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Controlled Polymerization
Published in Timothy P. Lodge, Paul C. Hiemenz, Polymer Chemistry, 2020
Timothy P. Lodge, Paul C. Hiemenz
In this section we take up the topic of controlled radical polymerization, which represents one of the most active fields in polymer synthesis in recent years. The combination of the general advantages of radical polymerization (a wide range of suitable monomers, tolerance to many functional groups, characteristically rapid reactions, relatively relaxed polymerization conditions) with the unique features of a living polymerization (narrow molecular weight distributions, controlled molecular weights, end functionality, block copolymers, and other complex architectures) has tremendous appeal in many different areas of polymer science. In this section we outline first in general terms how this combination is achieved, and then give some specific examples of the mechanistic details. We choose the term “controlled” rather than “living” in this section, because irreversible termination reactions cannot be rigorously excluded.
Poly(Alkyl Cyanoacrylate) Nanoparticles for Delivery of Anti-Cancer Drugs
Published in Mansoor M. Amiji, Nanotechnology for Cancer Therapy, 2006
R. S. R. Murthy, L. Harivardhan Reddy
Essentially, the initiation of a monomer for polymerization requires an initiator that can generate ions or radicals to start the polymerization process. If the nucleation of the monomer is due to ions, then the mechanism is called “ionic polymerization.” Depending on the type of ions produced, the ionic polymerization may be anionic or cationic. If the radical nucleates the monomer, then the mechanism is known as “radical polymerization.”10
Modeling of Polymerization Processes
Published in E. Robert Becker, Carmo J. Pereira, Computer-Aided Design of Catalysts, 2020
Free-radical polymerization is widely used in the polymer industry because many vinyl monomers (e.g., styrene, vinyl chloride, methyl methacrylate, etc.) are readily polymerized by free-radical mechanisms and they are available in large quantities from the petrochemical industry. Moreover, free-radical polymerization is relatively less sensitive to impurities than ionic polymerizations and thus is favorable for industrial polymerization.
Liquid crystal elastomers: an introduction and review of emerging technologies
Published in Liquid Crystals Reviews, 2018
Sabina W. Ula, Nicholas A. Traugutt, Ross H. Volpe, Ravi R. Patel, Kai Yu, Christopher M. Yakacki
The synthesis of side-chain elastomers can be classified into two main categories: radical polymerization and hydrosilylation polymerization. Radical polymerization involves using an initiator to start the polymerization process (Figure 5) [4,6,45]. The initiator, which is often either a photo-initiator or thermal initiator, is combined with the mesogenic, spacer, and crosslinking monomers. When light (in the case of a photo-initiator) or heat (in the case of a thermal initiator) is introduced to the system, the initiator breaks apart, resulting in a free electron (i.e. a radical). This radical then attacks an unsaturated C=C double bond to disrupt an electron pair and form a new bond with one of the electrons. The remaining electron then serves as the radical for a new iteration of the reaction with an adjacent C=C double bond. This pattern of bond breaking and reforming propagates throughout the material until it is fully polymerized. The advantage of this type of reaction is that it is fast, simple, and can be carried out with a variety of starting materials, although acrylates and methacrylates are the most common ones [12,31]. The reaction can also be done at room temperature, which bypasses the need for heating or cooling. Radical polymerization is primarily controlled through the initiation of the free radical – once that has been triggered, polymerization occurs without any further action. However, this can also be a disadvantage, as the reaction generally cannot be stopped once it has started. This means that mesogen alignment must be established beforehand and maintained during polymerization, which can be difficult to achieve in bulk samples. Additionally, radical polymerization is sensitive to oxygen, meaning it can be disrupted by oxygen molecules; therefore, the environment in which the reaction occurs needs to be regulated to prevent any unwanted side reactions. Another disadvantage of the reaction is that it can only result in C–C backbones, thereby limiting the types of LCEs that can be synthesized to side-chain and end-on configurations, which have lower chain anisotropy than main-chain LCEs. This means that the transition from monodomain to isotropic states in these LCEs produces less actuation than it does in main-chain LCEs [46]. This may make side-chain LCEs less than ideal for specific applications as actuators and artificial muscles. Another potential drawback of this reaction is that it is not selective – that is, every molecule is free to react with every other molecule, which can result in heterogeneous network formation. Additionally, radical polymerization has a large reaction enthalpy, which restricts its use to the creation of thin films, in which suitable heat transfer can be ensured. This problem can be overcome by carrying out the reaction in a solution, but requires an extra de-swelling step after polymerization [31]. Finally, carrying out the polymerization reaction below Tg can be problematic, as vitrification results in decreased chain mobility, which in turn decelerates the crosslinking reaction [47].