk (or in the presence of a solvent), its rate decreases (Tab. 2). If tetrachloromethane having a rather high transfer constant is used as solvent, there is not only a decrease in the rate of polymerization but also a pronounced decrease in the average molecular weight of the forming soluble polymer and a decrease in the amount of the graft polymer, because the transfer occurs also on chains fixed on the matrix (gel) and further polymerization proceeds already in solution.

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Polymerization Initiated with Polymeric Compounds

Tab.2. Polymerization of MMA in presence of MA-ST-DVB gel and properties of the product obtained. Reaction temp. 80°C, MA content in gel 2.30mmol/g, MMA content 0.94g, solvent 1 ml, amount of gel 0.1 g.

Solvent Time Conver- a, x Free Grafting Grafting (h) sion polymer (%) efficiency

(%I (%) (%I

H 2 0 2 19.8 1.6 39.0 61 .O 1 14.3 H 2 0 4 99.2 2.4 49.0 51.0 473.4 Dioxan 2 6.4 - 43.0 57.0 34.7 Dioxan 4 12.6 1.4 47.2 52.8 63.0 cc14 2 6.4 0.2 71 29 17.4 CC14 4 9.5 0.5 71 29 26.2

A special position is occupied by water which according to some author^^,^-^ is necessary for the given polymerization type. The results indicate however that there is no marked difference between bulk polymerization and polymeri- zation in the presence of a large amount of water. The effect of the decrease in rate by dilution with monomer is not operative in this case, because the latter is not soluble in water to any important degree, and no positive effect of water on rate can be seen either. There is a theoretical possibility however that both effects just compensate each other.

Unlike addition of water, polymerization can be considerably accelerated according to Gaylord' with glycerylaldehyde (Tab. 3), which in itself does not virtually initiate the polymerization. Macroporous copolymer of acrylalde- hyde with ethylene dimethacrylate (A-EDMA) was used as the initiating polymer. Comparison shows that the polymerization acceleration is due to

Tab. 3. Polymerization of MMA in presence of copolymer A-EDMA and cyclohexanol or glycerylaldehyde. Monomer 1 ml, temp. 80"C, polymerization time 90 min, copolymer 0.1 g, A content 2.68 mmol/g.

A-EDMA Glycerylaldehyde Cyclohexanol Conversion (g) (€9 (g) (%)

0.1 0.1 0.1

- 0.1

-

0.1 -

21.0 20.0 90.0

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F. Svec, S. P. Vernekar, and J. KAlal

Tab. 4. Polymerization of MMA in presence of copolymer A-EDMA. Monomer 1 ml, A-EDMA 0.1 g, A content 2.68 mmol/g.

Reaction Tempera- Conver- Grafting Grafting time ture sion ( %) efficiency (h) ("C) ( %) (%I

2 40 2 50 2 60 2 70 2 80 4 40 4 50 4 60 4 70

0.8 3.0 5.5

15.2 98.8 2.6 5.7

29.8 69.3

58.2

76.2 92.1 92.2 74.9 83.1 85.0 86.3

- 8.3

46.5 141.2 854.1 23.0 46.0

281.1 583.4

-

aldehyde and not to ketone, which suggests that the presence of the -CH=O group may be necessary. It cannot of course be ruled out that the positive effect of glycerylaldehyde is also due to the hydroxylic groups being present in the aldehyde molecule at the same time.

Polymerization in the presence of A-EDMA was investigated in greater detail. Tab. 4 shows a pronounced temperature dependence of the polymeriza- tion rate characteristic of radical polymerizations.

Tab. 5 summarizes results obtained with various amounts of initiating copolymer. Comparison with the results obtained by using MA-ST-DVB reveals a higher polymerization rate. Because of its markedly larger surface area, on which radicals can be formed macroporous A-EDMA makes pos-

Tab. 5. Polymerization of MMA in presence of copolymer A-EDMA. Monomer 1 ml, temp. 80°C, polymerization time 2 h, A content 2.68 mmol/g.

