polymers 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 first firstfirst case, case, alkyl alkyl hydrogenhydrogen