Progress in Organic Coatings 63 (2008) 100–109

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Progress in Organic Coatings

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Preparation and photopolymerization behavior of multifunctional thiol–ene

systems based on hyperbranched aliphatic polyesters

Qi Fu, Jianhua Liu, Wenfang Shi ∗

Department of Polymer Science and Engineering, Joint Laboratory of Polymer Thin Films and Solution,

l basbrancorbor-indusynthith P

dingg thene syome

h lessd its rquatithiol–

University of Science and Technology of China, Hefei, Anhui 230026, PR China

a r t i c l e i n f o

Article history:Received 23 December 2007Received in revised form 1 March 2008Accepted 22 April 2008

Keywords:Multifunctional thiol–eneHyperbranchedPolyesterPhotopolymerizationUV-curing

a b s t r a c t

A multifunctional polythioand two multi-ene hyperfunctionalized H20 and nspectral analysis. The UVincluding those oligomerswas real-time monitored wsion were evaluated accordecreased with decreasinization behavior of thiol–eof the oligomers and monether monomers even witof a monomer determinealso calculated from the ecomplex factors affecting

1. Introduction

The thiol–ene photopolymerization systems have been attract-ing much attention for scientific research and industrial applicationover the last century [1]. Almost any type of ene monomers, includ-ing norbornenes, acrylates, vinyl ethers, allyl ethers, vinyl estersand alkenes, can be introduced into thiol–ene photopolymeriza-tion systems [1–2]. Moreover, thiol–ene systems have shown manysignificant advantages, such as insensitivity of oxygen inhibitioneffect, relatively high conversion and thermo-stability. Therefore,thiol–ene photopolymerization systems are widely studied, aswell commercialized in many fields in recent years, such as clearcoatings, inks, adhesives, photoresists, optical materials and bio-materials [2–6].

The presence of oxygen in traditional formulations often causesthe inhibition effect of polymerization and leads to low curingrate, low conversion of monomers and successive flaws of curedproducts. The oxygen in polymerization system will consume thehighly reactive free radicals and form peroxide radicals that havequite low reactivity [7,8]. However, for thiol–ene photopolymer-

∗ Corresponding author. Tel.: +86 551 3606084; fax: +86 551 3606630.E-mail address: wfshi@ustc.edu (W. Shi).

0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.porgcoat.2008.04.014

ed on hyperbranched polyester BoltornTM H40 (thioglycolic actate of H40),hed polyesters based on BoltornTM H20 and P1000 (allyl ether terminalnene end-capped P1000) were synthesized and characterized by 1H NMRced photopolymerization behavior of multifunctional thiol–ene system,esized above, a tri-functional thiol, a tetra-allyl ether and a tri-norbornene,hoto-DSC in air. The polymerization rate (Rp) and functional group conver-to the Photo-DSC exotherms. It was found that the Rp and the conversionmolar ratio of [SH]/[C C] from 1:1 to 1:10. Moreover, the photopolymer-stem was greatly influenced by the type of ene group and the functionalityrs. The norbornene monomer exhibited much higher reactivity than allylfunctionality. And for the same type of thiol–ene system, the functionalityeactivity. The apparent rate constant for photopolymerization (kapp) wason Rp = kapp [SH]x[C C]y to generally evaluate the integrative effect of allene photopolymerization.

© 2008 Elsevier B.V. All rights reserved.

ization systems, which proceed through a step-growth free radicalmechanism, the thiol groups react with the peroxide radicals andgenerate highly reactive thiyl radicals after hydrogen-abstraction

by the peroxide radicals. Thus the thiol–ene photopolymerizationcan achieve high photopolymerization rate and high conversion[9,10].

Another unique advantage of thiol–ene systems is that thepolymerization proceeds steadily with the addition of much lessamount of photoinitiator than traditional photo-curing systems.It is even polymerizable at the absence of photoinitiator [2,10,11].In order to counteract the oxygen inhibition effect and raise thecuring speed and conversion, the addition of excessive amount ofphotoinitiator is a routine solution in non-thiol photopolymeriza-tion systems. However, the relatively high content of photoinitiatorwith small molecular weight not only increases the product cost,but also results in some problems of cured films. The remnantof photoinitiator will lead to yellowing effect of the cured filmsunder daylight exposure. This problem can be effectively solved inthiol–ene photopolymerization systems. Most photoinitiators areirritant or toxic, and a trait of photoinitiator pieces is unable tocross-link with the polymer network. Thus during long-term use ofthe photo-cured products the migration of fragments of photoini-tiators may cause some problems, such as irritant volatile, contactstimulative and potential harm for health. However, in UV-curable

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drous MgSO4. After the removal of MgSO4 by filtration and

Q. Fu et al. / Progress in Org

thiol–ene systems, whereas thiols completely get cross-linked withmultifunctional ene monomers, small molecule of photoinitiator isfixed in the polymer network without migration from the UV-curedproducts.

