CROSS-LINKED MULTICOMPONENT COPOLYMERS WITH
MACROMONOMER PEROXYINITIATORS (MMPI)
BAKI HAZER1,2* and SEVIL SAVAS°KAN3
1Zonguldak Karaelmus University, Department of Chemistry, Zonguldak 67100, Turkey,2
TuÈbitak-Marmara Research Center, Food Science and Technologies Research Institute, P.O. Box 21, Gebze 41470 Kocaeli, Turkey and3Karadeniz Technical University, Department of Chemistry, Trabzon,
61080, Turkey
(Received 2 April 1996; accepted in ®nal form 10 April 1997)
AbstractÐMacromonomer peroxy initiator, MMPI, was synthesized by the reaction of polytetrahydro-furandiol (Mw1000 g/mol) with isophoran diisocyanate (IPDI), 2,5-dimethyl-2,5-dihydroperoxyhexane
(Luperox 2,5-2,5) and isocyanato ethyl methacrylate (IEM). Copolymerization of methyl methacrylate, (MMA), with MMPI gave poly(THF-b-MMA) cross-linked block copolymers. The overall polymeriz-ation rate constant, k, was calculated to be 2.46 10ÿ4 (mol/L)1/2 secÿ1. The cross-linked copolymer
samples showed the peroxide decomposition at 1758C and glass transition at approximately 548C. Poly(THF-b-MMA) cross-linked block copolymers containing undecomposed peroxide groups initiated the thermal polymerization of styrene; S. Poly(THF-b-MMA-b-S) cross-linked block copolymers with a Tgof 808C were obtained. By measuring the degrees of swelling of the polymer networks, the mechanism
of crosslinking polymerization has been investigated. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
Macro intermediates such as macroinitiators, macromonomers and macrocross-linkers are im-portant in polymer modi®cation leading to block and graft copolymers [1±3]. New kinds of macro termediates known as macromonomeric azo in-itiators (macroazoinimers) have recently been reported by Hazer et al. These behave as macromo-nomers, macroinitiators and macrocross-linkers at the same time [4±8]. Macroazoinimers can be syn-thesized by the vinylation of the polyazoesters [9± 10] with isocyanatoethylmethacrylate [5], methacry-loyl chloride [6] or 4-vinylbenzyl chloride [7]. Polymerization of vinyl monomers with macroazoi-nimers produces branched or cross-linked block copolymers depending on the macroazoinimer con-centration and polymerization time.
The present paper describes the copolymerization kinetics of methyl methacrylate with MMPI to obtain cross-linked poly(THF-b-MMA) block copolymers having undecomposed peroxy groups. These are capable of initiating the polymerization of a vinyl monomer to yield multicomponent crosslinked poly-mer.
EXPERIMENTAL
Instrumentation
NMR and FT-IR spectra of the polymer samples were recorded by using a Bruker 200 MHz NMR and a Perkin± Elmer 1600 FT-IR spectrometer, respectively. Thermo-gravimetric analysis (TGA) of polymer samples was carried out on a Du Pont 910 thermal analyser at a heating rate 108C/min. Dierential scanning calorimetry (DSC) was
per-formed using a Perkin±Elmer 7 Series thermal analysis sys-tem. Samples were prepared by annealing at 1508C for 5 min followed by quenching to ÿ1008C. The heating rate was 208C/min.
Materials
Materials used and suppliers are listed as follows: isophor-andiisocyanate, IPDI: Fluka AG; poly(tetrahydrofuran)-diol, p-THF-diol (Mw: 1000 g/mol, OH]112): BASF;
2,5-dimethyl 2,5-dihydroperoxyhexane, Luperox 2,5-2,5: Lucidol Div. Penvalt Corp.; 2-isocyanato ethyl methacry-late, IEM: Polyscience; Dibutyltindilaurate, DBTDL: Cincinatti Milacron Chemical Inc.; Pyridine and CH3COOH: Merck AG; S and MMA were dried over CaCl2
and then distilled on CaH2under reduced pressure before
use. Bromine, concentrated sulphuric acid and sodium thio-sulfate (Merck AG) were used without further puri®cation. Synthesis of MMPI
MMPI was synthesized by the reaction between Luperox 2,5-2,5, IPDI, p-THF-diol and IEM according to the cited literature [11]. The molecular weight of MMPI obtained, (g/ mol): 3008 (Mn, by GPC), 2880 (calculated from its
for-mulae) and peroxygen content, wt%: 1.8 (by iodometry), 2.0 (theoretical).
