IONIC POLYMERIZATION
PHOTOPOLYMERIZATION
CONTROLLED RADICAL POLYMERIZATION METHODS
BLOCK and GRAFT COPOLYMERIZATION
NANOCOMPOSITES
SYNTHESIS and MODIFICATION of POLYMERS
PHOTOPOLYMERIZATION
A. Photoinitiated Free Radical Polymerization
When polymerizations are initiated by light and both the initiating species and the growing chain ends are radicals, we speak of radical photopolymerization. In far the most cases of photoinduced polymerization, initiators are used to generate radicals. One has to distinguish between two different types of photoinitiators.See animation of the photopolymerization of the different type monomers. Photopolymerization of monofunctional monomers, Photopolymerization of bifunctional monomers
Type I Photoinitiators: Unimolecular Photoinitiators. These substances undergo an homolytic bond cleavage upon absorption of light. Benzoin and its derivatives are the most widely used photoinitiators for radical polymerization of vinyl monomers
Type II Photoinitiators Bimolecular Photoinitiators. These photoiniating systems consist of a photoinitiator such as benzophenone or thioxanthone and a coinitiator such as alcohol or amine. Radicals are generated in a bimolecular process by the reduction of photoexcited aromatic carbonyl compound by hydrogen abstraction or electron transfer reactions
B. Photoinitiated Cationic Polymerization
Direct Acting Systems
Upon photolysis, these thermally and hygroscopically stable initiators undergo irreversible photofragmentation to produce cation radicals and Brønsted acids. Reactive species thus produced photochemically with onium salts initiate the cationic polymerization of suitable monomers as illustrated below.
Indirect Acting Systems
For practical applications, onium salts should absorb light appreciably at wavelengths longer than 350 nm where the commercially available medium and high-pressure mercury lamps emit much of their radiation. The indirect actions of these systems, which extend their spectral sensitivity to longer wavelengths, are shown in Figure 1.
Several indirect ways to expand wavelength range have been described [11]. All of these pathways involve (i) electron transfer reactions either with photoexcited sensitizer or free radicals (iii) and with the electron donor compounds in the excited charge transfer complexes (ii)
(i) Photosensitizer
Many aromatic hydrocarbons such as anthracene, phenothiazine, and perylene are able to sensitize the decomposition of onium salts via electron transfer. The irradiation of the sensitizer is followed by the formation of a complex between excited sensitizer molecules and ground state onium salt. In this complex, one electron is transferred from the sensitizer to the onium salt giving rise to the generation of sensitizer radical cation as a result of homolytic cleavage of the corresponding onium salt. The radical cations themselves initiate the polymerization of appropriate monomers or, alternatively, interact with hydrogen donor constituents of the polymerization mixture (such as solvent or monomer) resulting in the release of Brønsted acid.
(ii) Charge Transfer Complex
Pyridinium salts are capable of forming charge transfer (CT) complexes with electron rich donors such as methyl- and methoxy-substituted benzene. It was found that the CT complexes formed between pyridinium salts and aromatic electron donors act as photoinitiators for the cationic polymerization of cyclohexene oxide and 4-vinyl cyclohexene oxide. The mechanism illustrated in equations 19 and 20 for the initiation of the cationic polymerization has been suggested [22].
(iii) Free Radical Promoted Cationic Polymerization
Among the indirectly acting initiating systems, free radical promoted cationic polymerization is the most flexible route, since free radical photoinitiators with a wide range of absorption characteristics are available. Many photochemically formed radicals can be oxidized by onium salts. The cations thus generated are used as initiating species for cationic polymerization according to the following reactions.
CONTROLLED RADICAL POLYMERIZATION METHODS
Recent developments in controlled/living radical polymerization provided possibility to synthesize well-defined telechelic polymers with controlled functionality also with radical routes. As it will be shown below, all the three standard methods for controlled/living radical polymerization, namely, Atom Transfer Radical Polymerization (ATRP), Stable Free Radical Mediated Polymerization (SFRP), also called as Nitroxide Mediated Polymerization (NMP), and Reversible Addition-Fragmentation Chain Transfer Polymerization (RAFT) were used for the preparation of telechelic polymers.