Furthermore, because of the thiol ether linkage structure formedby thiol–ene free radical addition in polymerization, the thiol–enebased cured products show some unique chemical and mechani-cal properties. It was reported by Woods et al. that the thiol–enebased cured films showed good water absorption resistance andhigh thermo-stability in air [12]. Besides low shrinkage of photo-cured films, the thiol–ene based cured products also exhibitimproved mechanical properties according to Wei et al.’s study[13].

On the other hand, the application of thiol is greatly limitedbecause of its unpleasant odor, especially for small molecu-lar thiol compound. However, macromolecular multifunctionalpolythiols are almost odorless. The hyperbranched (HB) poly-mers have shown several unique properties [14]. They havea large number of tunable surface functional groups that arefacile for tailored modification. Their highly branched struc-tures with relatively large molecular weight bring them specialrheological and mechanical properties. BoltornTM H20 andH40 are commercial hyperbranched polyesters based on 2,2-bis(hydroxymethyl) propionate structures. BoltornTM P1000 is ablend of hyperbranced polyether and polyester. In order to syn-thesize multi-thiol polymer, the modification of hyperbranchedpolyester is expected to be a facile, low-cost and efficientmethod. In this work, a multifunctional polythiol (TAH40) wassynthesized through the esterification of BoltornTM H40 withthioglycolic acid, which has been proved as a highly efficientroute [15]. A multifunctional allyl ether terminal functional-ized BoltornTM H20 (AEH20), and a norbornene-2-carboxylicester of BoltornTM P1000 (NCP1000) were also synthesizedvia surface modification of hyperbranched BoltornTM H20 andBoltornTM P1000. Moreover, tris(hydroxymethyl)propane thiogly-colic acetate (TMPT), as a tri-functional thiol, trimethylolpropanediallyl ether succinic diester (TAESD), as a tetra-functional allylether monomer and tris(hydroxymethyl)propane tri-norbornene-2-carboxylic ester (NCTP), as a tri-functional norbornene monomerwere synthesized for comparison. The products were all character-ized with 1H NMR spectra.

To monitor the polymerization process, Photo-DSC applied inthis work was an effective way for real-time monitoring theheat flow generated during the photopolymerization; thus the

polymerization rate and the general functional group conver-sion can be calculated [16–19]. Moreover, the effects of thiolgroup content and the functionality of multi-ene monomers onthe photopolymerization kinetics of thiol–ene system were alsoinvestigated.

2. Experimental

2.1. Materials

Dicyclopentadiene and thioglycolic acid were purchased fromAldrich Chemical Co. Tris(hydroxymethyl)propane, succinic anhy-dride, 4-dimethylamino pyridine (DMAP) and p-toluene sulphonicacid (PTSA) were purchased from Shanghai First Reagent Co.(China). 1-Hydroxycyclohexyl phenyl ketone (HCPK, Irgacure 184)was offered by Ciba Specialty Chemicals. BoltornTM H20 andBoltornTM H40 (defined as H20 and H40), full acrylate end-capped BoltornTM P1000 and trimethylolpropane diallyl ether(2,2-bis(allyloxymethyl)-1-butanol) were purchased from PerstorpAB (Sweden). Tris(hydroxymethyl)propane triacrylate (TMPTA)

oatings 63 (2008) 100–109 101

was supplied by Sartomer Co. (USA). All chemicals were used asreceived without further purification.

2.2. Measurements

The 1H NMR analysis was carried out on a Bruker 300 MHznuclear magnetic resonance instrument (Bruker Bio-Spin Co.,Switzerland) with CDCl3 as a solvent and tetramethylsilane (TMS)as an internal reference, and the chemical shifts were expressedin parts per million (ppm). Gel permeation chromatography (GPC)analysis of the polymers was performed on a GPC apparatus(Waters, USA) at 25 ◦C. DMF was used as solvent with an elutionrate of 1.0 mL/min. The calibrations were based on standard PSt forsamples. The polymer solution was ultrafiltered using 220 nm PTFEfilters before GPC analysis. Molecular weights of the samples wereanalyzed with EmpowerTM GPC software obtained with the GPCapparatus. The photopolymerization kinetics was real-time moni-tored by a CDR-1 differential scanning calorimeter (DSC) (ShanghaiBalance Instrument Co., Shanghai, China) equipped with a BHG-250UV-spotcure system (Mejiro Precision Co., Japan). The incident UVintensity at the sample pan was measured to be 2.4 mW cm−2 witha UV power meter.

2.3. Synthesis

2.3.1. Synthesis of multifunctional thiols2.3.1.1. Synthesis of hyperbranched thioglycolic acetate of H40(TAH40). 4-Dimethylaminopyridine p-toluenesulfonate (DPTS) as acatalyst was prepared according to the literature reported by Mooreand Stupp [20]. The equivalent amount of DMAP and PTSA were,respectively, dissolved in hot toluene and then mixed together.After cooled to room temperature, the sulphonate was filtered anddried under vacuum. The obtained product was a white crystallinepowder (yield: 99%).

