Riboflavin-mediated radical polymerization – Outlook for eco-friendly synthesis of functional materials

Highlights

• Riboflavin as a driving force in polymerization processes.

• Riboflavin mediates both free and controlled radical polymerization techniques.

• Eco-friendly and easily scalable approaches for synthesis of functional polymers.

Abstract

Radical polymerization techniques driven by naturally occurring substances are rapidly developed towards more sustainable, thus eco-friendly and industrially relevant approaches. Riboflavin as a multifunctional molecule due to its unique architecture and biological activity is an excellent target in the context of radical polymerization, considering both the versatile properties of isoalloxazine ring – a characteristic feature of the flavin family, and the ribitol tail – susceptible for facile modification to incorporate additional functional groups. Three-ring part of riboflavin is essential for redox processes, UV absorption, and photosensitivity – the beneficial features to maintain control during radical polymerization, while a ribitol side chain with hydroxyl groups is easily modified to include several functions in one molecule. This review highlights the recent advances in the development of free radical polymerization and controlled polymerization techniques mediated by riboflavin structure, to create environmentally friendly and easily scalable approaches for the preparation of functional polymer materials with versatile use.

Introduction

The development of polymer chemistry is strongly focused on investigating polymerization techniques that are environmentally friendly and easy to implement in the industry [1], [2], [3]. Moving in this direction, naturally-derived substances are increasingly used as substitutes for organic compounds, playing the role of factors controlling the polymerization processes, thus completely eliminating or decreasing the number of organic compounds in the reaction system. An application of the substances that are easy to find in nature and often biologically active as well, thus biocompatible, is privileged due to the cost-effectiveness of this type of processes and an ability implemented these approaches for the preparation of the polymer materials with potential use in biomedicine [4], [5]. Naturally-derived substances are commonly known to have a wide range of beneficial functionalities, that can be utilized in radical polymerization to mediate the process e.g. redox activity [4], [6], [7] and photoinitiation characteristics [8], [9], [10]. Considering these features the rapidly developed and ecological aspect in polymerizations is the use of light as an external stimulus to control the whole process [11], thus a need to search for efficient and eco-friendly photoinitiators as a promising alternative to widely used organic ones. In photopolymerization, a photoactive compound with strong absorption in the wavelength emitted by the source of the light absorbs incident light to form polymerization-initiating active species [12]. In recent years, visible light photopolymerization as a high-energy ultraviolet (UV) light counterpart is increasingly investigated. It is connected with a series of disadvantages connected with UV irradiation e.g. harmful to cells and tissues, and low penetration depth [13]. Visible light is characterized by low costs, safety, high-penetration, and a wide range of applications including dentistry [14], laser-induced 3D curing [15], and optoelectronic products [16]. For these reasons, naturally occurring visible light photoinitiators are intensively sought for effective and eco-friendly radical polymerization, providing widespread polymer materials with an enormous application field. In this consideration, natural dyes may act as highly efficient photoinitiators, thus as widely abundant, often low toxicity substrates for receiving functional biomaterials [17]. Radical photopolymerization may be controlled by type I and type II photoinitiators depending on their chemical structure, and thus decomposition mechanism. The first one covers the one-component photoinitiation system with generation of a radical directly after irradiation in the process of its hemolytic photodissociation. While two-component system (type II) uses an additional co-imitator such as an electron donor or acceptor or a hydrogen donor. The excited photoinitiator reacts with co-initiator molecules providing radicals or radical-ions [18], [19]. In relation to the described photoinitiation mechanism, the efficiency of photopolymerization is directly connected with the structure of the selected photoinitiator. The characteristic parameters that determine the choice of the photoinitiator are maximum absorption wavelength and a molar extinction coefficient [20].

Riboflavin (vitamin B₂) is a naturally occurring dye in the form of orange-yellow powder with absorption maxima in both UV and visible light due to its unique chemical structure [21]. It is a type II photoinitiator, thus creates a two-component photoinitiation system with an electron donor as a co-initiator [9], [22], [23]. It composed of two basic components – 7,8-dimethyl-10-alkylisoalloxazine ring responsible for biological and chemical properties of riboflavin as a photosensibility and redox properties, and ribitol tail with four reactive hydroxyl groups prone to facile modification [21], [24], [25]. A three-ring moiety of vitamin B₂ has a great potential as a photoinitiator and reducing agent in radical polymerization, while a ribitol tail can be easily modified to incorporate additional functionalities, improving the reaction setup.

Up to now, riboflavin was extensively used as a photoinitiator in free radical polymerization in the preparation of functional polymer materials, usually used in a two-component initiation system – with the use of co-initiator as an electron donor [9], [26]. Among organic co-initiators (tertiary amine structures), also biocompatible structures were applied, creating a fully environmentally-friendly reaction system [23], [27]. Controlled radical polymerization methods including reversible deactivation-radical polymerization (RDRP) techniques utilize riboflavin in different roles, providing an eco-friendly reaction setup in the synthesis of precisely-defined polymer materials with a predetermined structure proved by low dispersity, thus with a great potential for specialized use [7], [8], [28], [29]. Reversible addition-fragmentation chain transfer (RAFT) polymerization with two-component vitamin-inspired biocompatible and oxygen tolerant photopolymerization benchtop enables the polymerization of a wide range of monomer families i.e. acrylamides, acrylates and methacrylates [8]. While in atom transfer radical polymerization (ATRP) technique riboflavin was used as an ATRP initiator by facile modification of a ribitol tail with bromide molecule, providing ATRP initiation sites [28], [29], going to a dually functional molecule – ATRP initiator and a reducing agent [29] or photoinitiator [30] simultaneously in one molecule, and a triple-functional substance – ATRP initiator, reducing agent and oxygen scavenger – included in one molecule [7].

This review intends to drive the attention to an application of riboflavin as a widely available, eco-friendly, and biocompatible molecules in the radical polymerization techniques as a driving force, creating environmentally friendly and easily scalable approaches for the preparation of functional polymer materials with specialized use. Following the introduction, a brief discussion of riboflavin chemical structure, and thus functionalities are discussed. After this, an overview of the use of riboflavin in radical polymerization, starting from free radical polymerization, going to controlled polymerization techniques has been made, with the objective of the riboflavin functionalities and prepared functional polymer materials, emphasizing ease of use.