benzoyl peroxide-initiated copolymerization of citronellol and vinyl acetate

Benzoyl Peroxide-Initiated Copolymerization of Citronelloland Vinyl Acetate

PRACHI PANDEY, A. K. SRIVASTAVA

Department of Chemistry, H. B. Technological Institute, Kanpur, 208 002, India

Received 20 July 2001; accepted 10 January 2002Published online 00 Month 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pola.10173

ABSTRACT: The radical copolymerization of citronellol with vinyl acetate (VA) in xyleneat 60 � 0.1 °C for 90 min in the presence of benzoyl peroxide follows ideal kinetics andresults in the formation of an alternating copolymer as demonstrated by the values ofthe reactivity ratios [r1 (VA) � 0.02 and r2 (citronellol) � 0.0002], which have beencalculated with the Kelen–Tudos method. The overall activation energy is computed tobe 75 kJ/mol. The IR spectrum of the copolymer shows the presence of bands at 3400cm�1 due to an alcoholic group and 1750 cm�1 due to a �CAO group. The values of theMark–Houwink constants for this copolymer system have been determined with gelpermeation chromatography to be � � 0.375 and K � 2.4 � 10�4. The glass-transitiontemperature, determined with differential scanning calorimetry, is 68.32 °C. The mech-anism has been elucidated. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40:1243–1252, 2002Keywords: citronellol; vinyl acetate; copolymerization; reactivity ratios

INTRODUCTION

Block copolymers of vinyl acetate (VA) withmethyl methacrylate, acrylonitrile, and vinyl pyr-rolidinone have been prepared by copolymeriza-tion in viscous and poor solvents for the VA mac-roradical. Copolymers of VA containing cyclicfunctional groups in the polymer chain have beenprepared by the copolymerization of VA with N,N-diallyl cyanamide and N,N-diallyl amine. VA andvinylidene cyanide form highly alternating co-polymers.1 VA has been grafted to polymers otherthan poly(vinyl alcohol), such as atactic polypro-pylene2 and casein.3

A search of the literature reveals that the ap-proach of polymer chemists to examining terpeneshas been limited to only terpene hydrocarbons,mostly bicyclic monoterpenes such as �- and�-pinenes.4,5 However, no attention has been given

to acyclic monoterpenoids such as citronellol andlinalool. Citronellol is a typical acyclic monoterpe-noid, containing one double bond and one alcoholicgroup; it was first prepared by Dodge6 in 1889:

Because of a lack of data for the radical poly-merization of citronellol, it is very interesting toinvestigate the copolymerization of citronellolwith VA in the sequence of our continuingwork.7,8 Therefore, attempts have been made tostudy the details of the kinetics, mechanism,and characterization of the copolymerizationcitronellol with VA in xylene initiated by benzoylperoxide (BPO) at 60 � 0.1 °C. Furthermore, thecopolymers formed are significant because of theirnice fragrance, functional properties, and opticalactivity.

Correspondence to: A. K. Srivastava (E-mail: [email protected])Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 40, 1243–1252 (2002)© 2002 Wiley Periodicals, Inc.

1243

EXPERIMENTAL

Materials

VA (Merk–Schuchardt) and other solvents werepurified by usual methods.9,10

Citronellol11 [mass � 156.27 g/mol, 1 lt � 0.86kg, bp � 103 °C, refractive index (nD

20) � 1.4562,specific rotation ([��]D

20) � �4–5°] was used asreceived. BPO was recrystallized twice frommethanol and then dried in vacuo.

Polymerization Procedure

A dilatometric technique (diameter � 2 mm,length � 9.5 cm, capacity � 2.5 mL) was used forfollowing the copolymerization runs under oxy-gen-free conditions. The polymerization mixturewas prepared with both comonomers and BPO.Polymerization was continued up to 90 min sothat the conversion range would be limited be-tween 5 and 18% at 60 � 0.1 °C in xylene. Thepolymer was isolated with acidified methanol anddried in vacuo. It was then treated with toluenefor the removal of poly(vinyl alcohol) when noweight loss was observed. Finally, the copolymerwas dried to a constant weight, and the conver-sion percentage was calculated. The rate of poly-merization (Rp) was calculated from the slope ofthe graph between the conversion percentage andtime.