Thioglycolic acid (10.14 g, 0.11 mol), H40 (11.43 g, 1.56 mmol,0.10 mol of –OH) and DPTS (0.5 g) were mixed with 100 mLof toluene in a three-neck flask with a Dean-Stark trap. Thenthe mixture was stirred and kept refluxing overnight at 110 ◦Cunder nitrogen atmosphere. Thereafter, the toluene was removedunder reduced pressure. The crude product was diluted with50 mL of CH2Cl2, washed with saturated NaHCO3 aq. (3× 100 mL)and then distilled water (3× 100 mL), and dried with anhy-

CH2Cl2 by evaporation under reduced pressure, the product(TAH40) as a colorless viscous oil was obtained with a yield of87%.

1H-NMR (CDCl3): ı (ppm) 1.09–1.34 (–CH3, 180H), 2.03–2.06(–SH, 46H), 3.28–3.31 (–CH2–S–, 116H), 3.47–3.73 (–CH2–OH,–CH2–CH2–, 28H), 4.29–4.39 (–CH2–COO–, 240H).

2.3.1.2. Synthesis of trimethylolpropane thioglycolic acetate. Thesynthetic procedure was described in the literature reportedby Mayadunne et al. [15]. Thioglycolic acid (10.14 g, 0.11 mol),tris(hydroxymethyl) propane (4.03 g, 0.03 mol) and PTSA (0.5 g,3 wt.%) were stirred in 100 mL of toluene in a flask with a Dean-Stark trap. The reaction was refluxed overnight at 110 ◦C undernitrogen atmosphere. Then toluene was removed under reducedpressure and the crude product was washed with saturated NaHCO3aq. (3× 100 mL) and then distilled water (3× 100 mL). After dryingover anhydrous MgSO4, the product was obtained by filtration ascolorless oil with a yield of 92%.

1H-NMR (CDCl3): ı (ppm) 0.89–0.93 (–CH3, 3H), 1.51–1.58(–CH2–, 2H), 1.99–2.05 (–SH, 3H), 3.26–3.29 (–CO–CH2–S, 6H), 4.11(–CH2–O–, 4H).

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multi-thiol, multifunctional TAH40 and tri-functional TMPT wereused. And four kinds of ene oligomers and monomers includingAEH20, NCP1000, TAESD and NCTP were used for studying the influ-ence of the type of ene and the functionality of oligomer/monomeron thiol–ene photopolymerization. Their ideal schematic molecularstructures are shown in Fig. 1.

3.1. Characterization

H40 is a hyperbranched polymer with 64 primary hydroxylgroups and a molecular weight of 7316 g/mol, theoretically [14].After esterified with thioglycolic acid (Schemes 1–3), from the1H NMR spectrum of TAH40 in CDCl3, the signal for thiol pro-ton is observed at the chemical shift of 2.03–2.06 ppm. Thesignals for methylene proton of hydroxymethyl group of 2,2-bis(hydroxymethyl) propionate (BMPA, the structural unit of H40)shift to higher ı value, as shown in Fig. 2. Although according tothe integral results of the NMR peaks, the number of thiol groupof TAH40 was less than theoretical value, the integral result of the

102 Q. Fu et al. / Progress in Org

2.3.2. Synthesis of multifunctional allyl ether oligomer andmonomer2.3.2.1. Synthesis of allyl ether terminal functionalized hyperbranchedpolyester: trimethylolpropane diallyl ether with succinic link to H20(AEH20).

(1) Synthesis of 2,2-bis (allyloxymethyl)-1-butanol succinicmonoester

In a typical synthesis, a mixture of succinic anhydride (10.07 g,0.1 mol), 2,2-bis (allyloxymethyl)-1-butanol (21.43 g, 0.1 mol)was stirred in 100 mL of acetone at 40 ◦C for 1.5 h. Thereafter,the product was obtained after acetone was evaporated.

1H-NMR (CDCl3): ı (ppm) 0.83–0.85 (–CH3, 3H), 1.36–1.48(–CH2–, 2H), 2.60–2.69 (–CO–CH2–CH2–CO–, 4H), 3.23–3.33(–CH2–O–, 4H), 3.43–3.57 (–CH2–OOC–, 2H), 3.92–3.98 (–CH2–,4H), 5.13–5.23 (–C CH2, 4H), 5.81–5.95 (–CH C, 2H), 9.36(–COOH, 1H).

(2) Synthesis of AEH202,2-Bis (allyloxymethyl)-1-butanol succinic monoester

(12.52 g, 40 mmol), H20 (3.50 g, 2 mmol) and PTSA (0.5 g,3 wt.%) were mixed in 100 mL of toluene in a flask with aDean-Stark trap and refluxed overnight. Then toluene wasevaporated under vacuum and the residue was extracted with50 mL of CH2Cl2. The organic layer was washed with saturatedNaHCO3 aq. (3× 100 mL) and distilled water (3× 100 mL) inturn, and then dried with anhydrous MgSO4. Thereafter, theCH2Cl2 was evaporated to obtain colorless oil (yield: 84%).

