Grafting on polybutadiene with polytetrahydrofuran macroperoxyinitiators. Postpolymerization studies

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Grafting on polybutadiene with polytetrahydrofuran

macroperoxyinitiators. Postpolymerization studies

Hu¨lya Macit, Baki Hazer

*

Zonguldak Karaelmas University, Department of Chemistry, 67100 Zonguldak, Turkey Received 11 April 2007; received in revised form 13 June 2007; accepted 14 June 2007

Available online 28 June 2007

Abstract

Grafting reactions of polybutadiene with macro peroxy initiators and postpolymerization were studied. The cationic polymerization of tetrahydrofuran (THF) initiated by the cationic species derived from bis-(4-bromomethylbenzoyl) per-oxide (BBP) or bis-(3,5-dibromomethylbenzoyl) perper-oxide (BDBP) gave the PTHF macroperoxy initiator (MPI). PTHF-b-PMMA macroperoxy initiator (MPIb) was also obtained by the redox polymerization of methyl methacrylate initiated with the hydroxyl ends of PTHF and Ce(IV) salts without decomposing the peroxide groups in the middle. Macroperoxy initiators thermally grafted on cis-polybutadiene (PBD) with thermal curing to yield graft copolymers containing cross-linked and soluble parts, which were separated by the sol–gel analysis. FTIR spectra of the crosscross-linked samples indicated the characteristic signals of the PTHF, PBD and PMMA blocks. The crosslinked copolymers decomposed at around 470C. Postpolymerization of the crosslinked products indicated the increase in crosslinking density which has been fol-lowed by measuring the gradual increase of swelling values. Postpolymerization crosslinking was estimated as a ﬁrst order reaction rate.

Keywords: Postpolymerization; Macroperoxyinitiator; Grafting reaction; Polytetrahydrofuran

1. Introduction

Free radical polymerization has a great interest because of the robust, economical process and wide selections of polymers[1–3]. Macroinitiators among the free radical initiators are useful intermediates to prepare block and graft copolymers via free radical mechanism [4–17]. For example, linear and star

block copolymers have been synthesized via free radical polymerization of some vinyl monomers by the macroperoxyinitiators derived from bromo-methyl benzoyl peroxide[18–22].

Macroinitiators can also be used in the grafting reactions onto the polymers containing unsaturated units. In this manner, grafting reactions are carried out via the hydrogen abstraction[23]or addition to the unsaturated units of macroradicals induced thermally from the macroinitiator[24–28].

In free radical polymerization, post polymeriza-tion is one of the main subjects, which means that the polymerization continues after free radical

* _{Corresponding author. Tel.: +90 372 257 40 10/1372; fax: +90}

372 257 41 81.

E-mail addresses: bhazer2@yahoo.com, bkhazer@ karaelmas.edu.tr(B. Hazer).

European Polymer Journal 43 (2007) 3865–3872

www.elsevier.com/locate/europolj

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source removal[29–36]. Postpolymerization is espe-cially important for methacrylated dental resin’s photopolymerization in the dental curing applica-tions [30]. (At the same time, postpolymerization has also been used to mean functionalization of the polymers as a useful methodology for the gener-ation of new materials with wide range of applica-tions[37]).

Usually, in case of photopolymerization, free rad-icals generated by the irradiation initiate the poly-merization; after cessation of the light source, the polymerization continues [31–37]. In these works, postpolymerization kinetics have been analyzed and as a result, rate of polymerization for postpoly-merization was found to be less than that of photo-polymerization. In addition, the polymerization of several acrylate monomers after initiating the reac-tion during diﬀerent illuminareac-tion times, the mono-mer concentration in the dark was studied by using infrared spectroscopy and electron paramagnetic resonance measurements [29]. Apart from this, to observe postpolymerization, polystyrene radicals stripped from the unreacted styrene monomer are also used to initiate polymerization of methyl meth-acrylate (MMA) to obtain related block copolymers by means of postpolymerization [38]. Similarly, postpolymerization causes crosslinking in the graft copolymerization of MMA onto a microbial polyes-ter containing unsaturated side chains. In this situation, after polymer was precipitated, the poly-merization advances to crosslinked polymer form from the soluble branched copolymer [39]. In a recent work, the study of postpolymerization reac-tions of photopolymerized mono- and dimethacry-lates in poly(styrene-b-butadiene-b-styrene) triblock copolymer matrix has been reported [40]. Thus, crosslinked structures were obtained and radicals involved in postpolymerization reactions were evalu-ated using electron spin resonance spectroscopy.