Block copolymerization of MMA with MMPI
In a pyrex tube, in which MMA and a given amount of MMPI were charged separately, nitrogen was purged through a needle into the tube. The tightly capped tube with a rubber septum was placed in an oil bath thermostated at 808C. After the required time of period of polymerization, the polymer produced was placed in chloroform to separate sol±gel fractions. The soluble part was extracted from the network polymer mixture by soaking in chloroform over-night at room temperature.
Swelling ratio, qv. The crosslinked polymer samples
(0.2 g) were soaked in 50 mL of a solvent (CHCl3) for
24 hr at 208C. Swelling ratio, qv, was calculated by the
fol-lowing equation [12]:
# 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0014-3057/98 $19.00 + 0.00
PII: S0014-3057(97)00193-6
*To whom all correspondence should be addressed.
qvVdry polymerV Vsolvent
dry polymer ; 1
where Vdry polymer is the volume of dry polymer and
Vsolvent is the volume of the absorbed solvent at
equili-brium swelling. Tables 1 and 2 also contain the swelling ratios of the cross-linked polymer samples.
Polymerization of styrene with active polymer networks, poly(THF-b-MMA)
A series of poly(THF-b-MMA) with various compo-sitions were prepared in bulk polymerization at 808C. As a typical polymerization procedure: 0.2 g of poly(THF-b-MMA) block copolymer sample swollen in styrene mono-mer was kept at 908C for 8 hr under N2. Cross-linked
mul-ticomponent polymer samples obtained by this way were separated from the soluble part by soaking in solvent (CHCl3). The puri®ed polymer network was dried at 508C
under vacuum for 2 weeks.
RESULTS AND DISCUSSION
MMPI was synthesized according to the literature [10]. The number of vinyl groups per MMPI mol-ecule was determined to be 1.94, which con®rms the vinylization of both ends to be completed as indi-cated in the following formulae:
The FT-IR spectrum of MMPI showed the characteristic bands at 1100 (C0O0C), 3440 (0NH0CO0) and 1750 cmÿ1 (0C1O). The
con-densation reaction was also followed by disappear-ance of the diisocyanate absorbdisappear-ance peak at 2200 cmÿ1 in FT-IR. TGA traces of MMPI
exhib-ited decomposition maxima at approximately 1608C (peroxy), 3408C (a shoulder, probably decompo-sition of ester and urethane groups) and 4008C (p-THF). Quantitative analysis of vinyl groups in MMPI was performed by bromine addition to the double bonds according to the procedure in the cited literature [13].
The copolymerization of MMA with MMPI was carried out in two ways. In the ®rst series of exper-iments, time conversion data were obtained by keeping the MMPI concentration constant in monomer solution (Table 1). Figure 1 shows the amount of cross-linked polymer formed as a func-tion of the polymerizafunc-tion time. This has a plateau after 40 min. The soluble part of the copolymer yield was founded to be less than 1 wt% of polymer yield when the polymerization time was long.
The polymer produced can be more random and allow for the fact that the sequence of MMA or THF units have a wide distribution. The
cross-link-ing structure can be a mixture of poly(THF-co-MMA) and poly(THF-b-poly(THF-co-MMA). However, the initial mol ratio of vinyl monomer to MMPI is ap-proximately 100 and the conversion of monomer has generally been 100%. In these conditions, unde-composed MMPI and/or deunde-composed MMPI rad-ical fragments behave as cross-linking agents in MMA polymerization. Therefore, these cross-linked copolymers obtained by using macromonomeric in-itiators have been considered as block copolymers [4±7, 11].
The swelling ratio of cross-linked polymers in CHCl3 changes from 19.5 to 5.3 as polymerization
proceeds. Figure 2 shows the decrease in qv as the
polymerization time increases. These results suggest that the MMPI behaves as a macrocross-linker at the beginning of the polymerization of MMA. Cross-linked block copolymers with higher cross-linked density can occur if the peroxide groups react into the network, as shown in the following reaction (Scheme 1).