1H-NMR (CDCl3): ı (ppm) 0.83–0.88 (–CH3, 48H), 1.24–1.27(–CH3, 36H), 1.40–1.48 (–CH2–, 32H), 2.62 (–CH2–CH2–, 64H),3.31 (–CH2–O–C–, 64H), 3.92–3.94 (–C–CH2–OOC–, 64H),4.04–4.09 (–O–CH2–C C, 64H), 5.12–5.27 (–C CH2, 64H),5.82–5.91 (–CH–C C, 32H).

2.3.2.2. Synthesis of 2,2-bis (allyloxymethyl)-1-butanol succinicdiester (TAESD). In a typical route, succinic anhydride (10.07 g,0.1 mol), 2,2-bis (allyloxymethyl)-1-butanol (42.86 g, 0.2 mol) andPTSA (0.5 g, 3 wt.%) were added into 100 mL of toluene, stirred andrefluxed for 4 h. Then the reaction was cooled and toluene wasremoved under vacuum. The residue was washed with saturatedNaHCO3 aq. (3× 100 mL) and then distilled water (3× 100 mL),then dried with anhydrous MgSO4. The product was obtained afterfiltration as clear colorless oil with a yield of 92%.

1H-NMR (CDCl3): ı (ppm) 0.83–0.85 (–CH3, 6H), 1.36–1.48(–CH2–, 4H), 2.67 (–CH2–CH2–, 4H), 3.23–3.33 (–CH2–O–C–,

8H), 3.39–3.48 (–CH2–OOC–, 4H), 3.92–3.98 (–O–CH2–C C, 8H),5.13–5.23 (–C CH2, 8H), 5.81–5.95 (–CH–C C, 4H).

2.3.3. Synthesis of multifunctional norbornene monomers2.3.3.1. Synthesis of norbornene end-capped hyperbranchedpolyester: norborn-5-ene-2-carboxylic ester of P1000 (NCP1000).The cyclopentadiene was obtained by pyrogenation of dicyclopen-tadiene at 170 ◦C. In a typical synthesis, full acrylate end-cappedP1000 (25.80 g, 0.01 mol), cyclopentadiene (10.56 g, 0.16 mol) and4-methoxy-phenol (0.1 g, 0.3 wt.%) were dissolved in 50 mL of THF.The mixture was stirred in ice bath for 6 h, then slowly heatedto 50 ◦C and kept stirring for 48 h. After THF and the excessivecyclopentadiene were removed under vacuum, the product wasobtained as colorless oil (yield: 93%).

2.3.3.2. Synthesis of norborn-5-ene-2-carboxylic acid tris(hydroxymethyl) propane triester (NCTP). In a typical synthe-sis, cyclopentadiene (3.96 g, 0.06 mol) and 4-methoxy-phenol(0.03 g, 0.3 wt.%) were dissolved in 50 mL of diethyl ether, andthen cooled with an ice bath. Then TMPTA (5.93 g, 0.02 mol) wasadded into the solution. After stirred for 4 h the reaction was

oatings 63 (2008) 100–109

left to room temperature and continuously stirred for 48 h. Thecrude product was washed carefully with distilled water, and theorganic layer was dried with anhydrous MgSO4. Then the solventwas finally removed to obtain a colorless liquid with a yield of97%.

1H-NMR (CDCl3): ı (ppm) 0.85–0.92 (–CH3, 3H), 1.41–1.52(–CH2–, 2H), 2.31–2.36 (–CH2– of norbornene, 6H), 2.88–3.07(–CH– of norbornene, 9H), 4.09–4.11 (–CH2–, 6H), 6.14–6.19(–CH CH– of norbornene, 6H).

2.4. Preparation of samples

The multi-thiol and multi-ene were mixed with different molarratios of [SH]/[C C] as 1:1, 1:2, 1:4, 1:6, 1:8 and 1:10 in order to esti-mate the influence of thiol group content on photopolymerizationand the conversion in cured thiol–ene system. Every mixture wascarried on photopolymerization with addition of 0.5 wt.% HCPK as aphotoinitiator. The UV-induced photopolymerization behavior wasreal-time monitored with Photo-DSC at room temperature withoutadditional heating or nitrogen flow protection.

3. Results and discussions

To evaluate the effect of the functionality on the reactivity of

signal peaks for methylene proton of –CH2–SH at 3.28–3.31 ppmwas accurate. The signal peaks of the methylene proton of the

Scheme 1. Synthesis route of multifunctional thiol TAH40 and TMPT.