Polybutadiene is a well known engineering plas-tic, which can easily be crosslinked via free radical mechanism in order to obtain elastomer. Graft and block copolymers of polybutadiene could be of particular interest for novel elastomers [41,42]. As a continuing research in our laboratories includ-ing graftinclud-ing reactions of polystyrene[24]and poly-ethylene glycol [43] on polybutadiene, in this paper, we report the grafting reactions of polybuta-diene carried out thermally with PTHF-peroxy initi-ators to obtain PBD-g-PTHF comb type graft copolymers. The extent of crosslinking by means of the postpolymerization observed after the graft

copolymer precipitated is also analyzed in respect of swelling ratios.

2. Experimental part 2.1. Materials

Macroperoxy initiators were already obtained in our recent work [21]. Chemical structures and molecular weights of the PTHF and PMMA-b-PTHF macroperoxy initiators were given in Scheme 1 and inTable 1, respectively.

Other chemicals and organic solvents used in this study were purchased from Aldrich and used as received.

2.2. Grafting reactions on cis-polybutadiene with macroperoxyinitiators

The procedure cited in the literature was used [24,43]. In a typical grafting procedure, a solution

Table 1

Molecular weights of the MPIs synthesized in our recent work

[41]

Entry MPI Molecular weight (GPC)

Type Mn· 104 MWD HM-5 PTHF 5.2 1.46 HM-6 PTHF 2.1 2.68 HM-10 PTHF 0.9 2.41 B4-9 PTHF(HM-9)-b-PMMA 7.6 1.10 B2-10 PTHF(HM-10)-b-PMMA 7.5 1.11 O HO-[(CH2)4O]m-CH2- -C-O-}2 HM-10 O HO-[(CH2)4O]n-CH2 -C-O-}2 HO-[(CH2)4O]p-CH2 HM-5, HM-6, HM-9 O PMMA---[(CH2)4O]q-CH2 - -C-O-}2 PMMA---[(CH2)4O]q-CH2 B4-9

Scheme 1. Macroperoxy initiators used in the grafting reactions of polybutadiene.

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was prepared from a mixture of 0.25 g PTHF (or

PTHF-b-PMMA) and 0.5 g PBD in 15 mL

chloroform.

The solution was stirred for 24 h at room temper-ature and spread onto a Petri dish (/ = 10 cm) for air drying to obtain a polymer ﬁlm. Grafting reac-tions were carried out on this glass plate by intro-ducing it in an oven preheated to 80C for 2 h. For the sol–gel analysis, the polymer obtained was extracted with chloroform. Crosslinked and soluble parts were separately precipitated from methanol and dried under vacuum oven at 30C for 2 days. 2.3. Swelling measurements

Swelling degrees of polymers at equilibrium were determined by gravimetry at room temperature in chloroform, according to the procedure described elsewhere [44,45]. The qwswelling values for cross-linked polymers were calculated using Eq.(1) q_w¼mswollen polymer

mdry polymer

; ð1Þ

where qw, mswollen polymerand mdry polymerare swelling ratio, weight of swollen polymer and weight of dry polymer, respectively.

2.4. Characterization of the polymers

FTIR spectra were recorded of the KBr disc of polymer samples, by using a JASCO model 300E FTIR spectrometer.

Thermogravimetric analysis, TGA, measure-ments were carried out under nitrogen by using a PERKIN ELMER/Pyris1 model TGA.

2.5. Scanning electron microscopy, SEM

Scanning electron micrographs were taken on a JEOL JXA-840A scanning electron microscope (SEM). The specimens were frozen under liquid nitrogen, then fractured, mounted, and coated with gold (300 A˚ ) on an Edwards S 150 B sputter coater. The SEM was operated at 15 kV, and the electron images were recorded directly from the cathode ray tube on a Polaroid ﬁlm.

