Polydispersity and Molecular Weight Distribution of Hyperbranched Graft Copolymers via “Hypergrafting” of AB_m Monomers from Polydisperse Macroinitiator Cores: Theory Meets Synthesis

Abstract

The hypergrafting strategy designates the synthesis of hyperbranched graft copolymers (HGCs) in a grafting-from approach, using AB m monomers, from multifunctional, polydisperse macroinitiator cores by slow monomer addition. Hypergrafting leads to complex polymer topologies with defined molecular weight, degree of branching (DB), and polydispersity (PD). By a generating function formalism, a generally applicable equation for the PD of HGCs (PD = PD f + (m − 1)/ f ̅) is derived, where PD f is the polydispersity of the core and f ̅ its average functionality. In addition, the complete molecular weight distribution function has been calculated for varied m and f ̅ as well as for a given distribution of initiator functionalities f. For comparison of the theoretical predictions with experimental results, a series of novel linear polyglycerol-graf t-hyperbranched polyglycerol (linPG-g-hbPG) HGCs (M n = 1000−4000 g mol −1 ) were synthesized and characterized as a model system. An increase in polydispersity occurred as a consequence of the hypergrafting process, confirming the theoretical predictions of the novel equation. Moreover, the model system allows for the determination of the DB of hbPG prepared by hypergrafting from linear polyglycerol macroinitatiors (DB = 0.59−0.61). The theoretical results presented are key to achieve control over the branch-on-branch topology of hyperbranched blocks in nonconventional polymer architectures, such as linear−hyperbranched block copolymers. ■ INTRODUCTIONOne of the current key challenges in polymer science is the controlled synthesis of multifunctional, complex polymer architectures. 1 Hyperbranched polymers are characterized by a branch-on-branch topology and can usually be synthesized in one reaction step. 2,3 Moreover, they are interesting building blocks for nonconventional copolymer topologies such as block and graft copolymers, which all exhibit properties that differ entirely from their linear counterparts. In this context, the hypergrafting strategy (grafting-f rom), i.e., the synthesis of a hyperbranched block copolymer by using a multifunctional macroinitiator core for the polymerization of AB m monomers, is a useful synthetic method to attain control over the key parameters molecular weight, polydispersity (PD), and degree of branching (DB). 4a Scheme 1 shows different hyperbranched graft copolymer architectures with linear or hyperbranched macroinitiators used for the hypergrafting reaction, exemplified for an AB 2 monomer.Glycidol is an important AB 2 monomer, which can be polymerized in a controlled manner by ring-opening multibranching polymerization (ROMBP) to give hyperbranched polyglycerol (hbPG), 4b,5 which has been used as a building block for several hyperbranched graft copolymers (HGCs).If hyperbranched macroinitiators are employed for hypergrafting, hyperbranched-graf t-hyperbranched copolymers (HHGCs) can be generated, which are potentially interesting as core−shell structures for biomedical transport applications. 6,7 In a work by our group, low molecula...

Statistical thermodynamic properties of a new self-condensing vinyl polymerization system

“…In essence, PI is another important polymeric quantity to characterize the distribution of HBPs with different DPs. It is often used to measure the uniformity of HBPs in the system, hence an intense interest has been attracted in numerous experimental and theoretical works [29][30][31][32][33][34][35]. Theoretically, PI is closely related to the number-and weight-average DPs.…”

“…It is well known that many physical and chemical properties of HBPs are mainly determined by their structures [30][31][32][33][34][35]. Hence, all sorts of new materials are being pursued for the design of promising products with desired structures.…”

“…In practice, the numbers of these four quantities can be used to characterize the molecular structure of HBPs, and they are closely related to the properties of resultant HBPs and hence play an important role in both experimental and theoretical works [30][31][32][33][34][35]. Experimentally, the NMR technique is usually applied to characterize the molecular structures of HBPs because different molecular structural units will cause different chemical shifts, which can distinguish them from one another [34,35]. Theoretically, the numbers of these distinct structural units can be calculated in a straightforward manner and can be expressed as an explicit function of conversion x.…”

“…Thus, how to regulate the DB has become one of the major aspects in the design of macromolecules and functional material. For this purpose, some considerable effort has been made [29][30][31][32][33][34][35].…”

“…As far as the branched polymer is concerned, its DB is usually considered [30][31][32][33][34][35] because it is closely related to the number of branched units. For convenience, the definition of DB proposed by Fréchet is adopted, which is defined as DB=(N b.u.…”