SYNOPSlSA number of norbornene-functionalized resins were prepared and their photoinitiated crosslinking reaction with a multifunctional thiol was studied. Access to these novel resins can be gained by several synthetic routes. The most general of these routes is the [ 47r + 2x1 cycloaddition reaction (the Diels-Alder reaction) of the corresponding multifunctional acrylate ester with cyclopentadiene ( CPD) monomer. The photocrosslinking reaction of these resins with multifunctional thiols such as pentaerythritol tetramercaptopropionate (PETMP) is quite rapid and sensitive to low UV dose as we11 as dose rate. FT-IR cure studies also indicated that the cure rate slows down dramatically as the conversion approaches the calculated gel points for the systems under study. Mechanical properties of cured thin films of these materials exhibited a wide range of tensile and dynamic mechanical properties, which depended on the structure of starting materials used in the preparation of the norbornene resin and the crosslinking thiol. I NTRODUCTIO NThe photoinitiated polymerization reaction between a multifunctional olefin and a multifunctional thiol (thiol-ene reaction) is a useful and versatile method for the preparation of ultraviolet (UV) curable adhesives, sealants, and coatings that find wide application in industry.' Because of the utility of this reaction, it has received a great deal of study. It is well established that the thiol-ene polymerization is a step growth process that propagates by chain transfer and that the conversion kinetics can be predicted by the Carothers e q u a t i~n .~.~ Of the many unique features of this type of photopolymerization mechanism, the feature that has set this process apart from other UV-initiated processes is the observation that thiol-ene polymerizations are not inhibited by ambient oxygen or m o i s t~r e .~.~ In fact, dissolved oxygen is actually incorporated into the polymer by a complex series of steps that have been described by Kharasch and co-workers6 and more recently by Szmant and his co-w~rkers.~-l~ This process is illustrated below.

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A number of novel organic norbornene and norbornene siloxane polymer precursors have been synthesized as part of an on-going research project to produce a non-acrylate, UV-curable adhesive system. These precursors (monomers or oligomers) are di-, tri-and tetra-functional. The crosslinking agent is a multifunctional thiol.The organic norbornene systems, formulated to stoichiometry, have been characterized using thermal, static and dynamic mechanical analysis. We have found that norbornene ester systems have a range of physical properties, with tensile moduli ranging from 820-2300 MPa (1 18-350 kpsi), tensile strengths of 17-61 MPa (2.5-8.8 kpsi) and elongations of 4-100%. Glass transition temperatures range from 30-71°C for samples cured at room temperature with a dose of 620 mJ/cm2. Norbornene siloxanes crosslinked with thiol siloxanes have been formulated with increasing levels of a high surface area reinforcing agent. Tensile properties of filled films show that filler loading to 30 wt% significantly increases peak stress and elongation to break above that of 25 wt% filler. The addition of filler did not appear to increase the T, of the films in this series monotonically. Increased levels of filler were found to produce films with a broadened cold crystallization range and an increased melting temperature.

SYNOPSISThe spectroscopic properties and photopolymerization activities of a mono-and tetraperester derivative of benzophenone are examined and compared with those of benzophenone. Their photopolymerization activity in methyl methacrylate ( MMA ) and a n ethoxylated bis-phenol-A diacrylate have been studied using a combination of gel permeation chromatography (GPC) , and real time Fourier transform infrared spectroscopy (RTFTIR) as well as the commercial pendulum hardness test. Using GPC analysis on the poly (methyl methacrylate) samples the tetra-t-butyl perester derivative of benzophenone is found to give a higher weight average ( M u ) and number average ( M , ) molecular weight polymer than that produced using the mono perester. For the two peresters of benzophenone photopolymerization efficiency using real time FTIR increases with increasing initiator concentration, and no self-termination is observed up to 0.75% w/w concentration with the mono perester derivative being the more efficient initiator. Similar results were obtained using the commercial pendulum hardness tester with a triacrylate/epoxyurethane acrylate resin with benzophenone exhibiting the lowest activity. Photopolymerization activities of the initiators correlate well with their spectroscopic properties. The phosphorescence quantum yield is higher for the monoperester than the tetraperester derivative and is consistent with a shorter lifetime and lower photolysis quantum yields in 2-propanol. Compared with benzophenone, phosphorescence analysis indicates that the perester groups impart a degree of charge-transfer content to the molecule which is consistent with the degree of substitution. Ketyl radical formation on microsecond flash photolysis follows the order benzophenone > mono-> tetraperester derivative and is consistent with the phosphorescence quantum yields. On nanosecond laser flash photolysis in nitrogen-saturated acetonitrile, triplettriplet absorption is extremely weak for both the perester derivatives, being stronger for the monoperester. I NTRO DUCT10 NWe recently examined the photochemistry and photopolymerization activity of t-butylperester derivatives of fluorenone.'.2 These compounds were found to initiate photopolymerization through homolytic scission at the peroxy link to give aroyloxy and tbutyloxy radicals. Analysis of the polymer by spec- troscopic methods showed that the former species was responsible for initiating free radical polymerization. Detailed studies on the photopolymerization activity of compounds of this type have shown that their efficiency depends on the absorption characteristics of the associated chromophore, 3-5 and this was confirmed in our previous study.' Using secondorder derivative UV absorption spectroscopy, poly ( methyl methacrylate) samples prepared using the t-butyl perester derivatives of fluorenone were found to contain residual photoinitiator fragments. Further studies using laser flash photolysis showed 1169

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