Recent studies in photoinitiated polymerization
Authors: Aliwi S. M. , Bamford C. H. , Mullik S. U. 
Published: June 2010

Two types of photoinitiating systems are considered in this paper: (i) Group VII transition metal carbonyls in the presence of fluoro‐olefins and (ii) vanadium chelates and their derivatives.

(i) The photoinitiating activities of Mn2(CO)10 and Re2(CO)10 in association with CF2 :CFCl, CF2 :CHF, CF2 :CH2, or CHF:CH2 have been studied. Methyl methacrylate was the monomer and the incident wavelengths were 436 nm and 365 nm for the manganese and Re2 (CO)10 at relatively low [T]. The kinetics of photoinitiation have been investigated. With rhenium derivatives, respectively. Of the above fluoro‐olefins, only CF2 :CFCl forms a photoinitiating system with Mn2(CO)10 while all are active with Re2(CO)10.

In all the active systems the rate of polymerization ω increases with increasing fluoro‐olefin (T) concentration but finally reaches a plateau value; plateau conditions are achieved with Mn2(CO)10/CF2:CFCl (the system studied in most detail) the “sharpness” of the ω ‐ [CF2:CFCl] relation increases as the incident intensity decreases and the rate of consumption of manganese carbonyl is equal to the rate of initiation over the whole range of [CF2 :CFCl]. Quantum yields of photoinitiation have been determined for the various systems; they range from unity for Re2 (CO)10/CF2:CHF to 0.35 for Re2 (CO)10/CF2:CH2.

Polymers prepared with the aid of these photoinitiators carry terminal groups in which the metal is covalently attached to the polymer chain through a unit derived from the fluoro‐olefin; for example with Mn2(CO)10/CF2:CFCl the terminal groups are (CO)5MnCF2CFCl. The initiating radicals are therefore formed by addition of an activated metal carbonyl species to the fluoro‐olefin and not by halogen abstraction as in systems such as Mn2(CO)10/CCl4, Mn2(CO)10/CBr4.

On the basis of these and other results a mechanism is proposed in which photolysis of manganese carbonyl in methyl methacrylate solution leads reversibly to dissimilar fragments, only one of which forms an initiating radical with the fluoro‐olefin.

The activity of fluoro‐olefins in these reactions (compared to the inertness of common vinyl monomers) is attributed to their ability to form relatively strong transition metal‐carbon bonds.

(ii) Chloro‐oxobis‐(2,4‐pentanedionato) vanadium (V), VO(acac)2Cl, irradiated with light of Λ = 365 nm, initiates free‐radical polymerization of methyl methacrylate with a quantum yield of 0.02. The primary act is scission from the molecule of a chlorine atom, which is the initiating species. In the presence of an electron‐donor D (e.g., dimethylsulfoxide) ionic complexes a− are formed and photoiniate with much higher quantum yields (29‐fold in the case of dimethylsulfoxide). We believe that in these systems the primary act is electron‐transfer leading to D+ and VO(acac)2; subsequent reactions of D± then generate initiating radical species.

Photolysis of the chelate VO(Q), OMe, in which Q iepresents the 8‐quinolyloxo ligand, generate initiating radicals which comprise both CH, O and CH, OH. These results provide to route to photosensitive polymers with side‐chains carrying vanadium chelates which may be used to synthesize graft copolymers and polymer networks. Preliminary experiments have indicated the feasibility of these processes.