Gelation in the copolymerization of methyl methacrylate with trimethylolpropane trimethacrylate

Eur. Polym. J. Vol. 25, No. 4, pp. 385-389, 1989

0014-3057/89 $3.00 + 0.00 Copyright © 1989 Pergamon Press plc

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GELATION IN THE COPOLYMERIZATION OF METHYL METHACRYLATE WITH TRIMETHYLOLPROPANE TRIMETHACRYLATE AKIRA MATSUMOTO, HIROYUKI ANDO and MASAYASHI OIWA Department of Applied Chemistry, Faculty of Engineering, Kansai University, Suita, Osaka 564, Japan

(Received 21 July 1988) Abstract--Three-dimensional network formation in the copolymerization of methyl methacrylate with trimethylolpropane trimethacrylate (TMPMA) is discussed. In order to clarify structural features of the copolymer, the cyclization constant of TMPMA was estimated to be Kc = 4.2 mol/l and thus, the TMPMA units incorporated into the primary chain consisted of 18% noncyclic and 82% monocyclic structures for a typical copolymer. The gel points obtained under various polymerization conditions depended more significantly on dilution than on the amount of crosslinker. The molecular weight and intrinsic viscosity data suggest the occurrence of intramolecular cyclization leading to the formation of loop structures which was confirmed by GPC-LALLS measurement. Solution properties of the resulting copolymers are discussed in connection with the occurrence of intramolecular crosslinking which becomes more significant with the progress of polymerization.

INTRODUCTION As an extension of our research program concerned with the gelation in monovinyl-divinyl copolymerization [1-3], we attempted to explore the gelation behavior in the copolymerization of methyl methacrylate ( M M A ) with trimethylolpropane trimethacrylate ( T M P M A ) . N o detailed basic study of the polymerization of T M P M A has been published, although the preparation of macroporous polymers for use as adsorption material and ion-exchange resins has been attempted for the homopolymerization of T M P M A in various solvents [4, 5] and the copolymerization of T M P M A with M M A in toluene or ethyl acetate [6]. EXPERIMENTAL

MMA monomer, 2,2'-azobisisobutyronitrile (AIBN), and 1,4-dioxane were purified by conventional methods. TMPMA monomer (Kyoeisha Chemical Co. Ltd) was used without further purification, although the purity was checked by ~H NMR spectroscopy. Polymerization was carried out as described previously [7]. After a definite time of reaction, the polymer was precipitated by pouring the reaction mixture into a large amount of methanol. The gel fraction of polymer samples obtained at conversions beyond the gel point was separated by extracting the sol fraction with tetrahydrofuran (THF). The weight-average molecular weight (Mw) was measured by light scattering (LS). The measurements were carried out in THF at 30 + 0.1°C in a Union Giken LS-601 automatic laser scattering photogoniometer over the angular range between 30 and 150°, using unpolarized light of wavelength 632.8 nm. The intrinsic viscosity [r/] of the polymer was measured in THF solution at 30°C in an Ubbelohde viscometer. GPC-LALLS measurements were carried out with a Waters Associates ALC/GPC 244 apparatus at room temperature under the following conditions: solvent, THF; ~u styragel column combination, 10,2 500, I0,3 104 and 105 A (Waters designation); polymer concentration, 0.1% (w/v); and flow rate, 1 ml/min. The dual detector system consisted of a

LALLS and a differential refractometer in sequence in the flow direction. The LALLS was LS-8000 (TOSOH Corp.) where the laser beam of wavelength 632.8 nm was focused on a 30 #1 flow cell. The scattering angle was always 5°. ~H NMR 100-MHz spectra were recorded with a JEOL JNM-PS-100 instrument. RESULTS AND DISCUSSION

Structural features o f M M A - T M P M A

copolymer

Prior to discussion of the gelation in M M A - T M P M A copolymerization, the structural features of the copolymers obtained at an early stage of copolymerization, the primary chain, should be explored in detail. The T M P M A units incorporated into the copolymer chain may have three different types of structures, including noncyclic (I), monocyclic (II) and bicyclic units. Here the formation of a bicyclic ring would be impossible from the careful inspection of a molecular model. Thus, we tried to estimate first the cyclopolymerizability of T M P M A to form a monocyclic ring according to the following kinetic treatment: the five reactions shown below are considered as the propagation reactions, M. + M - , M . 3kp[M .] [M] M'~Mc'

