Study on MMA and BA Emulsion Copolymerization Using 2,4-Diphenyl-4-Methyl-1-Pentene As the Irreversible Addition–Fragmentation Chain Transfer Agent
Article Study on MMA and BA Emulsion Copolymerization Using 2,4-Diphenyl-4-methyl-1-pentene as the Irreversible Addition–Fragmentation Chain Transfer Agent
Zuxin Zhang , Daihui Zhang , Gaowei Fu, Chunpeng Wang, Fuxiang Chu and Riqing Chen *
Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry; National Engineering Laboratory for Biomass Chemical Utilization; Key Laboratory of Chemical Engineering of Forest Products, National Forestry and Grassland Administration; Key Laboratory of Biomass Energy and Material, Nanjing 210042, China; [email protected] (Z.Z.); [email protected] (D.Z.); [email protected] (G.F.); [email protected] (C.W.); [email protected] (F.C.) * Correspondence: [email protected]; Tel.: +86-025-8548-2519
Received: 14 December 2019; Accepted: 1 January 2020; Published: 2 January 2020
Abstract: As a chain transfer agent, 2,4-diphenyl-4-methyl-1-pentene (αMSD) was first introduced in the emulsion binary copolymerization of methyl methacrylate (MMA) and butyl acrylate (BA) based on an irreversible addition–fragmentation chain transfer (AFCT) mechanism. The effects of αMSD on molecular weight and its distribution, the degree of polymerization, polymerization rate, monomer conversion, particle size, and tensile properties of the formed latexes were systematically investigated. Its potential chain transfer mechanism was also explored according to the 1H NMR analysis. The results showed that the increase in the content of αMSD could lead to a decline in molecular weight, its distribution, and the degree of polymerization. The mass percentage of MMA in the synthesized polymers was also improved as the amounts of αMSD increased. The chain transfer coefficients of αMSD for MMA and BA were 0.62 and 0.47, respectively. The regulation mechanism of αMSD in the emulsion polymerization of acrylates was found to be consistent with Yasummasa’s theory. Additionally, monomer conversion decreased greatly to 47.3% when the concentration of αMSD was higher than 1 wt% due to the extremely low polymerization rate. Moreover, the polymerization rate was also decreased probably due to the desorption and lower reactivity of the regenerative radicals from αMSD. Finally, the tensile properties of the resulting polyacrylate films were significantly affected due to the presence of αMSD.
Keywords: 2,4-diphenyl-4-methyl-1-pentene; irreversible addition–fragmentation chain transfer; emulsion polymerization; MMA; BA
1. Introduction Due to the growing environmental concerns, emulsion polymerization has become a vital method for polymer production with its environmentally friendly process, high polymerization rate, high monomer conversion, and excellent heat dissipation performance [1]. Many engineering efforts have also been put on the multiscale modeling of this process [2,3]. Chain transfer agent (CTA) as an important component, including reversible addition–fragmentation chain transfer (RAFT) and traditional CTAs, have been widely employed to control the molecular weights of synthesized polymers during the emulsion polymerization. Although many works about RAFT CTAs have recently been conducted, the complex processes and strict operation conditions of RAFT CTAs limit their wide applications. [4–6] For traditional CTAs, including mercaptans [7–10], alcohols [11], and halides [12],
Polymers 2020, 12, 80; doi:10.3390/polym12010080 www.mdpi.com/journal/polymers Polymers 2020, 12, x FOR PEER REVIEW 2 of 14 Polymers 20202020,, 12,, x 80 FOR PEER REVIEW 22 of of 14 14 halideshalides [12],[12], severalseveral disadvantagesdisadvantages havehave alsoalso bbeeneen observed,observed, suchsuch asas anan unpleasantunpleasant smellsmell forfor mercaptansmercaptansseveral disadvantages andand seriousserious have environmentalenvironmental also been observed, pollutionpollution such forfor as haha anlides.lides. unpleasant InIn thethe casecase smell ofof foralcohols,alcohols, mercaptans thethe amountamount and serious usedused asasenvironmental aa molecularmolecular pollution weightweight regulatorregulator for halides. inin Inthethe the polymeripolymeri case of alcohols,zationzation isis the typicallytypically amount largelarge used (10%–20% as(10%–20% a molecular relativerelative weight toto monomermonomerregulator inmass),mass), the polymerization andand itit isis alsoalso notnot is considered typicallyconsidered large toto bebe (10%–20% anan environmentallyenvironmentally relative to monomer friendlyfriendly agent.agent. mass), and it is also not consideredAnAn irreversibleirreversible to be addition–fragmentationaddition–fragmentation an environmentally friendly chainchain agent. trantransfersfer (AFCT)(AFCT) agentagent hashas beenbeen developeddeveloped toto solvesolve thethe aboveaboveAn irreversible problems.problems. ItIt addition–fragmentation displayeddisplayed higherhigher chainchain chain trantransfersfer transfer efficiency,efficiency, (AFCT) lowerlower agent toxicity,toxicity, has been andand lowerlower developed dosage.dosage. to TheThesolve firstfirst the reportreport above aboutabout problems. thethe AFCTAFCT It displayed agentagent waswas higher prespresentedented chain inin transfer 1988,1988, whenwhen efficiency, MeijsMeijs loweretet al.al. [13][13] toxicity, usedused and αα-benzyl--benzyl- lower oxy-styreneoxy-styrenedosage. The asas first aa reportchainchain abouttransfertransfer the agentagent AFCT forfor agent thethe was preparationpreparation presented ofof in 1988,poly(methylpoly(methyl when Meijs methacrylate)methacrylate) et al. [13] usedandand polystyrenepolystyreneα-benzyl-oxy-styrene withwith lowlow molecular asmolecular a chain transferweights.weights. agent Then,Then, for varivari theousous preparation newnew typestypes ofofof poly(methylAFCTAFCT agentsagents methacrylate) werewere reportedreported and inin researchpolystyreneresearch paperspapers with [14–21][14–21] low molecular andand patentspatents weights. [22–24].[22–24]. Then, TheThe various commcommonon new structurestructure types of ofof AFCT thethe irreversibleirreversible agents were AFCTAFCT reported agentagent inisis shownshownresearch inin papers SchemeScheme [14 1.1.– C=ZC=Z21] and isis aa patentsdoubledouble bond [bond22–24 thatthat]. The isis vulnerablevulnerable common structure toto bebe attackedattacked of the byby irreversible thethe chainchain AFCTradical.radical. agent R-XR-X isis a showna singlesingle in bondbond Scheme thatthat1 .Ccancan= bebeZ isbrokenbroken a double easily,easily, bond andand that YY isisis vulnerableaa groupgroup toto to activateactivate be attacked thethe reaction.reaction. by the chain TheThe generalgeneral radical. mechanismmechanismR-X is a single forfor bond thethe irreversibleirreversible that can be AFCTAFCT broken agentagent easily, mainlymainly and Y containscontains is a group threethree to activatesteps.steps. Firstly,Firstly, the reaction. thethe chainchain The radicalradical general isis addedaddedmechanism toto thethe for unsaturatedunsaturated the irreversible doubledouble AFCT bondbond agent ofof C=Z.C=Z. mainly ThThen,en, contains thethe additionaddition three steps. productproduct Firstly, offersoffers the aa chain leavingleaving radical groupgroup is radicalradicaladded to(R•)(R•) the byby unsaturated thethe decomposition.decomposition. double bond Eventually,Eventually, of C=Z. Then, thethe novelnovel the addition radicalradical productreinitiatesreinitiates off theersthe apolymerizationpolymerization leaving group process.process.radical (R ) by the decomposition. Eventually, the novel radical reinitiates the polymerization process. • RXRX CC ZZ
YY SchemeScheme 1. 1. GeneralGeneralGeneral chemicalchemical chemical structurestructure structure ofof of thethe irreversible irreversible addition–f addition–fragmentationaddition–fragmentationragmentation chainchain transfer transfer (AFCT)(AFCT) agent. agent.