Q. Fu et al. / Progress in Organic Coatings 63 (2008) 100–109 103

Fig. 1. Ideal schematic representation of the multifunctional thiol and ene oligomers anNCTP; 6, TAESD).

disulfides (–CH2–S–S–CH2–) with higher chemical shift value wereabsent, which proved that the thiol groups were not apparently oxi-dized. According to the 1H NMR analysis, the conversion of –OH ofH40 was calculated to be 96%, theoretically. It demonstrated thatalmost all hydroxyl groups at the surface of H40 were esterifiedexcept for some internal unreacted ones. Moreover, the structureof TMPT was also proved by 1H NMR results (Fig. 2(B)). After ester-ification the resonances of methylene proton of hydroxymethylgroup shifted to 4.10 ppm, approximately, and the signal peaks

d monomers applied in this study (1, TAH40; 2, AEH20; 3, NCP1000; 4, TMPT; 5,

of methylene proton of –CH2–S were found at 3.26 ppm, approxi-mately. It was demonstrated that the conversion of hydroxyl groupof tris(hydroxymethyl) propane was 98% according to the integralresults of NMR peaks, and the functionality of TMPT was calculatedto be 3.

H20 is theoretically characterized by a hydroxyl number of16 and an average molecular weight of 1750 g/mol [14]. The1H NMR spectrum of AEH20 indicates that H20 was completelymodified after esterification with excessive amount of succinic

anic Coatings 63 (2008) 100–109

104 Q. Fu et al. / Progress in Org

Scheme 2. Synthesis route of multifunctional allyl ether AEH20 and TAESD.

monoacid linked with trimethylolpropane diallyl ether (Fig. 3). TheNMR signal peaks of methyl proton and methylene proton occur,respectively, at 1.24–1.27 and 4.02–4.09 ppm. The signal peaks ofmethylene proton of original terminal hydroxymethyl group of H20all shift to higher ı value at approximately 4.0 ppm after esterifica-tion. Based on the integral results of the NMR peaks, it is calculated

that 99% groups on the surface of H20 have been functionalized(Fig. 3).

BoltornTM P1000 is a blend of hyperbranched polyester con-taining 2,2-bis(hydroxymethyl) propionate structural units andhyperbranched polyether containing trimethylolpropane etherstructural units. Therefore, its NMR signals of backbone structureare too complicated to be accurately analyzed for 16-functionalacrylate of P1000. However, by comparing with the NMR spec-tra of the reactant and the product from the Diels–Alder reaction,it is obviously shown that the signal peaks for ene proton ofacrylic group approximately at 5.8, 6.1 and 6.4 ppm are absent afterreacting with cyclopentadiene. However, the signal peaks for nor-bornene proton are found approximately at 1.8, 3.0, 4.2 and 6.2 ppm(Fig. 4). Therefore, it can be concluded that the acrylate groups ofP1000 are completely modified to norbornene-2-carboxylic groups.And it also proves that the Diels–Alder reaction is a mild and highlyyielding reaction for synthesizing norbornene derivatives.

The characterization of Boltron polyesters with NMR spec-troscopy and size exclusion chromatography (SEC) had beenreported in some early literatures [14,21–23]. From the GPC resultsshown in Fig. 5 it was indicated that the polydispersity (PDI) of the

Scheme 3. Synthesis route of multifunctional norbornene NCP1000 and NCTP.

Fig. 2. 1H NMR spectra of TAH40 (A) and TMPT (B) in CDCl3.

chemical modified product (TAH40 and AEH20) were, respectively,lower than that of initial commercial polyester polyol core (H40and H20). As listed in Table 1, the polydispersity was calculated tobe 1.85 for H40 and 1.57 for H20, which was close to the result inearly report [14]. By using polystyrene standards, the weight aver-age molecular weight (Mw) was determined to be 8550 for H40 and2470 for H20, after chemical modification it increased to 16600 forTAH40 and 8930 for AEH20. And the number average molecularweight (Mn) of modified hyperbranched polyester also increased to5863 for AEH20 and 9252 for TAH40. Based on the average molec-ular weight (Mn and Mw) determined with the GPC and the NMRanalysis, the number average degree of polymerization (DPn) percore molecule could be calculated as 12.0 for H20 and 38.5 for H40on the assumption that all the hyperbranched structural units werelinked to the core molecule (Eq. (1)) [21].

Mn = mBMPA − mH2O + mcore = 115(DPn − 1) + 308 (1)

Table 1GPC data of the initial Boltron hyperbranched (HB) polyesters and the modifiedproducts

HB polyester Mtheora(g/mol) Mw (g/mol) Mn (g/mol) Mw/Mn

H20 1,748 2,471 1574 1.57AEH20 6,486 8,927 5863 1.52H40 7,316 8,550 4619 1.85TAH40 12,059 16,598 9252 1.79

a Mtheor is theoretic molar mass of the molecule based on a dendrimer-equivalentideal model.

Q. Fu et al. / Progress in Organic Coatings 63 (2008) 100–109 105

Fig. 4. 1H NMR spectra of NCP1000 (A1), 16-functional acrylate of P1000 (A2) andNCTP (B) in CDCl3.

the reactivity of different type of thiol groups follows the order:Aryl–SH > HOOCCH2–SH � RCH2–SH > R1R2CH–SH > R1R2R3C–SH[26]. Therefore, thioglycolic acetate is relatively highly reactive tobe applied in this work. As H40 owns a relative large number oftunable surface hydroxyl groups and is facile for chemical modi-fication, the tailored modified hyperbranched polyester polythiolbased on H40 with high density of terminal thiol group will showhigher reactivity than small molecular thiol of low functionality.

