Gérard Hild

Synthesis of ring-shaped macromolecules

Abstract Ring-shaped macromolecules have been synthesized by the reaction of a bifunctional “living” polystyrene with a stoichiometric amount of dibromo- p -xylene, at very low concentration. The reaction is carried out step by step, leading both to the expected cyclic polymer and to a “polycondensate”. Fractionatal precipitation and one-step fractionation lead to well defined macrocycles of known size.

Cyclic macromolecules. Synthesis and characterization of ring-shaped polystyrenes

Abstract A method has been developed to synthesize cyclic polystyrene molecules, by reacting bifunctional “living” polystyrene with α,α′ dibromo- p -xylene. The ability of the method to yield cyclic polymers over a molecular weight range from 7000 to 250,000 has been thoroughly investigated and discussed. Characterization of the cyclic macromolecules with respect to linear homologues of the same molecular weight has been performed by means of viscometry. The effect of cyclization tends to decrease as the molecular weight increases.

Hydroxyalkyl methacrylates: Kinetic investigations of radical polymerizations of pure 2-hydroxyethyl methacrylate and 2, 3-dihydroxypropyl methacrylate and the radical copolymerization of their mixtures

Abstract Hydroxyalkyl methacrylates such as 2-hydroxyethyl methacrylate (HEMA) and 2, 3-dihydroxypropyl methacrylate (DHPM) have been prepared. An efficient method has been developed yielding a quantitative purification in order to eliminate any trace of crosslinking agent in these monomers. Kinetic investigations of the radical polymerization and of the radical copolymerization of their mixtures have been performed by measuring, at various times, the monomer consumptions, using gas-liquid chromatography (g.l.c.). It has been established that the radical copolymerization of DHPM-HEMA couple works efficiently without excessive fluctuations in the composition of the formed chains. The corresponding radical copolymerization ratios have been precisely determined and the obtained results to demonstrate that DHPM-HEMA system leads to an ideal copolymerization.

Hydroxyalkyl methacrylates: hydrogel formation based on the radical copolymerization of 2-hydroxyethylmethacrylate and 2,3-dihydroxypropylmethacrylate

Abstract The synthesis of hydrogels by radical copolymerization of mixtures of 2,3-dihydroxypropylmethacrylate (DHPM) and 2-hydroxyethylmethacrylate (HEMA) has been performed using ethylene dimethacrylate (DME) as crosslinking agent, in the presence of 2,2′-azobisisobutyronitrile (AIBN) or of a redox system as initiator. The hydrophilicity of the obtained materials increases as the proportion of 2,3-dihydroxypropylmethacrylate increases. The maximum swelling degree of poly (HEMA) hydrogels is limited by thermodynamic effects whereas the equilibrium swelling degree of materials with high DHPM unit content is structure dependent. The gels obtained have been characterized by their equilibrium swelling degrees in water or in organic solvents and by the determination of their elastic modulus by uniaxial compression measurements. All the materials are of great interest as potential biomaterials especially for soft contact lens manufacture.

Investigation of the radical copolymerization of styrene and divinylbenzene

Abstract A kinetic investigation of the early steps of the radical copolymerization of styrene and divinylbenzene (DVB) has been performed in the presence of a transfer agent, meant to delay the onset of the gel point. It is confirmed that the consumption of DVB is faster than that of styrene. However, in the early stages of the reaction, pendant double bonds are formed to a large extent as a result of DVB incorporation. In later stages, the probability for these pendant unsaturations to be involved in intermolecular linking increases as the concentration of the monomers decreases. This process is responsible for gelation and it continues after the gel point, to build more and more crosslinks. This effect can be followed by measurements of the equilibrium swelling degree, which decreases and reaches a limiting value, depending upon the DVB content of the monomer mixture. The results prompt us to reconsider some of the commonly accepted ideas about the structure of crosslinked polymers synthesized by radical copolymerization.