(i) Nitroxide Mediated Polymerization
In NMP systems, stable free radicals, such as nitroxides, are used as reversible terminating agents to control the polymerization process. Dormant chains are generated by reversible deactivation of the growing chains through covalent bond formation. At high temperatures, the bond undergoes a homolytic cleavage to produce the active growing chain and the nitroxide radical. Activation is followed by a rapid deactivation, whereby a few monomer units are incorporated to the propagating chain. See animation of the NMP
(ii) Atom Transfer Radical Polymerization
Atom Transfer Radical Polymerization, pioneered by Matyjaszewski and Sawamoto, is based on a continuous and reversible halogen transfer (pseudo halogen) between a dormant propagating species, Pn, and a transition-metal (eg. Cu), complexed by a ligand (eg. bipy), in its lower oxidation state (Scheme 6). Halogen transfer is accompanied by a one-electron oxidation of the transition metal, whereby propagation takes place by the addition of monomers to the activated chains. Homogeneity of the polymeric chains is controlled by the fast and simultaneous initiation as well as rapid deactivation of the growing chains. Here, the rate of deactivation should be faster than the propagation rate for an effective control. Low concentration of the active centers, maintained by deactivation, suppresses termination and chain transfer reactions at later stages. See animation of the ATRP
Reverse ATRP differs from ATRP in its initiation process, where a conventional radical initiator, such as AIBN (2,2’-Azobisisobutyronitrile), is used. As shown below, in the initiation step, once generated, the initiating radicals or the propagating radicals, I. Or I-P., can abstract the halogen atom X from the oxidized transition-metal species, XMtn+1, to form the reduced transition-metal species, Mtn, and the dormant species, I-X or I-P1-X. In the subsequent steps, the transition-metal species, Mtn, promotes exactly the same ATRP process as normal ATRP where R-X/Mtn/Lx are used as the initiation system. Instead of first activation of a dormant species, R-X, with Mtn, as in the case of normal ATRP, reverse ATRP originates from the deactivation reaction between radicals, I. or I-P., and XMtn+1. See animation of the RATRP
(iii) Reversible Addition-Fragmentation Chain Transfer Polymerization
Reversible Addition-Fragmentation Chain Transfer Polymerization (RAFT), developed by Rizzardo and co-workers in the late 1990s, utilizes a chain-transfer- active thiocarbonylthio moiety for the exchange between active and dormant chains. The mechanism is illustrated in below.
The active species, such as the radicals stemming from the decomposition of the initiator and propagating radicals (Pn.), are transferred to the RAFT-agents, e.g. the thiocarbonylthio moiety. Meanwhile, an intermediate radical is formed, and undergoes a fast fragmentation reaction, giving a polymeric RAFT agent and a new radical. The radical reinitiates the polymerization. The equilibrium is established by successive chain transfer-fragmentation stages. Z and R groups in the thiocarbonylthio compound represent the activating group and homolytically leaving group, respectively. These, in turn, determine the rates of addition and fragmentation. As a matter of fact, the choice of RAFT agent for a specified monomer is rather significant and affects the degree of control. See animation of the RAFT
BLOCK and GRAFT COPOLYMERIZATION
Block and graft copolymers are the most demanded advanced materials because of their diverse copolymer structures. In most cases, the corresponding homopolymers do not form homogeneous phase. However, linear arrangement of the blocks by chemical bonds results in the realization of a stable structure with two phases separated. Each segment exerts its character or function to the bulk of the copolymers. This way various properties are improved or combined to give possibility of using block copolymers as compatabilizers, impact modifiers, surface modifiers, coating materials, antistatic agents and adhesives.
Block Copolymers
According to the most common methodology, monomer A is polymerized completely, after which, monomer B is introduced in the mixture and its polymerization proceeds upon initiation by the active site of the first block. This approach is referred to as sequential monomer addition in controlled/living polymerization methods. For obtaining a well-defined block copolymer, the living site of the first monomer must be effective in initiating the polymerization of the second monomer, that is, simultaneous initiation of all growing B chains must be ensured and the rate of the crossover reaction must be higher than the rate of propagation of monomer B.
Another route involves the use of a difunctional initiator and has been employed in the preparation of ABA symmetric triblock copolymers. In this methodology, a compound possessing two initiating sites is utilized in the formation of the middle block first, followed by the polymerization of the second monomer to synthesize the first and the third blocks. This allows the preparation of the ABA blocks in two, instead of three steps without fractionation or other purification steps. The difunctional initiator must be chosen so as to initiate the polymerization with the same rate from either direction.
Moreover, AB diblock or ABA triblock copolymers can be prepared by coupling two appropriately end-functionalized chains of A and B homopolymers or AB blocks, respectively. In the latter case, block B should initially contain half the molecular weight of the final desired block B. The efficiency of coupling will be high if the reaction is rapid and the living diblock copolymer is used in excess.
Graft Copolymers
Graft copolymers can be obtained with three general methods: (i) grafting-onto, in which side chains are preformed, and then attached to the backbone; (ii) grafting-from, in which the monomer is grafted from the backbone; and (iii) grafting-through, in which the macromonomers are copolymerized.
Grafting onto methods involve reaction of functional groups (Y) located at the chain ends of one kind of polymer with another functional groups (X) which distributed randomly on the main chain of the other polymer
Chemical initiation has also been used in grafting from method. In this case, a polymer backbone made that contains some thermally cleavable bonds such as azo and peroxide linkages (See below). If the polymers are heated in the presence of a second monomer, initiation takes places. In such systems, homopolymer formation is unavoidable because of the concomitant formation of low molar mass radicals.
Macromonomers are short polymer chains possessing a polymerizable group at the one termini. A great variety of methods involving living polymerization techniques, chain transfer reactions and end chain modifications, have been developed to synthesize such species.
NANOCOMPOSITES
In situ synthesis of poly(methylmethacrylate) (PMMA) nanocomposites by photopolymerization using organophilic montmorillonite (MMT) as the layered clay is reported. MMT clay was ion-exchanged with N-phenacyl, N,N-dimethylanilinium hexafluoro phosphate (PDA) which acts as both suitable intercalant and photo initiator. These modified clays were then dispersed in methylmethacrylate (MMA) monomer in different loading degrees to carry out the in situ photopolymerization.