3. Results and discussion

3.1. The graft polymerization of PTHF and cis-polybutadiene (PBD)

The cationic polymerization of THF was carried out by a mixture of bromomethyl benzoyl peroxide and AgSbF6to obtain a poly-THF macroperoxyini-tiator HM-5, -6, -9 and -10 T according to the pro-cedure in our recent article [21]. Poly-THF macroinitiators, HM-9 or HM-10 with hydroxyl end groups are used to initiate the redox polymeri-zation of MMA in the presence of Ce(IV) salt to give poly(THF-b-MMA) macroperoxyinitiators. Scheme 1 shows the macroperoxyinitiators.

Free radical crosslinking on PBD was carried out by PTHF or PTHF-b-PMMA macroperoxy initia-tors listed inTable 1. In the ﬁrst step of the grafting reactions, macroradicals are thermally induced from the macroinitiators and then they attack polybutadi-ene to form graft copolymers. Hydrogen abstraction and addition to the double bonds can cause the sol-uble and the crosslinked graft copolymers. The for-mation reaction of PTHF-g-PBD graft copolymers

Poly-THF….-OO-…poly-THF , 80oC 2 poly-THF. , PMMA-g-poly-THF….-OO-…..poly-THF-g-PMMA 2 PMMA-g-poly-THF. (macroradical) 80oC

Macroinitiator 2 Macroradical (R.)

i. Addition to double bond

R. + ----CH=CH---- ----CH CH.---- R

ii. H-abstraction

R. + ----CH=CH CH2--- ----CH=CH CH. ---- + RH

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has been illustrated in Scheme 2. The conditions and results of the graft polymerization reactions have been presented inTable 2. The crosslinked part with sol–gel analysis and the soluble part have been sepa-rated from each other and analyzed separately. FTIR spectra indicate the characteristic signals at 1110 cm1 for PTHF segments, 740 cm1 for PBD segments and 1730 cm1for both PMMA and ester

groups of PTHF segments as shown inFig. 1. Sur-face topography of the crosslinked graft copolymers was studied by SEM; a continuous homogeneous matrice was observed. Decomposition temperatures of the graft copolymer samples were observed at around 470C.

3.2. Postpolymerization

After sol–gel analysis of the graft copolymers (Table 2), soluble parts got crosslinked the follow-ing day at room temperature (Table 3). The entrapped radicals continued polymerization in the solid polymer matrix, indicated in Scheme 2, leading to the crosslinked structure. It was also observed that these entrapped radicals are still alive so that the crosslinking reaction proceeds for 1–3 weeks and the crosslinking densities increase. Naturally post polymerization reaction was faster in the begin-ning but later becomes slow.

The polymer samples which dissolved in CHCl3 straight after the reaction did not dissolve in CHCl3 after a while and were crosslinked. This shows that

Table 2

Grafting reactions of PBD with MPIs at 80oC for 2 h and sol–gel analysis of the graft copolymers obtained after precipitation on the ﬁrst day

Run # MPI PBD Graft copolymer

Type (g) (g) Yield (g) Crosslinked (wt%)

PTHF-g-PBD 1 HM-6 0.25 0.51 0.75 49 PTHF-g-PBD 2 HM-5 0.25 0.50 0.74 24 PTHF-g-PBD 3 HM-10 0.25 0.52 0.75 Not determined PTHF-g-PBD 4 HM-5 0.11 0.50 0.60 24 PTHF-g-PBD 5 HM-5 0.51 0.50 0.74 Not determined PTHF-g-PBD 6 HM-6 0.11 0.50 0.58 22 PTHF-g-PBD 7 HM-6 0.51 0.50 0.99 38 PTHF-b-PMMA-g-PBD-1 B4-9 0.26 0.51 0.72 56

PTHF-b-PMMA-g-PBD-2 B2-10 0.26 0.51 0.75 Not determined

Fig. 1. FTIR spectra of the graft copolymers: (a) PMMA-b-PTHF-g-PBD-1, (b) PTHF-g-PBD-1.