Mechanism of network formation by radical copolymerization

Radical copolymerization of two monomers, one of them being bi— functional, has been widely used for the synthesis of polymer networks. However, the actual structure of such networks is still controversial, and it has never been related accurately with the copolymerization parameters. It has been assumed that large fluctuations of the length of the elastic chain originate, at least in part, from the fact that the consumption of the two monomers follow different rate laws. It occured to us that the decisive factor in that respect is not the consumption of the bifunctional monomer, but it is the ability of the pendant double bonds to undergo reaction with growing radicals and to form the actual branch points. This is clearly evidenced by recent works carried out on the system styrene—divinylbenzene in which it was shown that, once the gel point has been reached, cross— linking goes on; more links between individual chains are formed, whereby the crosslink density increases and the average length of the elastically effective network chain decreases. A kinetic investigation of the radical copolymerization was carried out on several systems of this type: styrene—divinylbenzene, styrene—diisopropenyl— benzene, styrene—ethylene dimethacrylate, methylmethacrylate—ethylene di— methacrylate. In some cases, a chain transfer agent was added to the system to delay the occurence of network formation. From the conversion curves of each individual monomer — that were obtained from vapor phase chromatography taken at regular time intervals — the instantaneous composition of the co— polymers formed were determined. Thus, the values of the radical reactivity ratios of the two monomers involved could be calculated. It was confirmed that the bifunctional monomer is more reactive and therefore more readily consumed than the monofunctional monomer. In addition, it was shown that the rate of polymerization of styrene is enhanced noticeably in the presence of a small amount of divinylbenzene. Also it was established that the rate of crosslinking does not parallel the rate of consumption of the divinyl monomer. To give account of all results it has to be assumed that in the early stages of the reaction most of the bifunctional monomer reacted gives yield to pendant double bonds (and, perhaps, to a much lesser extent, to cyclization). The reactivity of these pendant unsaturations is far lower than that of the monomers involved, consequently the pendant double bonds are still numerous, once the gel point has been reached. As these double bonds are slowly consumed in the later stages of the reaction (when not much monomer is left over), the network becomes tighter, its swelling ratio decreases and its modulus increases. These findings throw new light into the problem of the structural homogeneity of network synthesized by radical copolymerization. It cannot be stated anymore that the crosslink density is bound to be inhomogeneous in such networks, nor that once the bifunctional monomer is consumed pendant chains are formed solely. In fact it appears that these species are far more homogeneous in structure that it had been assumed previously, and the crosslink density can be related with the composition of the monomer mixture, provided the reaction was pursued until all double bonds have been consumed. INTRODUCTION Polymer networks are commonly obtained by radical copolymerization of two monomers, one of them being bifunctional (Ref. 1—3). It is a method of great industrial interest, which was applied to a large number of systems. However, even in the absence of syneresis (solvent expulsion during the

Diblock copolymers, triblock copolymers andmodel networks synthesized by sequential anionic polymerization of styrene and 2,3-epoxypropyl methacrylate

Abstract Diblock and triblock copolymers were synthesized, under standard conditions, by sequential anionicpolymerization of styrene and 2,3-epoxypropyl methacrylate or glycidyl methacrylate (GMA). In both cases, suitable initiators were selected. The ‘living’ carbanions originating from the polymerization of styrene are able to attack GMA. The reverse reaction (attack of styrene monomer by poly (glycidyl methacrylate) anions) cannot occur. Consequently, styrene was polymerized first, yielding well defined poly (styrene) ‘precursors’. The high reactivity of poly (styryl) carbanions was reduced by the addition of 1,1-diphenylethylene. The polymer thus, fitted at chain end(s) with diphenylmethyl anions, served as macroinitiator for the subsequent polymerization of GMA. The synthesis of poly (styrene)-b-poly (GMA) diblock copolymers were carried out using sec-butyllithium as the monofunctional initiator, in the presence of lithium chloride. Potassium-naphthylide was chosen as the bifunctional initiator whenever a poly (GMA)-b-poly (styrene)-b-poly (GMA) triblock copolymer is desired. The poly (styrene) ‘precursors’ and the block copolymers were characterized by size exclusion chromatography, proton nuclear magnetic resonance and analytical titration of the oxirane functions. These methods allowed us to determine the weight- and number-average molar mass of each block, the molar mass distribution and the copolymer composition. The above ‘living’ bifunctional species were used as polymeric ‘precursors’, leading to the formation of model networks by an endlinking process, upon addition of a bisunsaturated monomer, such as ethylene dimethacrylate.

New crosslinking processes

Synthesis of model-networks characterized by the quasi-constant length of the linear chain elements between two successive branch points has been carried out by anionic block copolymerization of two monomers, one being bifunctional. The influence of various parameters (concentration, temperature, number of molecules of bifunctional monomer added per active chain end) on the behaviour of the gels is discussed. This method can be applied to several systems.Another method was successfully used to synthesize networks: reaction of α, ω-difunctional linear polymer chains with tri- or tetrafunctional molecules, in stoichiometric amount was shown to lead to gels in which both the length of the linear chain elements and the functionality of the branch points are controlled. Reactions of terminal carbon metal bonds with electrophilic groupings of various kinds were used, as well as reaction of silane end groups with allylic double bonds. The gels obtained were characterized by their swelling behaviour, in relation to the molecular weight of the chain elements and the functionality of the branch points.