IR and 1H NMR spectral analyses were re-corded with a PerkinElmer 599B (with KBr pel-lets) and a Varian 100 HA JEOL 400 LA spectro-photometer, respectively. Thermogravimetricanalysis (TGA) was performed on a Stanton Red-croft instrument at a heating rate of 10 °C/min.Differential scanning calorimetry (DSC) was car-ried out on a DuPont V4.1C model 2000 at a

heating rate of 10 °C/min under a nitrogen atmo-sphere. The monomer reactivity ratios were de-termined with the Kelen–Tudos method.12

RESULTS AND DISCUSSION

Some monomers do not undergo homopolymeriza-tion because of steric hindrance,13,14 low stabili-zation energy between the monomer and free rad-icals in the transition state,15 excessive chaintransfer,16 or termination of cyclization, such as1,2-disubstituted ethylene,17 maleic anhydride,fumarates, and vinyl ethers. Citronellol also doesnot homopolymerize under experimental condi-tions.

The kinetics of copolymerization have beenstudied with variations in the BPO concentration(6.8 � 10�3 to 34.43 � 10�3 mol L�1), with theconcentrations of citronellol and VA kept constantat 0.96 and 1.74 mol L�1, respectively. The reac-tion proceeds with a short induction period ofabout 3 � 1 min. It is clear that Rp is a directfunction of the initiator concentration and theexponent value of the initiator, which is deter-mined from a linear graph of log Rp versus log[BPO] to be 0.5 � 0.01 (Table 1; Fig. 1).

The effect of the citronellol concentration on Rphas been studied with variations in the citronellolconcentration (0.38–1.73 mol L�1), with the VAand BPO concentrations kept constant at 1.74and 13.77 � 10�3 mol L�1, respectively. Rp isdirectly proportional to the citronellol concentra-tion. A plot of log Rp versus log [citronellol] is

Figure 1. Relationship between the Rp and BPO/citronellol and VA concentrations of 0.96 and 1.74 molL�1, respectively (copolymerization time � 90 min, co-polymerization temperature � 60 � 0.1 °C).

Table 1. Effect of the Initiator Concentration on Rp

of Citronellol and VAa

Sample[BPO] � 103

(mol L�1)Conversion

(%)Rp � 106

(mol l�1 s�1)

1 6.88 8.3 4.162 13.77 11.8 5.853 20.66 14.4 7.244 27.54 16.2 8.125 34.43 19.08 9.54

a [Citronellol] � 0.96 mol L�1; [VA] � 1.74 mol L�1; copo-lymerization time � 90 min; copolymerization temperature� 60 � 0.1 °C.

1244 PANDEY AND SRIVASTAVA

linear; the slope gives the following relationship(Table 2; Fig. 2):

Rp�[citronellol]1.0

The effect of the VA concentration on Rp has beenstudied with variations in the VA concentration(0.69–3.13 mol L�1), with the citronellol and BPOconcentrations kept constant at 0.96 and 13.77� 10�3 mol L�1, respectively. Rp is directly pro-portional to the VA concentration. A plot of log Rp

versus log [VA] is linear; the slope gives the fol-lowing relationship (Table 2; Fig. 2):

Rp��VA]1.0

Effect of Temperature

Rp increases with increasing temperature. Theoverall activation energy has been computed to be75 kJ/mol from the slope of an Arrhenius plot18 oflog Rp versus 1/T (Fig. 3).

The Mark–Houwink equation

�� K�M]�

Figure 3. Arrhenius plot of Rp versus the polymer-ization temperature ([BPO] � 13.77 � 10�3 mol L�1,[citronellol] � 0.96 mol L�1, [VA] � 1.74 mol L�1,copolymerization time � 90 min).