A-EDMA Conversion Grafting Grafting (g) (%I efficiency

( %)

0.02 25.3 85.0 1 01 0.0 0.05 59.3 89.7 998.6 0.10 98.9 92.2 854.1 0.1 5 97.8 93.5 572.8

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Polymerization Initiated with Polymeric Compounds

sible-even at an almost identical mean content of the aldehyde groups-for- mation of a major amount of radicals and at the same time an easier diffusion of free radical from the domain of bonded radical leading to a decrease in the fraction of recombination of primary radicals, and thus to an acceleration of polymerization. Unlike MA-ST-DVB, where porosity is very low, homopoly- merization via a free (unfixed) radical inside the pore can also proceed in this case, which enhances the probability of recombination with the fixed growing chain. Also-owing to the very high average molecular weights-phy- sical hampering of diffusion of the polymer molecule from the pore during extraction may occur here, i.e. a certain encapsulation of the soluble polymer in the porous gel. Both effects have as a result a pronounced increase in the value called, not quite justifiably in this case, grafting and the respective grafting efficiency.

Tab.6. Polymerization of MMA in presence of copolymers A-EDMA with various acrylaldehyde contents: Copolymer B 2.34 mmol/g, copolymer A 0.45 mmol/g; temp. 80°C, monomer 1 ml, polymerization time 1 h.

Copolymer Amount Conversion (g) (%I

A B B

0.1 0.02 0.1

1.1 14.8 24.6

As shown by Tab. 6, the polymerization rate is markedly dependent on the aldehyde concentration in the copolymer used and on the aldehyde concen- tration in the whole system (Tab. 1 and 5). If the rate depended only on the total concentration in the system, the conversion attained with 0.1 g of copolymer A should be the same as with 0.02 g of copolymer B. The results however are fairly different. It seems likely, therefore, that what plays a decisive role in the polymerization is not only the total concentration, but the local concentration of aldehyde groups on the surface on which they are available. Our earlier have shown that the aldehyde groups are able to react with each other while giving rise to various structures, such as tetrahydropyran cycles, the extent of which is proportional to the concentration of the aldehydic monomer in the copolymer. It cannot be ruled out, therefore, that the polymerization is initiated not by the aldehyde

209

F. Svec. S. P. Vernekar, and J. KBlal

groups directly, but by the product of their mutual reactions which as a rule proceed already during the polymerization of the aldehyde or in the separation of the product. Another possible explanation consists in assuming a co-operative effect of adjacent real aldehyde groups which may be operative only at a certain distance between them. From the standpoint of possible repeated use ofthe polymeric initiator and the possibility of a flow arrangement of the reaction connected with it, an attempt was made at recycling the MA-ST-DVB gel used in the experiment. The results are summarized in Tab. 7. There is no marked drop in activity in all three polymerizations carried out one after another. It should be borne in mind that from the original 0.1 g of the gel only 0.055g is used in the second and third step, because the remaining substance consists of graft poly(methy1 methacrylate). The results summarized in Tab. 1 make it clear that the polymerization rate in the second and third cycle is comparable with that of the original MA-ST-DVB for a given amount of gel and reaction time.

Tab. 7. Repeated use of MA-ST-DVB for polymerization of MMA. Temp. 80"C, MMA each time 1 ml (0.94 g), MA content in gel 2,3 mmol/g.

Cycle Initiator Time Conversion Grafting (g) (h) ( %) ( %)

1 0.1 8 2 0.3" 2 3 0.37b 2

91 .O 51.4 17.6 39.8 17.5 40.9

a Product of 1st cycle. Product of 2nd cycle.

This finding is important for the reaction mechanism, as will be shown below. Let us assume for the sake of simplicity that we initiate polymerization by 0.1 g ST-DVB-MA containing 0.23 mmol of aldehyde groups. The yield is 0.8 g poly(methy1 methacrylate) with M,= lo6 for the extractable fraction amounting to 50% (i.e. 0.4 8). Such an amount of PMMA represents 4 x lo-' mol=4 x mmol. Assuming a roughly 75% effectivity of the initiation reaction (25 X, of radicals formed do not initiate the polymerization, but are lost due to recombination of the primary radicals), then 5.3 x 1 0 - 4 m m ~ l of radicals are needed for the formation of 4 x mmol of chains. The same amount of radicals is bonded on the polymer matrix, giving rise to

210

Polymerization Initiated with Polymeric Compounds

a corresponding amount of graft PMMA. If it is assumed further that the aldehyde groups in an amount of 2.3 x lo-’ mmol are responsible for the formation of radical pairs, then only a small part is inactivated by initiation, and the polymer would be “exhausted” only after some 500 polymerizations. It is possible, of course, that with respect to e.g. the already mentioned co-operative effect of the aldehyde groups the polymer will not exhibit any initiation activity much sooner than expected. In spite of this, however, during the first three cycles there is no serious reason for reducing the polymerization rate. An exact answer to this question will be given after completion of long-term experiments in a flow-reactor. It is certain, however, that after several cycles the original initiating polymer will form only a negligible part of the total insoluble mass consisting mainly from graft PMMA (2.5% after ten cycles of the above example), which also can considerably alter conditions of further application.