2kc[M']

M c ' + M - ~ M . 3kcp[Mc. ] [M]

(1) (2) (3)

Mc'--* Mcc" kcc [Mc']

(4)

M c c ' + M--*M" 3kccp[Mcc'] [M],

(5)

where M - - T M P M A monomer; M . - - n o n c y c l i c radical; Mc.--monocyclic radical; and M c c ' - - b i c y c l i c radical. F r o m the above discussion, reactions (4) and (5) would be negligible. Thus, the rate of consumption of T M P M A m o n o m e r is given by - d [ M ] / d t = 3kp[M'] [M] + 3kcp[Mc'] [M]. 385

(6)

386

AKIRAMATSUMOTOet al. CH 3 (CH2-~[)

CH 3

CH3

(CH2~'~CH2--~)

o=c

.....

C----O ' 0

CHy--CH2

\/ C /\

CH 2

\/ C /\

CH~

CH 2

CH 2 . CH3---CH 2

CH~

O

O

O

r

,

C..----O

I

C-----O

I

CH 3

I

CH3

CH3

I

II

On the other hand, the rate of formation of pendant unreacted methacrylic groups (PM) in the polymer is given by d[PM]/dt = 6kp[M.] [M] + 3kcp[Mc'] [M].

(7)

If a steady state is assumed for Mc', equation (8) can be obtained: d[Mc']/dt = 2kc[M'] - 3kcp[Mc'] [M] = 0.

(8)

By using equations (6)-(8), we obtain: -d[PM]/d[M]( = X) = (6[M] + 2Kc)/(3[M] + 2Kc),

(9)

where K c ( = k c / k p ) denotes the cyclopolymerizability of the noncyclic radical M'. Thus, the Kc value is estimated thus: Kc = 3[M] (2 - X ) / 2 ( X - 1).

(9')

Figure 1 shows the ~H N M R spectrum of a typical TMPMA homopolymer; the polymerization was carded out in dioxane using 0.001 mol/1 of AIBN at [M] = 1 mol/1 and at 50°C in the presence of lauryl

mercaptan (13 mol% of TMPMA), which greatly delayed the gelation of TMPMA, and the polymer of number-average molecular weight (h4"n) 7400 obtained at 5.6% conversion were subjected to N M R analysis. The content of the pendant unreacted methacrylic groups X was estimated by the comparison of the peak area of 5.6 ppm corresponding to vinyl methylene protons with the aromatic protons of polystyrene of ~w 18,000 added as an internal standard. The X values were obtained as 1.263 and 1.416 at [M]= 1 and 2moi/1, yielding K c = 4 . 2 0 and 4.21 mol/1 from equation (9'), respectively. Thus the Kc value of TMPMA was found as 4.2 mol/1. Second, the contents of the structural units I and II were calculated according to the following cyclocopolymerization mechanism [8] using the Kc value obtained above. In the copolymerization of TMPMA (M j) having three equivalent methacrylic groups with MMA (M2), seven elementary reactions (10)-(16) can be given as the propagation reactions for cyclocopolymerization: Ml' + M i a M i • 3kll[Ml '] [Ml]

(10)

M t • + M ~ M 2. kl2[Ml .] [M~]

(11)

MI.--*Mc. 2kc[M l .]

(12)

Mc" + Ml ~ M j . 3kcl [Mc'] [MI]

(13)

Mc' + M2~M2' kc2[Mc'] [Ms]

(14)

M2. + MI--,M 1• 3k21[M~.] [Mr]

(15)

M2" + M:~M2" k::[M2'] [M:].

(16)

By assuming steady state conditions for the different types of radicals, equation (17) is derived: d[lI]/d[l + II] I 6

I 4

' 2

L 0

8 (ppm) Fig. 1. ~H NMR spectrum of a typical TMPMA homopolymer (100 MHz, in CDCI3, at 23°). Polystyrene was added a s a n internal standard.