2,4-Diphenyl-4-methyl-1-pentene2,4-Diphenyl-4-methyl-1-pentene (( (αααMSD),MSD),MSD), shownshown shown inin in SchemeScheme Scheme 22,2,, isisis aaa typicaltypicaltypical irreversibleirreversibleirreversible AFCTAFCTAFCT whichwhich can can be be prepared prepared by by αα-methylstyrene-methylstyrene dimerization dimerization in in the the presence presence of of an an aqueous aqueous sulfuric sulfuric acid acid catalystcatalyst in in the the liquid–liquid liquid–liquid mode mode of of operation operation [25,26]. [[25,26].25,26].
SchemeScheme 2. 2. ChemicalChemical structurestructure ofof 2,4-diphenyl-4-methyl-1-pentene2,4-diphenyl-4-methyl-1-pentene ( (ααMSD).MSD).
Currently,Currently, ααMSDMSD hashas has beenbeen been introducedintroduced introduced intointo into solutionsolution solution andand and bulkbulk bulk radicalradical radical polymerizationpolymerization polymerization inin order inorder order toto synthesizesynthesizeto synthesize polystyrenepolystyrene polystyrene withwith with decreaseddecreased decreased molecularmolecular molecular weightsweights weights andand and narrowednarrowed narrowed molecularmolecular molecular weight distributiondistribution [[27,28].[27,28].27,28]. For For thethe specificspecificspecific regulationregulationregulation mechanism mechanismmechanism of ofofα ααMSDMSDMSD in inin the thethe polymerization polymerizationpolymerization (Scheme (Scheme(Scheme3) 3)3)Fisher FisherFisher et etet al. al.al. [29 [29][29]] proposed proposedproposed that thatthat the thethe chain chainchain transfer transfertransfer would wouldwould proceed proceedproceed in inin two twotwo cases. cases.cases. InInIn thethe firstfirstfirst case, case, alkyl alkyl hydrogenhydrogen carryingcarrying aa freefree electron electron abstracted abstracted fromfrom an an ααMSDMSD was was combined combined withwith aa chain chain radical radical to to formform aa deaddeaddead chain chainchain (Scheme (Scheme(Scheme3a1), 3a1),3a1), whereas whwhereasereas the thethe residual residualresidual structure structurestructure of α ofMSDof ααMSDMSD carrying carryingcarrying a free aa electron freefree electronelectron could couldcouldreinitiate reinitiatereinitiate the polymerization thethe polymerizationpolymerization process processprocess (Scheme (Scheme(Scheme3a1). In 3a1).3a1). the second InIn thethe case,secondsecond a chain case,case, radicalaa chainchain was radicalradical directly waswas directlydirectlyadded to addedaddedαMSD toto and ααMSDMSD then andand formed thenthen formedformed a transient aa transienttransient state radical. statestate radical.radical. An alkyl AnAn hydrogen alkylalkyl hydrogenhydrogen radical radicalradical divided divideddivided from fromfromthe α MSDthethe ααMSD structureMSD structurestructure of the ofof latter thethe radicallatterlatter radicalradical was then waswas transferred thenthen transferredtransferred to a monomer toto aa monomermonomer to form toto a form newform radical. aa newnew radical.Afterwards,radical. Afterwards,Afterwards, the new thethe radical newnew wasradicalradical able waswas to reinitiateableable toto rere theinitiateinitiate polymerization. thethe polymerization.polymerization. Finally, Finally,Finally, the residual thethe residualresidual part of partpartthe transient ofof thethe transienttransient state radical statestate radical becameradical becamebecame a dead chain,aa deaddead and chain,chain, the andand end thethe group endend of groupgroup the dead ofof thethe chain deaddead was chainchain an internal waswas anan internalinternalolefin (Scheme olefinolefin (Scheme3(Schemea2). However, 3a2).3a2). However,However, according accordingaccording to the results toto thethe of resultsresults the thermal ofof thethe polymerizationthermalthermal polymerizationpolymerization of styrene, ofof styrene,Yasummasastyrene, YasummasaYasummasa et al. [30 ]etet concluded al.al. [30][30] concludedconcluded that the polymerthatthat thethe polymerpolymer chain radical chainchain would radicalradical first woulwoul bedd added firstfirst bebe to addedadded the terminal toto thethe terminalterminaldouble bond doubledouble of α MSDbondbond to ofofproduce ααMSDMSD to anto produce intermediateproduce anan intermediateintermediate radical, and then radiradi thiscal,cal, radicalandand thenthen would thisthis decompose radicalradical wouldwould into decomposedecomposea polymer chain intointo aa with polymerpolymer a terminal chainchain withwith double aa terminalterminal bond and doubledouble a cumyl bondbond radical. andand aa cumylcumyl Finally, radical.radical. the cumyl Finally,Finally, radical thethe wouldcumylcumyl radicalradicalreinitiate wouldwould the polymerization reinitiatereinitiate thethe (Schemepolymerizationpolymerization3b1) [ 31 –(Sch(Sch34].emeeme Furthermore, 3b1)3b1) [31–34].[31–34]. the regulation Furthermore,Furthermore, mechanism thethe regulationregulation study of mechanismmechanism studystudy ofof ααMSDMSD inin thethe radicalradical bulkbulk andand suspensionsuspension polymerizationpolymerization ofof styrenestyrene hashas beenbeen observedobserved toto supportsupport Yasummasa’sYasummasa’s theory.theory.
Polymers 2020, 12, 80 3 of 14
αMSD in the radical bulk and suspension polymerization of styrene has been observed to support Yasummasa’sPolymers 2020, 12, theory.x FOR PEER REVIEW 3 of 14
Scheme 3. TheThe two two chain chain transfer transfer mechanisms of αMSD in the polymerization of styrene. Mechanism of alkyl hydrogen suggested by Fisher et al.: transferring transferring to to chain chain radical radical ( (a1a1)) and and monomers monomers ( (a2a2).). (b1) Addition–fragmentation chainchain transfertransfer mechanismmechanism reportedreported byby YasummasaYasummasa etet al.al.
Although therethere have have been been some some developments developments for the radicalfor the bulk radical and suspension bulk and polymerization suspension polymerizationusing αMSD, its utilizationusing αMSD, in the its emulsionutilization polymerization in the emulsion has polymerization never been reported. has never In the been present reported. study, Indiff theerent present levels study, of αMSD different were incorporated levels of αMSD into were poly-(meth) incorporated acrylate into emulsion poly-(meth) via the acrylate semi-continuous emulsion viaemulsion the semi-continuous copolymerization emulsion of MMA co andpolymerization BA to investigate of MMA its eandffects BA on to the investigate properties its of effects the resulting on the polymerproperties emulsion, of the resulting such as molecular polymer mass, emulsion, polymerization such as molecular rate, and particle mass, size.polymerization The αMSD regulationrate, and particlemechanism size. was The also αMSD discussed. regulation mechanism was also discussed.