In last decade Bowman and coworkers have done some

Fig. 3. 1H NMR spectra of AEH20 (A) and TAESD (B) in CDCl3.

The degree of branching (DB) could be calculated based on theanalysis of the quantitative proton-decoupled 13C NMR spectra asFrechet’s definition [24]:

DB = D + T

D + T + L(2)

where D, L and T represent the fractions of dendritic, linear and ter-minal repeat units, respectively. Therefore, according to the reportsof Zagar and Zigon [21], the DB was 0.44 for H20 (D: 10.0%; L:56.5%; T: 33.5%) and 0.43 for H40 (D: 16.5%; L: 57.0%; T: 26.5%),respectively. As the main structural units of Boltron polyesters were2,2-bis(hydroxymethyl) propionate, belonging to AB2 system, thefunctionality of polyester was calculated to be 14.8 for H20 and42.4 for H40, respectively, based on the DPn and DB according tothe Eq. (3). Thus, considering of the chemical modification proce-dure and the conversion of reaction evaluated based on 1H NMRanalysis (Figs. 2 and 3), the functionality of modified HB polyesterswas calculated to be 29 for AEH20 and 42 for TAH40, approximately.

f

DPn

= L + 2T

D + L + T(3)

3.2. Photopolymerization mechanism

A number of investigations have given detailed explanations tothe step-growth radical mechanism since Kharasch first outlinedthe thiol–ene polymerization mechanism in 1938 (Scheme 4, steps1–7) [25]. According to early work reported by Kharasch et al.,

systematical research on kinetics and modeling of thiol–ene pho-topolymerization [27–30]. As described by Bowman et al. [2],

Fig. 5. GPC traces of the commercial starting materials (H20 and H40) and themodified products (AEH20 and TAH40).

106 Q. Fu et al. / Progress in Organic C

Scheme 4. Photopolymerization mechanism of thiol–ene system described by Kha-rasch et al. [25,26].

the photopolymerization is based on the thiol–ene step-growthaddition reaction. The photoinitiator is excited and split into freeradicals after absorbing the certain wavelength band of UV light.And the thiyl radicals are generated by the reaction between thiolgroups and the free radicals (steps 1–2). In the propagation andchain transfer period, a thiyl radical adds to a carbon–carbon dou-ble bond and generates a carbon radical (step 3), which reacts withanother thiol group to regenerate a new thiyl group (step 4). Thusthe addition and chain transfer reaction both cause the propagationof the polymerization system. The termination of the polymeriza-tion reaction occurs via radical recombination (steps 5–7).

Based on the mechanism described above, the photopolymer-ization rate (Rp) of thiol–ene system can be estimated by thefollowing relationship [11,29]:

Rp ∝ [SH]x[C C]y (4)

The x and y are the exponents depending on the reaction rate of

the relevant functional group and the ratio of the propagation rateconstant and chain transfer rate constant (kp/kct), which is a keyvalue indicating the polymerization kinetics, varying for differenttype of thiol–ene system.

According to the research of Cramer and Bowman, forthiol–norbornene system, kp/kct = 1.0 and x = y = 0.5; for thiol–allylether system, kp/kct = 10.0 and x = 1, y = 0 [28,29]. This indicated thatfor thiol–norbornene photopolymerization, the propagation stepand the chain transfer step equally develop, and both the concentra-tion of thiol group and ene group determines the Rp; for a thiol–allylether system, the propagation step is rate-determining, and thevariation of the concentration of allyl ether monomers slightlyeffects the Rp, because allyl ether is not prone to homopolymerize.

3.3. Photopolymerization kinetics

It is very important to study the photopolymerization kineticsof thiol–ene systems for understanding their UV-curing behavior,which are influenced by many complicated factors, such as theconcentration of thiol group, the type of ene monomer, and thefunctionality of ene and thiol oligomer/monomer. The impact of

oatings 63 (2008) 100–109

those factors can be estimated via analysis of the Rp and the con-version of the functional group.

The rate of polymerization (Rp) was calculated using followingformula (Ep is the experimental exothermic value from polymeriza-tion, �Hr is the theoretical molar enthalpy value for reaction, andms is the mass of sample). Therefore, the conversion of the func-tional group can be obtained via the integration of Rp ∼ t curve inthe time (t) interval.