In the second series of experiments, conversion of monomer was kept under 9 wt% in order to eluci-date the kinetics. Table 2 shows the conditions and results of the methyl methacrylate copolymerization with MMPI in low conversion. In a radical
polym-erization, it is well known that Rp is given by the
relation [14±19]: Rp kp f kkdI t 1=2 M 2 by taking k kp f kkd t 1=2 ; 3 We obtain Rp kMI1=2; 4 where [I] and [ M] are the MMPI and MMA con-centrations. k, kp, kt, kdand f are overall rate
con-stant, propagation rate concon-stant, termination rate constant, decomposition rate constant and initiator eciency, respectively. The overall rate constant k was calculated from the slope of linear part of the relationship illustrated in Fig. 3. At higher concen-trations of MMPI ([ M][I]1/2>1.4), linearity has
been disturbed because of autoacceleration of vinyl groups of MMPI. The time conversion curve pre-viously shown in Fig. 1 shows an induction period of approximately 5 min. Similarly, Fig. 3 also shows linearity with an induction period starting at around 0.1. The overall rate constant k for MMA polymerization was calculated to be 2.46 10ÿ4 L1/ Table 1. Copolymerization of MMA with MMPI at 808C
Copolymer yield
total Cross-linked
Run no. MMA (g) MMPI (g) Polymerization time (min) (g) (wt%) (wt%) qvin CHCl3
1 1.405 0.500 20 0.574 30.1 25.5 19.5 2 1.429 0.501 40 1.289 66.8 61.9 8.45 3 1.423 0.500 60 1.334 69.4 67.3 8.03 4 1.481 0.501 90 1.436 72.4 70.9 7.40 5 1.425 0.600 110 1.485 73.3 73.0 7.31 6 1.418 0.500 120 1.475 76.9 75.7 6.67 7 1.428 0.500 125 1.508 77.0 75.5 6.32 8 1.462 0.500 140 1.667 84.9 94.2 5.78 9 1.419 0.501 145 1.724 89.6 88.3 5.26
Table 2. Polymerization of MMA with MMPI at 808C Polymer yield
MMPI total Cross-linked
Run
no. MMA(g) Amount(g) wt% inMMA Polymerizationtime (min) (g) (wt%) (wt%) (mol/L[M]ÿ1) [I] 1/2 (mol/L1/2) [M][I] 1/2 (mol/L3/2) Rp10 3 (mol/Lÿ1s) 10 2.899 0.008 0.276 190 0.139 4.8 0.4 9.543 0.028 0.270 0.038 11 2.872 0.019 0.662 100 0.229 7.9 0.8 9.411 0.045 0.421 0.119 12 3.013 0.051 1.692 30 0.076 2.5 0.2 9.768 0.074 0.723 0.131 13 2.971 0.054 1.817 25 0.041 1.4 0.1 9.623 0.077 0.745 0.083 14 2.922 0.073 2.498 20 0.099 3.3 0.7 9.404 0.089 0.941 0.254 15 2.919 0.081 2.774 20 0.117 3.9 1.0 9.371 0.095 0.889 0.294 16 2.922 0.117 4.004 15 0.084 2.8 0.3 9.273 0.110 1.016 0.279 17 2.904 0.138 4.752 15 0.010 0.3 0.0 9.153 0.118 1.083 0.346 18 2.987 0.186 6.226 10 0.069 2.2 0.1 9.274 0.138 1.278 0.331 19 2.957 0.213 7.203 5 0.030 0.9 0.3 9.101 0.148 1.350 0.286 20 2.509 0.366 14.582 20 0.293 8.9 3.2 8.548 0.190 1.622 0.679 21 2.899 0.505 17.422 10 0.122 3.6 2.7 8.181 0.219 1.792 0.543
2molÿ1/2sÿ1 from the slope of the line in Fig. 3.
This compares to k of 1.0 10ÿ4(mol/L)1/2sÿ1 for
the copolymerization of styrene with MMPI [11]. From the comparison of k values for some peroxide initiators displayed in Table 3, it can be seen that the MMPI in this work is typical of other peroxide initiators.
When comparing the swelling ratios of the cross-linked block copolymers in Table 1, qv values
decrease as the polymerization time increases. The FT-IR spectra of the soluble part of poly(MMA-b-THF) block copolymersis exempli®ed in Fig. 4: Absorptions at 1730 (carbonyl bands of MMA) and 1100 cmÿ1 (etheric band of p-THF) are prominent.
Fig. 1. Cross-linked polymer yield as a function of MMA polymerization time.