2. Materials and Methods 2. Materials and Methods α Reagents: αMSDMSD (Shanghai (Shanghai Dibo Dibo Chemical Chemical Technology, Technology, Shanghai, China), MMA MMA and BA (monomers), nonylphenol polyoxyethylene ether ether (NP- (NP-10,10, nonionic emulsifier), emulsifier), alkyl phenol ether sulfosuccinate sodiumsodium salt salt (OS, (OS, nonionic nonionic emulsifier), emulsifier), sodium sodium dodecyl dodecyl sulfate (SDS, sulfate anionic (SDS, emulsifier), anionic disodium hydrogen phosphate dodecahydrate (Na2HPO4 12H2O, stabilizer, Sinopharm Chemical emulsifier), disodium hydrogen phosphate dodecahydrate ·(Na2HPO4·12H2O, stabilizer, Sinopharm ChemicalReagent, Shanghai,Reagent, Shanghai, China), and China), ammonium and ammonium persulfate persulfate (APS, initiator, (APS, Nanjinginitiator, Reagent,Nanjing Reagent, Nanjing, Nanjing,China) were China) used were as received. used as Distilledreceived. deionized Distilled deionized water (DDW) water was (DDW) used as was the used reaction as the medium. reaction medium.Emulsion polymerization: The emulsion polymerization of (meth) acrylates was carried out in a 500 mLEmulsion 4-neck glasspolymerization: reactor equipped The emulsion with a mechanical polymeriza stirrer,tion of a (meth) reflux condenser, acrylates was a nitrogen carried gas out inlet, in a 500and mL a thermometer. 4-neck glass Thesereactor following equipped substances with a mechanical and amounts stirrer, were a reflux mixed condenser, into the DDW a nitrogen (158.40 gas g): SDS (2.60 g), NP-10 (0.65 g), OS (0.65 g), and Na HPO 12H O (0.40 g), followed by mechanical inlet, and a thermometer. These following substances2 and 4amounts· 2 were mixed into the DDW (158.40 stirring to form an aqueous solution. The (meth) acrylate monomers MMA (65 g) and BA (65 g) were g): SDS (2.60 g), NP-10 (0.65 g), OS (0.65 g), and Na2HPO4·12H2O (0.40 g), followed by mechanical α stirringhomogeneously to form an mixed aqueous with solution. various The amounts (meth) of acrylateMSD monomers under a mechanical MMA (65 g) stirring. and BA The (65 g) formed were homogeneouslyorganic phase was mixed then with added various dropwise amounts into theof α aqueousMSD under solution a mechanical to form anstirring. emulsion The solution. formed Aorganic total ofphase 10 g was of the then resulting added solutiondropwise and into 0.55 the gaqueous of initiator solution solution to form (0.05 an g emulsion APS in 0.50 solution. g DDW) A weretotal of poured 10 g of into the the resulting glass reactor solution and and then 0.55 heated g of initiator to 75 ◦C. solution Finally, a(0.05 residual g APS pre-emulsion in 0.50 g DDW) and were 6.6 g pouredof initiator into solution the glass containing reactor and 0.60 then g APS heated were to added75 °C. Finally, dropwise a residual simultaneously pre-emulsion into the and reactor 6.6 g toof polymerizeinitiator solution at 75 ◦ containingC for 4 h under 0.60 ag nitrogenAPS were atmosphere. added dropwise simultaneously into the reactor to polymerizeCharacterization: at 75 °C for Monomer4 h under a conversion nitrogen atmosphere. was determined by a gravimetric analysis of samples withdrawnCharacterization: from the glass Monomer reactor duringconversion polymerization, was determined and these by a samples gravimetric were analysis quenched of and samples dried withdrawnat 120 ◦C for from 2 h. Thethe conversionglass reacto calculationsr during polymerization, were based on theand total these amount samples of solid were of quenched each emulsion and driedsample at taken120 °C at for the 2 beginningh. The conversion of the polymerization. calculations were The based number on the and total weight-average amount of solid molecular of each emulsion sample taken at the beginning of the polymerization. The number and weight-average molecular weights of the polymers were obtained by the gel permeation chromatography (GPC) via a Malvern Viscotek3580 system, which was calibrated using standard narrow molecular weight polystyrene samples. The dried polymer films were dissolved in GR-grade tetrahydrofuran eluent and filtered through a polytetrafluoroethylene (PTFE) filter (0.22 μm) before the injection. Analysis
Polymers 2020, 12, 80 4 of 14 weights of the polymers were obtained by the gel permeation chromatography (GPC) via a Malvern Viscotek3580 system, which was calibrated using standard narrow molecular weight polystyrene samples. The dried polymer films were dissolved in GR-grade tetrahydrofuran eluent and filtered through a polytetrafluoroethylene (PTFE) filter (0.22 µm) before the injection. Analysis was conducted with a flow rate of 1 mL per minute at 40 ◦C. Average particle size (dp, light intensity-weighted) and particle-size distribution index (PDI) were measured by a laser light-scattering method using a Malvern ZETASIZER Nano at 25 ◦C. All latex samples for the measurement were diluted 50 times with DDW. The PDI was a dimensionless coefficient denoting the broadness of the distribution diameter, and it was produced by a second-order method of cumulant analysis [35]. The 1H NMR spectra of the resultant polymers were measured with a Bruker DMX 300 NMR. The polymer samples were dissolved into CDCl3 (10–15 wt/vol %) at 25 ◦C. The tensile properties of the dried polymer films were measured on electronic tensile machine CMT7504 with a crosshead speed of 50 mm/min and a load cell of 250 N at room temperature. The stress–strain curves of the polymer films were plotted to estimate the films’ strength and toughness variations.
3. Results and Discussion
3.1. Effect of αMSD on the Physichemical Properties of Polymers Different levels of αMSD were introduced into the emulsion polymerization of (meth) acrylate in order to effectively regulate the physical properties of the resulting polymers. Its effect on average molecular mass, polymerization degree, and monomer conversion is shown in Table1.
Table 1. Effects of αMSD concentration on average molecular mass, degree of polymerization, and monomer conversion. Note: CTA = chain transfer agent.
1 4 2 Run CTA (CTA/M )/%Mn/10 Ð (Mw/Mn) Xn Conversion/% 1 - 0.0 40.56 3.37 3612 100.0 2 αMSD 0.1 28.58 2.69 2545 97.8 3 αMSD 0.3 12.15 1.96 1082 97.6 4 αMSD 1.0 5.44 1.69 479 88.2 5 αMSD 3.0 1.79 1.61 122 47.3 1 2 M is the total weight of methyl methacrylate (MMA) and butyl acrylate (BA); Xn is the number-average degree of polymerization.