Rp = Ep

�Hrms(5)

3.3.1. Effects of the thiol–ene molar ratio and the type of enesFrom the results of Photo-DSC shown in Fig. 6, the photopoly-

merization exotherms indicated that the thiol group content greatlyaffect the Rp and functional group conversion. The thiol–enephotopolymerization has been proved to proceed through a step-growth radical addition mechanism. According to the Photo-DSCdata, when the thiol–ene molar ratio decreases from 1:1 to1:10, the Rmax

p correspondingly decreases approximately from0.31 to 0.09 mol g−1 s−1 for thiol–norbornene and from 0.26 to0.04 mol g−1 s−1 for thiol–allyl ether. The results can be explainedby the following equation [30]:

Rp = kct[SH][–CH2–CH•] = kp[–S•][C C] (6)

The high concentration of thiol group in the system leads tohigh concentration of thiyl radical, which reacts with ene group,producing the carbon radical and contributing to the propagationand chain transfer for polymerization. So when the molar ratioof [SH]/[C C] is as high as 1:1, the peak of Photo-DSC exothermperforms sharper, whereas the peak for lower thiol group contentperforms more flater.

Moreover, the conversion also decreases with decreasing thethiol–ene molar ratio. The relatively high conversion is achievedwhile the [SH]/[C C] is as high as 1:1–1:4 (for TAH40/NCP1000:96–88%; for TAH40/NCTP: 94–73%; for TMPT/NCP1000:92–74%; for TMPT/NCTP: 84–65%; for TAH40/AEH20: 94–70%;for TAH40/TAESD: 86–67%; for TMPT/AEH20: 77–54%; forTMPT/TAESD: 75–44%). In multifunctional thiol–ene polymer-ization system, the chain transfer reaction effectively contributesto overcome the reduced diffusion rates of free radicals whenthe polymer network forms. Thus, the remaining thiol and enegroups still can be polymerized and the conversion is achievedrelatively higher. It shows great advantage compared with tradi-

tional photopolymerization systems by multifunctional acrylates,the conversion of which often is much lower than theoretic valuebecause of early gelation.

By comparing the Rmaxp value of thiol–norbornene with

thiol–allyl ether system having the same thiol–ene molar ratio,it obviously shows that the multi-norbornene monomers per-form much higher reactivity than multi-allyl ether monomers(Fig. 7). According to the expression (4) and Eq. (6) [11], fornorbornene monomers, Rp ∝ [SH]1/2[C C]1/2; for thiol–allyl ethersystem, Rp ∝ [SH][C C]0 = [SH]. It is found that for the multi-functional thiol–ene system with high functionality, there is smalldeviation from the model because of the limited diffusion of freeradicals and some other complex factors, such as the variation ofviscosity during the polymerization. However, the above relation-ship is still approximately consistent with the results in this work.The allyl ether monomers are not prone to homopolymerize, sothe Rp shows direct proportional relationship with the concentra-tion of thiol group. When the ene group of norbornene monomerreact with thiol group or free radicals, the addition of thiyl radi-cal to the carbon–carbon double bond of the norbornene ring leadsto the formation of a new thiol ether substituted norbornene ring.

Q. Fu et al. / Progress in Organic C

Then the high reactive new-born carbon-centered radical abstractsthe thiol hydrogen to generate a thiyl radical very rapidly. Fur-thermore, the carbon–carbon double bond of norbornene, whichis sp2-hybrid and has planar structure, is converted to a sp3-hybridcarbon–carbon single bond, thus the relief of high ring strain affordsadditional drive for polymerization. Therefore, the norbornenemonomers usually exhibit very high reactivity in thiol–ene poly-merization.

3.3.2. Effects of the functionality of multi-thiol and multi-eneoligomer/monomer

Based on the Photo-DSC results, it can be concluded thatpolythiol TAH40 generally exhibits much higher reactivity thantri-functional TMPT. In thiol–ene photopolymerization, the mul-tifunctional thiols and ene monomers are cross-linked to formthe polymer network, so both the functionality of the multi-thioland multi-ene oligomer/monomer determine the reactivity of thepolymerization system. From Figs. 6 and 7, it can be seen that

Fig. 6. Photo-DSC exotherms and conversion curves of multifunctional thiol–ene photop1:6, 1:8 and 1:10 ((a) TAH40/NCP1000; (b) TAH40/NCTP; (c) TMPT/NCP1000; (d) TMPT/N

oatings 63 (2008) 100–109 107

the functionality of the ene monomer also significantly influencesthe Rp and the functional group conversion under the same molarratio of [SH]/[C C]. The results are consistent with the photopoly-merization reactivity of the norbornene monomers and allyl ethermonomers. The multifunctional monomer with high functional-ity will produce much higher local concentration of radical aroundthe multifunctional molecule than the average level of the wholesystem. Thus, the surface modified polythiol exhibits higher effi-ciency of initiation and addition reaction than di-thiol or tri-thiolmolecules usually applied.

3.3.3. Integrative effect by other complex factorsBesides the influence of thiol–ene molar ratio, the ene type,

and the functionality of multi-thiol and multi-ene, the thiol–enephotopolymerization system is also affected by many other com-plex factors, such as the variation of the volume and viscosity, theenvironmental temperature, the intensity of UV irradiation, theabsorption band of the photoinitiator and the molecular structure

olymerization systems with different molar ratios (r) of [SH]/[C C] as 1:1, 1:2, 1:4,CTP; (e) TAH40/AEH20; (f) TAH40/TAESD; (g) TMPT/AEH20; (h) TMPT/TAESD).

