Thermoreversible Nonlinear Diels-Alder Polymerization of Furan/Plant Oil Monomers Carla Vilela, 1 Armando J. D. Silvestre, 1 Alessandro Gandini 1,2 1 CICECO and Chemistry Department, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal 2 Sa ˜ o Carlos Chemistry Institute and Materials Engineering Department, University of Sa ˜ o Paulo, 13566-590 Sa ˜ o Carlos, Brazil Correspondence to: A. Gandini (E-mail: agandini@iqsc.usp.br) Received 13 November 2012; accepted 17 January 2013; published online 19 February 2013 DOI: 10.1002/pola.26610 ABSTRACT: Novel trifunctional monomers based on renewable resources were prepared and subsequently polymerized via the Diels-Alder (DA) polycondensation between furan and malei- mide complementary moieties. Three basic approaches were considered for these nonlinear DA polycondensations, namely the use of (i) a bisfuran monomer in combination with a trisma- leimide (A 2 þ B 3 system) and (ii) a trisfuran monomer in conjunction with a bismaleimide (A 3 þ B 2 system) leading to branched or crosslinked materials, and (iii) the use of monomers incorporating both furan and maleimide end groups (A 2 B or AB 2 systems), which lead to hyperbranched structures. The applica- tion of the retro-DA reaction to the ensuing polymers confirmed their thermoreversible character. V C 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2260–2270 KEYWORDS: biomaterials; crosslinking; Diels-Alder click/unclick polymerization; hyperbranched; nonlinear polycondensation; renewable resources; thermoreversible polymers INTRODUCTION The dwindling of fossil resources in con- junction with their price fluctuations raises serious questions about the long-term pursuit of their exploitation. Hence, the dramatic acceleration of interest in materials derived from renewable resources in the last couple of decades. 1 Within this context, the field of furan polymers is blooming as a potential alternative to petroleum-based counterparts, because of the ubiquitous availability of their renewable saccharide-based first-generation precursors, furfural and hydroxymethylfurfural. 2 The peculiar chemical properties of the furan ring catapulted its exploitation mainly into 2 (i) electrophilic substitutions to generate reactive end groups for block and graft copolymers, (ii) the incorporation of photosensitive chromophores into various macromolecular structures, (iii) the synthesis of conjugated oligomers and polymers, and, more recently, (iv) its proneness to intervene as a diene in one of the best examples of click-chemistry known as the Diels-Alder (DA) reaction, applied to original polymer syntheses. The thermally reversible character of this click interaction is definitely its most attractive feature since it provides a straightforward means of regenerating the monomeric or macromolecular reagents, thus opening the way to synthe- sise polymers with unique properties such as self-mendabil- ity and network recyclability through reversible crosslink- ing. 3 The fact that the forward DA reaction gives rise to both endo and exo isomeric adducts, whose proportion varies as a function of the specific structure of the reagents and the sys- tem conditions, does not play a significant role in these mac- romolecular syntheses, since both participate in the chain growth. The DA polycondensation applied to polymer synthe- sis based on furan/maleimide 3 is essentially dominated by two different approaches, that is, (i) polycondensation reac- tions calling upon complementary bifunctional or polyfunc- tional monomers, including AnBm-type structures and (ii) reversible crosslinking of linear polymers bearing pendant furan or maleimide moieties, based on the temperature sen- sitivity of the DA equilibrium. 3 Among the other numerous renewable resources used to prepare novel macromolecular materials, 1,2 plant oils occupy a forefront position, as demonstrated by the growing research activities in the last several years, 4–12 focussed particularly on the application of original mechanistic approaches. These materials include polyurethanes, 13 poly- esters, 14 and other macromolecular structures, 4–12 wherein the long aliphatic segments play a significant role in confer- ring properties such as hydrophobicity and low thermal transitions, while the allied polar moieties can favor biodegradability. Our interest in plant oils and their fatty acids stems from the idea that they are appealing substrates for appending furan and maleimide functions that can be used for DA poly- merizations. In a previous study, we demonstrated that unsaturated plant oil derivatives can be converted into V C 2013 Wiley Periodicals, Inc. 2260 JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 2260–2270 ARTICLE WWW.POLYMERCHEMISTRY.ORG JOURNAL OF POLYMER SCIENCE