As the addition of αMSD increased from 0% to 0.1%, the number-average molecular weights were evidently decreased from 40.56 104 to 28.58 104 g/mol, whereas monomer conversion underwent × × a slight decrease to 97.8%. When αMSD concentration increased from 0.1% to 1%, both molecular weights and monomer conversion were significantly decreased, as the presence of αMSD was able to effectively interfere with the polymerization process. In terms of the dispersity (Ð) of the molecular weight, the addition of αMSD contributed to the gradual decrease of the dispersity to nearly 1.60 due to a mathematical artefact in the definition of dispersity [36]. However, despite its efficiency in adjusting molecular weights and Ð, only 44.6% monomer conversion was accomplished when the concentration of αMSD was 3%. These results indicate that the utilization of αMSD as a chain transfer agent is effective in the adjustment of the molecular weights and the dispersity Ð of synthesized polymers, but a further addition of αMSD might compromise the monomer conversion. Figure1 was obtained by plotting the change in logarithm of the concentration of αMSD (ln [αMSD]) as a function of the logarithm of Mn (ln [Mn]). It showed that the influence of αMSD 0.797 concentration on Mn was estimated to be Mn [αMSD] , and the fitting linear expression was ∝ − y = 7.458 0.797x (R-Square = 0.983, y-ln [Mn], x-ln [αMSD]). The fitting result quantitatively described − the effect of αMD on molecular weight of synthesized polymers, which could be used to estimate the appropriate CTA dosage according to different mass requirements. PolymersPolymers2020 2020, 12, 12, 80, x FOR PEER REVIEW 5 of5 of 14 14
13
12
11 ln(Mn)
10
-7 -6 -5 -4 -3 ln[αMSD]
FigureFigure 1. 1.A A log–log log–log curve curve of of M Mn andn andα MSDαMSD concentration. concentration.
In addition, X in Table1 was obtained from Equation (1) [ 37]. As MMA was expected to In addition, nX n in Table 1 was obtained from Equation (1) [37]. As MMA was expected to polymerizepolymerize first, first, followed followed by by the the polymerization polymerization of of BA BA [38 [38],], Equation Equation (1) (1) is is shown shown as as follows. follows. ! x x1− x1 x XMnnX=+n = Mn + −, , (1)(1) m m mm121 2
0 0 −−f1()(1 C) f1 F f=111− Cf− (2) F1 = 1 C (2) C !α !β 0 !γ α f1 fβγ2 f1 δ C = 1 0 − (3) fff− f 0 f 0 f−δδ C =−1 1211 2 1 − (3) 00 −δ where x is the mass fraction of MMA in theff12 synthesized copolymers; f 1 F1 as the mole fraction of MMA in the copolymer is calculated by Meyer’s [37] copolymerization equations (Equations (2) and (3)); m and where x is the mass fraction of MMA in the synthesized copolymers; F1 as the mole fraction of1 MMA m f 0 f in2 are the the copolymer relative molecular is calculated masses by Meyer’s of MMA [37] and copolymerization BA, respectively; equations1 and 1 (Equationsare the mole (2) fractions and (3)); of m1 MMAand m in2 are the the monomer relative phasemolecular at the mass beginninges of MMA and and after BA, a certainrespectively; conversion f10 and of f1 theare the polymerization, mole fractions 0 respectively;of MMA in fthe2 andmonomerf 2 are thephase mole at the fractions beginning of BA and at diafterfferent a certain stages; conversionC is a monomer of the polymerization, conversion; α, β,respectively;γ, and δ are coef20 andfficients f2 are relating the mole to fractions the properties of BA of at monomers, different stages; which C are is a shown monomer in Table conversion;2. α, β, γ, and δ are coefficients relating to the properties of monomers, which are shown in Table 2. Table 2. Formulas and values of α, β, γ, and δ.
Coefficients Tableα 2. Formulas and β values of α, β γ, γ, and δ. δ
r2 r1 1 r1r2 1 r CoefficientsFormulas α = ββ = γ = γ − = δ2 1 r2 1 r1 (1 r )(1 r ) δ 2 r− r − − 1 2 1 2 Values 0.52 2.08 1−0.56−rr − 2.44− −− r2 r1 γ 12 1 r2 Formulas α= β= − = − δ=− Note: r1 and r2 are reactivity1−r ratios of1 MMA−r and BA()()11 (shown−−rr in Table4), respectively.2−−rr 2 1 12 12 The relativeValues variation tendencies0.52 of x and−2.08 1-x could be seen−0.56 in Figure 2. As the− amounts2.44 of initiator and αMSD inNote: the polymerizationr1 and r2 are reactivity process ratios were of MMA very and small, BA (shown their proportions in Table 4), respectively. in the polymer were ignored. It is evident that x increased gradually with increasing amounts of αMSD. This indicated that αMSDThe affected relative not onlyvariation the molecular tendencies weights of x and ofpolymers, 1-x couldbut be alsoseen its incomposition. Figure 2. As the amounts of initiator and αMSD in the polymerization process were very small, their proportions in the polymer were ignored. It is evident that x increased gradually with increasing amounts of αMSD. This indicated that αMSD affected not only the molecular weights of polymers, but also its composition.
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1.0
MMA 0.8 BA
0.6
x, 1-x 0.4
0.2
0.0 0.00.51.01.52.02.53.0wt% αMSD content FigureFigure 2.2. Variation tendenciestendencies ofof thethe averageaverage qualityquality proportionproportion of ofMMA MMA( (xx)) and and BA BA (1-(1-xx)) inin aa polymerpolymer inin thethe presencepresence ofofα αMSD.MSD.
ToTo analyze analyze the the potential potential reason, reason, the reactivitythe reactivity ratios ratios were calculatedwere calculated according according to a semi-empirical to a semi- formula,empirical equation formula,Q equation-e (Equations Q-e (Equations (4) and (5)). (4) and (5)).
=−−Qa Qa reeeaaabra =exp[exp()[ ea(ea ]e,b )],(4) (4) Qb Qb − − Q Qb b =−−rb = exp[ eb(eb ea)] (5) reeebbbaexp[Qa ()− − ] (5) Qa where Q and e represent the conjugation degree and polarity of monomer substituents, respectively. rwherea and Qrb andare thee represent reactivity theratios conjugation of monomers degree anda and polarib, respectively.ty of monomer In substituents, the case of respectively.ra, a higher valuera and meansrb are the alarger reactivity self-polymerization ratios of monomers trend. a and Alternatively, b, respectively. a lowerIn the valuecase of indicates ra, a higher a larger value copolymerizationmeans a larger trend.self-polymerization trend. Alternatively, a lower value indicates a larger copolymerizationDuring the calculation, trend. the Q and e values for αMSD were estimated using Q and e of alpha methyl styreneDuring (AMS), the as calculation,αMSD was the a dimer Q and of e AMS.values The forspecific αMSD Qwereand estimatede values ofusing AMS, Q MMA,and e of and alpha BA weremethyl listed styrene in Table (AMS),3, and as the αMSD calculated was a reactivity dimer of ratiosAMS. ofThe the specific three monomers Q and e values were given of AMS, in Table MMA,4. and BA were listed in Table 3, and the calculated reactivity ratios of the three monomers were given in Table 4. Table 3. Q–e values of alpha methyl styrene (AMS), MMA, and BA.