108 Q. Fu et al. / Progress in Organic C

Fig. 6. (Continued) .

of the multifunctional hyperbranched polymer. For example, as thehyperbranced aliphatic polyester has flexible structural units, themultifunctional thiol–ene system with high functionality showsmuch less shrinkage after photo-cured than a typical multi-acrylatephotopolymerization system [31]. Moreover, the flexible thiol ether

Fig. 7. Maximum rates of polymerization (Rmaxp ) of different multifunctional

thiol–allyl ether (a) and thiol–norbornene (b) systems with different molar ratios of[SH]/[C C] as 1:1, 1:2, 1:4, 1:6, 1:8 and 1:10, respectively.

oatings 63 (2008) 100–109

Table 2Apparent rate constant for photopolymerization (kapp) obtained via curve fit forthiol–norbornene and thiol–allyl ether two-component systems

Thiol–ene system kapp (L mol−1 g−1 s−1)

Fit curve: Rp = kapp

[SH]x[C C]y

TAH40/NCP1000 0.55x = 0.5 TAH40/NCTP 0.48y = 0.5 TMPT/NCP1000 0.44

TMPT/NCTP 0.41

TAH40/AEH20 0.51x = 1 TAH40/TAESD 0.50y = 0 TMPT/AEH20 0.47

TMPT/TAESD 0.46

The parameter x and y were set as reported in literature [11].

linkage of high content in polymer network also affords higher con-version. Therefore, it is necessary to set an apparent and generalparameter to evaluate the integrative effect of all those complexfactors. According to the expression (4), it is a facile way to set anapparent rate constant for photopolymerization (kapp) and buildthe equation:

Rp = kapp[SH]x[C C]y (7)

Based on the experimental results above, the kapp can be calculatedby fitting the experimental curve (Rp ∼ [SH]/[C C]) with Eq. (7), aslisted in Table 2. The so-called apparent rate constant factually isnot a strict definition for accurate description of the mechanismof polymerization. However, it is practical to generally evaluatethe photo-curing reactivity of different thiol–ene polymerizationsystems. And according to the results listed in Table 2, the reactivityof different multifunctional thiol–ene systems follows the order:TAH40/NCP1000 > TAH40/NCTP > TMPT/NCP1000 > TMPT/NCTP >TAH40/AEH20 > TAH40/TAESD > TMPT/AEH20 > TMPT/TAESD.

4. Conclusions

It has been proved to be an effective way to synthesizea multifunctional thiol with high molecular weight and highfunctionality in order to raise the reactivity of multi-thiol and min-imize its volatility. Therefore, a multifunctional polythiol basedon hyperbranched polyester H40 (thioglycolic acetate of H40,TAH40) was synthesized. And a multifunctional allyl ether oligomerbased on H20 (AEH20) and a norbornene oligomer based on

P1000 (NCP1000) was also synthesized for investigating the pho-topolymerization behavior of multifunctional thiol–ene systems.Moreover, a tri-functional thiol (TMPT), a tetra-allyl ether monomer(TAESD) and a tri-norbornene monomer (NCTP) were introduced tothis work for comparison.

The multifunctional thiol–ene systems with different combina-tion of thiols and enes above, with different thiol–ene molar ratiosas 1:1, 1:2, 1:4, 1:6, 1:8 and 1:10, were UV-cured and real-timemonitored with Photo-DSC. The Rp value and conversion of everythiol–ene system were calculated according to the Photo-DSC data.The results indicated that the thiol–ene molar ratio determined theRp and the conversion of the thiol/ene group, which was consistentwith the relationship: Rp ∝ [SH]x[C C]y (for thiol–norbornenesystem: x = y = 0.5; and for thiol–allyl ether system: x = 1, y = 0).It was found that the thiol–norbornene system exhibited muchhigher reactivity than thiol–allyl ether system in photopolymer-ization with the same thiol–ene molar ratio. Furthermore, thefunctionality of multi-thiol and multi-ene monomer also is animportant factor affecting the reactivity of the thiol–ene system.After highly functionalized TAH40, AEH20 or NCP1000 was addedinto the thiol–ene system, the Rp and conversion of the functional

anic C

[[

[

Q. Fu et al. / Progress in Org

groups were lifted up to a higher level. And the apparent rateconstant for photopolymerization (kapp) was calculated fromthe equation Rp = kapp [SH]x[C C]y to evaluate the integrativeeffect of all complex factors. According to the results obtainedabove, the order of reactivity of photopolymerization of the

multifunctional thiol–ene two-component systems is as follows:TAH40/NCP1000 > TAH40/NCTP > TMPT/NCP1000 > TMPT/NCTP >TAH40/AEH20 > TAH40/TAESD > TMPT/AEH20 > TMPT/TAESD.

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (grant no. 50233030) and granted by china NKBRSFProject (no. 2001CB409600).

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