The thermal analysis also shows that cross-linked block copolymers still contain undecomposed per-oxy group in the cross-linking structure (weight loss at around 1708C due to decomposition of peroxide group, Fig. 5). A single Tgwas obtained at
approxi-mately 548C which may result from the overlap of the pure p-THF Tgat ÿ408C and a PMMA mixed
phase. A single melting transition, Tmwas observed
at 648C resulting from poly-THF segments because they have a very fast crystallization. Notably the poly-THF content in the cross-linked block copoly-mer was calculated as 30±35 wt% from 1H NMR
spectra and the calculated Tg (Tgof pure poly-THF
is ÿ408C, Tgfor pure PMMA is 1108C) of the
net-work copolymer was very close to the observed value (Tg051±588C).
Peroxide decomposition in cross-linked poly(THF-b-MMA) block copolymers was also observed at 1708C. Therefore, we used these samples as the initiator in the polymerization of styrene to obtain cross-linked multicomponent
poly-mers. In the DSC curve of such samples, the Tg
value increased to 808C, which is likely to result from the insertion of the stier styrene segments into the block copolymer structure. However, a melting point at 648C, related to the crystallization of poly-THF blocks, was also observed in the ®rst scan at the same temperature (648C) of that of the original di-block copolymers. Table 4 includes the results and conditions in the copolymerization of styrene with di-block copolymers at 808C. When comparing the degrees of swelling of di-block copo-lymers (in Table 1) with poly (THF-b-MMA-b-S) (in Table 4) the latter were found to be lower. As polymerization time increases, it is likely that the peroxide bonds in the poly (THF-b-MMA) network cleave and initiate the polymerization of styrene to form the molecular weight between crosslinks. It is also possible that pendant vinyl [22] groups of poly (THF-b-MMA) are consumed during the polym-erization of the second vinyl monomer to reduce the chain length between crosslinks. In order to evaluate this idea we observed the thermal de-composition of poly (THF-b-MMA) samples (run no.4 in Table 1) swollen in chloro benzene at 908C for 8 hr. The degrees of swelling of decomposed polymer network was lower than the original (qv=3.8 and 7.4, respectively). This suggests that
pendant vinyl groups remain in the polymer net-works and these pendant vinyl groups are reacted with free radicals formed to yield denser cross-link-ing structure. Additionally, it could be possible that chain transfer to polymer could occur later in the
Fig. 3. The eect of monomer and initiator concentrations on the rate of polymerization MMA polym-erization with MMPI at 808C.
Table 3. Overall rate constant, k, for MMA with some macroper-oxyinitiators at 808C
Macroperoxide k, (L1/2molÿ1/2sÿ1) Reference
Polymeric peroxcarbamate 4.82 10ÿ4 [19]
Ollgoperoxide 3.47 10ÿ4 [20]
Luperox 2,5-2,5 1.33 10ÿ4 [21]
Benzoyl peroxide 11.50 10ÿ4 [21]
Fig. 4. FT-IR spectru m of poly(T HF-b-MM A ) cro ss-linked block copo lymer (run no. 16 in Ta ble 2).
polymerization where polymer concentration is high, leading to further cross-linking.
CONCLUSION
The copolymerization of MMPI with MMA revealed that the reactivity of MMPI was
compar-able with common peroxide initiators. Cross-linked multicomponent copolymers, poly(THF-b-MMA-b-S), can be prepared by using macromonomeric per-oxyinitiators. As polymerization of styrene with poly(THF-b-MMA) proceeds, the remaining per-oxide bonds in poly(THF-b-MMA) network cleave and initiate the polymerization of styrene to form multicomponent cross-linked copolymer.
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Table 4. The copolymerization of styrene with poly(THF-b-MMA) at 908C to obtain poly(THF-b-MMA-b-S) cross-linked multicompo-nent copolymer
Multicomponent cop. yield
Block copolymer: Cross-linked
Run no. (in Table 1)Run no. Amount(g) Swollen inS (g) copolymer in Sqvof block Total(g) (g) (wt%) qvin CHCl3
22 1 0.200 2.433 15.4 2.011 1.500 82.7 2.06 23 2 0.206 2.365 14.6 1.549 1.378 65.5 2.96 24 3 0.201 2.333 14.7 1.794 1.498 76.9 2.99 25 4 0.204 1.821 11.3 1.221 1.209 67.1 1.80 26 8 0.201 1.817 11.3 1.326 1.031 73.0 1.19 27 9 0.200 1.687 9.74 1.476 1.330 87.5 1.10