= 2 r j K c / ( 2 r j K c + 3rl[Ml] + [M2]), (17)

where r 1 = kH/k12 and K c = k c / k l l By substituting rl ffi 1 and Kc=4.2mol/l into equation (17), the content of monocyclic structural unit II was estimated to be 82% for the initial

Copolymerization of MMA with TMPMA

387

40

j

20

6

J I 0

1

2

3

Time (hr] Fig. 2. Time-conversion curve for the copolymerization of M M A with T M P M A : ©, total polymer; O, gel polymer.

0

I

I

I

5

10

15

Conversion (%)

M M A - T M P M A copolymer obtained under the polymerization conditions of Fig. 2.

Determination of gel point Figure 2 shows the time vs conversion curve for the solution copolymerization of MMA with TMPMA, I mol% of which was employed as a crosslinker for MMA, in 1,4-dioxane (volume ratio total monomer: 1,4-dioxane = 1:4) using 0.04 mol/1 of AIBN at 50°C. These polymerization conditions are set as the same as in previous work [1] on the copolymerization of MMA with oligoglycoi dimethacrylates (OGMA). The percentage of gel polymer obtained by the sol-gel separation is also plotted vs time in Fig. 2. The gel point was estimated to be the conversion at the time at which gel starts to be formed. The gel point was determined as 21.0%, which is lower than 28.0% for the MMA-ethylene dimethacrylate (EDMA) copolymerization [1]. No Trommsdorff effect [9] was again observed.

Molecular weight and intrinsic viscosity Figure 3 shows the dependence of /~'w on the conversion for the M M A - T M P M A copolymerization system, with the homopolymerization result for MMA. The initial weight-average degree of polymerization (P,.0), the primary chain length, was estimated by extrapolation of the curve to zero conversion to be 4460; this value is quite high compared with 2240 for MMA homopolymerization. This kind of increase in primary chain length was also observed in our previous paper [1] concerned with the copolymerization of MMA with OGMA, in which P,.0 increased with increasing number of oxyethylene units in OGMA; this is ascribed to the occurrence of intramolecular cyclization leading to the formation of loop structures which induce the sterically hindered suppression of the intermolecular termination between growing polymer radicals [2]. Supporting evidence for the formation of loop structures comes from the intrinsic viscosity data for the resulting polymers compared with those of the presumed linear polymers having corresponding mo-

Fig. 3. Dependence of M, on conversion. Dotted line corresponds to MMA homopolymerization. lecular weights [2]. In this connection, the dependence of [r/] of the resulting polymers on conversion are plotted in Fig. 4; [r/] increased with conversion as a reflection of the enhanced occurrence of intermolecular crosslinking with the progress of reaction. The intrinsic viscosity [~/]0 of the primary polymer chain having no crosslinkage was estimated to be 0.54 by extrapolating the curve to zero conversion and compared with [r/]L of the presumed linear polymer calculated by using the experimental ~t,, 0 and the following equation: [r/]L=4.24x lh~q,- 5 ' ~ 0 7,6v l w , 0 , although this equation was determined for linear poly(MMA) in THF at 30°C [2]. Thus [r/]0/[r/]L was obtained as 0.64, obviously suggesting the occurrence of intramolcular cyclization. The extent of formation of loop structures in the primary chain is compared with those for the M M A - O G M A copolymerzation systems as shown in Fig. 5; it is between n = 2 and 3. The presence of loop structures in the primary

-O.1

I-J

-0.3

o

I

1

5 lO conversion (%)

Fig. 4. Dependence of [~/] on conversion.

I 15

388

AKIRAMATSUMOTOet al. structures is sterically suppressed to yield a highermolecular-weight polymer [2]. However, no intensified LS was observed for the M M A - E D M A copolymer having a linear primary chain ([r/]0/[r/]L = 0.98) [2] as expected. In connection with this discussion, the occurrence of branching may be considered as an alternative explanation. At an extremely early stage of polymerization extrapolated to zero conversion, the intermolecular reaction between growing polymer radical and prepolymer, leading to the formation of branched structures, should be negligible and thus, the possibility of incorporation of branching into the primary chain can be ruled out.

