To overcome this problem, branching can be used in the structure by designing macromolecules such as dendrimers or hyperbranched polymers, which will introduce hierarchical relaxation dynamics.6,7,8,9 Dendrimers are ball-shaped macromolecules that have a perfectly controlled structure thanks to a thorough step-by-step synthesis process including protections and deprotections.10 They are composed of a polyfunctional core (a polyol is often used) (Ax with x ≥ 2), elongation units (AA or AB monomers), branching monomers (ABx with x ≥ 2) and end-groups. It has been shown that dendrimer-like structures have a lower viscosity when compared to their linear counterparts because their ball-shaped structure prevents the entanglement.11 Indeed, for the same molecular weight, a dendrimer or an HBP will have a lower viscosity than its linear counterpart.12 This effect is caused by the absence of entanglement in the HBP, as long as the arms are not too long as they could themselves be subject to this effect. According to the Flory-Stockmayer theory13,14, it will occur if the Mw of the branch exceeds a certain mass of entanglement (Me) without branching. For this reason, HBP are usually restrained to a Mw between 2 and 10 kDa where each branch has a Mw well under the Me.15,16 Yan et al. studied the limits of branch length in HBP structures and were able
5 to obtain high Mw poly(acrylated epoxidized soybean oil) (up to 390 kDa) thanks to their understanding of the inter- and intramolecular reactions impact on the relaxation processes and viscosity, allowing them to avoid crosslinking.17
Another major advantage of using dendrimer-like structures is the greater number of functional groups at the periphery of the structures. Indeed, a dendrimer is composed of many arms that are all terminated by a functional group, their number is greater than for a linear polymer and more easily accessible as they are at the end of the chains. It is also possible to synthesize dendrons separately and bind them in a final step to obtain heterofunctional dendrimers (with two types of end groups on each side of the structure) which can lead to even more applications (principle used for the self-healing polymers where a cationic end-capped and an anionic end-capped dendrons are combined to form a heterofunctional dendrimer) (Figure 4).
Figure 4 Representation of the structure of a dendron and homo- or hetero-functional dendrimers.
Dendrimers can be synthesized using two main approaches: the divergent (core first) or the convergent (arm first). For the divergent route, the core is reacted with branching units and/or elongation units to build the dendrimer and finally with end groups. For the convergent route, dendrons are synthesized from the end groups and are linked to the core in a final step (Figure 5).
However, dendrimers are difficult to synthesize as numerous steps are required making their production costly, energy and time consuming, and they also often have a poor atom economy as the activation step is often the result of a deprotection. When the target molecule does not need to have a very precise structure and a low dispersity in molecular weight, their synthesis is too complicated in comparison to more common linear synthesis and the benefits in terms of properties is often not worth synthesizing these macromolecules. Thus, the interest
6 has considerably grown towards the synthesis of less well-defined dendrimer-like structures, so-called hyperbranched polymers (HBP). HBPs are macromolecules with a ball-like shape very close to the dendrimer structure but the synthesis pathway is less precise (no more activation or deprotection needed) leading to a higher Mw dispersity and irregular structure but it has been shown that their properties are very similar to dendrimers and for most applications, the difference is negligible.18 Furthermore, the less thorough production allows for a more economical accessibility without impairing the properties much.
Figure 5 Synthesis of dendrimers through divergent or convergent routes.
The most common branching unit for the synthesis of hyperbranched polyesters is 2,2-bis(hydroxymethyl)propionic acid or bMPA, an AB2 building block also used for the synthesis of the commercial HBPs and dendrimers Boltorn™.18,19 When several branching units are added, the layers of branching units are called generations and the term pseudo-generation is used for HBP due to the uncontrolled nature of the reaction steps. In the course of this study, no dendrimers will be synthesized and the term generation, will be used to talk about pseudo-generation.
7 Figure 6 a. PFD.G3 dendrimer third generation and b. H30 HBP third generation from Perstorp.
Dendrimers based on bMPA are usually synthesized step by step to grow the generations one by one using bMPA acetonide (protected as a cyclic acetal) which requires the deprotection to grow the next generation (referred as activation step in Figure 5). If we compare a dendrimer and a HBP that have the same core and the same number of generations (or pseudo-generations for HBP), even though the structure is less defined, the properties will be very similar and they both have the same number of functional groups at the periphery of the structure (Figure 7).
Figure 7 Shape of dendrimers and HBP with number of functional groups (red).
Giving the fact that dendrimers and hyperbranched polymers are composed of several building blocks, their properties can be widely different depending on the nature of the selected monomers, thus allowing their use for a wide range of applications with various desired properties.20,21,22 This ability to tune the structure and the final properties made them good candidates for scaffolding macromolecules with a difference in lipophilic or lipophobic behavior between the internal and external layers of the structure. They can help with the transportation and controlled diffusion of drugs23 for a better targeting of active agents for
8 example, thus showing great potential as drug carriers.24 Other end-capping allowed their use as membranes or self-assembly macromolecules, self-healing materials25 or even optical polymers.26 HBPs usually show better solubility than linear polymers and lower viscosity. For UV-curing applications such as additive manufacturing, coating or dental composites when biocompatible, they possess a higher reactivity and lower shrinkage.27 This increased reactivity is due to the fact that the functional end groups are more accessible and more abundant than in a linear chain. The end-groups also have a strong impact on the viscosity: an OH-terminated HBP will be more viscous due to the strong inter- and intramolecular H-bonding whereas an alkyl substituted HBP will possess a lower viscosity.