In order to prove the necessity of presence of the aldehyde group for a successful polymerization of MMA, a number of polymeric crosslinked mate- rials containing various groups such as hydroxyl, epoxide, or without functional groups were tested. Tab. 8 shows that the presence of an aldehyde group is absolutely necessary. It confirms the activity of the aldehyde group in a number of various polymeric materials. If we disregard the fact that the copolymers used differed in their physical structure and the aldehyde content, it can still be said that the polymerization of MMA occurs in all cases.

Tab. 8. Polymerization of MMA in presence of various cross-linked copolymers of ethylene dimethacrylate (EDMA) or divinylbenzene (DVB). Monomer 1 ml, temp. 80”C, polymerization time 2 h, copolymer 0,l g.

Copolymer Aldehyde Conversion content ( %) (mmol/g)

EDMA - glycidyl methacrylate EDMA - 2-hydroxyethyl methacrylate DVB - styrene DVB - styrene - methacrylaldehyde EDMA - acrylaldehyde EDMA - MMA - acrylaldehyde EDMA - MMA - methacrylaldehyde EDMA - 3-phenylacrylaldehyde EDMA - crotonaldehyde

0 0 0 2.3 2.1 2.4 1.6 0.01 0.03

0 0 0.2

24.1 98.8 16.1 24.6 3.9 5.9

21 1

F. Svec, S. P. Vernekar, and J. KAlal

It is reported in some p a p e r ~ ~ * ~ . s > ~ that acrylonitrile (AN) and styrene polymerize relatively reluctantly. We therefore carried out a number of experi- ments with various monomers, especially based on acrylates and methacrylates (Tab. 9), and with styrene. The results show that not'all types of monomers can polymerize by initiation with a polymer containing aldehyde groups, yet that this is still possible for a broad range of types, including AN, with good conversions.

Tab. 9. Polymerization of various monomers in presence of copolymer A-EDMA. Monomer 1 ml, temp. 80°C, copolymer 0.1 g, A content 2.7mmol/g.

Monomer Reaction time Conversion (min) ( %)

Styrene Methacrylaldeh yde Acrylonitrile Methacrylic acid Methyl methacrylate Methyl acrylate Glycidyl methacrylate Acrylic acid

120 120 1 20 120 120 70 35 15

2.2 3.4

22.7 21.3 98.9 89.2 64.2 78.2

The discussion of the results presented in this paper is based in principle on the view that the polymerization is initiated with a pair of radicals, of which one moves freely and the other is fixed to the polymeric matrix, without knowing the route by which the radicals are formed. Although the existing literature offers some views concerning the mechanism of radical f ~ r m a t i o n ~ . ~ , and this paper also contains some suggestions in this respect, the elucidation of the problem will undoubtedly come as a result of extensive experiments involving various methods which are now in progress.

' S. Kimura, T. Takitani, M. Imoto, Bull. Chem. SOC. Japan 35 (1962) 2012 M. Imoto, K. Takemoto, M. Kondo, Makromol. Chem. 98 (1966) 74 M. Imoto, Y. Iki, M. Kimoshita, K. Takemoto, Makromol. Chem. 122 (1969) 287 M. Imoto, Y. Iki, Y. Kawabata, M. Kimoshita, Makromol. Chem. 140 (1970) 281

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Polymerization Initiated with Polymeric Compounds

N. G. Gaylord, S. Maiti, S. S. Dixit, J. Polym. Sci. Part B 10 (1972) 855 M. Imoto, K. Takemoto, T. Otsuki, Makromol. Chem. 104 (1967) 244

' M. Imoto, M. Oishi, T. Ouchi, Makromol. Chem. 175 (1974) 2219 M. Imoto, M. Oishi, T. Ouchi, Makromol. Chem. 176 (1975) 3287 F. Svec, J. Hradil, J. eoupek, J. Klilal, Angew. Makromol. Chem. 48 (1975) 135

l o F. Svec, M. Houska, M. MyslivcovA, J. KAlal, Makromol. Chem. 177 (1976) 777 F. Svec, J. KAlal, to be published

l 2 J. Klilal, M. KlimovA, F. h e c , Collect. Czech. Chem. Commun. 38 (1973) 2719 l 3 J. Klilal, M. Houska, 0. SeyEek, P. Adlimek, Makromol. Chem. 164 (1973) 249

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