Table 2. Effect of the Comonomer Concentration on Rp with VAa

Sample[Citronellol]

(mol L�1)[VA]

(mol L�1)Conversion

(%)Rp � 106

(mol L�1 s�1)

6 0.38 1.74 7.3 4.467 0.57 1.74 9.2 5.012 0.96 1.74 11.8 5.858 1.34 1.74 13.9 6.459 1.73 1.74 16.1 7.58

10 0.96 0.69 5.3 2.6911 0.96 1.04 8.6 3.312 0.96 1.74 11.8 5.85

12 0.96 2.44 13.6 6.3013 0.96 3.13 17.82 6.91

a [BPO] � 13.77 � 10�3 mol L�1; copolymerization time � 90 min; copolymerization temper-ature � 60 � 0.1 °C.

Figure 2. Relationship between Rp and (F) thecitronellol concentration with a constant VA concentra-tion of 1.74 mol L�1 and (H) the VA concentration witha constant citronellol concentration of 0.96 mol L�1

([BPO] � 13.77 � 10�3 mol L�1, copolymerization time� 90 min, copolymerization temperature � 60 � 0.1°C).

BENZOYL PEROXIDE-INITIATED COPOLYMERIZATION 1245

relates the intrinsic viscosity ([�]) of a polymer toits molecular weight ([M]), which is used to cal-culate the empirical constants K and �. A typicalgel permeation chromatography (GPC) curve isshown in Figure 4. The values of K and � aredetermined from the intercept and slope of theplot of log Mv versus log [�] for the copolymers,where Mv is the viscosity-average molecularweight (Fig. 5). The values of K and � are 2.4� 10�4 and 0.375, respectively (Table 3).

Characterization of the Copolymer

IR Spectroscopy

The IR spectra of the copolymer shows bands at3400 cm�1 for an alcoholic group of citronellol andat 1750 cm�1 for a �CAO group of VA (Fig. 6).

NMR Spectroscopy

The chemical shifts of protons, attached to ele-ments other than carbon such as OOH, ONH,andOSH, to a greater extent or lesser extent areinfluenced by related phenomena of intermolecu-lar exchange and hydrogen bonding. The signalsappearing in the NMR spectra that are due toOOH (hydroxyl protons) with species of small

Figure 4. GPC graph of the copolymer (sample 2).

Figure 5. Plot of log [�] versus log Mv.


molecular weight, where intermolecular associa-tion is not hindered, generally resonate in theregion of � � 3–5.5 ppm (hydroxyl protons ofCH3OOH appear at � � 3.3, whereas those ofC2H5OH appear at � � 5.4).19 However, withmany large molecules, the hydroxyl protons oftenresonate near � � 8 ppm, even at relatively highconcentrations, partially because of steric ef-fects20 and partially because of resonance stabili-zation. Therefore, we have assigned the peaks ofthe OOH group in the range of � � 7–7.7 in theNMR spectra of citronellol and the copolymer ofcitronellol and VA.

The copolymer of citronellol and VA containspeaks at � � 7–7.7 due to hydroxy protons and �� 1–1.4 due to acetoxy protons in the NMR spec-tra (Fig. 7).

DSC

The DSC curve indicates that the glass-transitiontemperature (Tg) of citronellol-alt-VA is 68.32 °C.(Fig. 8).

The values of the initial temperature (Ti), onsettemperature (To), and peak temperature (Tp) ofthe endotherms for various copolymers are pre-

Table 3. GPC Parameters of the Copolymerization of Citronellol with VA and with BPO as an Initiatora

Sample [�] Mv Mn Mw Mz

12 2.86 � 10�4 71,646 67,002 68,918 70,81413 3.38 � 10�4 125,734 34,416 71,999 121,3199 3.87 � 10�4 196,801 141,836 167,518 190,0942 3.73 � 10�4 174,858 39,115 94,850 166,6983 2.94 � 10�4 78,628 76,590 77,409 78,231

a [BPO] � 13.77 � 10�3 mol L�1; copolymerization time � 90 min; copolymerization temperature � 60 � 0.1 °C; viscositytemperature � 25°C.

Figure 6. IR spectrum of the copolymer (sample 2).


Figure 7. NMR spectrum of the copolymer (sample 2).

