I will prepare some materials 'till or by' the end of this week.
Is there any difference in the meaning? Let us imagine today is Monday. Will you have different expectations because of this?
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Till a certain time means the verb is continuously happening until the end of that time period. For example, I will work till sun down means I will continue to work until sunset. The emphasis is on the subject, the person that does the verb.
By means a job or task will be finished a the end of the time period, the emphasis is on the task/verb. For example, The job will be done by sun down.
If you say "I will be preparing some materials till the last day of week." it means you will be working all week with little or no breaks.
If you say "The materials will be prepared by the end of this week.", it means those items will be ready by the weekend.