Dependence of molecular weight of polystyrene on initiator

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II

Paul Ander ~niversi~ Soulh Orange, New Jersey 07079

seton

Dependence of Molecular Weight of Polystyrene on Initiator Concentration An introductory physical chemistry experiment

M u c h hnn been written about plncing grcntcrstrr~s1111 the fundnmentnls of polymer chemistry in tho unclrrgr:iclo:~te chemistry currirulum. Slore nttention is bring givrn to this subject in the Inborntory portions of p11ysir:~lt ~ n dorg:mir rhemistry conrsrs. \A simple cxpcrinirnt which could introducr the stoclent to mncromolrrulrs is the eRwt of the eoncentrntion of n free rtulicnl initintor, sw11 RS h e n ~ o y lperoxide. on the molec111:ir weight of ludystyrene. T h e experiment clrscrihwl helow, which the nuthor l~crforninl IIR n stu,lent, hns hren s~~ccrssful with our undergrnduntc l~hysirnlcltemistn studrnts. I bulk polymrriantioo of styrene initintrcl by sm:~ll nmoonts of frw rnclicnl forming substnncrs (commonly though inrorrectly rrfcrrnl to ns r n t n l y r t ) rnn b r r e p m c n t n l by n simple mnhnnism (1-4) whirh lends to .k, . IN1 ~

*

1'1'

-

, k ; k , p ,l],,,

where is thc nvrrnge d e p w of polymerirntion of lhc polyntrr formml; k,. $, nnd k, nrr the spwilir rntr ronstnntsof t h r init intion, pmp:~gt~tiori. nnd trrn~inntion stelisof the postul:~trrlmechnnisn~;[ \ I ] is the monomer ro~~eentrntion; nnd [ I ] is t h r initintor ronrn~trntion. (Statr 1111 the nswrnptions used in tleriving q n . (I).) Iwi n a I W I I ~ W of pwtlrml dry iw n ~ t dwhile- wmriog ~ n f v t y~I$L