Table 3. Q–eMonomer values of alpha AMS methyl styrene MMA (AMS), BAMMA, and BA. Q 0.97 0.74 0.50 Monomere 0.81AMS MMA 0.40 BA 1.06 Q − 0.97 0.74 0.50 Table 4. Reactivitye ratios−0.81 of AMS,0.40 MMA, 1.06 and BA.
TableMMA 4. /ReactivityBA MMA ratios/AMS of AMS, MMA, AMS and/BA BA. r = 1.93 r = 0.47 r = 0.43 1MMA/BA MMA/AMS3 AMS/BA5 r2 = 0.34 r4 = 0.49 r6 = 0.07 r1 = 1.93 r3 = 0.47 r5 = 0.43 r2 = 0.34 r4 = 0.49 r6 = 0.07 Where r1 and r3 are the reactivity ratios for MMA to BA and AMS; r2 and r6 are the reactivity ratios for BA to MMA and AMS; and r and r are the reactivity ratios for AMS to MMA and BA, respectively. where r1 and r3 are the reactivity4 ratios5 for MMA to BA and AMS; r2 and r6 are the reactivity ratios for As given in Table4, r > r revealed that MMA had a greater polymerization activity than BA, BA to MMA and AMS; and1 r4 and2 r5 are the reactivity ratios for AMS to MMA and BA, respectively. which contributed to a higher x (>0.5) in the early polymerization period. For the sample without As given in Table 4, r1 > r2 revealed that MMA had a greater polymerization activity than BA, αwhichMSD additioncontributed (Run to 1), a BA’shigher mass x (>0.5) fraction in the in the early polymers polymerization gradually period. increased For up the to sample 50% in thewithout later polymerizationαMSD addition period. (Run 1), However, BA’s mass this fraction process in would the polymers be terminated gradually earlier increased in the presence up to 50% of α inMSD. the Becauselater polymerization of the chain transfer period.reaction Howeve ofr, αthisMSD, process the propagation would be terminated process was earlier terminated in the significantly presence of earlierαMSD. with Because an increase of the inchain the amounttransfer ofreactionαMSD. of As α aMSD, result, the the propagation growth of BA’s process mass was fraction terminated in the significantly earlier with an increase in the amount of αMSD. As a result, the growth of BA’s mass
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Polymersfraction2020 in, 12the, 80 polymer was terminated earlier and earlier. Finally, MMA’s mass fraction in7 ofthe 14 polymer increased, whereas the mass fraction of BA was lowered with an increase in the amount of αMSD. polymerIn addition, was terminated r1 > r3 indicated earlier and that earlier. MMA had Finally, a greater MMA’s tendency mass fraction to copolymerize in the polymer with AMS increased, than α whereasBA. Similarly, the mass r2 > fractionr6 indicated of BA that was BA lowered tended withto copolymerize an increase inwith the AMS amount rather of thanMSD. MMA. In other words,In addition,both MMAr1 and> r3 BAindicated were more that likely MMA to had copolymerize a greater tendency with AMS to copolymerize(αMSD). This could with AMSindirectly than BA.explain Similarly, why αr2MSD> r6 effectivelyindicated that controlled BA tended the tomolecular copolymerize weights with of AMS the polymer. rather than For MMA. AMS, In r4 other ≈ r5, words,there was both an MMA almost and equal BA werepossibility more likelyof AMS to to copolymerize copolymerize with with AMS MMA (αMSD). or BA. This could indirectly explain why αMSD effectively controlled the molecular weights of the polymer. For AMS, r r , there To compare the regulation reactivity differences of αMSD as a chain transfer agent to4 MMA≈ 5 and wasBA more an almost accurately, equal possibilityits transfer of coefficients AMS to copolymerize (Ctr) to each with single MMA monomer or BA. were calculated with the followingTo compare expression the regulation (Equation reactivity (6)). diIt ffwaserences derived of αMSD by as Alfrey a chain [39] transfer for agentthe tocase MMA of andbinary BA morecopolymerization accurately, its in transfer the presence coefficients of small (Ctr) toamounts each single of CTA, monomer based were on the calculated assumption with thethat following the only expressioneffect of the (Equation chain transfer (6)). Itreaction was derived was to by terminate Alfrey [39 one] for growing the case molecular of binary copolymerizationchain and start another, in the presencei.e., ignoring of small cases amounts in which of a CTA,chain basedtransfer on also the assumptionleaded to the that termination the only eofffect the of kinetic the chain chain. transfer reaction was to terminate one growing[][]A molecular++BAB chain and [][] start another, i.e., ignoring cases in which a chain transfer also leadedKAC=+ to thetr,a termination of BCthe tr,bkinetic chain. , (6) []A ++rBab [] rA [][] B [A] + [B] [A] + [B] where A and B are mole fractionsK = ACof tr,amonomer A and+ BC monotr,b mer B, respectively,, in the polymer; [ (6)A] [A] + ra[B] rb[A] + [B] and [B] are the mole fractions of monomer A and monomer B in the whole monomers; ra and rb are wherethe reactivityA and B ofare the mole two fractions monomers; of monomer and K is A andthe slope monomer of a B,Mayo-like respectively, plot in (i.e., the polymer;1/ X n vs. [A ]total and [monomerB] are the concentration, mole fractions Equation of monomer (7)) [40,41]. A and monomer B in the whole monomers; ra and rb are the reactivity of the two monomers; and K is the slope of a Mayo-like plot (i.e., 1/X vs. total monomer 11 []C n concentration, Equation (7)) [40,41]. = + K , (7) XXnn01 , 1 []M [C] = + K , (7) [ ] Xn Xn,0 M where [C] and [M] are concentrations of the CTA and whole monomers, respectively; Xn is the whereaverage [C degree] and [M of] arepolymerization concentrations for of the the polymer CTA and at whole a given monomers, [C]; and respectively;X n,0 is for theX npolymeris the average in the degreeabsence of of polymerization CTA. for the polymer at a given [C]; and Xn,0 is for the polymer in the absence of CTA.In this work, MMA and BA were regarded as monomers A and B, respectively, as αMSD was usedIn as thisthe CTA. work, On MMA the basis and BA of the were above regarded analysis, as monomers the values Aof andthe individual B, respectively, transfer as αcoefficientsMSD was used(Ctr,a and as the Ctr,b CTA.) for On a particular the basis ofmonomer the above ratio analysis, were thesucce valuesssfully of obtained the individual by the transfer two simultaneous coefficients (Cequationstr,a and C(Equationstr,b) for a particular (6) and (7)). monomer ratio were successfully obtained by the two simultaneous equationsFigure (Equations 3 displayed (6) the and fitting (7)). result of the Mayo plot, and K was derived from the slope of the plot: Figure0.56 (R-Square3 displayed = 0.986). the fitting K remained result of theunchanged Mayo plot, as andlong Kaswas the derivedmonomer from ratio the of slope MMA of theand plot: BA 0.56was (R-Squarea constant.= As0.986). a consequence,K remained Ctr,a unchanged and Ctr,b could as long be ascalculated the monomer through ratio Equation of MMA (6) and by BAchoosing was a constant.any two runs As a (including consequence, αMSD)Ctr,a andin TableCtr,b 1,could such be as calculated Run 2 and through Run 4, Equationand the result (6) by was choosing shown any in twoTable runs 5. (including αMSD) in Table1, such as Run 2 and Run 4, and the result was shown in Table5.