Table 3

Postpolymerization of the soluble graft copolymers at room temperature for the following day

Run no. Soluble polymer Crosslinked polymer, Run no (g) (g) (wt%) PTHF-g-PBD 1s PTHF-g-PBD 1 0.38 0.38 100 PTHF-g-PBD 2s PTHF-g-PBD 2 0.56 0.56 100 PTHF-g-PBD 3s PTHF-g-PBD 3 0.75 0.32 72 PTHF-g-PBD 4s PTHF-g-PBD 4 0.45 0.45 100 PTHF-g-PBD 5s PTHF-g-PBD 5 0.93 0.93 100 PTHF-g-PBD 6s PTHF-g-PBD 6 0.45 0.45 100 PTHF-g-PBD 7s PTHF-g-PBD 7 0.62 0.62 100

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polymerization continued after the reaction (post polymerization).

Swelling experiments had been conducted on the crosslinked part and the part which dissolved in CHCl3 after polymerization and which crosslinked straight after. The Swelling ratios (qw) of the poly-mers have been calculated using Eq. (1). The advances of the crosslinking have been observed by measuring the gradual change of qwvalues. This procedure had been repeated until qw became con-stant. The values of the crosslinking polymers in

the ﬁrst separation have been presented in Table 2. The soluble fractions given inTable 2crosslinked the following day. They were coded PTHF-g-PBD 1–7 s and the same swelling experiments had also been applied to this part. The results are presented inTable 3.

The graphs of the gradual change of the swelling ratio (qw) of polymer samples from the part which crosslinked in the ﬁrst separation are presented in Fig. 2. As can be observed from these graphs the swelling ratio decreases gradually, maintaining a

Fig. 2. The gradual change of the qwvalue of the parts crosslinked in the ﬁrst separation of the PTHF-g-PBD graft copolymer (Tables 1

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Fig. 3. The observation of the gradual crosslinking of the parts of the graft copolymer PTHF-g-PBD dissolved in the ﬁrst separation (Table 2).

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certain value. Thus, this shows that crosslinking continues for a while after the polymerization reac-tion and then comes to a halt. As crosslinking increases the swelling ratio decreases.

The graphs of the gradual change of the swelling ratio (qw) of the parts which dissolved in CHCl3in the ﬁrst separation and however then crosslinked after polymerization are presented in Fig. 3. The case is the same for these graphs. Here also the crosslinking continues after the polymerization reaction and then comes to a halt.

The post polymerization reaction lasts for one to two weeks in both the situations. Some reactions go smoothly which means that reaction rate decreases gradually; while some follow random pathways which means that crosslinking reaction stops for a while and then decreases sharply. When we consider smooth reactions of PTHF-g-PBD-1, -2 and -3, by using ﬁrst order reaction rate equation [46] given below (Eq. (2)), we may expect the reaction order of the post polymerization.

logA0

A ¼

kt

2:303 ð2Þ

When we consider A’s as the swelling values for each day, we obtain a line in the plot of logA0/A ver-sus t (day), which we attribute to the ﬁrst order reac-tion rate.

3.3. Graft copolymerization of PTHF-b-PMMA macroperoxyinitiator and cis-PBD

PMMA-b-PTHF macro radicals induced ther-mally in the mixture of PTHF-b-PMMA macroper-oxyinitiator and cis-PBD attack double bonds of PBD to give PTHF-b-PMMA-g-PBD comb type copolymer. The crosslinked part and the soluble part have been separated from each other by sol– gel analysis and analyzed separately. Conditions and results of graft copolymerization have been pre-sented in Table 3. As the eﬃciency of the graft copolymer represented as (PTHF-b-PMMA)-g-PBD-2 in this chart is very low sol–gel analysis has not been conducted. The graft copolymer with the code (PTHF-b-PMMA)-g-PBD three dissolved in CHCl3in the ﬁrst separation. A swelling experi-ment has been conducted on the crosslinked part of the graft copolymer (PTHF-b-PMMA)-g-PBD-1. The swelling ratio (qw) has been calculated using Eq. (1). The advance of crosslinking has been observed by measuring the gradual change of the

qw value. This procedure has been continued until qw is ﬁxed (Table 3). Further crosslinking after the polymerization reaction comes to a halt in the (PTHF-b-PMMA)-g-PBD 1 graft copolymer just as in the PTHF-g-PBD copolymer. The graph in Fig. 3 clearly illustrates this. As can be observed here the swelling ratio gradually decreases and attains a ﬁxed value.

Acknowledgment

This work was ﬁnancially supported by Zongul-dak Karaelmas University Research Fund and TU¨ B_ITAK Research Project No. 104M128. References

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