Figure 8. DSC curve of the copolymer (sample 2).


sented in Table 4. Ti is the temperature at whichthe curve deviates from the baseline. It is a measureof the initiation of the reaction. To is obtained at theintercepts of the tangents to the baseline at thelower temperature side of the endothermic peak. Tp

is the temperature at which the bulk of the polymerhas undergone a dehydration reaction, and the dif-ference between Tp and To (Tp � To) is a measure ofthe overall rate of reaction.21 The smaller the dif-ference is, the greater the rate of reaction is. An-other important observation made for the DSC scanis an enthalpy change associated with the endother-mic peak, which varies with the change in the com-position of the copolymers.

Thermal Analysis

The thermogravimetric results (Fig. 9) of the co-polymer sample are stable up to 354.78 °C andbegin losing weight above this temperature.Rapid decomposition begins at 410 °C.

Copolymer Composition and Values of theReactivity Ratios

To calculate the reactivity ratios r1 and r2, weused the citronellol content from the peak area ofthe hydroxy protons and the VA content from thepeak area of the acetoxy protons. The Kelen–Tudos approach has been used for the evaluationof r1 and r2 for the monomer pair as follows:

Table 4. Thermal Properties

Sample Copolymer Composition Tg (°C) To (°C) Tp (°C) Tp � To (°C)

2 1.36 68.32 181.57 182.77 1.2

Figure 9. TGA curve of the copolymer (sample 2).


� � r1� �r2

�1 � �

where � � G/(� � H) and � � H/(� � H). Thetransformed variables G and H are given by

G ��M1�/�M2��d�M1�/d�M2� � 1�

d�M1�/d�M2�

H ��M1�/�M2�

2

d�M1�/d�M2�

We have calculated the parameter � by taking thesquare root of the product of the lowest and high-est values of H for the copolymerization series. Agraphical evaluation of VA/citronellol yields r1� 0.02 and r2 � 0.0002 (Fig. 10; Table 5).

The product of r1r2 is nearly zero, which is asign of alternating copolymerization.

Elemental Analysis of the Copolymer

Found (by inductively coupled plasma spectros-copy): C, 32–52%; H, 60.63%; O, 6.6%. Calcd.: C,32.55%; H, 60.6%; O, 6.8%.

Mechanism

The oxidation of citronellol with KMnO4 hasyielded results of considerable value in the eluci-dation of the constitution and nature of alcohol.Tremann and Semmler22 observed that, whencitronellol was oxidized with a 1% solution ofKMnO4 and then a chromic acid mixture, acetonewas always found among the oxidation products,a fact regarded as evidence in favor of the break-ing of the double bond.

Furthermore, Kotz and Steche23 studied withgreat care the oxidation of citronellol; theytreated this alcohol with benzoyl hydroperoxideand obtained in this manner, after hydration ofthe oxide, a glycerol, which is represented by for-mula 1. The products can be shown as follows:

Figure 10. Kelen–Tudos plot of citronellol and VA forthe determination of the reactivity ratios.

Table 5. Composition of the Copolymers

Sample[VA]/[Citronellol] Molar Ratio in the

Monomer FeedConversion

(%)[VA]/[Citronellol] Molar Ratio in the

Copolymer Composition

2 1.81 11.8 1.0367 3.02 9.2 1.0608 1.29 13.9 1.025

11 1.088 8.6 1.02112 2.53 13.6 1.05


On the basis of the aforementioned facts, the proposed steps of the speculative mechanism are as follows.

Initiation

Propagation

Termination

CONCLUSIONS

Citronellol can be successfully copolymerized withVA and a radical initiator; the result is the for-mation of an alternating copolymer with a Tg

value of 68.32 °C. The copolymer contains a pen-

dant alcoholic group, which is significant for afunctional and optically active polymer.

The authors thank Dr. K. P. Singh, Director of theHarcourt Butler Technological Institute (Kanpur, In-dia), for providing the necessary facilities. A. K. Sriv-astava thanks DST (New Delhi, India) for sanctioning


the research project “Synthesis and Characterization ofCopolymers of Terpenes with Vinyl Monomers” (SP/S-1/H-26/2000).

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benzoyl peroxide-initiated copolymerization of citronellol and vinyl acetate

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