0.008
0.006
0.004
0.002
0.000
0.000 0.004 0.008 0.012 0.016 [C]/[M]
Figure 3. Mayo plot of 1/ 1/XXnn vs. vs. total totalmonomer monomerconcentration concentration forfor MMA–BAMMA–BA copolymerizationcopolymerization in the presence ofof ααMSD.MSD.
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α If this polymerizationTable system 5. Transfer was treated coefficients as a (terpolymerizationCtr) measured for ofMSD. MMA, BA, and αMSD, the two Ctrs of αMSD could be calculated by the reactivity ratios of the three monomers on the basis of Monomer CTA Ctr Reference Q-e values. According to the definition of Ctr, the Ctrs values of αMSD should be kinetically equivalent MMA αMSD 0.62 39 to the reciprocals of the reactivity ratios of any two monomers. The calculation results were shown BA αMSD 0.47 39 in Table 5. MMA αMSD 2.12 Q-e BA αMSD 14.28 Q-e Table 5. Transfer coefficients (Ctr) measured for αMSD.
If this polymerization systemMonomer was treated CTA as a terpolymerization Ctr Reference of MMA, BA, and αMSD, the MMA αMSD 0.62 39 two Ctrs of αMSD could be calculated by the reactivity ratios of the three monomers on the basis of Q-e BA αMSD 0.47 39 values. According to the definition of Ctr, the Ctrs values of αMSD should be kinetically equivalent to the reciprocals of the reactivityMMA ratios of anyαMSD two monomers.2.12 Q- Thee calculation results were shown BA αMSD 14.28 Q-e in Table5. As shown above, the Ctrs values of αMSD determined by different methods were obviously As shown above, the Ctrs values of αMSD determined by different methods were obviously different. Ctr,MMA and Ctr,BA were 0.62 and 0.47 according to Alfrey’s theory, respectively. However, different. Ctr,MMA and Ctr,BA were 0.62 and 0.47 according to Alfrey’s theory, respectively. However, when using Q-e method, the former was 2.12 and the latter was increased to 14.28. The difference could when using Q-e method, the former was 2.12 and the latter was increased to 14.28. The difference be explained from two aspects: First, the C calculations based on the Q-e method utilized AMS’s could be explained from two aspects: First,tr the Ctr calculations based on the Q-e method utilized reactivityAMS’s reactivity ratios to representratios to representαMSD’s. αMSD’s. Nevertheless, Nevertheless, as a dimer as a dimer of AMS, of AMS,αMSD’s αMSD’s structure structure was was more complexmore complex than AMS, than and AMS, the former and the had former more steric had emoreffects steric when iteffects reacted when with it other reacted monomers. with other Thus, theirmonomers. reactivity Thus, ratios their as wellreactivity as Ctr ratioss should as well be di asff erent.Ctrs should Second, be different. there were Second, some there inherent were errors some in calculatinginherent errors the reactivity in calculating ratios the using reactivity the Q-e ratiosmethod. using the Q-e method. InIn contrast contrast to to the theQ-e Q-emethod, method,the the substitutionsubstitution of AMS AMS for for ααMSDMSD was was avoided avoided in inAlfrey’s Alfrey’s theory, theory, soso the theCtr Cstr valuess values of ofα MSDαMSD should should be be more more accurate. accurate. InIn addition,addition, the almost same same values values of of CCtr,MMAtr,MMA andandCtr,BA Ctr,BAindicated indicated that thatα αMSDMSD hashas almost same chain chain transfer transfer possibilities possibilities with with both both MMA MMA and and BA, BA, whichwhich was was consistent consistent with with the the above above analysis analysis ofof reactivityreactivity ratios in Table Table 44.. AsAs described described above, above, few few studies studies have have been been conducted conducted to to use useα MSDαMSD as as a a chain chain transfer transfer agent agent in in the emulsionthe emulsion polymerization. polymerization. Therefore, Therefore, we have we used have NMR used analysis NMR analysis to reveal to its reveal potential its mechanism.potential In themechanism. polymerization In the process,polymerization chain propagation process, chain would propagation be terminated would by the be irreversible terminated AFCT by agent.the Therefore,irreversible a part AFCT of theagent. polymer’s Therefore, terminal a part of structures the polymer’s consists terminal of the structures AFCT agent. consists On of this the basis, AFCT the mechanismagent. On forthisα basis,MSD the could mechanism be revealed for α byMSD a polymer could be terminalrevealed structureby a polymer analysis. terminal The structure1H NMR analysis. The 1H NMR analysis of the resulting polymer using 1% αMSD showed that two signals of analysis of the resulting polymer using 1% αMSD showed that two signals of similar intensities at similar intensities at approximately 5.0, 5.1, and 5.6 ppm were observed as the characteristic peaks of approximately 5.0, 5.1, and 5.6 ppm were observed as the characteristic peaks of the two syntonic the two syntonic hydrogen atoms of the terminal double bonds (Figure 4). Moreover, multiple signals hydrogen atoms of the terminal double bonds (Figure4). Moreover, multiple signals at ~7.0 ppm at ~7.0 ppm were present, which were ascribed to the substituted benzene ring from the αMSD. were present, which were ascribed to the substituted benzene ring from the αMSD. Accordingly, these Accordingly, these results confirmed that emulsion polymerization of acrylates using αMSD as a α resultschain confirmed transfer agent that emulsionfollowed polymerizationYasummasa’s mechanism. of acrylates The using detailedMSD regulation as a chain mechanism transfer agent is followedillustrated Yasummasa’s in Scheme 4. mechanism. The detailed regulation mechanism is illustrated in Scheme4.
1 FigureFigure 4. 4.1HNMR HNMR spectrum spectrum of of thethe resultingresulting polymer in in the the presence presence of of ααMSDMSD (content: (content: 1%). 1%).
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O O O m O initiation m n n αMSD m n m O n initiation m n O O αMSD m n O O OO O O OO O O OO O MMA BA O OO O MMA BA
x MMA + y BA x MMA + y BA x y reinitiation x y reinitiation O OO O O OO O
Scheme 4. Chain transfer mechanism of αMSD in MMA and BA emulsion copolymerization. Scheme 4. Chain transfer mechanism of αMSD in MMA and BA emulsion copolymerization.