0.8

gtla

o 0.4

0

I

I

10

20

Number of oxyethtene units

Fig. 5. Comparison of [r/]0/[r/]L of (0) MMA-TMPMA copolymer with those of (O) MMA43GMA copolymers plotted against the number of oxyethylene units in OGMA. chain was also checked by GPC-LALLS; Fig. 6 shows the GPC-LALLS curve of the M M A - T M P M A copolymer obtained at 7% conversion under the polymerization conditions of Fig. 2, along with that of the M M A - E D M A copolymer. The intensity of LS at a low elution volume for the M M A - T M P M A copolymer was enhanced compared with the calculated value (dotted line) assuming a linear polymer. This is in line with the view that bimolecular termination between growing polymer radicals having loop

(LALLS]

Intramolecular cyclization and crosslinking as reflected in solution properties Figure 7 shows the relationship between ~kt"wand It/] based on the data in Figs 3 and 4, along with those for M M A - E D M A copolymerization and MMA homopolymerization; plots of both M M A - T M P M A and M M A - E D M A copolymerizations were placed below that of MMA homopolymerization and the slopes were quite low, although the slope for M M A - E D M A copolymerization were much lower. These findings are interpreted by considering the occurrence of intramolecular cyclization and intramolecular crosslinking, the latter becoming more significant with the progress of polymerization, i.e. with increase in 3~w. Here it is well known that the intramolecular crosslinking is closely related to the greatly delayed gelation in divinyl homopolymerization [10-12] and monovinyl--divinyl copolymerization [13-16]. Furthermore, the significance of intramoleeular cyclization at an early stage of polymerization and of intramolecular crosslinking enhanced with conversion were observed again as GPC-LALLS curves shown in Fig. 8 for the M M A - T M P M A copolymers obtained under the following polymerization conditions: TMPMA 1.5 mol%, in 1,4-dioxane (volume ratio monomer: 1,4-dioxane= 1:7), [AIBN]=0.04mol/I, at 50°C. These conditions were set because the polymer

o.I I

I B //~A

I I

-o.t

(RI]

II/ i I/

-a3

/i 5

25

i

30 ELutlon volume (mL)

I 6

} 7

LoQ /I~w

35

Fig. 6. GPC-LALLS curves for (A) MMA-EDMA and (B) MMA-TMPMA copolymers.

'

7. Relationships between [~/3 and -~]w in the copolymerizations of MMA with (O) TMPMA and (0) EDMA. Dotted line corresponds to MMA homopolymerization. Fig.

Copolymerization of MMA with TMPMA

389

-3-

\

(LALLS)

2

-4--

5%

-5

I 6

I Z

Log Mw Fig. 10. Double logarithmic plots of A2 and .b~w in the copolymerizations of MMA with (O) TMPMA and (O) EDMA. 25

30

35

Elution volume (ml) Fig. 8. Variation of GPC-LALLS curves with conversion.

obtained at a high conversion under the previous conditions in Fig. 2 does not go through 0.45 # m micropore filter, i.e. it cannot be subjected to G P C - L A L L S measurement. Here it should be noted that the gel point was obtained as 36.5% under the new polymerization conditions, in which a high content of T M P M A as a crosslinker and a high dilution were employed; the delayed gelation from 21.0 to 36.5% demonstrates the greater significance of monomer concentration than the added amount of crosslinker. This would be relevant to the enhanced occurrence of intramolecular cyclization in a diluted solution. In this connection, the gel point was estimated to be 16.0% under the following conditions at a low dilution and a small amount of crosslinker: T M P M A 0.5mo1%, in 1,4-dioxane (volume ratio monomer: 1,4-dioxane = 1:2), [AIBN] = 0.04 mol/1, at 50°C. 3.5

./ % v

3.0

¢ln

_o

5

I 6

I 7

Log '~w Fig. 9. Double logarithmic plots (s:)~/2 and .b7w in the copolymerizations of MMA with (©) TMPMA and (O) EDMA.

Figures 9 and 10 show double logarithmic plots

of r.m.s, radius of gyration (($2)~/2) and second virial coefficient (A:) vs 3~', for M M A - T M P M A and M M A - E D M A copolymerizations. The upper M M A - T M P M A plot in (s2)~ ,'2 may be ascribed to the broad molecular-weight distribution arising from the occurrence of intramolecular cyclization because the value of (s2)~/2 is z-averaged. The large decrease in A2 with increasing M , , i.e. with the progress of polymerization, would be relevant to the occurrence of intramolecular crosslinking leading to the shrinkage of the molecular size of the resulting copolymer. REFERENCES

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