3.2.3.2. EEffectect of αMSD on the PolymerizationPolymerization RateRate 3.2. Effectff of αMSD on the Polymerization Rate As αMSD has an obvious influence on polymer molecular weights, it was necessary to study if As αMSD has an an obvious obvious influence influence on on polymer polymer molecu molecularlar weights, weights, it itwas was necessary necessary to tostudy study if ifits its presence presence would would also also affect affect the the polymerization polymerization rate (Rp).). In In the emulsion polymerization,polymerization, thethe its presence would also affect the polymerization rate (Rp). In the emulsion polymerization, the constant speed stage was typically used as the indicator of total polymerization rate, and it could be constant speed stage was typically used as the indicator of total polymerization rate, and it could be calculated from the conversion–time curve. Figure 5 showed the monomer conversion versus reaction calculated from the conversion–time curve.curve. Figure 5 5 showedshowed thethe monomermonomer conversionconversion versusversus reactionreaction time in the presence of ααMSD at various concentrations. The slopes of the curves directly reflected time in thethe presencepresence ofof αMSDMSD atat variousvarious concentrations.concentrations. The slopes of the curves directly reflectedreflected RRpp. It It was clear that the presencepresence ofof ααMSDMSD couldcould decreasedecrease thethe RRpp asas comparedcompared toto samplessamples withoutwithout Rp. It was clear that the presence of αMSD could decrease the Rp as compared to samples without ααMSD.MSD. Moreover,Moreover, asas thethe increaseincrease inin thethe quantitiesquantities ofof ααMSD,MSD, RRpp was decreased more significantly.significantly. For αMSD. Moreover, as the increase in the quantities of αMSD, Rp was decreased more significantly. For example, the monomer conversion only reached 0.4 after 250 min when 3% of αMSD was added, example, the monomer conversion only reached 0.4 after 250 min when 3% of αMSD was added, whereas its conversion approached 100% without ααMSD addition. Furthermore, the effect of ααMSD whereas its conversion approached 100% without αMSD addition. Furthermore, the effect effect of αMSD concentrationconcentration ([([ααMSD])MSD]) on onR pRwasp was further further correlated correlated viaconstructing via constructing a log–log a log–log plot of Rplotp and of [ αRMSD],p and concentration ([αMSD]) on Rp was further correlated via constructing a log–log plot of Rp and 0.945 as[αMSD], shown inas Figureshown6 .in As Figure expected, 6. theAs resultexpected, was quantitativelythe result was estimated quantitatively to be Restimatedp [αMSD] to be [αMSD], as shown in Figure 6. As expected, the result was quantitatively estimated∝ to− be (R-SquareRp∝[αMSD]= −0.905).0.945 (R-Square = 0.905). Rp∝[αMSD]−0.945 (R-Square = 0.905).
1.0 1.0
0.8 0.8
0.6 0.6
0.4 0.4 Conversion Conversion 0.2 0.2
0.0 0.0 0 50 100 150 200 250 0 50 100 150 200 250 Time (min) Time (min) 0 0.1% 0.3% 1% 3% 0 0.1% 0.3% 1% 3% FigureFigure 5.5. Monomer conversion as a functionfunction ofof reactionreaction timetime atat variousvarious molarmolar ratiosratios betweenbetween ααMSDMSD Figure 5. Monomer conversion as a function of reaction time at various molar ratios between αMSD andand acrylateacrylate monomersmonomers (see(see TableTable1 ).1). and acrylate monomers (see Table 1).
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-5
-6
-7 ) p
ln(R -8
-9
-10 -7 -6 -5 -4 -3 -2 ln[αMSD] FigureFigure 6.6.Rate Rate ofof polymerizationpolymerization asas aa function function of of various variousα αMSDMSD concentrations. concentrations. SlopeSlope ofof linearlinear fit fit= = −0.9450.945 ((R-SquareR-Square == 0.905). − α TheThe potentialpotential reasons for the the decrease decrease in in RRp inp in the the presence presence of ofαMSDMSD may may be explained be explained from from two twoaspects. aspects. First First of all, of the all, overall the overall rate constant rate constant of the of scission the scission process process of the of leaving the leaving group group may be may lower be lowerthan propagation than propagation [20]. [The20]. Theefficient efficient transfer transfer reacti reactionon of ofan an irreversible irreversible AFCT AFCT agent agent required thatthat transitionaltransitional radicals, radicals, created created by by the the addition addition reaction reac oftion the of irreversible the irreversible AFCT agentAFCT and agent chains and radicals, chains couldradicals, easily could fragment easily fragment to generate to newgenerate radicals new to radicals reinitiate to reinitia the polymerizationte the polymerization process. Asprocess. a result, As retardationa result, retardation would happen. would In happen. fact, the In novel fact, radical the no (isopropylvel radical benzene (isopropyl radical, benzene seen inradical, Scheme seen4) in in thisScheme paper, 4) hadin this less paper, access had to reinitiateless access polymerization to reinitiate polymerization than the primary than radical the primary (sulfate radical radical (sulfate anion) dueradical to its anion) steric due hindrance. to its steric Thus, hindrance. the macroscopic Thus, the reaction macroscopic rate decreased. reaction Second, rate decreased. radicals derivedSecond, fromradicals both derived the initiator from andboth leaving the initiator groups and might leaving escape groups from micelles might escape or latex from particles micelles (desorption) or latex andparticles then be(desorption) terminated and with then radicals be terminated in either thewith water radicals phase in oreither polymer the water particle phase (reabsorption). or polymer Thisparticle desorption (reabsorption). phenomenon This desorption had been successfully phenomenon verified had been in the successfully emulsion polymerization,verified in the emulsion and the relatedpolymerization, theoretical and model the hadrelated been theoretical previously model established had been [42–44 previously]. The sulfate established radical anion [42–44]. is more The hydrophilicsulfate radical than anion the isopropyl is more benzene hydrophilic radical. than As athe result, isopropyl the desorption benzene numberradical. of As the a former result, was the muchdesorption greater number than latter, of the and former the latter was tendedmuch greater to stay inthan the latter, original and particles. the latter This tended led to to serious stay in loss the oforiginal the high-active particles. free This radicals, led to andserious in addition, loss of thethe remaining high-active free free radicals radicals, in the and latex in particlesaddition, were the notremaining active enough, free radicals which in further the latex exacerbated particles were the decline not active in R penough,. which further exacerbated the decline in Rp. 3.3. Effect of αMSD Particle Size and Relevant Parameters
3.3. EffectThe influence of αMSD of ParticleαMSD Size concentration and Relevant on Parameters latex particle size(dp), its distribution (PDI), and particle N numberThe ( influencec) were alsoof αMSD explored, concentration and the results on latex were particle shown size( ind Tablep), its 6distribution. The particle (PDI number), and particle of the resultingnumber ( emulsionNc) were wasalso calculatedexplored, and using the an results Equation were (8). shown in Table 6. The particle number of the resulting emulsion was calculated using an Equation (8). 6m 21 Nc = 10 , (8) 6m 3 ×21 c =×πρdp N 3 10 (8) πρdp where m is the solid content of the dispersed phase (containing, monomer and polymer at a given 1 conversionwhere m is andthe expressedsolid content in g of/mL the− ),dispersed and ρ represents phase (containing the particle monomer density. and polymer at a given −1 conversionAs shown and in expressed Table6, d inp declined g/mL ), fromand ρ 109.38 represents to 46.88 the nmparticle as α MSDdensity. dosage was varied from 0% 17 17 1 to 3%,As whereas shown inNc Tablewas enhanced6, dp declined from from 1.67 109.3810 toto 46.88 20.79 nm 10as αMSDmL .dosage Moreover, was variedPDI started from 0% to × × − dramaticallyto 3%, whereas increase Nc was when enhanced the αMSD from dosage 1.67 was× 1017 higher to 20.79 than × 1%.1017 mL−1. Moreover, PDI started to dramaticallyAccording increase to recent when reports the [α45MSD,46], thedosage exit andwas entryhigher rates than of 1%. radicals and monomers into particles have strongAccording effects to onrecent miniemulsion reports [45,46], polymerization the exit kinetics.and entry In rates this study, of radicals isopropyl and benzene monomers radicals into derivedparticles from haveα MSDstrong increased effects theon wholeminiemulsion radical concentrationpolymerization in thekinetics. particles, In this which study, contributed isopropyl to thebenzene greater radicals desorption derived of sulfate from radicalαMSD anions.increased Thus, the thesewhole sulfate radical radical concentration anions were in the resorbed particles, by otherwhich micelles, contributed and moreto the micelles greater coulddesorption be transformed of sulfate intoradical polymer anions. particles. Thus, these This sulfate explained radical the anions were resorbed by other micelles, and more micelles could be transformed into polymer particles. This explained the increase in Nc and the decrease in dp. Generally speaking, the increase in
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Polymers 2020, 12, x FOR PEER REVIEW 11 of 14 increase in Nc and the decrease in dp. Generally speaking, the increase in Nc should be accompanied Nc should be accompanied by inhibition of the termination reaction, leading to a higher reaction rate. by inhibition of the termination reaction, leading to a higher reaction rate. However, the actual Rp was However, the actual Rp was decreased. In fact, actual variations of Nc and dp were not necessarily decreased. In fact, actual variations of Nc and dp were not necessarily related to Rp. A mass transfer related to Rp. A mass transfer resistance occurred during the transfer process of sulfate radical anions resistance occurred during the transfer process of sulfate radical anions among the micelles, water, and among the micelles, water, and particles. This would counteract the slight increase in Rp caused by particles. This would counteract the slight increase in Rp caused by the lower degree of termination. the lower degree of termination. As a result, the variations of Nc and dp failed to improve Rp. As a result, the variations of Nc and dp failed to improve Rp.
Table 6. Collection of latex particle size (dp) and its distribution (PDI) and particle number (Nc) with Table 6. Collection of latex particle size (d ) and its distribution (PDI) and particle number (N ) with αMSD at different concentrations. p c αMSD at different concentrations. Run (CTA/M)/% dp (nm) PDI Nc (1017 mL−1) Run (CTA/M)/% d (nm) PDI N (1017 mL 1) 1 0.0 p109.38 0.068 c 1.67 − 12 0.00.1 109.3893.75 0.079 0.068 2.68 1.67 2 0.1 93.75 0.079 2.68 3 0.3 78.15 0.045 4.58 3 0.3 78.15 0.045 4.58 44 1.01.0 62.5062.50 0.135 9.07 9.07 55 3.03.0 46.8846.88 0.230 20.79 20.79
3.4. The Effect Effect of αMSD on Tensile PropertiesProperties ofof PolymerPolymer FilmsFilms
The tensile properties (E, σp, and εb) of the latex films prepared from polyacrylate emulsion at The tensile properties (E, σp, and εb) of the latex films prepared from polyacrylate emulsion at variousvarious ααMSDMSD concentrationsconcentrations werewere determined,determined, and and the the stress–strain stress–strain curves curves were were shown shown in in Figure Figure7. The7. The results results were were summarized summarized in Table in 7Table. As expected, 7. As expected,αMSD addition αMSD addition had an impact had an on theimpact mechanical on the mechanical properties of the latex film. With the increase in αMSD from 0% to 1%, E and σp of the properties of the latex film. With the increase in αMSD from 0% to 1%, E and σp of the resultant films resultant films were reduced by nearly 75% and 50%, respectively, whereas εb was only slightly were reduced by nearly 75% and 50%, respectively, whereas εb was only slightly affected. Furthermore, theaffected. obvious Furthermore, yield point the of the obvious resultant yield polymers point of gradually the resultant became polymers less obvious gradually when became the amount less ofobviousαMSD when was abovethe amount 0.3%, of indicating αMSD was that above the polymer0.3%, indicating films changed that the from polymer hard films to soft changed [47]. When from hard to soft [47]. When the amount of αMSD reached to 3%, E and εb were only 2.00 and 0.23 MPa, the amount of αMSD reached to 3%, E and εb were only 2.00 and 0.23 MPa, respectively, and εb had decreasedrespectively, to 467%.and εb The had toughness decreased of to the 467%. resultant The filmstoughness was significantly of the resultant decreased, films was and significantly the film was softdecreased, and weak. and Thethe decreasefilm was insoft the and molecular weak. The weights decrease of polymers in the molecular was the mainweights reason of polymers to lead to was the poorthe main mechanical reason to properties. lead to the poor mechanical properties.
8
1 6 2
4 3
4 Stress(MPa) 2
5 0 0 200 400 600 800 Strain(%) Figure 7. Stress–strain curves of latex filmsfilms with didifferentfferent ααMSD concentrations.concentrations.
Table 7. Summary of Young’s modulus (E), tensile strength (σp), and elongation at break (εb) of films prepared using emulsion polymerization with αMSD as the CTA.
Run (CTA/M)/% E (MPa) σp (MPa) εb (%) 1 0.0 57.65 5.33 798 2 0.1 52.53 5.06 643 3 0.3 44.28 3.77 564 4 1.0 14.84 2.65 658 5 3.0 2.00 0.23 467
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Table 7. Summary of Young’s modulus (E), tensile strength (σp), and elongation at break (εb) of films prepared using emulsion polymerization with αMSD as the CTA.
Run (CTA/M)/% E (MPa) σp (MPa) εb (%) 1 0.0 57.65 5.33 798 2 0.1 52.53 5.06 643 3 0.3 44.28 3.77 564 4 1.0 14.84 2.65 658 5 3.0 2.00 0.23 467
4. Conclusions In this study, it was shown that αMSD was a highly effective CTA to control the molecular weights in MMA and BA emulsion copolymerization. Molecular masses and their distributions, as well as the mass proportion of MMA in the polymers, were gradually decreased when increasing the dosage of αMSD. Meanwhile, the final monomer conversion was also declined as the αMSD concentration increased above 1%, indicating its apparent inhibition ability of the polymerization. The chain transfer constant of αMSD to MMA was approximately the same with αMSD to BA. The αMSD regulation mechanism was verified by NMR analysis, which was consistent with Yasummasa’s theory. In addition, the Rp and dp of the acrylate latex were decreased as the increase in the dosage of αMSD, while the Nc of the emulsion was increased. Furthermore, the PDI was also affected when αMSD dosage was more than 1%. Finally, the decrease of the tensile properties of the resulting polyacrylate films was mainly due to the reduction in the molecular weights of synthesized polymers after the addition of αMSD.
Author Contributions: Conceptualization, Z.Z. and C.W.; Methodology, Z.Z. and F.C.; Validation, R.C., and F.C.; Formal analysis, Z.Z. and G.F.; Investigation, Z.Z., G.F., and D.Z.; Resources, R.C.; Data curation, C.W. and R.C.; Writing—original draft preparation, Z.Z.; Writing—review and editing, D.Z. and R.C.; Supervision, R.C.; Project administration, R.C.; Funding acquisition, R.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by special funds for basic scientific research operating expenses of central public welfare research institutions of Chinese Academy of Forestry, grant number CAFYBB2018ZB008. Acknowledgments: This work was financially supported by special funds for basic scientific research operating expenses of central public welfare research institutions of Chinese Academy of Forestry (CAFYBB2018ZB008). Conflicts of Interest: The authors declare no conflicts of interest.
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